It is frequently possible to control the selectivity of dimerization vs. Read the latest chapters of Advances in Organometallic Chemistry at. Stone, Robert West. Advances in Organometallic Chemistry, Volume 15 by F. Herein we report the structure and Sn NMR of this compound. Borrow eBooks, audiobooks, and videos from thousands of public libraries worldwide.
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Groups 1 and 2 or any of the heavier elements in groups 13 through Kirillov; Marina V. Advances in Organometallic Chemistry, Volume 70, contains authoritative review articles of worldwide known researchers in the field of organometallic chemistry. Inorganic Chemistry,. COMC- III provides a clear and comprehensive overview of developments since and attempts to predict trends in the field over the next ten years. Advances in Organometallic Chemistry is an essential reference work for the academic and industrial chemist and will provide up- to- date material at the cutting edge of chemistry research.
A novel and relatively stable complex resulted from the reaction of tin II chloride with a dithioether diallyl ether ligand created as a side product from other research in our lab. Advances in Organometallic Chemistry, Volume Stone, Robert West eds. Series Editors: Beller, M. Organometallic chemistry is the study of organometallic compounds, chemical compounds.
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Tetrahedron Lett. Orgunomeiullics , 13, Organomeiullics , 14, Organometullics, , 15, Organomeiullics , 10, Thesis, University of Kaiserslautern, Thesis, Universitat Kaiserslautern, c Haas, J. Thesis, Universitat Kaiserslautern, d Haas, J. Diplomarbeit, Universitat Bonn, Synthesis , Thesis, University of Miinster, Organornetallics , 15, Thesis, Universitat Kaiserslautern Thesis, Universitat Kaiserslautern, Phosphorus, Sulfur, Silicon , 77, 1.
Thesis, Universitat Stuttgart, Syntheses of Rings and Cages 67 79 Julino, M. Nova Acta Leopoldina, Neue Folge , 59, Phosphorus, Sulfur, Silicon , 77, 5. United Kingdom. Structures, Spectroscopy, and Bonding. Ligand Substitution. Cationic Species. Radical Species. Trinuclear Clusters. Higher Nuclearity Complexes. This review will present the chemistry of polynuclear cobalt-alkyne complexes, with emphasis on the more recent developments. Synthesis Although many metal-alkyne complexes are known, probably the most thoroughly studied are those in which the alkyne is coordinated with a dicobalt unit.
When this area was reviewed in , more than examples 69 Copyright 0 by Academic Press. All rights of reproduction in any form resewed. WENT were known, and many hundreds of new complexes have been described since then.
Advances in Organometallic Chemistry, Volume 20
This has given rise to applications in protecting the alkyne functionality and allows the release of products after transformation at a remote functional group or a dicobalt-mediated transformation see Section VI. Other preparative routes for [Co, p-alkyne CO ,] complexes have been described,' but are of little general utility, with the exception of a recent report of an attractive alternative that avoids the use of octacarbonyldicobalt.
Reduction of CoBr2 in anhydrous THF with zinc dust in the presence of alkynes while CO is bubbled through the reaction mixture affords the corresponding alkyne complexes. Attempts to isolate this complex by removal of the solvent under vacuum resulted in formation of the unsaturated air-sensitive dinuclear compound [Co2 p-Me3SiC2SiMe3 Cpz] 3 containing a cobalt-cobalt double bond. Although heterobinuclear alkyne complexes have been less intensively studied, a number of routes involving cobalt as one of the metals have been developed.
Compounds with two different metals are expected to have reactivity patterns different from those of the respective homonuclear species and also have applications in the preparation of chiral complexes, In a manner analogous to Eq. Structures, Spectroscopy, and Bonding The crystal structures of in excess of 70 hexacarbonylalkyne-dicobalt complexes have been determined by X-ray diffraction. The carbon-carbon bond of the alkyne is in a perpendicular orientation relative to the cobalt-cobalt bond, as opposed to the parallel orientation which is also observed in dinuclear complexes with bridging alkyne ligands.
The coordination around the cobalt atoms is distorted octahedral and the two tricarbonylcobalt moieties are eclipsed. There appears to be little correlation between cobalt-cobalt and carbon-carbon distances, as illustrated in Fig. The average angle at the alkyne carbon atoms is These data are plotted in Fig. The average cobalt-carbon distance is 1. There is some correlation between cobalt-carbon distance and the carbon-carbon-substituent angle, as illustrated in Fig. WENT 2. Cobalt-cobalt bond length alkyne CO ,] complexes. Figure 5 contains illustrations of these orbitals obtained using the CACAO program Hoffmann found that the perpendicular orientation is preferred over the parallel orientation.
The cobalt-carbonyl distances average 1. The exact nature of the product depends on the phosphine and the reaction conditions. Thermal ligand substitution with dppm or dppe affords the phosphine-bridged isomers, whereas electron transfer catalysis gives the chelated or bridged complexes. Products and yields were dependent on method of initiation, solvent, temperature, and ligand. The thermal reaction of 14 with dppm in hexane affords the dppm-bridged complex, Even at K 17 rapidly converts to 16 in all solvents.
Better yields of 19 are obtained in refluxing hexane, and a selective conversion to 19 is achieved by a cathode-induced reaction at The structure of 19 was determined by X-ray crystallography and confirms the chelating nature of the dppe coordination Fig. The initial product of the BPK-catalyzed reaction of 14 with ttas is the labile derivative 22,which can be converted into the thermally stable, fully ligated product 23 Scheme 8.
Compound 23 is the only product obtained from a thermal reaction in refluxing toluene, but at room temperature in THF traces of 21 which convert to 23,presumably via 22,are also obtained. The and. Substitution of hexacarbonylalkyne-dicobalt complexes can also be activated at room temperature using oxidative decarbonylation with Me3N0. Isomerization studies suggest consecutive reaction pathways in which the initially chelating isomer is an intermediate in the formation of the bridging isomer. The effects of the one-electron oxidation are dramatic. The cobalt-cobalt bond length is reduced by almost 0.
The observed value of 12" for a one-electron oxidation i. Also, the singly occupied molecular orbital of the cation would be expected assuming a MO pattern similar to that of the hexacarbonyl derivatives to be cobaltcobalt n--antibonding in character see Fig. Substitution reactions to afford chiral complexes have been of particular interest because they offer the possibility of performing asymmetric reactions and catalysis.
If the alkyne has a chiral center, then diastereomers will exist. The basic substitution reactions are described in this section, and their application to organic synthesis is amplified in Section VI. Significantly, the diastereomeric mixtures can be separated chromatographically and exhibit considerable configurational stability. This suggests that phosphine dissociation and readdition may account for the isomerization.
Crossover experiments support this hypothesis, but intramolecular isomerization such as an alkyne twist [Eq. However, with less reactive alkenes, which necessitate longer reaction times, poorer results are obtained because of the Reactions of Polynuclear Cobalt-Alkyne Complexes 91 easy loss of phosphine and hence interconversion of the two diastereomeric forms of starting material. D'Agostino et a l l 8 have shown that bornyl and menthyl complexes such as 5 react with phosphines with poor enantioselectivity. Also discouraging is the reaction of homochiral hexacarbonyl cholesteryl propargyl ether dicobalt with P OMe 3, which proceeds without stereoselectivity.
CF3 C0 6]with 2,2'-azobis 2methylpropionitrile. This transformation is readily reversed by treatment with CO, indicating that the Co-S bond is relatively weak. Thioether coordinated complexes have been postulated as intermediates in Pauson-Khand reactions of sulfur-substituted substrates see Section w. Cationic Species 1.
Synthesis of Monocationic Species The chemistry and synthetic utility of cobalt complexed propargyl cations have been demonstrated by Nicholas in an impressive series of papers, and the area was reviewed in Two general routes for the synthesis of dicobalt-propargylium complexes have been developed. The most commonly used method is the treatment of an alkynic ether or alkynic alcohol-hexacarbonyldicobalt complex with a Lewis or Bronsted acid [Eq.
WENT The reaction is typically performed in diethyl ether, and the dication is precipitated as a red-brown solid or oil. The greater reactivity and ready preparation of the dicobalt systems have led to their extensive use in derivatization of the coordinated alkyne, whereas the greater stability of Group 6-containing compounds has facilitated their more detailed structural and spectroscopic characterization. The molecular structure of [MoCo p. WENT have been performed by Schrieber, who established the existence of two fluxional processes, as depicted in Scheme The two processes are i a low-energy ca.
The transition state for the enantiomerization may resemble the upright structure 26,which is capable of maintaining partial delocalization of the carbon p-orbital into the hybrid d-orbitals on the neighboring cobalt atoms. This latter proposal is in accord with the observation that the activation energy barrier for this second fluxional process is lower for tertiary than for secondary cations because the more stable tertiary carbocation has less need for anchimeric assistance from a cobalt center.
In the heterobimetallic molybdenum-cobalt stabilized propargylium ions, only one fluxional mechanism is involved in isomerization. The cobalt-carbon interaction is replaced by a direct interaction with the heteroatom of the nucleophile. A similar effect has been observed in a study of charge dispersal in iminium-substituted alkynes. In the structure of 28 the iminium character of the alkyne is preserved, and significant charge delocalization onto the dicobalt unit does not occur; see Eq.
Complexation of the triple bond circumvents isomerization to allenic products. For example, although alkynyl aldehydes undergo crossed aldol condensation with trimethylsilyl enol ethers with little stereoselectivity, their hexacarbonyldicobalt derivatives react with moderate to excellent syn diastereoselectivity? This stereoselective reaction has been successfully applied to the synthesis of p-lactam antibiotics. Monitoring some of the isomeric mixtures in acetone revealed a gradual change in the ratio of the isomers, approaching values of nearly one at equilibrium.
The isomerization is a relatively high-energy process with an activation energy of kcal mot'. However, the reactivity of this system was so reduced that reactions with carbon nucleophiles were unsuccessful. It has been shown that the diastereomer ratios for metal-stabilized cationic clusters can be directly correlated with the ionization process, and it is proposed that the elimination of water is anchimerically assisted by I H H H t I H H SCHEME WENT nated alkyne to reduce the ring strain during the cyclization step. The bending associated with alkyne complexation has been employed by a number of research groups to facilitate the cyclization of acyclic alkynes.
A bis 9-fluoreniumy1 ethyne ditetrafluoroborateethyne-hexacarbonyldicobalt complex 29 has been reported, but is unstable at room temperature. See Formula Alternatively, nucleophiles can be used to stabilize dication species in a manner similar to the monocationic species. Secondary cations undergo coupling only, whereas tertiary cations with methyl substitutents show only H-atom abstraction and electrophilic coupling. The intramolecular variant of this reaction is particularly useful for the efficient synthesis of medium-sized 1,5-diynesand has been applied to the synthesis of hexacarbonyldicobalt-complexed 1,5-cycloctadiynes31 and a cyclooct5-ene-1,5-diyne derivative 32 Scheme Reaction of 34, illustrated in Eq.
WENT Triangular trinuclear systems in which the alkyne bridges two of the three metals have been prepared by reaction of preformed clusters with alkynes, as in Eq. See Formulae 37 and Mixed-metal triangular alkyne clusters containing cobalt also favor the parallel, as opposed to the perpendicular, alkyne coordination mode. If the alkyne is regarded as a four-electron donor, this type of compound has 60 cluster valence electrons and is therefore formally electron deficient, as the predicted value for a butterfly geometry is 62 electrons.
However, an alternative view is to regard the alkyne as two 3-electron donor methylidyne units, in which case the electron count is 62 in accord with a closo-octahedral cluster. PPh2 C0 9]. The product contains two alkynes triply bridging Pt2Co faces, and the two platinum atoms in the hinge positions are joined by a short Pt-Pt bond Fig. W FIG. Both are butterfly clusters with a wingtip cobalt atom and a hinge rhodium atom, but they differ in the sites occupied by the molybdenum and ruthenium centers.
The intramolecular Pauson-Khand reaction is an effective way of preparing bi- and polycyclic systems, and the cyclization of heptenyne derivatives to give bicyclo[3. WENT in the use of this group to protect the acetylenic triple bond. The dppm imparts the extra stability necessary for the dicobalt unit to withstand treatment with [Bu4N]'F- to remove the Si 'Pr 3 groups.
It is possible to draw analogies between alkynes coordinated to molecular species and those to surfaces. The chemistry of hexacarbonylalkyne-dicobalt complexes is the most developed of that of the various systems discussed in this review and is finding many applications in organic synthesis. Considerable progress has been made with chiral systems involving mixed-metal cores or asymmetric phosphines, but there are many problems still to be addressed.
The detection of radical intermediates has led to coupling reactions applied to the high-yield synthesis of cyclic enediynes. Coupling of several dinuclear radical species offers the possibility of organometallic polymer formation. Continuing expansion of the chemistry of polynuclear cobalt-alkyne complexes is expected, especially in the areas of reactivity and synthetic applications, in the coming years. Denisov, V. USSR ,2, Borman, S. News , Aug 28, Magnus, P. Organornetallic Reagents in Organic Synthesis , 1. Seyferth, D. Dellaca, R.
Unpublished results, Tetrahedron, , 50, Organornetallics , 5 , Acta , 88, L5. Organometallics ,5, Organometallics ,9, Acta Crystallogr. Organometallics ,3, A , Organometallics , 1, New J. Organometallics ,3, and references therein. Organometallics , 14, Organometallics Acta , , Manning, A. Inorg, Chim. Organometallics , 6, Organomerallics , 3, Organometallics , 2, Organornetallics , 13, Organometallics , 8, A,; Caple, R. Organometallics , 10, Organometallics , 11, A,; El Amouri, H.
A,; Gruselle, M. Organometallics , 4, Organometallics ,4, Organornetallics , 4, Organomerallics , 10, Tetrahedron , 41, Organic Reactions , 40, 1. Jaouen, G. Savignac, M. Bioconjugnte Chem. Rubin, Y. Diederich, F. Nature ,, Chemical Design Automation News , 8, 1 and Catalysis of Alkene Epoxidation. Mechanisms for 0-Atom Transfer. History of the Mechanistic Problem. Migration to an 0 x 0 Ligand. Cycloreversion of Rhenium Diolates. Several books have summarized the state of the art. The Group 7 metals technetium and rhenium have not been applied to the problem of oxidation chemistry to the level of their Group 6 and Group 8 counterparts.
This is understandable in light of their relative scarcity. Technetium is a synthetic element, recovered as a fission by-product from uranium. Rhenium is not plagued by either issue, yet it is still a relatively rare element, present at only an estimated 0. It is Copyright D by Acsdemlc Press. All rlghts of reproduction in any form reserved. Also, as this chemistry has developed it has become clear that there may be unique applications of these two metals, particularly for rhenium.
This review will outline the new mechanistic understanding of organic oxidation chemistry that has been generated over the past dozen years by using highvalent metal 0x0 compounds of technetium and rhenium. In general, the question of mechanism in this oxidation chemistry centers around several issues. First, what chemical species make up the catalytic cycle? Second, what is the nature of the interaction between the catalytic metal complex es and the substrate?
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Specifically,is there a metal-substrate bond that forms at some point, or are all C - 0 bond formations occurring remote from the metal? Finally, what is the timing of C-0 bond formation relative to other bonding reorganization? Separate but related issues include the sequence of reactions e.
The compounds from whence it arose were at that time rare examples of stable alkyl metal 0x0 systems. An important practical consideration is the choice of the anhydride; trifluoroacetic anhydride is quite readily available, but the byproduct trimethyltin trifluoroacetate is approximately as volatile as MTO, and the usual purification sublimation can be quite difficult. However, use of perfluoroglutaric anhydride results in a relatively nonvolatile tin perfluoroglutarate, allowing for a straightforward isolation of MTO.
MTO is an off-white crystalline solid which is stable to the ambient atmosphere. It is soluble in common organic solvents and in water, although it decomposes in water at high pH to form methane" see later discussion. It is easily sublimed and should be stored in a sealed container, It is reported to have a high photoreactivity in aqueous solutions.
Generally, nitrogen bases coordinate trans to the methyl group. Bidentate ligands such as bipyridyl lead to the facMe N-N Re03 isomer of the octahedral complex. Table I summarizes some of the structural information from X-ray crystallographic and infrared studies. Catalysis of Alkene Epoxidation Although the initial report of MeTc03 included the observation of its oxidation of tetramethylethylene to form a diolate complex see Section , the first report of an organic oxidation mediated by MTO was not published until The viable substrates included simple olefins: terminal, cis, truns-1,2 disubstituted, trisubstituted, and tetrasubstituted; both r bonds in dienes were usually oxidized; ester, alcohol, aldehydes, and carbonyls all allowed selective oxidation of the alkene without overoxidation.
Conditions used were 0. Under the conditions reported, many of the epoxides opened to form the trans-vicinal diol. The chemistry of MTO with hydrogen peroxide has been the topic of a long series of papers from both the Herrmann group in Munich and the Espenson group at Iowa State University. Espenson first characterized by UV a pair of new complexes 3 and 4,17which appeared to be kinetically competent in the oxidation of substrates such as thiolatocobalt complexes.
Both new adducts were found to absorb at this wavelength, but a careful analysis of absorption vs stoichiometry and nonlinear least squares fitting allowed determination of both molar absorptivities and equilibrium constants for formation of each species. Kinetic analysis of the rate of formation of each species was consistent with the UV determination of binding constants.
The Herrmann group considered an extensive array of rhenium species as potential alternatives to or competitors with these species," but found in all cases that these were either completely inactive or converted to the peroxo complexes under the reaction conditions. Herrmann proposed a catalytic cycle in the paper16 that involved both a mono-HZOz adduct and a bis adduct, and suggested the latter was responsible for epoxidation. However, the Espenson group proposed the kinetic scheme outlined in Scheme 1 to sort out the equilibrium and kinetic behavior of MTO with Hz;this made no initial assumption about which species was active.
A,; Fischer, R. With a knowledge of the molar absorptivities and formation constants for each rhenium species, it became a routine matter to choose conditions under which one or the other peroxide complex predominated. The directly measured molar absorptivities seen for the latter coincide nicely with those obtained from the least-squares fit to Eq. The confirmation of structure for one of these required X-ray crystallography of a diglyme complex Fig. Solid-state structure of the diglyme complex of 4.
The remaining terminal 0x0 ligand and a water ligand are bound axially, with the water hydrogen bonded through each hydrogen to a molecule of diglyme. Several of the subsequent papers from the Espenson group further established the behavior of MTO and Hz02under different reaction conditions. Table I11 shows results reported under several conditions.
The Herrmann group originally claimed through l29l3 little or no oxygen atom transfer reactivity for the monoperoxide adduct 3. However, substantial work by Espenson and co-workers capitalized on their knowledge of the equilibria to independently characterize the reactivity of both 3 and 4 knowing the forward and reverse rates for interconversion of 1, 3, and 4 allows one to rule out the possibility that all reactivity arises from a single species. Evaluation of mechanistic rate constants k3 and k4 from kobs is rather complex; the full derivation is described in Ref.
The full rate expression for formation of product SO is given by Eq. It is also necessary to account for any uncatalyzed oxidation of substrate by [H]that is competitive under the conditions; normally, this is shown to be negligible. Also, to confirm the validity of Eq. In all cases the peroxo complexes oxidize the substrate more rapidly than does Hz02alone.
Mechanisms for 0-Atom Transfer The assignment of the -peroxide structure to these reactive intermediates immediately raised several issues. One could surmise that the behavior would be similar to that of the so-called MoOPH reagents, which are also efficient alkene oxidants. A similar labeling experiment for the rhenium system confirmed that peroxide was the 0 atom source. One A is a direct 0-atom transfer. However, when proposed it has generally been viewed as a concerted atom transfer reaction without the degree of zwitterionic character Fig.
A second mechanism B which may become important for alkenes involves a net insertion into one of the M - 0 single bonds, as proposed by MimounF5 The cyclic peroxo structure that then forms would have to undergo a fairly deep-seated rearrangement, but one can imagine that ionic or radical paths might lead as well to the same type of end product as provided by the direct atom transfer. This mechanism posits a more direct role for the metal, requiring as it does substrate binding prior to insertion.
GABLE alkene complexes is not possible for d o metals, and without this interaction it is debatable how strong the alkene-metal interaction will bez6 and whether it is strong enough to induce this type of mechanism. Further, given the similar kinetic behavior of 3 and 4, it seemed unlikely that the two species oxidize substrate via substantially different mechanisms though this cannot be rigorously excluded.
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Also, steric influences were seen to be negligible, arguing against substrate coordination to rhenium. Tri- otoly1 phosphine and tri- p-toly1 phosphine have similar electronic behaviors, but markedly different cone angles " vs ". This suggests the mechanism is remaining consistent for each substrate, and that the degree of positive charge buildup at phosphorus is fairly small at the transition state.
Similar methodology was used in the investigation of styrene oxidation by 3 and 4F9 Although many substrates were analyzed for both rate constants, k4 reaction of 4 received heavier focus. Steric effects were also small cis vs trans P-methylstyrene, Table IV. Activation parameters were measured for k4 in the oxidation of 4-methoxystyrene; between 3. These indicate a bimolecular rate-determining step with a significant bond-cleavage component, though significant C - 0 bond formation was inferred. The data in toto were interpreted as being entirely consistent with a concerted O-atom transfer from peroxide to alkene, and less consistent with the stepwise formation of an intermediate metallacycle.
A subsequent series of investigations of aliphatic alkene oxidation similarly argued for a concerted transfer of oxygen from peroxide to alkene. Electron-withdrawing groups C02R, CN significantly retarded the reaction. The comparison of methanol to 1: 1 aqueous MeCN an increase in polarity and binding ability of the solvent accelerates the reaction. Here again there is a close parallel between trends seen for the reactivity of 4 and the reactivity of dimethyldioxirane. The authors further explored the ramifications of the concerted oxygen transfer by considering two slightly different transition state structures Fig.
The first has a planar interaction of atoms with the oxygen undergoing transfer, whereas the second has tetrahedral oxygen. The former might be expected to show somewhat more differentiation between cis and trans disubstituted alkenes because of a steric clash with methyl and coordinated water ligands. The rate ratio of 1.
The structure at rhenium is similar to 4, with the exception that the methyl group is replaced by an ReRe link. However, it suffers extreme water sensitivity, decomposing irreversibly to perrhenic acid in organic solvents containing as little as 12 ppm H 2 0. Comparison of planar and spiro transition state geometries for oxidation of cisand rrans-alkenes. Solid-state structure of H4Re20L3. In nonaqueous solvents i. Slow base-catalyzed decomposition to MTO and 0 2 is observed for 4. This might entail nucleophilic attack by hydroxide on the methyl group, with subsequent or concomitant fragmentation of the v2-peroxide to form terminal 0 x 0 s.
However, the authors favored the latter mechanism, based on the observation that the observed rate constants suggested that kA or kMTOhad to be near the diffusion-controlled limit, coupled with the known higher nucleophilicity of the hydroperoxide vs hydroxide anions. Dioxygen was detected using an oxygen electrode; quantitative GC analysis showed less than 2. The rate of O2evolution varied inversely with [H], whereas the final yield of O2varied linearly with peroxide.
Thus the stoichiometry in Eq. Some insight into the divergence in the behaviors of the two peroxides was gained by noting the stability of the products formed in each instance, as opposed to the instability of putative products formed by imposing one decomposition path on the other peroxide. GABLE rhenium. In the case of reduced Re V systems, the atom transfer can be turned around to effect a deoxygenation of epoxides in two instances. The reaction rate law was half-order in dimer, consistent with this supposition. However, it was not possible to distinguish two possible mechanisms.
A concerted 0-atom transfer probably via an intermediate 0coordinated epoxide would be similar to the epoxidations via 3 and 4 shown earlier. Such an effect might be expected for the concerted process, given presumed competition for weak binding to the metal between THF and the epoxide. An alternative would be for the d2 metal center to effect an oxidative followed by fragmentation of the alkene from the Rhenium and Technetium 0 x 0 Complexes intermediate metallaoxetane see Section Radical intermediates were excluded on the basis of the observed stereospecificity.
A related process is seen for MTO systems. Espenson and co-workers observed that MTO induced a ring expansion of epoxides4'; condensation of diol with MTO a common reaction for rhenium oxides48 gives a dioxodiolate complex R Diolate 10 was apparently not formed by hydrolysis of the epoxide, but was in fact a result of rearrangement that occurs within the coordination sphere of the rhenium. In the presence of phosphine, 0x0 transfer to Ph3P presumably creates a Re V diolate that rapidly extrudes alkene.
The mechanism for the latter was not discussed.
This was later proposed33to proceed via epoxidation of the electron-rich arene ring, followed by ring opening and further oxidation of the hydroquinone. GABLE rationalized on the basis of an initial epoxidation, followed by ring opening to the phenol. Preference was for placement of the new C - 0 bond at the ortho position 2 : l ,although this can be perturbed somewhat with addition of water or methyl f-butyl ether. A kinetic analysis was performed; the rate law is reported to be first-order in substrate and in rhenium, though the expected dependence on [H]was not reported.
However, radicals are probably not involved, as retention of stereochemistry is observed for both cis- and trans decalin and for cis- and trans-1,2-dimethylcyclohexane. Typical conditions used fert-butanol as solvent, temperatures of 4O-6O0C, and reaction times of h. Larger ring ketones proceed more slowly, although selectivity remains high. Complex 4 was presumed to be the active species. Little related work on the technetium analog has been completed.
No chemistry with H z 0 2has been reported; it is expected to behave in a similar manner to MTO, unless the change in the metal-oxo bond strength perturbs the overall thermodynamics. Histow of the Mechanistic Problem For much of this century, osmium tetroxide and potassium permanganate were alone in their ability to bishydroxylate alkenes.
Such reactions are as Rhenium and Technetium 0 x 0 Complexes attractive as epoxidations: They create two new functional sites in the molecule. Likewise, it is possible for the reaction to create new stereocenters with total control of the relative configuration, and recent advances have revealed methods for absolute control of stereochemistry?
In , Sharpless proposed the intermediacy of a four-membered metallacycle in the oxidation of cyclododecene by Cr02C The following years saw many attempts either to find support for this mechanism or to discredit it. However, the stepwise mechanism proposed a second step requiring the migration of an alkyl group to an 0x0 ligand.
This has very little precedent. Prior to , there were but a handful of possible examples, and in none of these were the 0x0 alkyl and the subsequent alkoxide directly observed KEVIN P. GABLE see later discussion. It appeared that osmylation was either a unique example of a metallapericyclic process, or a stepwise process for which one step was largely without precedent. In recent years work on Tc and Re complexes has provided insight into the viability of the latter possibility. Migration to an 0 x 0 Ligand Specific examples of alkyl migration between a metal and an 0x0 ligand have been rare.
The absence of much thermodynamic data also made it difficult to predict whether these new complexes owed their stability to such a kinetic barrier, or whether the alkoxide was inherently less stable than the 0x0 in any particular case. The latter is almost certainly true for early transition metals, and by the late s, a small number of examples of the reverse process had been observed, particularly for tantalum.
Mayer and co-workers prepared the tris-acetylene hydroxide complex 11, as well as the 0x0 hydride Upon standing at room temperature for 5 days in benzene solution, the hydroxide spontaneously forms the 0x0 hydride with loss of one equivalent of acetylene, as shown in Eq.
A primary kinetic isotope effect of approximately 5 was seen for the deuteroxide at K. No rate inhibition was seen in the presence of added alkyne, leading to the conclusion that preequilibrium loss of alkyne did not occur and that the rate-determining step was in Rhenium and Technetium 0 x 0 Complexes fact rearrangement of Exchange of external alkyne into the starting hydroxide was shown to be slower than the rearrangement.
However, in contrast to the solution behavior of the hydroxide, this two-step reaction is not clean. Finally, the 0x0 anion rapidly abstracts iodine from p-iodotoluene, and this reagent has no effect on the rearrangement of Analogous alkoxy and aryloxyrhenium complexes do not show the same reactivity. However, Brown and Mayer discovered soon thereafter that a different rhenium complex could mediate C- 0 bond formation. The role of pyridine was unclear, because other tertiary amines provided no such improvement in yield.
Substituted benzenes showed a preference for para activation over meta; fluorobenzene also gave a significant amount of ortho activation. However, UV photolysis in the presence of pyridine slowly caused conversion of this compound to the rhenium II1 phenoxide The ethyl compound produces the ethoxide 16, although the yield is lower and accompanied by significant amounts of ethane and ethene. Addition of O2 or PhSH suppresses formation of the ethoxide. These data strongly suggest that the alkyl migrations proceed via a radical pathway involving C-Re homolysis.
It is conceivable that the phenyl migration is also a radical process occurring within a cage, though one would expect the more stable phenyl radical to show a higher propensity for cage escape. The authors suggest it seems more likely that participation of the aryl T system circumvents the homolytic cleavage of the C-Re bond in these compounds. Clues to the nature of the barrier to alkyl-to-oxo migration were seen in work by Brown and Mayer. Aryl complexes TpRe 0 Ar2 17 prepared by alkylation of the corresponding dihalide react selectively with I2 at short reaction times to form TpRe 0 ArI However, weak Lewis bases such as dimethyl sulfoxide displace the triflate in solvents such as CD2C12;the equilibrium constant was estimated to be 4.
This reactivity points to the aryldioxo Re VI1 complex 22 as the key intermediate; this can be observed by switching to pyridineN-oxide as the 0-atom donor. Phenoxy pyridine complex 23 is formed, presumably from carbon migration followed by trapping with pyridine. A phenoxy chloride is also present, rationalized by C1- abstraction from solvent. Finally, a catecholate complex 24 is formed, presumably from subsequent oxidation of the initial phenoxy rhenium V cation; the proportion of this species increases with increasing concentrations of the N-oxide.
Identities of products 23 and 24 were confirmed by independent synthesis. This thus represents the first system where alkyl-to-oxo migration can be directly observed. GABLE 23 24 The authors point out that there are a number of cases where such a transformation is thermodynamically feasible but apparently kinetically unfavorable. Their rationale centers on the electronic nature of the 0x0 ligand. An analogy can be drawn between alkyl-to-oxo migration and the migratory insertions observed for alkyl metal carbonyls. In this case, the electrophilicity of the 0x0 ligand in TpReOzPh is documented by its reaction with organic sulfides.
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In general, one expects a metal 0x0 to have a metal-centered LUMO in which the 0x0 ligand is more nucleophilic, which would explain the general absence of this kind of reactivity. One key observation was that the analogous rhenium complexes were not effective oxidants, and because the Re V diolates underwent cycloreversion, the reactivity was thermodynamically controlled. Such diol deoxygenations were precedented, though the intermediate metal diolates were usually not isolated or characterized.