Hydride shifts reductive elimination

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

The 0-hydride shift followed by a reductive elimination produces acetaldehyde, while the resulting Rh fragment is trapped by cod or ethylene to give... 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

The mechanism of the C—H and C—C bond activation of bare Fe+ with n-heptyltrimethylsilane has been elucidated with the help of extensive labeling studies71. The system was found to display a rather rich chemistry. Loss of neutral tetramethylsilane from the ion-molecule complex (equation 11) was explained by an initial insertion of the metal ion into the Cl— C2 bond to form 21, and a subsequent fi-H shift giving rise to the iron-hydride complex 22. This ion can then lose a tetramethylsilane molecule via reductive elimination. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

The biopterin product is recycled by elimination of water, reduction using NADPH as the reagent, and reaction with molecular oxygen. The other product, the phenylalanine oxide, rearranges with a hydride shift followed by the loss of a proton to give tyrosine. 

Hydride shift, as in olefin oxidation in aqueous medium, forming carbonyl compounds (see eq. (20) in Section 2.4.1) is completed under conditions in which, instead of vinyl compounds, ethylidene diacetate or acetals are formed, since using deuterated acids or alcohols, e. g., AcOD or ROD, the respective products do not contain any deuterium [3]. According to eqs. (13) and (14) with R = OAc", 0-alkyl , the step leading to these products can be interpreted as reductive elimination. 

The reduction of ketone 559 by LiAlH results mainly in the endo alcohol 556. The thermodynamic equilibrium of exo and endo alcohols leads mainly to exo alcohol 560. Hence, the formation of endo alcohol 556 upon solvolysis of tosylate 558 cannot be due to steric factors — the endo-side attack is sterically less favourable than the exo-side one the resulting alcohol 556 is not a thermodynamically controllable product. Consequently, the data obtained cannot be used to assume a classical structure of the intermediate cation 561. At the same time the participation of the C —C bond and the intermediate non-classical ion are quite compatible with these facts. The acetolysis of the optically active tosylate 558 is accompanied by complete racemization, the rate of the latter being 3 times as high as that of acid elimination (internal return). The completeness of racemization shows the reaction to proceed via a symmetrical trishomocyclopropenyl ion or rapid equilibration of unsymmetrical cations 561 or to be accompanied by a 1,3-hydride shift from to C . 

Initial oxidative coupling of the ligated Rh complex with both the alkyne and alkene gave the metallacyclopentene A, followed by olefin insertion to form metallacycloheptene B. Tricyclic compound 430 was obtained by reductive elimination of Rh from B when is not hydrogen. In contrast, when R is hydrogen, a 1,3-hydride shift with concomitant ring opening takes place to afford metallacycle D. Subsequent reductive elimination of Rh resulted in the formation of bicyclic compound 431. 

Hydride shift ensues to generate ruthenium haxa-l,3.5-triene III, which undergoes 671-electrocyclic ring closure and reductive elimination to furnish cyclopentadiene IV. Ultimately, the most stable regioisomer 211 is yielded via a [1,5]-hydrogen shift. 

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