Reductive elimination acylation

The reaction of benzoyl chloride with (Me3Si)2 affords benzoyltrimethylsi-lane (878)[626,749,750]. Hexamethyldigermane behaves similarly. The siloxy-cyclopropane 879 forms the Pd homoenolate of a ketone and reacts with an acyl halide to form,880. The 1,4-diketone 881 is obtained by reductive elimination of 880 without undergoing elimination of /7-hydrogen[751]. 

C-C bonds can be formed by reaction with alkyl iodides or more usefully by reaction with metal carbonyls to give aldehydes and ketones e.g. Ni(CO)4 reacts with LiR to form an unstable acyl nickel carbonyl complex which can be attacked by electrophiles such as H+ or R Br to give aldehydes or ketones by solvent-induced reductive elimination ... 

Reductive Elimination from Acyl-Metal-NHC Complexes... 

In studies involving the reaction of a cA-[PdCl(Me)(NHC)]j chloro-bridged dimer with CO it was demonstrated that reductive elimination is also extremely facile for NHC-Pd-acyl complexes, yielding 2-acylimidazolium salts and Pd(0) (Scheme 13.4) [22]. The product distribution was shown to depend on the stracture of the complexes from which reductive coupling took place (pathways A and B, Scheme 13.4). 

Scheme 13.4 Reductive elimination from acyl-Pd-NHC complexes...

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

Procedures for the synthesis of ketones based on coupling of organostannanes with acyl halides have also been developed.211 The catalytic cycle is similar to that involved in coupling with aryl halides. The scope of compounds to which the reaction is applicable includes tetra-u-butylstannane. This example indicates that the reductive elimination step competes successfully with (3-elimination. 

In all these reactions, the acylating reagent reacts with the active Pd(0) catalyst to give an acyl Pd(II) intermediate. Transmetallation by the organoboron derivative and reductive elimination generate the ketone. 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

The reaction may be reasonably explained by the smooth oxidative addition of benzylic and acyl halides to nickel to afford benzylnickel halides and acylnickel halides. The metathesis of these complexes could give the acylbenzylnickel complex, which upon reductive elimination would yield the benzyl ketone. 

Complex (5) undergoes methyl migration after oxidative addition of CH3I to afford the acyl complex (6) containing two Rh—O bonds. Heating (6) in the presence of CO results in the reductive elimination of Acl, which upon hydrolysis is transformed to AcOH.12... 

Scheme 7 comprises the following patterns First, a metallacycle gives rise to ketones by CO insertion and reductive elimination. Next, a nickel hydride inserts an unsaturated substrate L, followed by CO. The acyl intermediate can give rise to reductive elimination with formation of acyl halides or acids and esters by hydrolysis, or it can insert a new ligand with subsequent reductive elimination as before. Alternatively, there may be a new insertion of carbon monoxide with final hydrolysis. Third, an intermediate R—Ni—X is formed by oxidative addition. It can react in several ways It can insert a new ligand L, followed by CO to give an... 

Subsequent insertion of CO into the newly formed alkyl-ruthenium moiety, C, to form Ru-acyl, D, is in agreement with our 13C tracer studies (e.g., Table III, eq. 3), while reductive elimination of propionyl iodide from D, accompanied by immediate hydrolysis of the acyl iodide (3,14) to propionic acid product, would complete the catalytic cycle and regenerate the original ruthenium carbonyl complex. 

The most plausible mechanism proceeds through oxidative addition of the aldehyde to an active Ru(0) species to form (acyl)(hydrido)ruthenium(ll) complex 155. Insertion of the less-substituted double bond of the 1,3-diene into the Ru-H bond occurs to generate an (acyl)( 73-allyl)ruthenmm(ll) intermediate of type 156. Successive regioselective reductive eliminations between the acyl and the 73-allyl ligands provide the desired product with regeneration of the... 

A most significant advance in the alkyne hydration area during the past decade has been the development of Ru(n) catalyst systems that have enabled the anti-Markovnikov hydration of terminal alkynes (entries 6 and 7). These reactions involve the addition of water to the a-carbon of a ruthenium vinylidene complex, followed by reductive elimination of the resulting hydridoruthenium acyl intermediate (path C).392-395 While the use of GpRuGl(dppm) in aqueous dioxane (entry 6)393-396 and an indenylruthenium catalyst in an aqueous medium including surfactants has proved to be effective (entry 7),397 an Ru(n)/P,N-ligand system (entry 8) has recently been reported that displays enzyme-like rate acceleration (>2.4 x 1011) (dppm = bis(diphenylphosphino)methane).398... 

Thus, (i) electron transfer from Pd(0) to cyclohexenone, for example, (ii) Pd—allyl complex formation, (iii) transmetalation forming an acylpalladium complex, and (iv) reductive elimination of Pd(0), would give either a 1,2- or a 1,4-acylation product [26] (Scheme 5.21). The role of the triphenylphosphane ligand in the regioselective formation of a 1,2-acylation product may be explained by the preferred formation of a stereochemically less crowded intermediate complex A (Scheme 5.22) and subsequent reductive elimination of Pd(0). 

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

In order to study the reactivity of the nitrogen atom in saccharidic OZT moieties Rollin and co-workers13,55 explored some standard reactions IV-acylation, IV-sulfonylation, IV-vinylsulfonylation by Michael addition, reductive elimination, conjugated addition and cycloaddition. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

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