Acyl cuprate

Nicolas, E., Russell, K. C., and Hruby, V. J. (1993). Asymmetric 1,4-addition of organo-cuprates to chiral a, b-unsaturated N-Acyl-4-phenyl-2-oxazolidinones A new approach to the synthesis of chiral b-branched carboxylic acids. J. Org. Chem. 58, 766—770. 

Semmelhack et al. chose CuBr, together with either Red-Al or LiAl(OMe)3H in a 1 2 ratio, to afford presumed hydrido cuprates, albeit of unknown composition [llj. In THF, both the former Na complex and the latter Li complex are heterogeneous (and of differing reactivities), yet each is capable of 1,4-reductions of unsaturated ketones and methyl esters (Eq. 5.4). Commins has used a modified version, prepared from lithium tri-t-butoxy-aluminium hydride and CuBr (in a 3 4.4 ratio), to reduce a 3-substituted-N-acylated pyridine regioselectively at the a-site [12]. 

Electrophiles, which lead to high yields, are methyl iodide, trialkyltin- and trialkylsUyl chlorides, diphenylphosphinyl chloride, acid chlorides, aldehydes and carbon dioxide. Remarkably, though highly acidic ketones are formed on acylation, no deprotonation or racemization by excess of carbanionic species occurs. Other alkyl halides than methyl iodide react very sluggishly with low yields. Benzylic and aUylic halides lead to partial racemization, presumably due to single-electron transfer (SET) in the alkylation step. As very recently found by Papillon and Taylor, racemization of 42 can be suppressed by copper-zinc-lithium exchange before alkylation to 43 via the Knochel cuprates (equation 7) °. 

There were two more stereocenters to set. It was expected that cuprates would add to the open face of the strained cyclobutene. The control of the other stereocenter was more problematic. One solution was to prepare an a-sulfonyl lactone. To this end, the ketone was converted to the secondary carbonate. As hoped, conjugate addition was followed by intramolecular acylation, but the reaction continued to full acyl transfer, to give 10. Fortunately, desilylation of 10 proceeded with concomitant lactonization. Desulfonylation then gave 2, which could be brought to high by recrystallization. 

Acylcuprate reagents.1 The reaction of cuprates of the type R2(CN)CuLi2 with carbon monoxide at -110° results in carbonylated reagents, possibly with the composition (RCO)R(CN)CuLi2. In any case, these cuprates effect 1,4-acylation of a,p-enones and -enals to provide 1,4-dicarbonyl compounds. 

Zirconium(IV) propoxide, 352 with organocopper reagents Dilithium acyl(alkyl)(cyano)cuprates, 209... 

Less reactive silyl metal species such as lithium bis(triphenylsilyl) cuprate react with a variety of acyl chlorides to give the corresponding acyl silanes in moderate to good yields75. Dilithium bis(trimethylsilyl) cyanocuprate is particularly effective for the preparation of sterically hindered acyl silanes (Scheme 8), and appears to provide a good general... 

One general method for acyl silane synthesis particularly successful for a-cyclopropyl examples (and even an a-cyclobutyl example) involves treatment of acid chlorides with lithium tetrakis(trimethylsilyl) aluminum or lithium methyl tris(trimethylsilyl) aluminium and cuprous cyanide (vide supra, Section III.A.3)77. For example, cyclopropyl acyl silane (23) was obtained in 89% yield by this process. Improved procedures use lithium t-butyldimethylsilyl cuprate78 and a dimethylphenylsilyl zinc cuprate species79,80 as reagents. 

Acyl anions (RC(=0)M) are unstable, and quickly dimerize at temperatures >-100 °C (Section 5.4.7). These intermediates are best generated by reaction of organolithium compounds or cuprates with carbon monoxide at -110 °C and should be trapped immediately by an electrophile [344—347]. Metalated formic acid esters (R0C(=0)M) have been generated as intermediates by treatment of alcoholates with carbon monoxide, and can either be protonated to yield formic acid esters, or left to rearrange to carboxylates (R0C(=0)M —> RC02M) (Scheme 5.38) [348]. Related intermediates are presumably also formed by treatment of alcohols with formamide acetals (Scheme 5.38) [349]. More stable than acyl lithium compounds are acyl silanes or transition metal acyl complexes, which can also be used to perform nucleophilic acylations [350],... 

Ryu, I. Ikebe, M. Sonoda, N. Yamato, S.-Y. Yamamura, G.-H. Komatsu, M. Chemistry of ketone a,/ -dianions. Acylation reactions of dianion cuprates by acid chlorides. Tetrahedron Lett. 2002, 43, 1257-1259. 

Thus vinylsilanes may be regarded as masked carbonyl groups as exemplified by the two reaction sequences depicted in Equations Si5.22 and Si5.23. Note that in the second example the cuprate is acting as an acyl anion equivalent. 

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