Alkyne Reductive homologation

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

Enzymatic reduction of 23a with recLBADH and CPCR resulted in unsatisfactory results (60% and 49% ee) as well. The results mentioned above indicate that a bulky substituent at the alkyne moiety results in a higher selectivity of the reduction. Furthermore, Bradshaw et al. reported that Lactobacillus kefir ADH, an enzyme highly homologous to LB ADH, affords (R)-4-trimethylsilyl-3-butyn-2-ol [(R)-25j with an ee of 94% in 25% yield [39bj. In our investigations ketone 23b was reduced by recLBADH with almost quantitative conversion. The enantiomeric excess and absolute configuration of the product were determined by desi-lylation with borax converting alcohol (R)-25 into enantiopure (R)-3-butyn-2-ol [(R)-24j (Scheme 2.2.7.14). 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

PhS(0)(0Ph)Me + RLi and LiX - PhSOCH X + PhSO Me]/ or reduction with homologation (with RjCuLi and LiX)/ Standard sulphinylation procedures reported include electrophilic substitution of phenols with ArSOCl and AlClj and the preparation of optically active sulphoxides from resolved (—)-menthyl sulphinates with ( )-l-alkenylmagnesium bromides or arylthiomethyl- or arylsulphonylmethyl-lithium compounds/ A practical route to 1-alkenyl sulphoxides uses non-activated alkynes and sulphenic acids, generated in situ from the thermolysis of 2-cyanoethyl phenyl sulphoxides/ ... 

All possible stereoisomers of -4,5-dihydroxydec-2-enal 333 have been prepared from D- and L-arabinose, D-ribose and L-lyxose. Key reactions involve Cl Wittig homologation, reduction and C4,C5 oxidative cleavage. The eneyne 336, an intermediate for a synthesis of la,2S-dihydroxyvitamin D3, has been prepared from D-xylose via lactone 334 and 335. Lithioacetylene used for alkyne introduction and further elaboration provides 336 (Scheme 51). ... 

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