Stereochemistry reductive cross-couplings

It is known that electrochemical reduction of oximes in protic media occurs in two steps the N—O bond is first reduced to form an imine and the latter is then reduced to afford a primary amine1,29. Tallec has shown that the amine from oxime 33 can be trapped intramolecularly (equation 16)35. Interestingly, the SS diastereomer predominates the chiral pyrrolidine ring derivative serves to control the stereochemistry of formation of the new benzylic chiral center. Electrochemical reductive cross-coupling of O-methyl oximes with carbonyl compounds in isopropanol at a tin cathode affords adducts (equation 17) which can be reduced further to 2-amino alcohols36. In this fashion, menthone could... 

The control of reactivity to achieve specific syntheses is one of the overarching goals of organic chemistry. In the decade since the publication of the third edition, major advances have been made in the development of efficient new methods, particularly catalytic processes, and in means for control of reaction stereochemistry. For example, the scope and efficiency of palladium- catalyzed cross coupling have been greatly improved by optimization of catalysts by ligand modification. Among the developments in stereocontrol are catalysts for enantioselective reduction of ketones, improved methods for control of the... 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

Suzuki-Miyaura cross-coupling polymerization of 1,4-bis((Z)-2-bromovinyl)benzenes with aryl-bis-boronic acids. The interest has been in an alternative approach, where rather than building a PPV with a pre-ordained stereochemistry, a postpolymerization yyn-selective reduction on a poly(phenylene ethynylene) (PPE) is used [125]. This scheme has the advantage that high molecular weight PPEs can be synthesized using either Pd-catalysis or alkyne metathesis. This route could also potentially allow for the access to an additional array of PPVs that are uniquely accessible from PPEs. The transformation of the triple bonds in PPEs and other acetylene building blocks to alkenes has considerable potential. 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

The housefly (Musca domestica) uses the hydrocarbon (Z)-9-tricosene (318 in Scheme 58) as the major component of its sex pheromone. Simple syntheses involve Wittig coupling 153, 154) or alkylation of a terminal alkyne and subsequent reduction 155, 156) to afford the desired Z-isomer in ca. 95% purity. Erucic acid 157, 158) or oleic acid 159, 160) have served as starting material of known Z stereochemistry. The transition metal-catalyzed olefin cross-metathesis reaction has been applied by Rossi 161) to synthesize (318) as a mixture of /Z-isomers together with the other possible Cis and C28 olefins (Scheme 58). 

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