Catalytic bond formation alkene metathesis

As an alternative, iridium complexes show exciting catalytic activities in various organic transformations for C-C bond formation. Iridium complexes have been known to be effective catalysts for hydrogenation [1—5] and hydrogen transfers [6-27], including in enantioselective synthesis [28-47]. The catalytic activity of iridium complexes also covers a wide range for dehydrogenation [48-54], metathesis [55], hydroamination [56-61], hydrosilylation [62], and hydroalkoxylation reactions [63] and has been employed in alkyne-alkyne and alkyne - alkene cyclizations and allylic substitution reactions [64-114]. In addition, Ir-catalyzed asymmetric 1,3-dipolar cycloaddition of a,P-unsaturated nitriles with nitrone was reported [115]. 

Rather unusual hydrophosphination of alkenes was reported using calcium-catalyzed transformation [131]. The reaction was carried out in benzene at 25-75 °C for 13-36 h and resulted in 78-95% conversion. Anti-Markovnikov addition products were formed with high selectivity. Proposed reaction mechanism involves insertion of the C=C bond into the Ca-P bond followed by metathesis stage (Scheme 8.48). The metathesis stage was found as rate determining for the catalytic cycle. Thus, if an activated alkene is utilized in the reaction insertion a more rapid insertion stage compared to slow metathesis stage results in formation phosphine-terminated polymers [131]. 

Olefin metathesis enables the catalytic formation of C=C double bonds under mild conditions.1 After the development of well-defined catalysts,1 2 selective cross-couplings between functionalized terminal alkenes (CM) have been noted.2 A general problem... 

Olefin metathesis is a catalytic reaction in which alkenes are converted to new products via the cleavage and re-formation of carbon-carbon double bonds (Scheme 1). 

Summary Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of rathenium (e.g., Grabbs catalyst). 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

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