Alkenes, hydroboration hydrosilylation

Other catalytic hydroelementations, E-H (E = P, B, Si, H) addition to C C multiple bonds, are efficiently mediated by organolanthanide complexes, and many are highly diastere-oselective. Hydrophosphination/cyclization appears to follow the catalytic patiiway similar to hydroamination/cyclization, whereas hydroboration, hydrosilylation, and hydrogenation proceed via different catalytic cycles, in which a lanthanide-hydride is the intermediate that is generated from protonolysis of the precatalyst. The proposed catalytic cycle of hydrosilylation of alkene using Cp LnCH(TMS)2 is presented in Scheme 5. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

Hydroboration and hydrosilylation reactions ofalkynes, when they do not proceed spontaneously, can be catalyzed by numerous transition metals [17]. Metal vinyli-dene-mediated processes uniquely provide (Z)-alkenes via trans-addition . In 2000, Miyaura and coworkers discovered that both Rh(I)- and Ir(I)-complexes supported by bulky electron-rich phosphine ligands catalyze the hydroboration of terf-butylacety-lene to give alkenylboronate 41 with >95 5 Z/E selectivity (Equation 9.5) [18]. 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

The hydrosilylation-oxidation of simple unfunctionalized alkenes has not been widely used for the diastereoselective preparation of alcohols, probably because it would not in general be expected to give very different results to hydroboration oxidation. One example which has been reported is the exo-selective hydrosilylation of norbornene (1) with trichlorosilane and hexachloroplatinic acid6, followed by oxidation to c.vo-norbornanol (2)1. 

The catalytic reduction of alkenes and alkynes are important methods for the synthesis of alkanes. The hydroboration and hydrosilylation of alkenes are alternatives to catalytic methods. Again, both the alkene and alkyne may have played an important role in the construction of the hydrocarbon chain. 

Subsequent studies have demonstrated that the relatively robust and readily accessible rare-earth trisamide La N(SiMe3)2 3 can also be successfully employed to catalyze anft -Markovnikov hydroboration of alkenes (28) [82], The 1,2-addition product in the hydroboration of vinyl arenes constitutes the opposite 2,1-addition products commonly favored in vinyl arene hydrosilylations (Sect. 3.1), although both reactions are believed to proceed via analogous mechanisms. This fact may also be interpreted in terms of uncatalyzed addition of the borane to the alkene being competitive with the catalyzed process in some cases [82,83],... 

Hydroboration and hydrosilylation. A mixture of EtjSiH and BCI3 is used for hydroboration of alkenes, leading to alkyldichloroboranes. Conjugated dienes give (Zj-alkenyltriethylsilanes on reaction with EtjSiH photochemically in the presence of Cr(CO),. 

Related processes such as hydrosilylation, hydroboration and hydroamination are considered in this chapter. Hydroformylation is also considered, as well as hydroacylation and hydrocyanation reactions of alkenes. 

The examples in Equation 16.45 show that the regiochemistry of the metal-catalyzed hydroboration of alkenes contrasts with that of tlie metal-catalyzed hydroboration of vinylarenes. The predominant products from the metal-catalyzed hydroborations of 1-alkenes are terminal boranes. - " These terminal boranes can also be formed from internal alkenes, as shown in Equation 16.46. Similar to hydrosilylation witli Speier s catalyst, the... 

The mechanisms of these reactions are varied, but can still be categorized. The hydrocyanations, hydrosilylations, and many of the hydroborations, occur with late-metal catalysts. These reactions occur by oxidative addition of the H-X bond, followed by migratory insertion of the olefin into the M-H or M-X bond, and reductive elimination to form the final product. Hydrocyanation occurs by insertion of the unsaturated reagent into the M-H bond, while hydrosilylation and hydroboration have been shown to occur by insertion of the olefin into the M-H bond in some cases and into the M-X bond in others. HydrosUy-lations and hydroborations of alkenes and alkynes catalyzed by (P transition metal complexes and by lanthanides follow a different pathway because these complexes caimot undergo oxidative addition. The mechanism of the reactions catalyzed by these complexes involves u-bond metatheses. 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

Iridium-catalyzed hydrogenation of alkynes, hydrosilylation of alkenes, and hydroboration of alkenes ... 

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