Hydroboration metal-mediated process

One facile way to incorporate boryl moieties into diene-containing polymers is via hydroboration. The addition of a B—H bond to C=C bonds via hydroboration is not stricdy a metal-mediated process, but catalyzing it with transition metal complexes provides a number of benefits. For example, transition metal catalysts can alter selectivity of hydroboration and provide alternatives for manipulating regio-, stereo-, and chemoselectivity. Additionally, they can help accelerating the addition of boryl substituents when the reaction rates are very slow—e.g., addition of catecholborane. 

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]. 

The activation of oxygen in oxygen transfer reactions is usually mediated by a suitable transition metal catalyst which has to be sufficiently stable under the reaction conditions needed. But also non-metal catalysts for homogeneous oxidations have recently been of broad interest and several of them have been compiled in a recent review.2 Other examples for well known alkene oxidation reactions are the ozonolysis, hydroboration reactions or all biological processes, where oxygen is activated and transferred to the substrate. Examples for these reactions might be cytochrome P450 or other oxotransferases. Of these reactions, this contribution will focus on transition-metal mediated epoxidation and dihydroxylation. 

While the highly versatile rare-earth metal-catalyzed hydrogenation, hydroboration, hydrosilylation, hydroamination, and hydrophosphination has been studied over the last two decades, the rare-earth metal-mediated activation of the hydroxyl 0-H bond for an analogous hydroalkoxylation process [87,190] has been reported only recently. Homoleptic trisamides were reported to catalyze the smooth cyclization of allenyl alcohols [191] and alkynyl alcohols [191,192] to form the corresponding unsaturated ethers. 

In many catalytic processes and transition metal mediated reactions, a-bor-ane complexes have been shown to be intermediates. The bis(borane) complex Cp2Ti( 72-HBcat/)2 (HBcaT = HBcat-4-f-Bu) is a highly active catalyst for the hydroboration of vinylarenes [37]. A mechanism, shown in Scheme 3, has been proposed for the Ti-catalyzed hydroboration on the basis of a detailed mechanistic study [37]. Theoretical calculations provided further support to the proposed reaction mechanism and showed that the reductive elimination step, giving the product molecules, is rate-determining [38]. In the Cp2Ti(CO)2 catalyzed hydroboration of alkynes [36,37], the proposed reaction mechanism (Scheme 4) also involves a a-borane complex similar to 11 and 14. In the titanium-catalyzed decaborane-olefin hydroborations [47,48], a-borane complexes were also considered as intermediates. In the Cp2MH (M = Nb, Ta) mediated hydroboration reactions of olefins [39,41], Smith and his coworkers observed several interesting cr-borane complexes, such as 21-23 discussed above. 

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