Unexpected catalytic activity of simple triethylborohydrides in the hydrosilylation of alkenes

Abstract

The first example of sodium triethylborohydride-catalysed hydrosilylation of alkenes is reported. The hydrosilylation of certain alkenes, in particular styrenes, vinylsilanes and allyl glycidyl ether, with aromatic hydrosilanes proceeded in a highly regioselective manner to give Markovnikov products. It is significant that several protocols use NaHBEt as a reducing agent generating active catalysts in situ of other hydrosilylation reactions. An anionic mechanism of hydrosilylation is proposed.

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

“…The triphenylsilyl complex [Ca(SiPh 3 ) 2 (thf) 4 ] hydrosilylated styrene derivatives with anti‐Markovnikov selectivity as did related alkali metal silanide and hydrosilicate complexes . Although simple Na[HBEt 3 ] was reported to hydrosilylate styrene derivatives with aryl hydrosilanes, no activity was observed when aliphatic olefins like 1‐hexene or silanes such as Et 3 SiH were used …”

“…Higher substituted 1‐phenylcyclohexene as well as tri‐ and tetraphenylethylene did not show any conversion (Table , entries 10–12). Triphenyl(vinyl)silane gave a mixture of both regioisomers (Table , entries 13), formed through competing insertion to give either the sterically favored linear or the α‐silicon‐stabilized branched carbanion (Ph 3 SiCHCH 3 ) − …”

“…[10] Although simple Na[HBEt 3 ] was reported to hydrosilylate styrene derivatives with aryl hydrosilanes, no activity was observed when aliphatic olefins like 1hexene or silanes such as Et 3 SiH were used. [11] In contrast to olefin hydrogenation by molecular alkaline earth metal hydrides, [13] hydrosilylation of unactivated alkenes using aliphatic silanes by s-block metal catalysts remains elusive. [12] Herein we report on the hydrosilylation of ethylene, a-olefins, and styrene derivatives using a molecular calcium hydride cation [CaH] + stabilized by the macrocyclic polyamine ligand Me 4 TACD (1,4,7,10-tetramethyl-1,4,7,10tetraazacyclododecane).…”

“…NMR spectroscopy revealed full consumption of the primary hydrosilane prior to the second addition of ethylene. While the selectivity with Ph 2 SiH 2 was lower due to scrambling ( Table 1, entry 10), alkyl-substituted secondary silanes were fully converted within 15-20 min ( Table 1, entries [11][12][13][14]. Hydrosilylation of sterically more demanding i Bu 2 SiH 2 was incomplete after 60 min ( entries 16 and 17).…”

“…Hydrosilylation of 1,4hexadiene only gave the 4-alkenylsilane, and as for 2-hexene, no reaction of the internal double bond was detected even after prolonged reaction time or with an excess of hydrosilane (Table 2, entries 6 and 7). Likewise, the internal double bond [8][9][10][11] were not hydrosilylated. Allyl methyl ether was not hydrosilylated as the nucleophilic hydride was readily converted into the methoxy complex 4 under formation of propene (Table 2, entry 12).…”