Aluminum halides in ionic hydrogenation and alkylation by organosilanes

Ionic and Organometallic‐Catalyzed Organosilane Reductions

“…Treatment of secondary alkyl alcohols with trifluoroacetic acid and organosilicon hydrides results only in the formation of the trifluoroacetate esters; no reduction is reported to occur. 1,2 Reduction of secondary alkyl alcohols does take place when very strong Lewis acids such as boron trifluoride 126,129 or aluminum chloride 136,146 are used. For example, treatment of a dichloromethane solution of 2-adamantanol and triethylsilane (1.3 equivalents) with boron trifluoride gas at room temperature for 15 minutes gives upon workup a 98% yield of the hydrocarbon adamantane along with fluorotriethylsilane (Eq.…”

“…The reduction of alkyl halides has been accomplished with triethylsilane/aluminum chloride. Substrates that undergo reduction under these conditions include primary alkyl halides, 146,185,186 secondary alkyl halides, 146,187,188 gem-dihalides, 189 vicinal dihalides, 189 and tertiary alkyl halides. 187,188 As expected, haloarenes generally do not undergo such reductions, even under vigorous conditions.…”

“…187,188 As expected, haloarenes generally do not undergo such reductions, even under vigorous conditions. 146,190 An exception to the need for carbocation formation is found when the silyl hydride functional group is part of a valence-expanded hydrosiliconate species. For example, potassium tetraethoxyhydridosilicate 104 is capable of reducing primary alkyl and benzylic bromides and chlorides directly to the corresponding hydrocarbons without the need for additional catalysis (Eq.…”

“…192 No significant difference in reactivities is observed between alkyl chloride and bromide substrates, 186 but allyl halides are more reactive than 2-halopropanes, which, in turn, are more reactive than 1-halopropanes. 190,146 With halides having a strong propensity to undergo ionization, such as trityl halides, reductions may occur in the absence of added Lewis acids. 29,54 Otherwise, the presence of Lewis acids is required.…”

“…192 Then it is necessary to add more than one equivalent of the Lewis acid to effect reduction of the carbon-halogen function. 136,146 Skeletal rearrangements may occur during these reductions, as in the reduction of bromocycloheptane (Eq. 47).…”