Alumanyl silanides as multifunctional reagents for olefin cycloaddition, CO hydrosilylation, and reductive CO coupling

TL;DR

This paper explores the reactivity of a rare alumanyl silanide in various chemical reactions, including cycloadditions and CO hydrosilylation.

Contribution

The study reveals new reactivity patterns of alumanyl silanides in CO hydrosilylation and reductive CO coupling.

Findings

Alumanyl silanide undergoes [2+2]-cycloaddition with olefins and aldehydes.

The compound captures two CO2 molecules and one CO molecule in distinct bonding modes.

A cyclic carbene intermediate is identified as key in CO hydrosilylation.

Abstract

Alumanyl silanides represent a rare class of main group complexes, characterized by an anionic Al–Si bond, stabilized through an intimate ion pair. Computations revealed pronounced multiple-bond character in these alumanyl silanides, which is further enhanced upon counter ion sequestration. Despite these electronic features, such bonding motifs remain largely unexplored in experimental chemistry. In this work, we investigate the reactivity of a sodium alumanyl silanide, stabilized by bulky silyl groups and an N-heterocyclic imine (NHI), towards alkenes, alkynes, aldehydes, CO2, and CO. The addition of ethylene, styrene, or mesitylaldehyde to said alumanyl silanide affords the corresponding [2+2]-cycloaddition products, characterized by polarized Al–Si–C–C and Al–Si–C–O heterocycles, respectively. Furthermore, the investigated alumanyl silanide captures two equivalents of CO2. One molecule is inserted between the Al center and its adjacent NHI ligand, whereas a second molecule of CO2 adds across the central Al–Si bond. Moreover, the title compound selectively adds one equivalent of CO to the Al–Si core. This is followed by 1,2-hydrogen migration from the silicon center to the carbon in the formed Al–Si–C–O cycle, showcasing a rare main group-mediated hydrosilylation of CO. The mechanism for this formation is examined using DFT calculations, which reveal the generation of a cyclic carbene intermediate as the key step. At low temperatures, the intermediate is successfully trapped in the presence of additional CO and an N-heterocyclic carbene (NHC), yielding silanyl ethynolates via reductive coupling of two CO molecules.

This article reports on the reactivity of a previously described alumanyl silanide with a series of olefins, carbonyls, and CO2 by means of [2+2]-cycloaddition reactions in addition to the formal hydrosilylation of CO.

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