Carbonylation of Cyclotrisilenes

Abstract

Carbon monoxide (CO) is a common ligand in transitionmetal chemistry. In organic chemistry, it is frequently used to introduce a C1 unit, although carbonylations, such as hydroformylation or the Pauson–Khand reaction, generally require a transition-metal catalyst, even for strained (and thus highly reactive) cyclic compounds. Only a few reports on the reactivity of CO towards compounds of main-group elements other than carbon are available. Although known since the pioneering work by Schlesinger and Burg in the 1930s, borane–CO adducts are usually unstable at room temperature. Only recently, Piers et al. reported a stable adduct between the very Lewis acidic perfluorinated pentaphenylborole and CO. The hydroborating Lewis acid (F5C6)2BH incorporates CO assisted by a alkenyl-functionalized phosphine in a frustrated Lewis pair (FLP) mechanism. A B(C6F5)3-based FLP is capable of stoichiometric CO reduction with dihydrogen. A few coordinatively unsaturated and thus Lewis acidic organic species, such as transient triplet carbenes, and Bertrand s cyclic alkyl amino carbenes react with CO in the absence of transition metals, forming ketenes. Despite growing appreciation of the capability of main group species to activate small molecules, such as H2 and NH3, [10] reactions of stable low-valent compounds of the heavier elements with CO have not, to our knowledge, been reported. Transient silylene–CO adducts, however, have been observed in the gas phase and in cold matrices. We have recently shown that reactions of cyclotrisilenes with N-heterocyclic carbenes (NHCs) reversibly afford cyclotrisilene–NHC adducts, which in the case of 1a can undergo reversible ring-opening to form a highly unsaturated NHCstabilized disilenylsilylene. Isonitriles also react with cyclotrisilenes, and, mindful of the isoelectronic relationship to CO, we became interested in the reactivity of the latter. Herein, we report the facile reaction of carbon monoxide with cyclotrisilenes in the absence of a catalyst to form highly functionalized cyclic silenes. Exposure of a benzene solution of cyclotrisilene 1a to 1 atm of CO at 25 8C results in precipitation of a yellow powder within a few hours, which owing to its insolubility cannot be characterized spectroscopically. A reaction for several days without stirring under otherwise identical conditions afforded bright yellow single crystals. Elemental analysis confirmed the incorporation of one equivalent of CO per molecule of 1a. X-ray crystallography revealed the product to be the tricyclic Brook-type bis(silene) 2a, formally arising from two molecules each of 1a and CO (Scheme 1, Figure 1). With a p-donating oxygen atom at carbon, 2a is reminiscent of donor-stabilized silenes with inverse polarization,