Head-to-head versus head-to-tail dimerizations of transient silenes—the synthesis and reactivity of 2-(8-dimethylamino-1-naphthyl)- and 2-(4-dimethylamino-1-naphthyl)-1,1-bis(trimethylsilyl)silene

Abstract

In the absence of scavenger reagents, the transient 2-(8-dimethylamino-1-naphthyl)-1,1-bis(trimethylsilyl)silene ( 4), generated by a base-initiated elimination of trimethylsilanol from (8-dimethylamino-1-naphthyl)-tris(trimethylsilyl)silylmethanol ( 3), undergoes a spontaneous formal [2+2] head-to-tail cyclodimerization to give a substituted 1,3-disilacyclobutane 5. In contrast, the similarly produced 2-(4-dimethylamino-1-naphthyl)-1,1-bis(trimethylsilyl)silene ( 10) dimerizes in a head-to-head mode, finally affording the hexahydro-2,3-disilaphenanthren-9-on ( 12). The ketone 12 is the result of a formal [2+4] cycloaddition of the silene 10 producing an intermediate with an enamine unit ( 11), which is hydrolysed during the aqueous workup to give 12. A model, explaining the different dimerization regiospecificity of the two isomeric silenes is proposed. In the presence of BH 3·THF the generated silene 4 is trapped to afford 3-[bis(trimethylsilyl)silyl]-1,1-dimethyl-1-aza-2-bora-2,3-dihydrophenalene ( 6). All the compounds isolated, were fully characterised by IR, NMR and MS data. The compounds Z- 5, 8 and E- 12 were also examined by X-ray crystal structure analyses.