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

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(trimetylsilyl)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(trimethysily)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 BH3 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. (C) 1999 Elsevier Science S.A. All rights reserved.