Head-to-head versus head-to-tail dimerizations of transient silenes — the generation and dimerization behavior of 2-(2-dimethylaminoaryl)-1,1-bis(trimethylsilyl)silenes

The transient silenes (Me3Si)2SiCR1R2 (4: R1=H, R2=2-Me2N-5-Me–C6H3; 11: R1=Me, R2=2-Me2N–C6H4; 15: R1=3,5-Me2–C6H3, R2=2-Me2N–C6H4; 18: R1=Me, R2=2-Me2N-5-Me–C6H3) were generated following the mechanism of the sila-Peterson reaction. Thus, 4 was obtained by base-induced trimethylsilanol elimination from (2-dimethylamino-5-methylphenyl)tris(trimethylsilyl)silylmethanol (3). Addition of methyllithium or 3,5-dimethylphenyllithium, respectively, to the carbonyl group of 2-dimethylaminobenzoyl-tris(trimethylsilyl)silane (10) and subsequent elimination of Me3SiOLi led to 11 and 15, respectively. Similarly, (2-dimethylamino-5-methylbenzoyl)-tris(trimethylsilyl)silane (17) and MeLi gave 18. The silene 4 underwent a spontaneous head-to-tail cyclodimerization to give a 1,3-disilacyclobutane (5), whereas 11 afforded a linear head-to-head dimer (12). The dimerization rates of 15 and 18 proved to be slow, thus, under the conditions of the sila-Peterson reaction readdition products of eliminated trimethylsilanolate to the SiC bond of the silenes were obtained (16, 19). The structures of the compounds prepared were elucidated on the basis of comprehensive NMR and MS studies; for 5 also the results of an X-ray structural analysis are given. Possible reasons for the different behavior of the similarly structured silenes are discussed.