Reactive intermediates in the synthesis and chemistry of organosilacycles
The role of several organosilicon reactive inter. mediates in the synthesis and chemistry of organosilacycles will be illustrated with a variety of examples from our laboratory. Particular emphasis will be placed on silanones, siloxy radicals, and silenes. The discussion will be limited to four, five, and s ix-membered rings. INTRODUCTION It is interesting to note that although silicon is directly beneath carbon in the periodic chart, many of the systems which are called reactive intermediates in organosilicon chemistry have quite stable analogs in carbon chemistry. For example, s ilenes (R2Si=CR2), disilenes (R2 Si=S iR2), and silanones (R2Si=O) have countless stable counterparts in the olef ins and carbonyl compounds of organic chemistry, but are known only as fleeting intermediates. Indeed, it is only in the last decade that evidence has been accumulating which appears to demand the existance of (p_p)rr bonded silicon (Ref. 1). As an example of two of these intermediates, some years ago we established that the gas-phase flow pyrolysis of 1 resulted in aromatization through extrusion of the bridge as dimethylsilene (2). When 1 was copyrolyzed with benzaldehyde, there were obtained three maor products -styrene, hexamethylcyclotrisiloxane (D3), and octamethylcyclotetrasiloxane (D4) (Ref. 2). The generally accepted mechanism for this process is cycloaddition of the silene to the carbonyl group to form silaoxetane, , which decomposes to a molecule each of olefin and dimethylsilanone (k). Thus, the cyclosiloxanes, D3 and D4, are believed to be the result o± cyclic oligomerization of k. Indeed, insertion into an Si-O bond, as in the conversion of D3 to D4T appears from all studies to date to be the best trapping reaction of a silanone.