on the occasion of his 70th birthday For a long time, unsaturated silicon compounds with multiple bonds to silicon have been thought to be unisolable at room temperature. [1] This situation changed profoundly in 1981. [2,3] During the past 25 years, intriguing progress has been achieved to generate a wealth of isolable compounds with silicon-heteroatom double bonds (heteroatom: elements from Groups 13, [4] 14, [5] and 15, [6][7][8] and sulfur [9] ), and more recently even isolable compounds with a silicon-silicon triple bond have been reported [10] which represent unique and indispensable building blocks in organosilicon chemistry. However, the isolation of silanones (R 2 Si = O) that are stable at room temperature ("Kippings dream") [9d,e] has hitherto been unsuccessful. The absence of isolable silanones is probably a result of the lack of suitable synthetic methods as well as the difficulty in preventing oligomerization of the silicon-oxygen double bond. [11] Clearly, the pronounced polarity of the silicon-oxygen p bond, estimated by theoretical calculations, [12] accounts for the extraordinarily high tendency of silanones to undergo dimerization and trimerization. This proceeds with no barrier, in contrast to their carbon analogues or any other related silicon-heteroatom p system. Thus, taming the high polarity of the silicon-oxygen double bond is pivotal for the generation of an isolable silanone derivative.
Isolable silaformyl compounds R(H)Si=O (A) could be prepared by taking advantage of the tautomerization (hydrogen-atom migration) of suitable hydroxo silylenes RSi(OH) (B; i.e. RSi(OH)!R(H)Si = O) [12d] in the presence of donor and acceptor groups attached to the silicon and oxygen atom, respectively, which at the same time may reduce the polar nature and provide additional steric protection of the siliconoxygen double bond (Figure 1). In fact, our calculations of the model series B, [B(NMe 3 )], and [B(NMe 3 )(BF 3 )] (R = H, Me, SiH 3 , NH 2 ; see Figure 1) [13a] revealed that the desired hydro-