Mass spectrometric behaviour of (E)-1-silyl-2-alkoxyethenes
The behaviour of ionized organosilicon compounds upon electron impact has been extensively examined. 1 -5 Fragmentation of the organosilane molecular ions, e.g. [Me 3 SiCH CH 2 ] Cž , [Me 2 Si(CH CH 2 ) 2 ] Cž and [MeSi(CH CH 2 ) 3 ] Cž , has been established to involve primarily elimination of the methyl radical at the silicon atom followed by the loss of acetylene through transfer of a hydrogen atom from the ˇ-carbon to the silicon atom (ˇ-transfer). Soderquist and Hassner 6 found that the transfer of R to the silicon (˛-transfer) is an important process in the fragmentation patterns of substituted vinylsilanes, mainly R 0 3 SiC(R) CH 2 (R D D, Br, OMe, OSiMe 3 , Ph). Whereas ˛-and ˇ-substituted vinylsilanes (R 0 3 Si)(R)C CH 2 and R 0 3 SiCH CHR have become very common silyl reagents used in regio-and stereoselective organic synthesis, 7 there are only a few reports on the substituent effect at silicon and at carbon atoms on the fragmentation mechanism of these class of organometallics. Styrylsilanes, PhCH CHSiR 0 3 , are the only ˇ-substituted vinylsilanes whose detailed fragmentation has been resolved. 8 Therefore, the aim of this work was to elucidate the decomposition characteristics of (E)-1-silyl-2-alkoxyethenes (Scheme 1) as the first example of silyl(alkoxy)ethenes containing a functional group (ether) at a carbon atom (R 0 3 SiCH CHOR). These compounds were prepared using the recently developed synthetic method based on cross-disproportionation (silylative coupling) of vinyl-trisubstituted silanes with vinyl alkyl ethers catalysed by ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds. 9 The metastable ion spectrometry and highresolution data were used to complete the fragmentation pathways (Scheme 1).
Electron ionization (EI) mass spectra of the (E)-1-silyl-2alkoxyethenes and their labelled derivatives listed in Table 1 were studied.