1-Silaallenes

An example of particular interest is the two-fold introduction of M(CO)n moieties at silicon to give HMPA adducts of organometallic analogues of silaallene. It has been shown that this reaction proceeds through the dichlorosilylene complex as intermediate. Both the iron 22 and ruthenium 23 compound and also the bimetallic complex 24 are accessible. 

In the case of 2-silaallene, a bent structure also seems to be energetically favored. 

Several groups have reported ab initio calculations of C2H4Si isomers some of the results are listed in Table I. The most stable structure is ethynylsilane. Relative to this molecule, I -silapropadiene is less stable by about 25-30 kcal moF (Ref. 12(f) places the energy of the parent silaallene ca. 55 kcal moP above ethynylsilane) and 2-silapropadiene is even more unstable, lying ca. 50 kcal moF above ethynylsilane. This is eonsistent with the fact that 1-heteroallenes have been isolated, but 2-heteroallenes are still unknown. 

Experiments reported in 1982 " by tbe same group provided the first example of heteroallene dimerization. In this work, head-to-head dimerization of 1-silaallenes lla-c (Scheme 4) was observed, forming 1,2-disilacyclobutanes 12a-c with two exocyclic double bonds. 

When alkynyldisilanes 13a and b were photolyzed in the presence of freshly generated dimesitylsilylene (Mes2Si ), the silylene added to the Si=C double bond of 1-silaallenes 14a and b to form disilacyclopropanes 15a and b (Scheme 5). Even without the independently generated silylene, photolysis of 13b produced 15b in 8% yield, but compound 13a gave only traces of 15a. In the case of 15b, the dimesitylsilylene most likely originated from silacyclopropene 16. 

The Kumada/Ishikawa group also investigated thermolytic reactions of alkynyl-polysilanes and silacyclopropenes in the presence of nickel catalysts and implicated a 1-silaallene-nickel complex as an intermediate in the reaction pathway... 

Numerous other studies " by Ishikawa and co-workers, with or without nickel catalysts, have reinforced the importance of l-silaallenes and nickel-complexed 1-silaallenes as intermediates in the pathways of the photolyses and thermolyses of alkynylsilanes. 

Barton and co-workers" performed flash vacuum pyrolysis (FVP) on trimethyl-silylvinylmethylchlorosilane (30), resulting in the production of trimethylchlorosi-lane (30%), trimethylvinylsilane (11.5%), and most interestingly, ethynylmethyl-silane (34, 11.9%). A proposed mechanism for the synthesis of 34 (Scheme 10) begins with the lo.ss of trimethylchlorosilane to form silylene 31, which can rearrange either to silaallene 32 or to silirene 33, both of which can lead to the isolated ethynylsilane. 

Maier et alJ studied the FVP of another vinylsilane, 35 (Scheme II). They proposed that, instead of leading directly to the observed products, silaallene 37... 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

In 1997, the intermolecular addition of organolithium reagents to fluoro-alkynylsilanes was used to synthesize three novel, stable 1-silaallenes. In this... 

Compound 59a underwent intermolecular reactions characteristic of silenes (Scheme 19). Water added instantly across the Si=C double bond of the I -silaallene is expected to give vinylhydroxysilane 65 in 71% yield, and methanol was added... 

Silaallene 73"" was synthesized in an manner analogous to that of 70. Compound 73 was stable at room temperature over I month, but in the presence of any protic source (i.e., water, methanol), it underwent a rearrangement different than that observed for 70, inserting into a methyl C—H bond (74) on the octamethylfluorenyl moiety rather than into one of the groups on silicon (Eq. (6)). It is believed that the favored mode of rearrangement for these groups is that of silaallene 73, but 70... 

West et al. have recently described the synthesis and reactions of a 1-germaallene. Germaallene 76 (Eq. (7)) is analogous to silaallene 59a and is synthesized by intermolecular addition of f-butyllithium to precursor 75, followed by salt elimination at —78 C. This germaallene is not stable above 0 C in solution, but remains intact until heated above 90°C in the solid state. In either case, the... 

One other study of group 14 heteroallenes involving transition metals was reported in 1995. Jones et al. described the isolation of a ruthenium complex of a 1-silaallene (132—Scheme 32). The 1-silaallene also interacts with a hydrogen atom as well as the ruthenium metal center. Jones et al. describe this view... 

Malrieu predicted qualitatively that the l-silaallene framework would be nonlinear. but how much the moiety would deviate from 180 was unclear. The first quantitative values were seen with the isolation and structural determination of the two 1-silaallenes 56 and 59a, whieh have very similar Si=C=C angles of 173.5 and 172.0, respectively (Table III). This is an average deviation of 7.3 from linearity—significant, but relatively small compared to the deviations shown by the germanium and tin substituted allenes. 

The structures of two group 14 heteroallenes that are complexed to other atoms have been determined—a ruthenium complex of a 1-silaallene (132a) and a... 

Reiterating the idea that the -complexes 132a and 86a are not truly heteroallenes, one must consider the central carbon C NMR chemical shifts. Silaallene complex 132a starts to approach allene status with a chemical shift of 175.5 ppm, but the most deshielded carbon for alkylidenetelluragermirane 86a is only 153.04 ppm. 

Replacing a carbon with a nitrogen in the 3-position of a 1-silaallene causes a... 

Like the C NMR chemical shifts for the central carbon of heteroallenes. changing the composition of the allene fragment can have as much of an effect (if not more) on the Si chemical shift as does changing a substituent. Switching the carbon at position 3 to a phosphorus (silaphosphaallene 122) moves the " Si chemical shift downheld to 75.7 ppm, which is the most deshiclded " Si shift fora silaallene to date, even though it has two aryl groups on silicon, which should keep the shift uplield. 

How might this held develop in the future At this time, the chemistry of 1-silaallenes is fairly well understood, and that of 1-germaallenes is at least partially explored. Perhaps the series can be extended to 1 -stannaallenes, but these are predicted to have quite limited stability. The isolation of the 1,2.3-tristannaallcne shows that any combination of heteroatoms may be combined to form allenes... 

As yet there is no firm evidence for heteroallenes of the group 13 elements these are likely to be investigated in the future. Finally, the flash photolytic studies of silaallenes which have provided much insight into their formation (see Sect. IIIA2) will probably be continued and expanded to include other members of the heteroalicne family. 

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