Heteroallenes

Novel coordination compounds formed from CS2 and heteroallenes. H. Werner, Coord. Chem. Rev., 1982, 43,165-185 (53). 

Eichler, Barrett, and West, Robert, Chemistry of Group 14 Heteroallenes. 46 1... 

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. 

The first stable group 14 heteroallene, a l-stannaketenimine (99), was reported by Griitzmacher et al. in 1992." Compound 99 was synthesized in 9 % yield by adding diarylstannylenc 97 and mesityl isocyanide 98 in hexane (Eq. (12)). The bonding in 99 can be described as a stannylene-isocyanide adduct rather than a... 

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... 

The crystal structures of a variety of heteroallenes have been solved and provide valuable information toward the understanding of the bonding in this series... 

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... 

Possibly the most characteristic piece of information one can obtain to prove the existence of a I-heteroaallene is the central carbon C NMR chemical shift. This carbon chemical shift is very deshielded. typically being greater than 200 ppm, which stands out from most other carbon resonances in a normal organic molecule. Most of the group 14 I-heteroallenes listed in Table VII have shifts greater than 200 ppm. Also, as the heteroatom becomes larger, the resonance moves farther downfield. 

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. 

Another useful tool for analyzing the structure of heteroallenes containing silicon is Si NMR. Typically, sp"-hybridized silicon atoms have a chemical shift... 

Although it is not a group 14 heteroallene, arsaphosphaallene Mes As=C=PMes (+299..S ppm) over-emphasizes the point that the central carbons of heteroallenes are greatly deshielded. For reference, l-phosphaallene Mes P=C=CPh2 has a central carbon shift of +2.17.6 ppm and 1,-1-diphosphaallene Mes P=C=PMes has a shift of +276.2 ppm. 

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. 

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