Silacyclopropenes structure

The classical open /l-silylethyl cation 9 and /f-silylvinyl cation 11 are no minima at higher level of theory3,4. They collapse to the bridged protonated silacyclopropane 21 and silacyclopropene 22, respectively. On the basis of their calculated structures (Figure 3) both cyclic molecules are best described as -complexes between a silylium ion and ethene or acetylene, respectively. 

Finally, it has been revealed by ab initio calculations at various levels that electropositive substituents (Li, BeH, and BH2) on silacyclopropene do not form a bridged bond over the ring. Although in 1-lithiumsilacyclopropene 28 the lithium occupies a position on the top of the ring, this structure, however, should be more appropriately described as a silacyclopropene anion Li ion pair, as shown by the MOs and Wiberg indices <1997PCA8565>. 

Irradiation of l,4-bis(pentamethyldisilanyl)butadiyne 88 in methanol yields two 1 1 photoaddition products 90 and 91 as well as one 1 2 photoadduct 93. Adduct 90 is the primary photoproduct, while 91 and 93 are the secondary products. Structure 93 is formed via two silacyclopropene intermediates, 89 and 92 (Scheme 27). Irradiation of 88 with acetaldehyde and acetone in deareated methylene chloride yields only the 1 1 photoadduct with a l-oxa-2-silacyclopent-3-ene ring due to steric effects, preventing formation of a second silacyclopropene intermediate. Trapping experiments for the silacyclopropene intermediates with methanol were performed <19960M2182>. 

Although x-ray diffraction remains the most reliable method for ascertaining the structures of three-membered rings with one silicon or tin, l3C NMR and 29Si NMR can be quite useful (Table 2). In silacyclopropanes, the chemical shifts for the ring carbons range from 10-23 ppm for silacyclopropenes, the range is 147-195 ppm. 

The structures for silacyclopropenes (24)-(28) were calculated using CNDO/2. The small C—Si—C bond angle and the short C=C bond length suggest that the n bond interacts with acceptor orbitals on silicon. This interaction leads to a net stabilization, described as aromatic character <78JOM(i54)C33>. 

The synthesis, structural and spectroscopic properties, and some transformations of silacyclopropenes have been previously reviewed <87PAC993>. Reactions include the addition of alcohols, two-atom insertions, and silylene insertions. 

When the thermolysis of silacyclopropene (59) was carried out in the presence of excess phenyl-trimethylsilylethyne, (60) and (61) were obtained in 13% yield and 23% yield, respectively, along with cis- and /ra .s-7a-mesityl-l-phenyl-2,2a,7-tris(trimethylsilyl)cyclobutenosilaindan (62) and (63) in 20% yield and 26% yield, respectively (Equation (21)). The structure of (62) was determined by x-ray diffraction <920M597>. 

Stoichiometric reaction of tetrakis(triethylphosphine)nickel(0) with silacyclopropene (64) gave nickelasilacyclobutene (65) in quantitative yield (Equation (22)). The structure was supported by spectroscopic analysis and subsequent chemical reactivity <86JA7417,890M2050). 

In recent years Ishikawa and coworkers have described some very complex photochemical rearrangements of compounds derived from nickel-catalyzed reactions of silacyclopropenes with silylalkynes. As depicted in equation 93, it was shown that the disilacyclohexadiene A could be photochemically converted to the same products as were formed from the silacyclobutene B. In the absence of a trapping reagent the silaindene C was formed as a major product, via the proposed intermediates shown141. In the presence of methanol, two adducts were isolated whose structures can be understood in terms of the reaction of methanol with one of the proposed intermediates. While the structures of the products are confirmed by the X-ray crystal structure data, the mechanisms of the reactions observed must be considered speculative at this time. 

Irradiation of l-aryl-4-(pentamethyldisilanyl)-l,3-butadiynes (PDSB, MDSB, NDSB) with acetone yields site specific and regioselective 1 1 adducts (74-78) having l-oxa-2-silacyclopent-3-ene structure through two-atom insertion of acetone to silacyclopropene intermediates [96]. 

---