Silyl and Germyl Cyclopropanes

The rotational barrier of the SiH3 group is definitely larger in c-C3H5SiH3 than in MeSiH3, while there is only a little difference between the corresponding barriers for CH3 

TABLE 13. Structural parameters of cyclopropylsilanes and -germanes c-C3H5AX3 (140)  

TABLE 14. Rotational barrier (kJmol ) of the AH, group in R—AH3 molecules 

The double bond in bicyclopropylidene (144) is shorter than in 142 and in ethene. Bond distances (A) from XD (-40 °C) have large uncertainties because of poor crystal quality, 

The ring and methylene geometries253 in Feist s acid (151), Cl—C2 1.546 (4), Cl—C3 (mean) 1.463 and C=C 1.313 (4) A, are similar to those in 142 (Table 15, note the different numbering). In both molecules, the bond opposite to C=CH2 is elongated, and the adjacent bonds are shortened. The COOH groups in 151 adopt sc conformations, and are disordered as concerns the interchange of their C=0 and C—OH groups253. 

This class of cyclopropane derivatives is characterized by five bonds linking substituents. The molecules can formally be derived by replacing one pair of geminal bonds in substituted cyclopropanes (2) by an exocyclic double bond (3). (It would have been better to name these bonds semicyclic , but the term exocyclic has been accepted.) The analogy is more than formal. Just as the geminal F2 substituent (and, to a smaller extent, CI2), also the =CH2 and the substituents induce a relative lengthening ( 1) of the distal bond by about 0.06 A, and a shortening of the vicinal bonds (see also References 5 and 238). 

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The silyl and germyl groups are (x-donors and 7I -acceptors Calculated CC distances in silyl- and germyl cyclopropane suggest that the Ti-acceptor character is significantly more pronounced for a silyl than a germyl substituent. 

A recent example of the preparation of a silylated and germylated cyclopropanone has been provided by Russian workers who added diazomethane in ether or methylene chloride at —130 °C to an equimolar amount of trimethylsilyl and trimethylgermylketene. The authors were able to obtain PMR spectra of the parent ketones which show an ABC system of cyclopropane ring protons at 0.8-1.8 ppm. These products react with methanol to form hemiketals and undergo ring expansion with excess diazomethane to form the corresponding cyclobutanone derivatives. 

Although a great number of cyclopropanes bearing a trialkylsilicon, -germanium, -tin, or -lead substituent are known, few are prepared by an addition reaction of a metal-substituted carbene to a C-C double bond. The syntheses and reaction pathways of silyl-, germyl-, stannyl-, and plumbylcarbenes have been described in detail in Houben-Weyl, Vol. E19b, p 1410. 

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