SiMe, group

H NMR of pyrrolophane 30 shows two singlets for SiMe groups that do not coalesce in the temperature range of -80 to +110°C, in contrast to furanophane 28, which shows rapid anti/syn isomerization down to -78°C, and thiophenophane 29, with a coalescence temperature of 15°C. The rigidity of 30 is obviously due to the steric bulk of the 2V-methyl groups. 

This property of the — SiMes group has also been quite clearly demonstrated in an extremely elegant manner by Bedford et al. (77). It has been amply demonstrated that in an electron spin resonance spectrum the isotropic hyperfine coupling constant, an, of a hydrogen atom attached to an sp2 hybridised carbon atom having an unpaired electron in the 2p—orbital is given approximately by an Equation (3) due to McConnel (18)... 

One of the objections to these experiments is that they give no real measure of the extent to which the SiMes group conjugates. The system of degenerate levels is so finely balanced that it takes very little to tip it one way or the other. Indeed there is evidence that the effect is small. In... 

Chemical shifts (5) in ppm relative to TMS(29Si) and MeN02(15N). For the SiMe group. cIsotope shift in ppb. 

In the case of two different substituents at silicon, the stereochemistry of the final [4+2] cycloaddition product depends on steric repulsion. Thus, the Diels-Alder reactions of silole 51 gives mixtures of 7-sila-norbornenes 52 and of 7-sila-norbornadienes 53, respectively, where the major isomer is that in which the SiMe group is in ry -position with respect to the C(2)-C(3) double bond <2003JOM(665)196>. 

The two positively charged carbons of the allyl ipoiety in 31 are less deshielded than the corresponding carbons in the analogups carbocation with a CH3 instead of a SiMes group. This indicates that the positive charge is delocalized away from the allylic system 31a 31b towards the (J-trimethylsilyl... 

An iterative sequence of propargylation, enantioselective epoxidation of the resulting silylated skipped enynes, an endo selective hydroxyepoxide cyclisation and protodesilylation in which a SiMes group plays a pivotal role produces the tris-tetrahydropyran 9 in 18 steps <03OL2339>. The stereoselectivity of the endo selective oxacyclisations of 1,4-di- and... 

The poly(silylene-co-ethynylene)s, described in the literature, mainly contain SiPh- or SiMe-groups, rarely SiH- or SiVi-units. These polymers or oligomers are currently produced by the condensation of dihalosilanes with the di-Grignard reagent of acetylene [3, 4], dilithioacetylene [5-7] or disodiumacetylide [8]. 

The reaction between disubstituted 1,3-diynes such as 5,5-dimethyl-1-tri-methylsilyl-l,3-hexadiyne with metallocenes M(t -C5H5)2 studied by Baumann et al yielded a complex with the intact diyne when M = Ti and a CT,7c-alkynyl-bridged dinuclear complex when M = Zr. These two types of compounds could be easily discriminated by the use of 7hc couplings observed between the terminal carbon atoms of the 1,3-diyne moiety and the protons either of the t-Bu or the SiMes group. 

The PMR spectrum consisted of two unsplit signals of equal areas which, from their r values, are due to a SiCH2Si and a SiMe group (18)... 

Table XI). This shows the presence of structure (c) in which all SiCH2 and SiMe groups are equivalent and in which the ratio of the number of protons in each group is 1 1. 

The PMR spectrum shows two resonances with an area ratio of 6 1. The r value for the smaller signal corresponds with a SiCH i group and that of the larger with an SiMe group. The absence of further signals and the area ratio show the structure to be (/) (18). This result was confirmed by an X-ray structure determination on the compound (46). 

The availability of the SiMes group to protect the terminal alkyne group in a C5 intermediate such as 1 -trimethylsilylpenta-1,4-diyne or 1 -bromo-6-trimethy Isilylhexa-... 

Figure 10.9. Proposed structures of 6 (left) and molecular structure of 7 (right the SiMes groups have been omitted for clarity).

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