Syn-rotamer

It should be noted that this adduct is chiral and that the opposite enantiomer would result upon addition of the phosphine to the other CNO face, which corresponds basically to addition of the phosphine to the other side of the M=C bond. Predictably, the base is bound most strongly when the alkoxide is electron-withdrawing (e.g., OCMe(CF3)2), and is more strongly bound to W than to Mo. A relatively stable CNO adduct of the syn rotamer forms first when OR is OCMe(CF3)2, as it is virtually the only rotamer present (Kei-103), and as was shown later (see below), the rate of conversion of the syn to the anti rotamer... 

Relatively few simple benzo[6]furan derivatives have been studied by X-ray or electron diffraction techniques. (Their hydro derivatives are better known.) An aldehyde prepared by formylating 2,4,7,-trimethylbenzo[fr]furan could not be orientated by spectroscopic methods so was converted into the derivative (36) and found to have the dimensions shown (81CSC83). In the dibenzofuran (37) (78CSC629) the effect of an acetyl group on the bond lengths and angles can be assessed by internal reference and seems to be very small, yet the molecule is nearly planar and is the s-syn rotamer in conformity with the later discussion. 

This is interpreted in terms of a mechanism involving an equilibrium between the syn and anti rotamers of the initiator, in which the conversion of the dominant syn rotamer into the minor anti rotamer is rate-determining at high [M], with it = 6.1 x 10 5 s 1, while at low [M] the addition of M to the anti rotamer becomes rate-determining305. The value of k agrees well with the value determined directly in toluene by photochemical displacement of the equilibrium between the rotamers121,122. The tacticity of this polymer is mentioned in Section VIILA.5. 

A marked temperature dependence of the cis content of the polymer formed from 219 using 224 as initiator in THF (100% at — 35 °C, 24% at 60 °C) has been interpreted in terms of an equilibrium between syn and anti rotamers, with cis C=C formed mainly by addition to the THF-free syn rotamer, and trans C=C formed mainly by addition to the THF-free anti rotamer556 cf Section III.B.5. 

The reaction of 219 with 7 (R = Me) proceeds mainly via the less abundant but much more reactive anti rotamer of the initiating and propagating species. The first four propagating species may be distiguished in the H NMR spectrum for the longer chains the anti and syn rotamers are just resolved, in the ratio of 1 6. The corresponding 13C... 

The conformations of 3-substituted-2-benzothiazolinones have been studied by UV and IR spectroscopy and the quantum chemistry semiempirical method AMI <93Mi 306-06,93MI 306-07). 5-Acyl-2-(7V,7V-disubstituted amino)thiazoles unsubstituted or with a phenyl group at the 4-position, in solution exist predominantly as the carbonyl 0,S-syn rotamers (25a). On the other hand, a t-butyl group in the same position induces the 0,S-anti arrangement (25b) (Equation (3)) with the 4-methyl derivatives both forms are present <87JCS(P2)2305, 88JCS(P1)2209>. 

Fig. 4. The relative orientations of the substrate and nicotinamide lead to a correlation between rotamers and stereospecificity. When the substrate binds on top of the nicotinamide ring, then the syn rotamer will lead to pro-S specificity of hydride transfer (a) however, when the anti rotamer of the nicotinamide ring is bound, then the enzyme will transfer the pro-R proton (b).

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the... 

Efforts have been made to explain the high rate acceleration of Mizoroki-Heck reactions in ionic liquids. The formation of the dialkylimidazol-2-ylidene palladium complex under conditions similar to those employed for the Mizoroki-Heck reaction has been studied. The C2-H proton of the imidazolium cation exhibits high acidity and can be deprotonated to form a carbene species, behaving as a good ligand for transition metals. Therefore, in the presence of a palladium salt and a base, [bmim][Br] formed the dimeric carbene complex 89, which further evolved to the monomeric c/x-90 and trans-9Q complexes. Each of these exists as an anti and a syn rotamer owing to the sterically demanding (V-alkyl substituents (Scheme 35 only the anti-90 rotamers are represented). 

The cage structure 6 is composed of two face-to-face porphyrins linked at both sides via six hydrogen bonds to two triaminopyridine entities. The distance between the cofacial porphyrins is estimated to be 10 A. The initial nieso 5,15-diuracil-substituted porphyrin has two rotameric forms, a syn and an anti, due to the relative orientations of the two uracil groups with respect to the porphyrin plane. In the self-assembling process, the syn rotamer yielded the major component, the cage structure, whereas the anti conformer yielded a zig-zag strand structure. As for the properties expected by such systems, metalation of one of the porphyrins allows for the creation of a self-assembled donor-acceptor system while complexation of the porphyrins with zinc(Il) allows for the cage cavity to be used as a... 

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