Synperiplanar conformation

The selectivity for (/ ,/ )( ,S)-10 has been rationalized by invoking a synperiplanar enolate species whose conformation is enforced by a donor(enolate oxygen)- acceptor) peril uo-rophenyl) interaction depicted in structure N47. Infrared and variable temperature NMR spectroscopic studies of the neutral precursor complex 8 support the existence of such a donor-acceptor interaction. 

Phenyllithium and phenylcopper boron trifluoride yield different diastereomers of the reaction products, i.c., the sense of asymmetric induction is a function of the metal. These results are rationalized on the basis of antiperiplanar 6 and synperiplanar 8 reactive enoate conformations for additions of the copper and lithium reagents, respectively. 

Here again, the addition took place from the synperiplanar conformation 8. The conjugate addition of trichloromethyllithium, prepared from butyllithium and carbon tetrachloride at —110°C, also proceeded through the synperiplanar conformation, although with low diastereoselectivity. 

In /Bu2(zPr)PSeBr2 intermolecular Br- Br contacts (358.90 pm) between T-shaped moieties lead to wave-like chains (Figure 27) that are packed into layers in the crystal. The conformation of the H-C-P-Se moiety in /Bu2(/Pr)P-SeBr2 (involving the methyne proton from the /-propyl group) is synperiplanar (0 = —43°) the PSe bond is longer (230.39 pm) and the deviation of the BrSeBr... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

The part of the Sequence Rule (4) that deals with conformations provides terms that are either fully chiral (MIP = minus/plus) or fully achiral (ap/sp = antiperiplanar/synperiplanar), but none that are suitable if the difference is either only graphochiral or only pherochiral. The torsional difference between 29a and... 

It can be seen from Fig. 8 that the minimal basis set on the hydrogen atoms produces larger errors in the OP term and FC term of 1,2-difiuoroethane in the synperiplanar conformation (8) than in the antiperiplanar conformation (6). The same difference was also observed between the trans and cis couplings in the... 

We find that the OP is the most important term and the absolute values of the four contributions follow the order OP > SD > F C > OD for all the molecules studied here with the exception of the synperiplanar conformations of 1,2-difluoroethane and 1,2-difluoropropane, where the FC term is the most important... 

In a polarizable environment, the absolute minimum structure of HFIP carries the OH synclinal (sc) or almost synperiplanar (sp) to the adjacent CH (Fig. 9) [49]. On the basis of quantum-chemical considerations as well as single-crystal X-ray structures in which HFIP acts as hydrogen bond donor, HFIP always takes on such an sc or even sp conformation. In this conformation, the hydrogen bond donor ability of HFIP is significantly increased (Figs. 10 and 11) [49]. 

It remains to explain why the Felkin transition states are the most stable. During the reaction, the major interaction occurs between the nucleophile s HOMO and the substrate s LUMO. Therefore, the most reactive conformation of the substrate is that with the lowest LUMO. This corresponds to the geometry in which the C2-L bond is parallel to the tt system, as there is then a good overlap between the orbital and the lowlying ac2-L orbital, leading to a stabilization of the LUMO. The nucleophile may attack this conformer in an antiperiplanar or synperiplanar stereochemistry. The latter is disfavoured for two reasons ... 

Lactic acid has a preferred conformation in solution with the hydroxyl group synperiplanar with the carbonyl group. The conformational dependence of its chiroptical properties has also been examined in detail. Empirically based relationships69,70 between CD spectra and conformations have been supported68 by calculations. 

Analysis of the crystal structure of the p-nitrobenzoate and 2,4-dinitrobenzoate derivatives of ewrfo-3-(trimethylsilyl)-ewrfo-2-norborneol 47, in which the Si—C and C—O bonds are in the synperiplanar geometry, showed that there was no significant lengthening of the C—O bond in these cases64. By comparison, the cyclohexyl ester 48, in which the Si—C and C—O bonds are in the antiperiplanar conformation, did show a significant lengthening of the C—O bond compared to model compounds64. 

In principle a crx A — 0 Y-A charge-transfer interaction would also be possible when the two vicinal X-A and Y-A bonds adopt a synperiplanar conformation. However, in this latter conformation the overlap integral crx A — [Pg.17]

An unshared electron pair can usually only assist nucleophilic displacements if this lone pair and the antibonding (cr ) orbital between carbon and the leaving group are either antiperiplanar or synperiplanar. If such a conformation cannot be achieved, a-heteroatoms can even reduce the rate of Sn2 or SnI reactions. Scheme 4.31 shows some examples of conformationally constrained, tertiary tosy-lates for which hyperconjugation between the heteroatom lone pairs and the antibonding C-OTs orbital is not possible [129]. Thus, although a-alkoxyalkyl sulfonates... 

It might, at first glance, come as a surprise that, despite the ban on 5-endo-trig cyclizations, 5-endo-dig cyclizations are allowed. A possible reason for this observation is that intramolecular nucleophilic or electrophilic attack at an alkyne can proceed with the substrate in a flat conformation, because two orthogonal jt (or jt) orbitals are available. Similarly, hydroxymethyl allenes cyclize on treatment with a base (Scheme 9.7), because the terminal Jt orbital of the allene and the hydroxymethyl group can readily achieve a synperiplanar orientation. Intramolecular... 

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