Synperiplanar

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... 

Roth and coworkers23 reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking rr-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synperiplanar (cis or gauche) butadiene. 

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... 

The prefixes synperiplanar and antiperiplanar will often be abbreviated as spp and app in the following. 

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... 

Fig. 8. Dependence of the vicinal spp F-F coupling constant in synperiplanar-1,2-difluoroethane on the basis set used for the carbon and hydrogen atoms Deviation (in Hz) of the locally dense basis set result from the calculation with the apTJ/apTJ/pT basis set. Only the basis sets on the non-coupled atoms are used as labels for the basis set axis. The apTJ basis set was used for fluorine in all calculations.

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]. 

MONOD-WYMAN-CHANGEUX MODEL KOSHLAND-NEMETHY-FILMER MODEL SUBSTRATE SYNERGISM SYNERGISTIC INHIBITION Synperiplanar,... 

Note that the terms syn and anti alone are no longer used in this context. The chlorine atoms may be described as synperiplanar, synclinal, anticlinal or antiperiplanar to each other. 

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 ... 

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