Synperiplanar orientation

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... 

Reactions similar to this type of fragmentation have been encountered by Sukh Dev in longifolene chemistry. One noteworthy feature of the fragmentation leading to (85) is Ae synperiplanar orientation... 

A single conformer of trifluoroacetic anhydride, CF3C(0)0C(0)CF3, is indicated by the analysis of gas electron diffraction data, but there is some doubt about the precise structure of this conformer.29 The two C=0 bonds have the synperiplanar orientation, but it is not possible to say whether the equilibrium structure is planar, with C2V symmetry, or distorted to C2 symmetry. The refined effective dihedral angle C-0-C=0 is 18(4)°, reasonably consistent with values of 16.5 and 13.9° given by MP2 and B3LYP calculations with the 6-31G basis set. 

In phosphanyl ylides, /(P,P) couplings depend strongly on the orientation of the P(III) lone pair with the phosphonio group they are largest, 210 Hz, when both moieties are placed with a synperiplanar orientation (8). ... 

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... 

In order to determine the conformation, the substitution pattern at carbon atoms C2 and C3 must first of all be established. Three different groups are attached to each. Hence, the groups of highest priority on each are taken as the reference groups (shown bold in the diagram below), thus this must be the antiperiplanar conformation. In order to transform the formula into a Fischer projection formula, the part of the formula which is closer to the observer is rotated until an eclipsed conformation is obtained in which the groups forming the main chain are oriented synperiplanar to each other. 

Conformations about two bonds must be considered for fluorocarbonyl trifluoromethanesulfonate, FC(0)0S02CF3.33 There are two different arrangements of the FC(O) groups relative to the CF3 group, trans [67(8%)] and gauche, with the CSOC dihedral angle 72(6)°. In both of these the C=0 bond is orientated synperiplanar with respect to the OS single bond. This distribution... 

In the next step, the product-yielding fi-hydride elimination (step D) can occur only after an internal rotation (step C) around the former double bond, as it requires at least one fS-hydrogen to be oriented synperiplanar with respect to the halopalladium residue. In some cases, anti elimination was made possible by suitable substrates or conditions leading to an type mechanism [16, 17]. The subsequent syn elimination yielding an alkene and a hydridopalladium halide is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained when the coupling reaction is performed with a terminal alkene. Reductive elimination of HX from the hydridopalladium halide, aided by the added base, regenerates the active catalyst and thereby (step E) completes the catalytic cycle. 

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