Antiperiplanar alignment

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

This surprising inversion shows unequivocally the role of TB coupling involving the bridging ace and acc NBOs that are in favorable antiperiplanar alignments with both nN and n0. 

The main product is thought to arise from transition state (119 Scheme 19). (119) is stabilized through the antiperiplanar alignment of the allylic C—O orbital with respect to the approaching carba-nion. This geometry entails the formation of the syn product (117). The diastereomeric transition state (120) is caused by the alkyl substituent striving for the least hindered orientation with respect to the peri-cyclic transition state. (120) competes with the electronically favored (119) only if R is sterically demanding. 

The anomeric effect is also present in acyclic systems and stabilizes conformations that allow antiperiplanar alignment of the C—X bond with a lone pair orbital of the heteroatom. In addition to a-haloethers, anomeric effects are prominent in determining the conformation of acetals and a-alkoxyamines. 

The results in Table 26.2 reveal that the TT conformer with dihedral angles C-O-C-C of 180° is the most stable, followed by the TG one. The least stable structure is GG , where the two methylene groups are in antiperiplanar alignment with respect to the ether group plane. Based on IR absorption intensity temperature dependence, Wieser et al. [28] estimated the enthalpy difference between TT and TG as 1.1 kcal/mol. This agrees well with the result obtained in this study. 

Fragmentation reactions, which are similar to p-elimination reactions (Section 2.1), can be useful for the formation of alkenes, particularly from carbocyclic compounds. Fragmentation reactions occur most easily from conformationally locked 1,3-difunctionalized compounds, in which the breaking C—X and C—C bonds (highlighted) are aligned antiperiplanar (2.29). 

Generalized anomeric effect The stereoelectronic requirement that a lone pair n at a heteroatom X and C-Y bond in a YCH X moiety are aligned in an antiperiplanar geometry for maximizing the hyperconjugative n interaction also leads to interesting conformational preferences in acyclic systems (Figure 6.62). 

Rotation around C-X bonds in heteroatom-substituted alkenes The preferred orientations of substituent X in the R2C=CR -X systems relative to the vinyl moiety are also interesting. Whereas the flat conformation is preferred for X=0, S to allow the perfect alignment of the p-type lone pairs atX with the it-systan, the varying degrees of pyramidalization can be observed for X=N as already discussed. When the second lone pair is present (X=0, S), rotation around the 0-R and S-R bonds is controlled by stereoelectronics in a well-defined manner that arranges this lone pair antiperiplanar to the orbital. As with aldehydes, esters etc., this situation again illustrates the interplay of different effects that manifest themselves in different observed properties 7i-effects (n -) n ) are important for reactivity whereas sigma effects (n o ) define shape of the molecules. 

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