Heterochiral coupling

Wynberg studied stereochemistry of the McMurry reductive dimerization of camphor in detail (64). In Scheme 37, A and B are homochiral dimerization products derived by the low-valence Ti-promoted reduction, while C and D are achiral heterochiral dimers. The reaction of racemic camphor prefers homochiral dimerization (total 64.9%) over the diastereomeric heterochiral coupling (total 35.1 %). Similarly, as illustrated in Scheme 38, oxidative dimerization of the chiral phenol A can afford the chiral dimers B and C (and the enantiomers) or the meso dimer D. In fact, a significant difference is seen in diastereoselectivity between the enaritiomerically pure and racemic phenol as starting materials. The enantiomerically pure S substrate produces (S,S)-B exclusively, while the dimerization of the racemic substrate is not stereoselective. In the latter case, some indirect enantiomer effect assists the production of C, which is absent in the former reaction. Thus, it appears that, even though the reagents and reaction conditions are identical, the chirality of the substrate profoundly affects the stability of the transition state. 

The Dq value of the complex between Cr and 1-butanol (2.6 0.2 kcal mol ) well conforms to the approximate value of 2-3 kcal moP indirectly estimated for the dissociation energy of the complex between F/ = (l )-(- -)-2-naphthyl-1-ethanol and methanol. ° Concerning the diastereomeric complexes, the homochiral adducts are invariably more stable than the heterochiral ones. This trend extends to the corresponding Si excited complexes as well. This observation, coupled with the appreciable deviation from linearity of the corresponding Ap values (Fig. 9), corroborates the view that the interaction forces in these complexes are affected by steric congestion to a different extent. Their sensitivity to steric factors is demonstrated by the diverging observations that (Table 4) (i) in the diastereomeric... 

The easiest combination of two chiral guests in the cylindrical capsule comes by the double encapsulation of racemic species with adequate size, shape and polarity to be accommodated in pairs, giving two diastereoisomeric complexes, the homochiral couple (R) - (R)/(S) - (S) and the heterochiral (R) -(S) combination. ( )-trans-1,2-Cyclohcxancdiol has all the above requisites and showed a 1.2 ratio between the two complexes in favor of the heterochiral combination [60]. This observation may be related to the preference in nature for centrosymmetric crystals or, alternatively stated, the higher melting points of racemates vs. enantiopure compounds where the resolution is driven by the less soluble pair [61,62], In the cylindrical capsule a single couple of chiral molecules is extrapolated from the bulk and the interactions between the two is governed by the shape of the cavity and their goodness of fit within the cavity. 

SO as to push apart the adjacent turns in a heterochiral coil (Figure 13.7a) or draw closer together the adjacent turns in a homochiral coil (Figure 13.7b). Further evidence to support coupling between fibre torsion and coil tensile actuation is the near equivalence of work density in both modes 2.1 kj kg torsional work capacity was measured in twisted nylon-6,6 fibre and 2.5 kjkg was measured when the same fibre was formed into a coil and operated in the tensile contraction mode. 

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