Conformation crystal structure-solid state

To this point, all the examples presented have been ones in which the origin of the asymmetric induction has been unimolecular in nature, that is, the molecules adopt homochiral conformations in the solid state that favor the formation of one enantiomer over the other, usually through the close intramolecular approach of reactive centers bimolecular crystal packing effects appear to play little or no role in governing the stereochemical outcome of such reactions. This raises the interesting question of whether the soUd-state ionic chiral auxiUary approach to asymmetric synthesis could be made to work for conformationally unbiased reactants, i.e., those possessing symmetrical, conformationally locked structures. Two such cases are presented and discussed below. 

In the case of (43) (R = Me or Et cis- and trans-isomcis are formed in comparable quantities, and these can be separated by fractional crystallisation. These compounds all have relatively large and positive exo-P—N—P spin-coupling (typically 350-400 Hz), and as shown in Scheme 7, this can be attributed to the preponderance of conformers a. The individual spin-couplings for the AA XX -type spin system exhibited by the P- H n.m.r. spectra for the m-isomer where R = Me are shown in Fig. 4, (i). The crystal structure of the same compound shows that the molecule also adopts the same conformation in the solid state. When R = Pr or Bu , only cw-isomers of (43) are formed a surprising result, which... 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

While working on a related series of non-polymeric model compounds, we prepared the binaphthyl derivative 35 by the synthetic route depicted in Scheme 3.17 [34, 51] The yield was particularly poor (10%) but, nevertheless, a full characterization of 35 was achieved. The crystal structure of 35 has been elucidated from single crystals obtained from THF [51] and reveals that the molecule adopts a folded conformation in the solid state that is characterized by an almost parallel... 

Macrocycle 177 in which the 2,6-pyridino and the 1,4-piperazino moieties were incorporated into the macrocyclic framework has been reported. The synthesis of 177 (10%) was accomplished by treatment of 2,6-dichloropyridine with the dianion of Ar,Ar -W. (2-hydroxyethyl)piperazine in refluxing xylene. Attempts to prepare the cobalt(II) complex of 177 resulted in diprotonation of the macrocycle. The X-ray crystal structure determination analysis has been performed for both 177 and 178. According to the crystal structure analysis of 178, the piperazine rings are in the chair conformation in the solid state and the molecule is fairly rigid due to the imposed steric constraints of the imidate moieties l39). 

A single crystal X-ray structural analysis of the furan-containing macrocycle 6.20 revealed that it adopts a nearly planar conformation in the solid state (Figure 6.3.2). Indeed, the average deviation from the mean least-squares plane of all nonhydrogen atoms was found to be only 0.04 A. The planarity of 6.20 results in the... 

A single crystal X-ray structural analysis of the alkyne-containing product 6.26a revealed that the macrocycle adopts a near-planar conformation in the solid state (Figure 6.3.3). Deviation from planarity results, presumably, from the methyl-methyl steric interactions within the terpyrrolic subunit. The crystallographic data also served to support the contention that the macrocycle does indeed contain a true an alkyne-like (C-C=C-C) fragment. Such a bond isolation, although expected in the present instance, stands in contrast to what is seen in the case of Vogel s stretched porphycene precursor. In this latter instance cumulene-type (C = C = C = C) bond delocalization is observed. 

A separate single crystal X-ray structural analysis revealed that the partially reduced macrocycle 6.27b also adopts a near-planar conformation in the solid state (Figure 6.3.4). It also confirmed the anticipated trans-stereochemistry about the meso-C = C bond in that one of the protons was found to be directed into the core of the macrocycle, while the other points outward. Taken together these solid-state structural findings are thus consistent with the proposal that 6.27 is a true isomer of sapphyrin. 

The NMR spectra of the oxyanions generated by the action of BuLi on p-allylcalix[4]arene in DM SO suggest that the conformations of the mono- and tetra-anions are cone, while those of the di- and tri-anions are less clear " " (see ref. 1, pp. 124-126). However, X-ray crystal structures of the monosodium and dilithium salts of 4 , generated by the action of NaH or BuLi on the calixarene followed by treatment with MeOH, show both to be in the cone conformation in the solid state. "" ... 

In the solid state and in solution, six-membered tetrahedral phosphorus compounds generally adopt a chair as the most stable ground-state conformation. However, Bentrude has shown, by nmr analysis, that the barrier to chairs twist interconversion is low in some six-membered systems (Day et al, 1984 Bentrude et al, 1984 Beres et al., 1987 Nelson et al., 1987). The twist/chair activation barrier for 1,3,2-oxazaphosphorinanes is low. Compound [79] exists in the twist conformation in the solid state and solution. Compound [80] adopts a full boat conformation, from solution nmr spectroscopy and crystal-structure analysis. 

---