Chiral dimer structure

Of course, there are two possible dimeric structures of the 1 1 complex 68, as shown in Figure 1.11, namely 69 and 70. Both dimers of 68 should behave in similar fashion in 6Li NMR to the previous experiment. To differentiate those two structures, the nitrogen atom in the chiral modifier 46 was labeled with 15N. Ii-d (-0.42 ppm) would be a doublet if the intermediate is 69. However, Li-c (-1.2 ppm) would be a doublet if it is 70. 

From this discussion the clear similarity between the SmAPA and SmCsPA structures is easily seen. In addition, the suggestion of Brand et al.29 that a bilayer smectic with all anticlinic layer interfaces (the SmAPF) would produce an achiral ferroelectric smectic follows directly. The unanticipated tilt of the director in the tilt plane, leading to a chiral layer structure, seems to be a general response of the bent-core mesogens to the spontaneous nonpolar symmetry breaking occurring in these rigid dimer structures. 

In 1999, Nozaki and co-workers were the first to report an asymmetric copolymerization, catalysed by a chiral amino-alkoxide zinc complex 15 (Table 6) and producing optically active PCHC with 70% ee (measured by hydrolysing the copolymer and analysing the resulting diol using chiral GC) [138,148], The crystal structure of the catalyst, reported subsequently, showed a dimeric structure it was unclear whether the dimer was maintained during the copolymerization [148], In the solid state, the zinc-zinc distance in the catalyst was determined to be 3.00 A (vs. ca. 4 A, for the loosely bound BDI zinc dimers). 

Fig. 17 Examples of intrinsically chiral molecules that associate into dimeric structures A clip racemic molecule provides only heterochiral dimers, while B helicene-based molecules give only homochiral dimers...

Stoddart and coworkers40 have synthesized a chiral lithium amide with C2-symmetry and two chelating methoxy groups from the amine (R,R)-di(a-methoxymethylbenzyl)amine (7). This lithium amide was crystallized from a hexane solution and X-ray analysis revealed a dimeric structure where both lithiums are tetracoordinated, (Li-7)2. 

Hilmersson and Davidsson studied by means of intensive NMR investigations a mixed 1 1 complex of n-BuLi and the Hthiated methoxyamine 6 (Scheme 1) in diethyl ether at -80°C [68]. A fluxional exchange between tetrameric and dimeric structures of the chiral Hthiated pyrrolidine 14 and -BuLi is apparent in diethyl ether solution [69]. With the Hthiated amine 15, a derivative of 6,75% ee of (S)-l-phenyl-l-pentanol was obtained in n-BuLi additions to PhCHO, using a ratio of 1.0 0.45 0.25 in diethyl ether at -116°C [37]. The N-methyl derivative gave only 2% ee, while the N-isopropyl derivative 16 yielded 82% ee under the same conditions. This demonstrates Ae crucial role of the N-isopropyl substituent in 16. Addition of dimethoxymethane increased the enantioselectivity of 16 to 91% ee of (S)-l-phenyl-l-pentanol. With Hthiated 16, enantioselectivities of up to 98.5% ee were achieved in butylations of aHphatic aldehydes [70]. Hilmersson demonstrated that the mixed Hthium amide/ -BuLi aggregate alkylates aldehydes faster than the pure -BuLi oHgomers, Eq. (2) [71]. 

The structure of S, S-diazaaluminolidine (1), a chiral compound which has been used in enantioselective Diels-Alder reactions, has been determined, and forms a dimeric structure embodying an eight-membered (A1NS0>2 ring through intramolecular S=0 — A1 coordination. ... 

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