Chiral bridges

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

Although there are many reports on the enantioselective catalytic double bond isomerization of functionalized achiral alkenes, that of alkenes bearing an isolated double bond have had limited success. The use of a chiral bis(indenyl)titanium catalyst 5 containing a chiral bridging group realized the highly enantioselective isomerizations of unfunctionalized achiral alkenes with up to 80% ee (Equation (27)).90... 

Twistane, as an example of a non-natural product, which is a chiral, bridged polycyclic compound, completely deprived of functionality. 

Partial racemization, however, occurred in the deamination of optically active 2-aminobicy-clo[2.2.1]hept-5-ene-2-carbonitrile yielding a 1 2 mixture of the ring-contraction products 2-hy-droxybicyclo[3.1.1]hept-3-ene-l-carbonitrile (21, 40% retention of optical activity) and 4-hy-droxybicyclo[3.1.1]hept-2-ene-l-carbonitrile (22, 5% retention of optical activity). The chiral bridged cation gave only 21, while from the achiral allyl cation both 21 and 22 were formed.87... 

Asymmetric isomerization of 4-terr-butyl-l -vinylcyclohexane (6) catalyzed by bis(indenyl)-titanium complex (8) bearing a chiral bridging moiety afforded (S)-4-fcrr-buty 1-1 -ethy lidenecy-clohexane (7) with up to 80% ee (Scheme 3A.3).3,4... 

Discrimination can readily be observed between the two possible modes of attack on a carbonium ion (195 196 and 195 197 when the nucleophile is part of the substrate. In such cases, the phenomenon of neighboring group participation is observed (for a review, see ref. 69). For example, solvolysis of the erythro-tosylate isomer 202 in acetic acid gave largely the erythro-acetate isomer 204 vi a the chiral bridged ion 203, whereas the threo isomer 205 yielded a racemic mixture of threo products 207A and 207B via the achiral intermediate 206 (70). 

The chiral complex EBTHI—Ti is an excellent chiral catalyst [40]. This complex is a derivative of titanocene and used as the Kaminsky catalyst, which has brought epoch-making progress in polypropylene production. The chiral bridged titanocene complex is used for the production of optically active polypropylene arising from the helical structure of the polymer chain. The chiral complex also behaves as an excellent... 

Scheme 8 Asymmetric synthesis route to optically active overcrowded alkenes (CB = Chiral bridging unit).

Chirogenic supramolecular systems based on dimeric/oligomeric porphyrinoids can also be classified according to their complexity and structural features. First, the assemblies consisting of two or more monomeric porphyrins fixed in an asymmetrical fashion by chiral bridges via supramolecular binding will be examined. 

From a stereochemical point of view, tetrabenzophosphonine type systems 40 can be analyzed in terms of chiral bridged biphenyl systems, whose racemization... 

Kanomata N, Suzuki J, Kubota H, Nishimura K, Enomoto T (2009) Synthesis of planar-chiral bridged bipyridines and terpyridines by metal-mediated coupling reactions of pyridinophanes. Tetrahedron Lett 50 2740-2743... 

Current transfer shifts both charge and momentum from donor to acceptor. We have reviewed time-dependent [38] and steady-state [39, 40] descriptions of current transfer through chiral bridges [36, 37]. In the tight-binding models, current transfer arises from coherent interferences between resonant or tunneling paths. This kind of interference has received attention in recent studies of molecular wires and nanodots [45-50]. 

Cavitands III having chiral bridges, X, were studied in Langmuir monolayers and some pH-dependent enantiomeric recognition of amino acids in the subphase was reported.56... 

Enantiomerically pure hemicarcerands in which the two cavitands are connected by chiral bridges derived from l,T-binaphthalene 26a57 or from tartaric acid 26b,c58 should be mentioned here. They show chiral recognition in complexation as well as a chiral differentiation for guest release (Figure 3). 

Figure 10. Calixarene analogues with chiral bridges.

Figure 5.5 Synthesis of bis-NHC ligands with chiral, bridging linker units.

Another interesting approach to an NHC ligand with a chiral, bridging wingtip group was introduced by Perry et al. [45] and uses enantiomerically pure 1,2-diamino-cyclohexane as the scaffold. Reaction with chloroacetic acid chloride and subsequently with DIPP-imidazole yields the imidazolium salt that can be reacted with silver(I) oxide [46] to the respective silver(I) NHC complex. Subsequent carbene transfer to palladium(II) renders the chiral palladium(II) carbene transfer that can be used in catalysis (see Figure 5.9). 

Related Reagents. The synthesis of chiral diazenedicarboxylates as potential chiral electrophilic aminating agents has received little attention. A series of chiral bomyl, isobomyl and menthyl diazenedicarboxylates has been reported and their reaction with achiral enolates of esters and N,N-dimethyl amides afforded a-hydrazino acid derivatives with little or no selectivity. Incorporation of a chiral azodicarboxamide unit into a chiral bridging binaphthyl moiety afforded a-hydrazino acid derivatives with high stereoselectivity in reactions with achiral oxazolidinone anions. ... 

Dimeric sapphyrins 5.138 and 5.139, containing chiral bridging units have also been synthesized. These were obtained by treating the activated form of the sapphyrin mono-acid 5.109 with a chiral diamine (i.e., 5.135 or 5.136 Figure 5.6.3). This afforded the corresponding sapphyrin dimers 5.138 and 5.139 in yields as high as 60% (Schemes 5.6.8 and 5.6.9). The cyclic sapphyrin dimer 5.140 was also prepared from the chiral diamine 5.137 and bis-acid sapphyrin 5.109. However, in this latter instance a stepwise synthetic approach was required. It involved reacting two equiva-... 

The first chiral bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecific polymerization catalyst was racemic ethylenebis-(4,5,6,7-tetrahydro-l-indenyl)zirconium dichloride (see Structure 9) [45]. Ewen showed that the analogous ethylenebis(l-indenyl)titanium dichloride (a mixture of the meso form and the racemate) produces a mixture of isotactic and atactic polypropylene [46]. The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic site control in the case of the titanocene, the achiral meso structure causes the formation of atactic polymer. 

New hydrogenated tetrahydroindenyl and substituted Ind titanocene complexes have been synthesized in an effort to modify the steric environment around the titanium atom in complexes containing chiral bridges connecting two binaphthyl substituted Ind ligands (Scheme 658). The molecular structure of some of these complexes has been determined by X-ray crystallography, -symmetry is observed in the solid state, although in solution at room temperature the NMR spectra indicate 6 -symmetry.1678... 

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