Alkenes chiral complexes

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

The studies summarized above clearly bear testimony to the significance of Zr-based chiral catalysts in the important field of catalytic asymmetric synthesis. Chiral zircono-cenes promote unique reactions such as enantioselective alkene alkylations, processes that are not effectively catalyzed by any other chiral catalyst class. More recently, since about 1996, an impressive body of work has appeared that involves non-metallocene Zr catalysts. These chiral complexes are readily prepared (often in situ), easily modified, and effect a wide range of enantioselective C—C bond-forming reactions in an efficient manner (e. g. imine alkylations, Mannich reactions, aldol additions). 

Prochirality Planar molecules possessing a double bond such as alkenes, imines, and ketones, which do not contain a chiral carbon in one of the side chains, are not chiral. When these molecules coordinate to a metal a chiral complex is formed, unless the alkene etc. has C2V symmetry. In other words, even a simple alkene such as propene will form a chiral complex with a transition metal. So will trans-2-butene, but cis-2-butene won t. If a bare metal atom coordinates to cis-2-butene the complex has a mirror plane, and hence the complex is not chiral. It is useful to give some thought to this and find out whether or not alkenes and hetero-alkenes form chiral complexes. One can also formulate it as follows complexation of a metal to the one face of the alkene gives rise to a certain enantiomer, and complexation to the other face gives rise to the other enantiomer. 

When a chiral metal complex forms a complex with a prochiral alkene , either because it contains a chiral ligand or a chiral metal centre, the resulting complex is a diastereomer. Thus, a mixture of diastereomers can form when the chiral complex coordinates to both faces of the alkene. As usual, these diastereomers have different properties and can be separated. Or, more interestingly, in the catalytic reactions below, the two diastereomers are formed in different amounts and their reactivities are different as well. 

Carbohydrates remain an attractive source of chirality in preparation of ligands for asymmetric catalysis. Functionalized phospholanes, 192 [167], and chiral bisphosphinites 193 [168] with an attached crown ether unit were obtained recently from D-mannitol and from phenyl 2,3-di-0-allyl-4,6-0-benzylidene-p-D-glucopyranoside, respectively (Figure 18). Compounds 194 and 195 were obtained in the photochemical addition of H2P(CH2)3PPH2 onto the crresponding alkenes - Pd-complexes of these new bisphosphines were successfully applied as catalysts in the copolymerization of CO and... 

When the Pd bears chiral ligands, these reactions can be enantioselective.1448 ir-Allylmo-lybdenum compounds behave similarly.1449 Because palladium compounds are expensive, a catalytic synthesis, which uses much smaller amounts of the complex, was developed. That is, a substrate such as an allylic acetate, alcohol, amine, or nitro compound1450 is treated with the nucleophile, and a catalytic amount of a palladium salt is added. The rr-allylpal-ladium complex is generated in situ. Alkene-palladium complexes (introducing the nucleophile at a vinylic rather than an allylic carbon) can also be used.1451... 

The idea of a reversible alkene-copper complex had been used to explain the enantioselective alkylation of 2-cyclohexenone using a chiral auxiliary ligand.30 Interaction of the cuprate with the re face of C-3 in 2-cyclohexenone was presumed to be favored over complexation with the si face of C-3 for steric reasons (equation 4). 

The rhodium(II) catalysts and the chelated copper catalysts are considered to coordinate only to the carbenoid, while copper triflate and tetrafluoioborate coordinate to both the carbenoid and alkene and thus enhance cyclopropanation reactions through a template effect.14 Palladium-based catalysts, such as palladium(II) acetate and bis(benzonitrile)palladium(II) chloride,l6e are also believed to be able to coordinate with the alkene. Some chiral complexes based on cobalt have also been developed,21 but these have not been extensively used. 

The reduction of alkenes to alkanes is a reaction that is often used as a key part of a synthetic sequence. In some cases this reaction is performed in an attempt to introduce chirality into a molecule. The emphasis here is on the stereocontrolled reduction of alkenes in complex molecules. 

A stoichiometric procedure for the osmium-mediated, enantioselective aminohydrox-ylation of traws-alkenes RCH=CHR (R = Ph, Et, Pr1) has been developed employing chiral complexes between tert-butylirnidoosmium (BufN=0s03) and derivatives of cinchona alkaloids. The success of the reaction is dependent on a ligand acceleration effect corresponding diols are the by-products. The e.e. varies between 40 and 90%486,487. 

Remarkable advances have been achieved in the homogeneous asymmetric hydrogenation of prochiral alkenes by using chiral complexes of Rh and Ru, which... 

More recently, Pfaltz has reported high enantioselectivities for the cyclopropanation of monosubstituted alkenes and dienes with diazo carbonyl compounds using chiral (semicorrinato)copper complexes (P-Cu) (23-25), and Evans, Masamune, and Pfaltz subsequently discovered exceptional enantioselectivities in intermolecular cyclopropanation reactions with the analogous bis-oxazoline copper complexes (26-28). With the exception of the chiral (camphorquinone dioximato)cobalt(II) catalysts (N-Co) reported by Nakamura and coworkers (29,30), whose reactivities and selectivities differ considerably from copper catalysts, chiral complexes of metals other than copper have not exhibited similar promise for high optical yields in cyclopropanation reactions (37). 

Corma et al have anchored Rh(I), Ru (II), Co(II) and Ni(II) chiral complexes based on p-aminoalcohols such as (L) prolinol onto silica and modified USY-zeolites (scheme 3) to perform enantioselective hydrogenation of the same prochiral alkenes than shown in scheme 2.20,34... 

Porphyrin complexes such as (porph)MnCl or (porph)Mn=0 have been much studied for the oxidation of alkanes, alkenes, and other organic compounds using O-transfer agents such as PhIO, H202, or 02. Schiff base complexes have also been used since chiral complexes (e.g., of the salen type) can be readily made on a ton scale and the enantiomers separated in pure forms. Thus the epoxidation... 

Epoxidation of alkenes with complex of a chiral salen ligand and manganese(III), 1.69 or 1.70, is known as Jacobsen epoxidation ... 

Diene complexes containing alkene or diene substituents undergo Diels Alder reactions in good yields. Hetero-Diels Alder reactions have also been reported. Chirahty transfer is observed upon reaction of chiral diene iron tricarbonyl complexes. Reaction of the chiral complex (101) with cyclopentadiene in the presence of a Lewis acid give (102) with a relatively high chirahty transfer from the metal complex (Scheme 162). 

Determination of % ee via Pt NMR. The cis chiral complex has been used to determine enantiomeric purity of asymmetric allenes and allylic alcohols and ethers via complexation and Pt NMR spectroscopy. The complexes (3) and (4) are generated by displacement of ethylene from (1) by the alkene recovery of the alkene and (1) is effected by the reverse sequence employing excess ethylene. ... 

These titanium compounds can be described as an alkene 7r-complex or a metallacyclopropane, which is of practical importance. According to several computational studies, it has been concluded that the alkene titanium complexes are best represented as titanacyclopropane derivatives. The synthesis of titanium-alkyne complexes Ti(Me3SiC=CG6H13)(OR)2 from reaction between l-(trimethylsilyl)oct-l-yne with achiral or chiral alkoxo titanium compounds Ti(OR)4 has been described (Scheme 95).184 A series of organotitanium compounds (Scheme 96) are obtained by metathesis reactions.41... 

FIGURE 2.5 Asymmetric epoxidation of terminal alkenes with hydrogen peroxide catalyzed by Pt(ll) chiral complexes Ih-k. 

A poly(bmaphthyl metallosalen complex) 128 (Scheme 3.36) was prepared and used as a catalyst for the asymmetric epoxidation of alkene [72]. Although enantioselectivities obtained by using the polymeric catalyst were low, this represented a new type of polymeric chiral complex based on the main-chain hehcity. 

Cyclopropanation of alkenes was greatly improved by the use of a new generations of chiral copper complexes [60-62]. Some of the Hgands (16-18) are indicated in Scheme 11. Chiral complexes of rhodium (II) started to be developed by Doyle et al. [63], later giving enantioselectivities up to 89-90% ee in many cases. 

The catalytic asymmetric hydroboration of 1,1-disubstituted alkenes R R C=CH2 with pinacolborane has been attained (<92% ee) by using complexes of iridium with chiral phospine-oxazolidines as catalysts (2.5 mol%). The same class of compounds has been shown to facilitate hydroboration (<98% ee) catalysed by chiral complexes of CuCl with a bidentate A)-heterocyclic carbene (NHC) containing SOg" as an additional ligating group. ... 

If the MLn catalyst fragment is also chiral, then we can use one resolved enantiomer of the chiral complex as catalyst. Instead of forming two enantiomeric alkene complexes in equal amovmts, we will have diastereomeric alkene-catalyst complexes, and in principle could have unequal amounts. Once H2 is added to the face the metal was bound, we have unequal amounts of the two enantiomeric products. One enantiomer of the catalyst should preferentially give one enantiomer of the hydrogenated alkene, and the other enantiomer should give the other product. In favorable cases, enantiomeric excesses of >95% can be obtained (e.e. = amount of major isomer-amount of minor isomer / total of both isomers ). 

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