Asymmetric 2-phenylpropene

Enantioselective osmylotion of alkenes. Osmium tetroxide forms a 1 1 wine-red complex with the chiral diamine l2 that effects efficient enantioselective dihy-droxylation of monosubstituted, oww-disubstituted, and trisubstituted alkenes (83-99% ee) at -110° in THF. The enantioface differentiation in all cases corresponds to that observed with t/mr-3-hexene and the complex with (-)-l. Essentially complete asymmetric induction is observed with frans-l-phenylpropene (99% ee). 

The first examples of asymmetric hydrocarboxylation have been reported for various alkenes, converted in the presence of palladium(II) chloride and Diop, with a maximum ee of 14.2% 6 (for 2-phenylpropene). In this unsymmetrical addition reaction, similar to hydroformylation, both at- and -induction via C—C and C —H bond formation are observed. Styrene and 2-phenyl-l-propene (a-methylstyrene) are typical examples. 

Iodorhodium(IIl) porphyrins also efficiently catalyze the reaction of ethyl diazoacetate with simple alkenes. generally providing the cw-isomers as the major product77 79110. The cis( tram ratio increases when bulkier porphyrins, such as tetramesitylporphyrin (TMP), are employed. The mechanism of this rhodium-catalyzed cyclopropanation with diazoacetate is interpreted as proceeding via carbene complexes79 80 111,112. Based on these results, asymmetric cyclopropanation of alkenes with ethyl diazoacetate is achieved if catalyzed by a chiral wall porphyrin81. An earlier described binaphthyl-system of this type82113114, introduced as an iodorhodium(lll) complex, 6, forms an extremely active catalyst and leads to m-cyclopropanes (preferred over the rran.v-products) with moderate to poor enantioselectivities if styrene, 1- and 3-phenylpropene are used as substrates (10-60% ee)81. 

With the exception of norbomene, internal olefins do not undergo hydrosilylation. Hydrosilylation of 3-phenylpropene with PhSiDj forms a unique product and the process tolerates a variety of fianctional groups, halides, ethers and acetals, despite the well known strong Lewis acidity of the catalysts. Cyclisation/silylation of 1,5-dienes or 1,6-enynes has been reported to give a single product (Scheme 14) [26]. In the case of metallocene complexes bearing a menthyl substituent, ee values near 70% were obtained for the asymmetric hydrosilylation of 2-phenyl-but-l-ene [31]. 

As intriguing as the above is the fact that an optically active phase-transfer catalyst can induce asymmetry. Addition of dichlorocarbene to styrene in the presence of (62) results in a cyclopropane product with a rotation of 3.18 (Scheme 8). On removing the hydroxy-group and employing optically active (63) for the same addition, the optical purity of the product is dramatically reduced. Analogous results are obtained with frans-l-phenylpropene. Although the degree of asymmetric induction is low,... 

Molecular dynamics studies in combination with sequence analysis of twelve OYE homologues were carried out to predict the stereopreference for the asymmetric reduction of l-nitro-2-phenylpropene 32a based on the amino acid sequence and structure [73]. A proline residue in the third position of a sequence pattern motif in loop p2 should thereby lead to high S-selectivity. In contrast, moderately selective enzymes exhibit a nonpolar short-chain amino acid, and all R-selective OYE homologues were predicted to contain long polar and charged amino acids in this position (Table 18.1) [73]. 

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