Regioselectivity chiral alkenes

Addition of (TMS)3SiH to a-chiral ( )-alkene 7 was found to take place with a complete Michael-type regioselectivity (Reaction 5.8) [26]. A complete syn stereoselectivity was observed for R = Me, and it was rationalized in terms of Felkin-Ahn transition state 8, which favours the syn product similar to nucleophilic addition. 

TABLE 24. Regioselectivity and diastereoselectivity in the photooxygenation of chiral alkenes within zeoUte Na-Y and in solution (values in parentheses)... 

In regioselective addition to a chiral alkene, four diastereomers may be formed by cis or trans addition to both diastereotopic faces of the double bond. 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

A regioselective Rh-catalysed hydroformylation of (/ )-iV-phthalimido-vinylglycinol CH2=CHCH(NHPhth)CH20H and other a-chiral alkenes has been shown to be controlled by choice of the reaction conditions preferential formation of the Unear regioisomer has thus been maximized. ... 

Less common is the use of nbd, but it has been utilized to obtain crystals for the determination of the absolute configuration of chiral palladium(ll) complexes of the type palladatricycloheptane produced by regioselective oxidative cyclization of Ca-symmetrical chiral alkenes. The structures of 26 and 27 were determined.Previously, complexes of the racemic equivalent with E = C02Me with nbd and with COD were crystallized, and their X-ray structures solved. 

This regioselectivity is opposite to the one observed by the non-catalysed additions of BH3 THF or 9-BBN to the same alkene, or those catalysed by Rh and Ir catalysts. Chiral NHC ligands (generated from 84) on Cu under the same conditions proceed with high enantioselectivity (enantiomeric ratio 99 1) [71] (Scheme 2.12). 

The major problem remains control of regioselectivity in favor of the branched regioisomer. While aryl alkenes as well as heteroatom-substituted alkenes favor the chiral branched isomer, for aliphatic alkenes such an intrinsic element of regiocontrol is not available. As a matter of fact branched-selective and asymmetric hydroformylation of aliphatic alkenes stands as an unsolved problem. In this respect regio- and enantioselective hydroformy-... 

Alkylated diphosphines (R,R)-(92) and (93) were used as chiral ligands in the Pt-catalyzed hydroformylations of some alkeneic substrates. These ligands bring about a loss of catalytic activity with respect to the corresponding diphenylphosphine homolog, particularly in the case of the platinum systems. The regioselectivity favors the straight-chain (or less branched) isomer in the case of terminal alkenes with the exception of styrene the enantioselectivity is very low in all cases.320... 

The idea of Hoveyda with co-workers to employ their peptide ligands (e.g., 295) as chiral inductors in allylic substitutions with dialkylzincs turned out to be very rewarding.399-401 As a result of meticulous screening of numerous optically active ligands, copper salts, and substrates under various conditions, they achieved excellent results for aliphatic alkenes. Particularly, allylic substitution products with tertiary 297 and quaternary 299 carbon centers were obtained regioselectively and with 78-96% ee (Scheme 151).401... 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

Regio- and enantioselective Heck reactions of 2 3-dihydrofuran with aryl and alkenyl triflates in the presence of the chiral ligand (R)-BITIANP provides 2-substituted 23-dihydro-furans with complete regioselectivity, high enantioselectivity (86-96% ee) and good yields (76-93%) <99CC1811>. A catalytic oxyselenylation-deselenylation reaction of alkenes offers a stereoselective one-pot conversion of alkenes into 2 -dihydrofurans <99EF0797>. 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

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