Olefination stereoselective

Carbonyls. The stereochemistry of the Wittig olefin synthesis has been reviewed. /i-a/u-Stereoselective olefin synthesis via /3-oxido-ylides is possible only in the presence of soluble lithium salts. Protonation of jS-oxido-ylides prepared from salt-free ylides leads to mixtures of erythro-and r/jr o-betaines and hence to mixtures of cis- and rm/i5-olefins. 

In 1982, a new reaction was reported by Tamura and Ono namely, allylic nitro compounds undergo replacement of the nitro group by various nucleophiles in the presence of a palladium (0) catalyst.17a b 18a b The details of these reactions are discussed in Ref. 2b here, only some typical examples are presented. Carbon, sulfur, nitrogen, and phosphorous centered nucleophiles replace the nitro groups at the allylic positions. The reaction of allylic nitro compounds with triphenylphosphine is applied to the highly stereoselective olefination of aldehydes (Eqs. 7.15-7.18).19... 

Catalytic ring-closing metathesis makes available a wide range of cyclic alkenes, thus rendering a number of stereoselective olefin functionalizations practical. The availability of effective metathesis catalysts has also spawned the development of a variety of methods that prepare specially-outfitted diene substrates that can undergo catalytic ring closure. The new metathesis catalysts have already played a pivotal role in a number of enantioselective total syntheses. 

The design and development of a Z-selective CM reaction faces two major challenges first, the thermodynamic preference for the formation of -olefms renders their Z-counterparts difficult to prepare second, any inherent kinetic Z-preference demonstrated by a catalyst can erode via secondary metathesis if the -pathway is not blocked. Consequently, most efforts to effect stereoselective olefin formation have focused on the preparation of -olefins. 

Gillespie, A.M. and White, D.P. (2001) Understanding the steric control of stereoselective olefin binding in cyclopentadienyl complexes of rhenium an application of de novo ligand design. Organometallics, 20, 5149. 

Several years later, the group of Corma reported on a successful study on stereoselective olefin epoxidation with MTO using various chiral nitrogen bases. Although the conversion is low (10%), an enantiomeric excess (ee) of up to 36% can be obtained with cA-p-methylstyrene as the substrate and R-(+)-1 -phenylethylamine as base (Fig. 3b) [34], Also, the groups of Saladino and Crucianelli used /f-(+)- -phenylethylamine as chiral base in a 1 1 ratio with MTO, forming the corresponding perrhenate salt, but also here, very low conversion is obtained. In the same report, the use of Lrans-( R,2R)-, 2-diaminocyclohexane (Fig. 3c) in combination with MTO as... 

A synthesis of the pear ester worked out in our laboratory uses the (Z)- and the ( )-stereoselective Wittig olefination for the generation of the two double bonds in the molecule 232165). Reaction of 2-ethoxyvinyl-triphenylphosphonium bromide 234 with one equivalent sodium ethanolate yields 2,2-ethoxyethylene-triphenyl-phosphorane 235166,167). (Z)-Stereoselective olefination of 235 with hexanal 236... 

A syn-elimination of Ph2MeP=0 and simultaneous stereoselective olefin formation from an oxaphosphetane are shown in Figure 4.40 (note that this oxaphosphetane is... 

Warren continues to extend his phosphine oxide-based method of stereoselective olefin synthesis. The previously published method of acyl transfer followed by borohydride reduction to give selectively the threo-hydroxyalkylphosphine oxide intermediate has now been applied to more complex examples carrying other chiral centres.28 Optically active butyl(2-methoxyphenyOphosphine oxide (47) has been used to provide a chiral... 

HWE olefination Stereoselective olefination of aldehydes and ketones using phosphoryl-stabilized carbanions. 212... 

Circular Dichroism Spectra of Square Planar Complexes Containing Prochiral Olefins and Their Stereoselective Olefin Exchange... 

A Sterling-Winthrop Research Institute group in cooperation with Johnson3 applied the new scheme of biogenetic-like stereoselective olefin cyclization to the synthesis of 19-norsteroids and so gained access to a steroid type of importance in medicine. 

For a review on the stereoselective olefin polymerization with chiral metallocene catalysts see Brintzinger HH, Fischer D, Mulhaupt R, Rieger B, Waymouth R (1995) Angew Chem 107 1255 Angew Chem Int Ed Engl 34 1143... 

Stereoselective olefination of carboxylic eaters or synthesis of allylamines from a-amino acid esters mediated by benzotriazole(Bt) derivatives. 

A one-pot synthesis of 4-hydroxycyclopent-2-en-l-ones (150), involving (Z)-stereoselective olefination of a-diketones to give (149) followed by intramolecular aldol condensation, has been reported. Wadsworth-Emmons reactions of bis(2,2,2-trifluoroethyl)phosphono sulfoxides (151) with aromatic aldehydes give predominantly (Z)-a,p-unsaturated sulfoxides while similar reactions with the corresponding sulfides (152) give ( )- or lower (Z)-selectivity. In olefinations using (5)-dimethyl phosphorylmethyl p-tolyl sulfoxide (153) substantial racemisation at sulfur occurs when -butyllithium is used as base. ... 

Platinum complexes show none of the catalytic activity found in palladium and nickel complexes. This chemical inertness makes platinum a useful model for the more active catalysts. There has been a suggestion that platinum complexes of chiral a-diimines might lead to stereoselective olefin polymerization. Chiral camphor-based ligands have been employed in palladium complexes for ethylene polymerization, but there was no mention of stereoselectivity in hexene polymerizations. 

Terpenes.- The first total synthesis of the biologically active marine natural product metachromin A (241) has been achieved using an ( )-stereoselective olefination with the phosphonate (240) as the key step. The olefination reaction of the a-cyanophosphonate (242) with ( , )-farnesal (243) has been studied under a wide variety of conditions with a view to optimising (Z)-stereoselectivity.Excellent (Z)-stereoselectivity was achieved and the resulting procedure applied to a stereoselective synthesis of plaunotol (244). Epimerisation and P-elimination of base-sensitive substrates can cause problems in phosphonate- and phosphine oxide-based olefinations. Such reactions have been excluded in a recent report of enantiospecific syntheses of iridoid monoterpene lactones by intramolecular phosphonate-olefination through the use of mild bases such as DBU and di(wopropyl)ethyl amine/lithium chloride. 

Carbostannylation of alkynes generates cis-substituted alkenylstannanes and is one of the most useful reactions for stereoselective olefin synthesis. Treatment of tributyl-(phenylethynyl)tin 37 with a 1 2 mixture of [PdCl(7r-C3H5)]2-A -(2-diphenylphosphino benzylidene)-2-phenylethylamine (38) under an acetylene atmosphere in THF at... 

Three general geometries for stereoselective olefin polymerization catalysts operating by site control. 

Scheme 7.16 Stereoselective olefin synthesis through Eschenmoser-Claisen rearrangement.

It was found that polypropylene made by such complexes (cocatalyzed by MAO) tended to be isotactic, as opposed to the atactic polypropylene that had been hitherto made by metallocenes such as Cp2ZrCl2. The strong relationship between ligand structure and polymer tacticity is considered to be the most secure proof of the migratory insertion mechanism for polymerization. The lure of homogeneous stereoselective olefin polymerization is responsible for much of the diversity of ligand structures within the bis(Cp) family. 

FIGURE 2.1 Cyclopentadienide, indenide, and fluorenide are important ligands commonly found in stereoselective olefin polymerization catalysts. 

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