Transition-metal-catalyzed asymmetric

This article aims to review papers that were published in 2006 on reactions and syntheses of furans, benzofurans and their derivatives. Two reviews have summarized the syntheses of furans <06OBC1627> and tetrahydrofurans <06EJO1627>. Another review <06001613> records the progress of transition metal-catalyzed asymmetric ring opening of oxabenzonorbornadienes. 

In contrast to the amines, inversion of configuration for phosphines is generally negligibly slow at ambient temperature. This property has made it possible for chiral phosphines to be highly useful as ligands in transition metal-catalyzed asymmetric syntheses. 

In the past few years, new approaches for the enantioselective synthesis of / -benzyl-y-butyrolactones appeared in the literature. Some of these approaches involve the asymmetric hydrogenation of 2-benzyl-2-butenediols (j [34]), the radical mediated rearrangement of chiral cyclopropanes (r [35]), the transition metal catalyzed asymmetric Bayer-Villiger oxidation of cyclobutanones n [36]), or the enzymatic resolution of racemic succinates (g [37]). 

During cyclization with acetylene, the chiral center is maintained. This reaction has recently been extended to the synthesis of bipyridyl compounds having optically active substituents (75PC1) and provides access to chiral ligands of potential interest in transition-metal-catalyzed asymmetric catalysis. 

Transition Metal-Catalyzed Asymmetric Allylic Alkylations... 

Transition Metal-Catalyzed Asymmetric Arylation of Enolates. 167... 

Transition Metal-Catalyzed Asymmetric Aliylic Alkylations of Enolates... 

Asymmetric catalytic addition of dialkylphosphites to a C=0 double bond is a powerful method, and probably the most general and widely applied, for formation of optically active a-hydroxy phosphonates [258], The basic principle of this reaction is shown in Scheme 6.108. Several types of catalyst have been found to be useful. The transition-metal-catalyzed asymmetric hydrophosphonylation using chiral titanium or lanthanoid complexes was developed by several groups [259, 260], The most efficient type of chiral catalyst so far is a heterobimetallic complex consisting... 

Quinap (95) in a number of mutually unrelated transition-metal-catalyzed asymmetric reactions.50 Thus, 105 added to tert-butyl acrylate 106 using silver acetate and PINAP ligand 96, under the conditions described in Scheme 2.25, to provide the desired product 109 in excellent enantioselectivity and yield (see Scheme 2.6 and Table 2.5). Notably, the reaction required only 3 mol% of catalyst for complete conversion at -40°C within 36 h. 

An enormous amount of work in the area of transition-metal-catalyzed asymmetric allylic substitution has been done over the last few years. Since the publication of the original chapter [1], a number of books [2-5], reviews [6-12], and accounts [13,14] covering this field have been published. This update will cover the recent developments from 1999 to early 2003. 

Chiral phosphine ligands are the chiral auxiliaries extensively studied for transition metal-catalyzed asymmetric reactions. Most of the ligands were originally designed for asymmetric hydrogenation, but they also worked well in the conjugate addition of organoboronic acids to electron-deficient alkenes.951,952 The rhodium(i) complexes of 496-503953-966 have been successfully utilized for these asymmetric reactions (Scheme 37). 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

Enantiomerically pure phosphines are frequently employed as ligands in transition-metal-catalyzed asymmetric reactions. For this reason, various methods have been designed for their preparation.1 Many of them involve the use of borane adducts of trivalent phosphorus compounds,2 in which the borane moiety mainly acts as a protecting group. These Lewis adducts are easily prepared and stable to air, and several methods have been designed to cleave them. 

Hayashi, T. Chiral Monodentate Phosphine Ligand MOP for Transition Metal-Catalyzed Asymmetric Reactions, Acc. Chem. Res. 2000,33,354-362. 

Besides the transition-metal-catalyzed asymmetric addition reactions to prochiral olefins, the substitution reaction of a carbon nucleophile to allylic esters has been investigated using a variety of chiral transition-metal catalysts. Using the aforementioned sugar diphosphites... 

Scheme 7 displays a possibility of the synthesis of chiral 2-arylpropionic acids via the oxidative tranformation of (7 )-3-aryl-l-butenes. The requisite chiral olefins may be obtained by transition metal-catalyzed asymmetric coupling between a benzylic Grignard reagent and vinyl bromide (93 % optical yield) [28] or, more attractively, asymmetric hydrovinylation of an aromatic olefin with ethylene. The asymmetric combination of styrene and ethylene, giving the adduct 25 in 95 % ee, has been performed on a 10-kg scale with a dinuclear Ni catalyst formed from ( -allyl)NiCl2 and a unique chiral dimeric aminophosphine obtainable from (/ )-myrtenal and (5)-l-phenylethylamine [7a],... 

Recently, some extensive research has been devoted to exploring a diastereo-selective and enantioselective route for the synthesis of a-hydroxyaldehydes or a-hydroxyketones because they are important building blocks for the construction of complex natural products and biologically active molecules [91]. In parallel with the transition-metal-catalyzed asymmetric nitroso-aldol reaction [92], much interest has also been expressed towards the proline-catalyzed direct asymmetric a-aminoxylation of aldehydes or ketones for the synthesis of optically active a-hydroxyladehydes and a-hydroxyketones [93]. Wang [94] and Huang [95] independently reported an L-proline-catalyzed asymmetric a-aminoxylation reaction in ionic liquids, whereby it was found tliat aldehydes and ketones could undergo... 

According to the mechanism of transition metal catalyzed asymmetric hydro genation of N acetyl enamines, N acetyl group is considered indispensable for the substrates to form a chelate complex with the metal of catalyst, which is important for the enantiocontrol of reaction. However, there is no N acetyl group in N,N dialkyl enamines to form such a chelate complex in the catalytic asymmetric hydrogenation, resulting in a low enantioselectivity. 

The transition metal catalyzed asymmetric addition of aryl organometallic reagents to aldehydes, ketones, and imines has provided efficient access to chiral aryl alcohols or aryl amines [89]. Arylboronic acids are less toxic, stable toward air and moisture, and tolerant towards a variety of functional groups, and are ideal reagents for the addition to aldehydes. However, when Sakai et al. [90] attempted the enantioselective Rh-catalyzed addition of phenylboronic acid to naphthaldehyde, only 41% ee was obtained. Chiral spiro phosphite complex (S)-18c was found to be an efficient catalyst for asymmetric addition reactions of arylboronic acids to aldehydes, providing diarylmethanols in excellent yields (88-98%) with up to 87% ee (Scheme 30) [20c]. 

The transition metal-catalyzed asymmetric Michael addition reaction was first reported by Brunner employing the complex of Co(acac)2 and (S,S)-l,2-diphe-nyl-l,2-ethylenediamine (Scheme 12) [59, 60]. An enantiomeric excess of 66% was attained in the reaction of 6 and 7 giving R)-8 at -50 C in toluene. The dimeric copper complex 65 derived from salicylaldehyde and optically active (S)-hydroxyamines also promoted the reaction giving (S)-8 in 75% ee [61, 62, 63]. The second hydroxy group is considered to occupy the axial position of the monomeric intermediate. 

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