Alkenes rhodium chiral complexes

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 synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

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 use of chiral rhodium-BINAP complexes for the asymmetric isomerization of alkenes has been utilized in the industrial synthesis of menthol by Ryoji Noyori (winner of the 2001 Nobel Prize in Chemistry). This synthetic method was industrialized by Takasago International Corporation and provides (—)-menthol to pharmaceutical and food companies worldwide. In this case the catalyst [(S-BINAP)-Rh(COD)] or [(S-BINAP)2-RuC104 ] is used for the asymmetric isomerization of diethylgeranylamine (1.62) to 3-(R)-citronellalenamine (1.63) (Scheme 1.13). 

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 highly developed enantioselective hydrogenation of prochiral functionalized alkenes using chiral phosphine complexes of ruthenium or rhodium as catalysts has become very common in academic laboratories as well as in industry... 

Since its discovery by Roelen in 1938 [l],the hydroformylation process was exclusively based on cobalt as catalyst metal, until the development of rhodium-phosphine complexes in the late 1960s [2]. Industrial efforts have been focused on the preparation of norraaZ-aldehydes (linear aldehydes) from 1-alkenes. In contrast, asymmetric hydroformylation, which requires iso-aldehydes (branched aldehydes) to be formed from 1 -alkenes, was first examined in the early 1970s by four groups independently, using Rh(I) complexes of chiral phosphines as catalysts [3,4,5,6]. Since then, a number of chiral ligands have been developed for... 

The asymmetric hydroformylation of alkenes is an exceptionally atom-efficient method for the synthesis of enantiomerically-pure carbonyl-containing compounds.[1] The hydroformylation of vinylacetate, in particular, represents an excellent method for the preparation of ot-alkoxy aldehydes and, through their reduction, homochiral 1,2-diols. The use of the novel chiral ligand, ESPHOS (1),[2] in a rhodium(I) complex, results in hydroformylation of vinyl acetate in high branched linear selectivity and exceptional ee (Figure 12.1).[3]... 

The reactions of the halogenophospha-alkene (183) with complex metallo-anions have given a range of P-metallophospha-alkenes, e.g., (184), which are found to be metastable, undergoing gradual dimerisation and subsequent transformation. Mathey s group has shown that rhodium complex-promoted catalytic hydrogenation of prochiral complexes of phospha-alkenes offers a route to related complexes of chiral secondary phosphines. The reactivity... 

Optically active furyl cyclopropane 402 was prepared from acetylene dicarboxylate and alkenes (Scheme 1.187) [261]. The acetylene dicarboxylate underwent dimerization to form metallocyclopentadiene 403, which decomposes to give cyclopropanes containing rhodium carbene complex 404. A good level of chiral induction was achieved using Segphos ligand with Rh(cod)2Bp4. 

C2.7.6.2 CHIRAL HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

The first example of an enantioselective [5 + 2]-cycloaddition was reported for the tethered alkene-VCP 7a, which upon treatment with a chiral rhodium complex afforded the m-fused bicyclo[5.3.0]decene 8a in 80% yield and 63% enantiomeric excess (ee) (Equation (6)).39 A later study found that when a 2,2-bis(diphenyl-phosphanyl)-l,l-binaphthyl (BINAP)-modified rhodium(l) catalyst is used, good to excellent ee s and yields are achieved with a variety of substrates (Equation (7)).40... 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

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