Rhodium asymmetric version

Bis(pinacolato)diboron 14 and bis(neopentyl glycolate)diboron 15 have been used for rhodium-catalyzed conjugate addition to a,/ -unsaturated ketones giving /9-boryl ketones, though the asymmetric version has not been reported [15]. 

Murakami [27] and Lautens [28] independently reported the rhodium-catalyzed addition of arylboronic acids to oxanorbornenes (Scheme 3.12). Murakami reported that the reaction of oxabenzonorbornadiene 37 a with phenylboronic acid 2m, in the presence of a rhodium-P(OEt)3 catalyst in MeOH at reflux, gave an 86% yield of the ring-opened alcohol 38 am. Lautens reported the asymmetric version of the reaction, where high enantioselectivity was observed with chiral ferrocenylbisphosphine hgand 39 in a... 

The cationic rhodium complexes such as [Rh(cod)(MeCN)2]BF4/dppb were the most efficient catalysts for addition of arylboronic acids or Ph4BNa to both aromatic and aliphatic A-sulfonyl imines.993 994 The asymmetric version giving optically active amines was achieved by amidomonophosphine catalysts (Equation (225)).995... 

Rhodium-catalyzed addition of boronic acids to enone moiety 89 led to a rhodium-enolate 90 which can be trapped by addition to the adjacent carbonyl function giving functionalized cyclopentanes or cyclohexanes 91. An important feature of this methodology is that this process allows the creation of three contiguous stereocenters with a high level of stereoselectivity. An asymmetric version of this reaction has also been realized with a chiral ligand (BINAP) giving excellent enantiomeric excesses (77 to 95%) (Scheme 34). 

The C-H insertion reaction of aryldiazoacetates to furnish dihydrobenzofurans is best carried out with dimeric rhodium(ll) catalysts. Rh2(PTTL)4 has proven to be the catalyst of choice for the asymmetric version of this process, providing exclusively j-2-aryl-3-methoxycarbonyl-2,3-dihydrobenzofurans with an ee of up to 94% (Equation 142) <2002OL3887>. 

The greater part of this chapter is concerned with the Diels-Alder and hetero-Diels-Alder reaction. The asymmetric version of both of these reactions can be catalysed with metal-based Lewis acids and also organocatalysts. The catalytic asymmetric 1,3-dipolar cycloaddition of nitrones and azomethine ylides is also discussed. Again, most success in this area has been achieved using metal-based Lewis acids and the use of organocatalysts is begiiming to be explored. This chapter concludes with a brief account of recent research into the asymmetric [2+2]-cycloaddition, catalysed by enantiomerically pure Lewis acids and amine bases, and also the Pauson-Khand [2- -2- -l] cycloaddition mediated by titanium, rhodium and iridium complexes. 

Morken and coworkers [39b] developed the first asymmetric reductive aldol reaction with silanes as reductants in combination with a chiral rhodium catalyst. a,P-Unsaturated esters were reacted with several aldehydes to provide the corresponding aldol products 79 in good yields and enantio- and diastereoselectivities (Scheme 8.23). Both aliphatic and aromatic aldehydes could be converted into aldol products 79 under these conditions. Furthermore, the group reported an iridium-catalyzed asymmetric version that tolerated various protected hydroxyaldehydes [39aj. On the basis of this precedence, a highly enantio- and diastereoselective... 

There are only very few reports in the literature on the hydroformylation of partially unsaturated O- or N-containing six-membered ring heterocycles. Rather harsh conditions were applied (Co catalyst, 50-140atm, 130-150°C) [67]. Claver, Castillon, and Bayon [68] showed that tetrahydropyranes require, also with modified rhodium catalysts, much severe hydroformylation conditions than the corresponding dihydrofurans. Moreover, poor regioselectivity was noted. These features may be responsible for the fact that, up to now, no asymmetric version could be realized. A similar situation is faced with 1,2,3,4-tetrahydropyridin as substrate. It could not be converted with a Rh(Bisdiazaphos) catalyst at 70 °C and 140 psi (about 10 bar) pressure [43]. 

Unfortunately, thus far there has been only one example of the asymmetric version of rhodium-catalyzed asymmetric arylation of aldehydes. In this report, by Miyaura [44], a rhodium complex coordinated with axially chiral monodentate phosphine Ug-and, (S)-MeO-mop, catalyzed the addition of phenylboronic acid (2m) to 1-naph-... 

More recently, Cramer and coworkers reported an asymmetric version of this chemistry to access functionalized chiral dihydrobenzofurans that possess a quaternary stereocenter using chiral rhodium catalyst (Scheme 6.13) [23]. They found that O-tethered substrate 54 was deuterated more quickly than meta-methyl-substituted derivative 55. Furthermore, the reaction will take more time for both substrates when the chiral complex with a 1,2-disubstituted cyclopen-tadienyl ligand was replaced with achiral complex with a more hindered Cp ligand. These differences between substrate 54 and 55 emphasize the significance of the alkoxy substituent as a secondary directing group for rhodium-catalyzed reactions. 

Asymmetric versions of these reactions have been carried out by Tanaka [49] and Shibata [50] by using rhodium complexes of modified BlNAPs (Fig. 10.37). TolBinap and Hg-Binap were found to be appropriate chiral ligands for the cycloadditions of enynes with oxygen, carbon or nitrogen tethers. In the case of nitrogen-tethered substrates, the substituent of the alkyne unit has a crucial effect... 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

In contrast to the intramolecular carbenoid C-H insertion, the inter-molecular version has not been greatly developed and has been for a long time regarded as a rather inefficient and unselective process. In this context, Davies and Hansen have developed asymmetric intermolecular carbenoid C H insertions catalysed by rhodium(II) (5 )-A-(p-dodecylphenyl)sulfonylprolinate. " Therefore, these catalysts were found to induce asymmetric induction in the decomposition of aryldiazoacetates performed in the presence of cycloalkanes,... 

In 1997, Miyaura and co-workers reported the nonasymmetric version of 1,4-addition of aryl- and alkenylboronic acids to a,/ -unsaturated ketones using rhodium-phosphine complex as the catalyst.97 Later, Hayashi and Miyaura realized the asymmetric 1,4-addition with high catalytic activity and enantioselectivity.98 In the presence of ( y)-BINAP, the reaction of 2-cyclohexenone with 2.5 equiv. of phenylboronic acid gave (A)-3-phenylcyclohexanone with 97% ee (BINAP = 2,2 -bis (diphenylphosphino)-l,l -binaphthyl Scheme 29).99... 

Asymmetric catalytic version with Rh(i) under solvent-free conditions have also been reported by Shibata. Contrary to the previous results, a neutral rhodium(i) complex provided comparable enantioselectivities with high chemical yields. ... 

FIGURE 27 Self-assembly of diphosphine catalyst for asymmetric rhodium-complex-catalyzed hydrogenation the catalyst contains titanium as the assembly metal (96). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Doyle s chiral rhodium (II) carboxamidates have proved to be exceptionally successful for asymmetric C-H insertion reactions of diazoacetates and some diazoacetamides leading to lactones and lactams, respectively. With 2-alkoxyethyl diazoacetates and the Rh2(5S- and 5R-MEPY)4 catalysts, for example, highly enantioselective intramolecular C-H insertion reactions occur, the 5S-catalyst, Eq. (40), and 5R-catalyst furnishing the S- and R-lactone, respectively [58]. A polymer-bound version of Rh2(5S-MEPY)4 has also been applied to the cycliza-tion in Eq. (40) to yield the lactone with 69% ee (R=Me) the catalyst could be recovered by filtration and reused several times, but with decreasing enantiose-lection [59]. 

Although the first catalysts were copper-based, the insertion of metal-associated carbenes into carbon-hydrogen bonds has undergone a renaissance with the advent of rhodium(II) carboxylate catalysts [56]. Metal-catalyzed enan-tioselective C-H insertions of carbenes have not been studied in great detail. Most of the efficient enantioselective versions of this reaction involve chiral rhodium complexes and until recently, the use of chiral catalysts derived from metals other than copper and rhodium for the asymmetric C-H insertion of metal-associated carbenes are still unexplored. 

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