Functionalization dirhodium catalysts

In fact, the stepwise nature of [Ru2(hp)4Cl] catalyzed reaction accounts for preferential allylic C-H bond functionalization over alkene aziridination which is the favored product when dirhodium catalysts are used. The presence of a discrete diradical intermediate further assists the selectivity for allylic C-H amination using a diruthenium catalyst. 

As in the case of C-H insertion, Si-H bond functionalization in aryl- and vinyl-diazoacetates takes place in the presence of dirhodium catalysts, and chiral ones result in asymmetric induction [60a]. In pioneering studies by Doyle et al, enantioselective Si-H bond insertion with chiral dirhodium catalysts was achieved [90]. Recently, Ball etal reported innovative studies with bis-acetate dirhodium complexes, bearing chelating nona-peptides (see Section 9.2.3.2), which catalyzed the enantioselective carbenoid insertion into Si-H bonds (Scheme 9.18) [46]. The optimization of the peptide bound to the dirhodium unit or the presence of a phosphite additive significandy improved the enantiose-lectivity of the silane products [46]. 

Dirhodium(II) carboxylate catalysts have been used extensively for the catalysis of carbene insertions. In many cases, impressive selectivities have been achieved (19-21). In an effort to find selective catalysts for carbenoid insertions, Moody screened a series of dirhodium(II) carboxylate catalysts for their ability to catalyze carbenoid Si-H insertion (22). The authors surveyed the commercially available carboxylic acids, -10,000 of which are chiral. The members of this group that contained functionality that is incompatible to the reaction were culled out. The remaining chiral carboxylic acids (-2000 compounds) were then grouped into 80 different clusters. There is no discussion presented for the criteria used in the grouping of the acids. A representative acid from each cluster was then chosen for... 

Thus changing the ligands on dirhodium(II) can provide a switch which, in some cases, can turn competitive transformations on or ofT146. Other examples include the use of dirhodium(II) carboxamides to promote cyclopropanation and suppress aromatic cycloaddition146. For example, catalytic decomposition of diazoketone 105 with dirhodium(II) caprolactamate [Rh2(cap)4] provides only cyclopropanation product 106. In contrast, dirhodium(II) perfluorobutyrate [Rh2(pfb)4] or dirhodium(II)triphenylacetate [Rh2(tpa)4] gave the aromatic cycloaddition product 107 exclusively (equation 100)l46 148. Although we have already seen that rhodium(II) acetate catalysed decomposition of diazoketone 59, which bears both aromatic and olefinic functionalities, afforded stable norcaradiene 60 (equation 70)105, the rhodium(II) acetate catalysed carbenoid transformation within an acyclic system (108) showed no chemoselectivity (equation 101). However, when dirhodi-um(II) carboxamides were employed as catalysts for this type of transformation, only cyclopropanation product 109 was obtained (equation 101). ... 

Examples in which the cation of the ionic liquid contains the transition metal complex for catalysis have also been published. For example, Forbes and coworkers [63] synthesized a Rh-containing ionic liquid cation by replacing an acetate ligand at the Rh center by a carboxyhc acid functionalized imidazolium moiety. The modified dirhodium(ii) dimer of this kind was applied as an effective catalyst in the intermolecular cydopropanation reaction of styrene using ethyl diazoacetate. 

An easy deprotection of the isopropyhdene residue in 1 and glycolic cleavage of the diol 2 to the aldehyde 3, or glycolic cleavage followed by the oxidation to the carboxylic function and formation of the ester 4, provide particularly attractive synthons (Scheme 2). Dirhodium complexes derived from difluoro-azetidinones, obtained in this way, were used as chiral catalysts for enantioselective decomposition of diazocompounds and cyclopropanation, to show, however, a moderate selectivity (Scheme 3) [26]. 

Quite recently, Davies and co-workers developed a new class of sterically demanding dirhodium tetracarboxylate catalysts, especially Rh2(R-BPCR)4, that changed the site selectivity of the C(sp )—H bond insertion reaction. In the presence of catalytic amount of Rh2(R-BPCR)4, the primary C—H bond is the preferred reaction site of various substrates containing primary benzylic C—H bonds, allylic C—H bonds, or C—H bonds a to oxygen, which is complementary to Rh2(i -DOSP)4 which favors secondary C—H bonds (Scheme 1.17a-c). Moreover, the use of this methodology was further proved by the selective C—H bond functionalization of complex molecules such as (-)-a-cedrene (Scheme 1.17d). 

The functionalization of C—H bonds through a transition metal carbenoid insertion has been known for over a century and has become a powerful method to achieve new C—C bonds. In most cases, these transformations have been completed with dirhodium (II) carboxylate catalysts. The development of chiral dirhodium (II) complexes has allowed the enantioselective version of these reactions and has led to a straightforward method for the preparation of chiral natural products and dmgs. 

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