Catalytic cycloadditions diazo compounds

The best known of metal carbene reactions, cydopropanation reactions, have been used since the earliest days of diazo chemistry for addition reactions to the carbon-carbon double bond. Electron-donating groups (EDG) on the carbon-carbon double bond facilitate this catalytic reaction [37], whereas electron-withdrawing groups (EWG) inhibit addition while facilitating noncatalytic dipolar cycloaddition of the diazo compound [39] (Scheme 5). There are several reviews that describe the earlier synthetic approaches [1, 2,4, 5,40-43], and these will not be duplicated here. Focus will be given in this review to control of stereoselectivity. 

Based on his previous work on the catalytic double addition of diazo compounds to alkynes173 using Cp RuCl(COD),174 Dixneuf has developed an efficient one-step synthesis of alkenyl bicyclo[3.1.0]-hexane derivatives of type 163 from enyne precursors 162 (Scheme 43). The catalytic cycle starts with the formation of an Ru=CHR species. It then adds to an alkyne to form ruthenacyclobutene 166, which evolves into vinylcarbene 167. [2 + 2]-Cycloaddition of 167 gives ruthenacyclobutane 168. The novelty in this transformation is the subsequent reductive elimination to give 170 without leading to the formation of diene 169. This can be attributed to the steric hindrance of the CsMes-Ru group. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

In summary, the metal-catalyzed decomposition of diazo compounds results in a broad array of opportunities for the development of new asymmetric catalytic transformations. In the last few years considerable advances have been made in enantioselective intermolecular C-H insertion, novel cycloadditions, and tandem cyclization/cycloadditions. These new transformations offer new strategies for the rapid enantioselective construction of complex structures. 

Hodgson, D. M. Pierard, E. Y. T. M. Stupple, P. A. Catalytic Enantioselective Rearrangements and Cycloadditions Involving Ylides from Diazo-Compounds, Chem. Soc. Rev. 2001,30,50-61. 

Catalytic [3 + 2]-cycloaddition of the carbonyl and azomethine ylides 129 with olefins gives the five-membered heterocycles 130 (Scheme 45). Longmire et al. reported that the catalytic asymmetric [3 + 2]-cycloaddition of the azomethine ylides 131 with dimethyl maleate in the presence of AgOAc and a bis-ferrocenyl amide ligand 133 gave the pyrrolidine triesters 132 in excellent yields with very high enantiomeric excesses (Scheme 46).122 As described in section 8, the [3 + 2]-cycloaddition reaction of diazo compounds with olefins proceeds similarly through the formation of carbonyl ylides. 

Catalytic enantioselective rearrangements and cycloadditions involving ylides from diazo compounds 01CSR50. 

Finally, fluorinated carboxylic acids react with epoxides (trimethylaluminum-catalyzed),224 and conveniently with diazo compounds.19 225 227 Cycloaddition is observed with diazo-methane.228 Reaction of compound 7 with trifluoroacetic acid alone is relatively slow (Freon 113, reflux, 12 h, for completion of reaction) however, catalytic amounts of eopper(ll) perchlorate increase the rate of formation of ester 8 (rt, 5 min).229... 

D.M. Hodgson, F.Y.T.M. Pierard, PA. Stupple, Catalytic enantioselective rearrangements and cycloadditions involving yhdes from diazo compounds, Chettr Soc. Rev. 30 (2001) 50—61. 

The chemistry of copper carbenoids involved in the catalytic decomposition of diazo compounds and related tosylhydrazones has been reviewed. Many aspects of these catalytic transformations are covered including not only the classical cyclopropanation and X-H insertion processes but also a range of formal cycloaddition reactions, the reactions involving ylide formation, and the various coupling reactions of diazo derivatives. An account more focused on asymmetric metal-catalysed X-H insertion has been published. Through this review, the dependence on the nature of the metal and its i ligands can be evaluated for these 0-H, N-H, S-H, and Si-H insertions of carbenoids. 

With respect to the large number of unsaturated diazo and diazocarbonyl compounds that have recently been used for intramolecular transition metal catalyzed cyclopropanation reactions (6-8), it is remarkable that 1,3-dipolar cycloadditions with retention of the azo moiety have only been occasionally observed. This finding is probably due to the fact that these [3+2]-cycloaddition reactions require thermal activation while the catalytic reactions are carried out at ambient temperature. A7-AUyl carboxamides appear to be rather amenable to intramolecular cycloaddition. Compounds 254—256 (Scheme 8.61) cyclize intra-molecularly even at room temperature. The faster reaction of 254c (310) and diethoxyphosphoryl-substituted diazoamides 255 (311) as compared with diazoacetamides 254a (312) (xy2 25 h at 22 °C) and 254b (310), points to a LUMO (dipole) — HOMO(dipolarophile) controlled process. The A -pyrazolines expected... 

Despite the great synthetic utihty of diazocarbonyl compounds in the generation of carbonyl ylide intermediates, definitive mechanistic studies on the metal-catalyzed cycloaddition of carbonyl yhdes are scarce. Among the various metal catalysts, dirhodium(II) catalysts are the most effective and versatile for diazo decomposition. Because of the rapid catalytic tmnovers of these reactions, structural information about the intermediates is difficult to obtain. A reasonable mechanism can be rationahzed on the basis of product distribution, and especially on the basis of enantioselective outcome of various carbonyl yhde reactions [55-63]. 

Another successful catalytic enantioselective 1,3-dipolar cycloaddition of Qf-diazocarbonyl compounds using phthaloyl-derived chiral rhodium(II) catalysts has been demonstrated [ill]. Six-membered ring carbonyl ylide formation from the a-diazo ketone 80 and subsequent 1,3-cycloaddition with DMAD under the influence of 1 mol % of dirhodium(II) tetrakis[M-benzene-fused-phthaloyl-(S)-phenylvaline], Rh2(S-BPTV)4 101 [112], has been explored to obtain the cycloadduct 102 in up to 92% ee (Scheme 31). 

Rh-catalyzed [3 + 2] dipolar cycloaddition. Generally, this transformation is performed at elevated temperatures and affords furans directly in a single step without isolation or even observation of possible cyclopropene intermediate (Section 8.2.1.1). It was first demonstrated by D yakonov that the Cu(II)-catalyzed reaction between a-diazoesters and internal alkynes provided the corresponding 2-alkoxyfurans in moderate yields [201-205]. Consequently, several research groups further elaborated on this transformation in the presence of different transition metal catalytic systems for a variety of differently substituted a-diazo-carbonyl compounds and alkynes. 

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