Cyclopropanes reactions with transition metal complexes

Yet another type of SO2 insertion reaction occurs with transition metal complexes containing a cyclopropane ring. The reaction opens the ring to give a cyclic sulphone. 

The reaction of transition-metal complexes and strained carbocycles has been examined most extensively with Pt(II) complexes and cyclopropane derivatives. Chloroplatinic acid and cyclopropane react in acetic anhydride at RT to give a stable, polymeric compound, [PtCl2(C3H6)]4(I), in 50-70% yield ... 

Metal-Catalyzed. Cyclopropanation. Carbene addition reactions can be catalyzed by several transition metal complexes. Most of the synthetic work has been done using copper or rhodium complexes and we focus on these. The copper-catalyzed decomposition of diazo compounds is a useful reaction for formation of substituted cyclopropanes.188 The reaction has been carried out with several copper salts,189 and both Cu(I) and Cu(II) triflate are useful.190 Several Cu(II)salen complexes, such as the (V-f-butyl derivative, which is called Cu(TBS)2, have become popular catalysts.191... 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Among transition metal complexes, the ubiquitous dicarbonylcyclopentadienyliron (Fp) complexes are the first, and perhaps the best, representatives to demonstrate the utility of metal-halogen exchange reactions in metallacyclopropane synthesis. Thus, reaction of the readily available sodium dicarbonylcyclopentadienyliron [Cp(CO)2Fe]Na (FpNa) with the parent cyclopropyl bromide " and derivatives " gave in moderate yields the corresponding cyclopropane-Fp complexes (equation 3). 

Cyclopropanes can also be obtained in acidolysis reactions of cyclopropene-transition-metal complexes. This reaction has been used analytically to prove the eoordination of an intact three-membered ring to a metal eenter. For example, dichlorobis(> -cyclopentadienyl)niobium (10), upon treatment with sodium amalgam in toluene in the presence of eyclopropene, gives a moss-green eyclopropene eomplex 11 which can be isolated and characterized by its NMR spectrum. Treatment of complex 11 with hydrochloric acid results in the formation of almost pure cyclopropane, according to GC analysis. 

Various unsaturated cyclic 7t-ligands undergo, within a transition-metal complex, cycloaddition reactions and rearrangements with simultaneous formation of cyclopropane subunits. This is observed with cycloheptatriene ehromium and iron complexes such as 4 which give cycloaddition products, e.g. 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

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