Styrene cydopropanation

In a long-term research project, Hossain and coworkers investigated the usefulness of the CpFe(CO)2+ fragment [35-38] in the cydopropanation reaction of alkenes by a carbene transfer utilizing diazo esters as the carbene source (Scheme 9.17). The cydopropanation products of styrene derivatives could be obtained in good yields of up to 80% and excellent cis selectivity by using an excess of the alkene, whereas the cydopropanation of aliphatic alkenes was less effective, yielding the desired cyclopropane derivative in up to 51% yield. 

Fig. 1 Catalytic cydopropanation of ethyl diazoacetate and styrene with ruthenium complexes 1-6 [catalyst roading (mol%), reaction temperature, yield of the cydopropanacar-boxylate, ratio of trans-.cis]...

Cydopropanation reactions were carried out with ethyl diazoacetate as car-bene source and styrene, using petroleum ether as solvent (Scheme 15). After completion of the reaction, the mixture was filtered in order to separate catalyst and products. The heterogeneous system provided slightly higher yields (up to 90%) and similar lower diastereoselectivities compared to the homogeneous system. 

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. 

Figure 6.16 Microfluidic synthesis and conversion of diazomethane (28) in the Pd-catalyzed cydopropanation of styrene (31). (Courtesy of S. Ldbbecke.)...

In the next step, diazoraethane (28) was subjected to in situ conversion in the Pd-catalyzed cydopropanation of styrene (31, Figure 6.16). The homogeneous Pd catalyst (palladium(II) acetylacetonate) (lmol%) was added to the substrate solution and instead of benzoic add styrene was injected into the reaction mixture for the diazomethane quench under otherwise similar reaction conditions. Here, the conversion of styrene was detected. Complete conversion was achieved at flow rates of 1 ml/min and a temperature of 10 °C within 60 s. 

Another chiral auxiliary-based approach was documented by Davies (Equation 5) [8, 34]. These studies involved diazo compounds that incorporate vinyl and C=0 substitution and revealed that these stable carbene precursors participate in a variety of useful transformations, including diastereoselective cydopropanations. Importantly, Davies found that the cydopropanations with 33, in contrast with those of their simpler diazoacetate counterparts, are highly diastereoselective. The reaction with styrene and chiral diazo compound 33 is representative treatment of styrene with 33 in the presence of a simple Rh catalyst resulted in the formation of product 34 with high asymmetric induction dr 98.5 1.5) and in 84% yield. In this example, the panto-lactone auxiliaiy is a convenient, readily available chiral alcohol. 

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