Carbenoid cyclopropanation

The formulation of an additive for zinc carbenoid cyclopropanation that meets these criteria is severely compromised by the by the inherent Lewis acidity of the zinc atom. This Lewis acidity is required for methylene transfer and plays a major... 

Scheme 6.1 Catalytic cycle of metal-catalysed carbenoid cyclopropanation reactions with diazo compounds.

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

It is not known whether or not this transformation is catalyzed by the transition metal. However, the metal-catalyzed ring-opening reaction of (3-alkoxycyclopropane carboxylates yielding vinyl ethers (e.g. 50 -> 51 and 52 - 53) is well documented 97 120 . Several catalysts are suited [PtCl2 2 PhCN, Rh2(OAc)4, [Rh(CO)2Cl]2, [Ru(CO)3Cl2]2, Cu bronze, CuCl], but with all of them, reaction temperatures higher than those needed for the carbenoid cyclopropanation reaction are required. 

Enantioselective carbenoid cyclopropanation can be expected to occur when either an olefin bearing a chiral substituent, or such a diazo compound or a chiral catalyst is present. Only the latter alternative has been widely applied in practice. All efficient chiral catalysts which are known at present are copper or cobalt(II) chelates, whereas palladium complexes 86) proved to be uneflective. The carbenoid reactions between alkyl diazoacetates and styrene or 1,1 -diphenylethylene (Scheme 27) are usually chosen to test the efficiency of a chiral catalyst. As will be seen in the following, the extent to which optical induction is brought about by enantioselection either at a prochiral olefin or at a prochiral carbenoid center, varies widely with the chiral catalyst used. 

Copper chelates in which the ligands are rigid chiral p-diketonates of type 205 are responsible for the highest optical yields known in carbenoid cyclopropanation reactions 200). The cyclopropane 206 was even obtained enantiomerically pure from 2-diazodimedone and styrene in the presence of CuL (L = 205c). 

Aziridines have been synthesized, albeit in low yield, by copper-catalyzed decomposition of ethyl diazoacetate in the presence of an inline 260). It seems that such a carbenoid cyclopropanation reaction has not been realized with other diazo compounds. The recently described preparation of 1,2,3-trisubstituted aziridines by reaction of phenyldiazomethane with N-alkyl aldimines or ketimines in the presence of zinc iodide 261 > most certainly does not proceed through carbenoid intermediates rather, the metal salt serves to activate the imine to nucleophilic attack from the diazo carbon. Replacement of Znl2 by one of the traditional copper catalysts resulted in formation of imidazoline derivatives via an intermediate azomethine ylide261). 

Carbenoid cyclopropanation, 57, 1 58, 1 Carbohydrates, deoxy, synthesis of, 30, 2 Carbo/metallocupration, 41, 2 Carbon-carbon bond formation ... 

Other interesting examples of asymmetric syntheses involving chiral monoterpenoids include the Claisen reaction between (—)-menthyl phenylacetate and benz-aldehyde (optical purity is confirmed by microcalorimetry), a highly enan-tioselective carbenoid cyclopropanation catalysed by (4), ° and the crossed aldol... 

Zhao, C., Wang, D.-Q., Phillips, D. L. Density functional study of selected mono-zinc and gem-dizinc radical carbenoid cyclopropanation reactions observation of an efficient radical zinc carbenoid cyclopropanation reaction and the influence of the leaving group on ring closure. Journal of Theoretical Computational Chemistry 2003, 2, 357-369. 

Enantioselective carbenoid cyclopropanation of achiral alkenes can be achieved with a chiral diazocarbonyl compound and/or chiral catalyst. In general, very low levels of asymmetric induction are obtained, when a combination of an achiral copper or rhodium catalyst and a chiral diazoacetic ester (e.g. menthyl or bornyl ester ) or a chiral diazoacetamide ° (see Section 1.2.1.2.4.2.6.3.3., Table 14, entry 3) is applied. A notable exception is provided by the cyclopropanation of styrene with [(3/ )-4,4-dimethyl-2-oxotetrahydro-3-furyl] ( )-2-diazo-4-phenylbut-3-enoate to give 5 with several rhodium(II) carboxylate catalysts, asymmetric induction gave de values of 69-97%. ° Ester residues derived from a-hydroxy esters other than ( —)-(7 )-pantolactone are not as equally well suited as chiral auxiliaries for example, catalysis by the corresponding rhodium(II) (S )-lactate provides (lS, 2S )-5 with a de value of 67%. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or Zra ,s-2-butene and norbomene with vinyldiazomethane 2 Although the yields were low (20-38 %),... 

Several catalysts are suited [PtCl 2 PhCN, RhjCOAc), [Rh(CO)2Cl]2, [Ru(CO)3Cl2]2, Cu bronze, CuCl], but with all of them, reaction temperatures higher than those needed for the carbenoid cyclopropanation reaction are required. 

Cyclopropanation. Rhodium complexes have been extensively employed for catalytic carbenoid cyclopropanation of alkenes. In the presence of [Rh2(S-TBSP)4], treatment of 2-aryl-substituted 2//-chromenes with dimethyl diazomalonate furnished the desired cyclopropanes in good yields (eq 41). The cyclopropane product with a terf-butyl ester group underwent rearrangement upon treatment with Sn(OTf)2 to from a y-lactone. OMe... 

Computational studies of the carbenoid cyclopropanation reactions of methoxymethyl-lithium and intra- and intermolecular carbenoid reactions of lithiated oxiranes have been reported. Computations suggest that methoxymethyllithium reacts with ethylene exclusively by a stepwise carbolithiation mechanism. The intramolecular reaction of lithiated l,2-epoxy-5-hexene was found to proceed by both the carbohthiation and the methylene transfer pathways, but the former is expected to dominate at room and low temperatures because the free energy of activation is less than half that of the latter pathway. 

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