Stereospecificity cyclopropanations

Photolysis of phenyldiazomethane in cis- or trans-butene leads to nearly stereospecific cyclopropane formation, although some C—H insertion occurs<2W ... 

The simple reaction scheme outlined in Scheme 2 easily accommodates the results obtained in the investigation of XA. The basic scheme is just the same as it is for BA except that the relative energies of the singlet and triplet states of the carbene are reversed. All of the evidence points to a singlet ground state for XA. This includes the absence of an epr signal, the nearly diffusion-controlled formation of ether in direct and sensitized experiments, the stereospecific cyclopropanation, and the absence of a rapid reaction of the carbene with 02. 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

A widely exploited procedure for bringing about carbenoid reactions of organic mono- and fifem-dihalides is by use of lithium alkyls. Examples are given in equations (11) and (12). Dimeric olefin formation, stereospecific cyclopropane formation from olefins, and insertion into carbon-hydrogen bonds have all been observed in suitable cases, together with further reactions of these products with excess of the lithium alkyl. 

The observation of stereospecific cyclopropane formation requires not only that both reaction steps be stereospecific but also that there be no conformational equilibration in the intermediate adduct. Further, more subtle tests of equation (18) need to be devised. 

Treatment of either cis- or franx-2,4-diphenylthietane-l oxide (stereochemical notation refers to arrangement of phenyl groups) with potassium t-butoxide in dimethylformamide gives mainly the cw-l,2-diphenylcyclopropane derivatives, 124 and 125, via the anion 123. ° Stereospecific cyclopropane formation occurs on treatment of 3- -hexylthietane 1-oxide with lithium cyclohexylisopropyl amide. ... 

A class of compounds which have drawn some interest are the sulfonium salt complexes 416, prepared by alkylation of sulfide derivatives (415) (Table IV). These coordinated ylides have found use as stable, stereospecific cyclopropanating reagents yielding compounds of the class 417. 

Thermal decomposition of a tosylhydrazone salt has also been demonstrated as one of only few examples of methods for the generation of 1-substituted 1-vinylcyclopropanes from diazoalkenes. Heating the dilithiotosylhydrazone 26 in xylene in the presence of dimethyl fumarate generates the pentacyclic carbon skeleton 27 via two stereospecific cyclopropanations. ... 

The reactions are stereospecific. The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), mo-lybdenum(O) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state. 

Homogeneous and heterogeneous catalysis of bismethoxycarbonylcarbene addition to olefins and the copper- and silver-catalysed decompositions of dimethyl diazomalonate all result in stereospecific cyclopropanation, although three-membered ring formation is accompained by significant amounts of allyic C—H insertions pro-... 

Dimethyl maleate undergoes a stereospecific cyclopropanation in the presence of a zinc-cobalt couple, but a ciS trans mixture is obtained with zinc-nickel.238 Cyclopropanation of a phosphorus-phosphorus double bond was reported from diiodomethane and zinc to give a diphosphirane (Eq. 53) 239... 

Stereospecific cyclopropanation of a,(3-unsaturated amides 122 was achieved using these reagents generated from Cr(II) and CHI3 [100]. This is a useful strategy for the preparation of trisubstituted cyclopropanes 123 and 124 in a stereoselective manner (Scheme 1.59) [101]. 

Cyclobutene derivatives (66) have been synthesized from a diyne and an alkene via a novel Au(I)-catalysed reaction. A highly active vinylidene intermediate (67), formed by a dual Au(I)-mediated activation of the diyne precursor, is believed to act as an alkylidene Au(I)-carbenoid to effect stereospecific cyclopropanation of the alkene the resulting methylenecyclopropane (68) converts to (66) via an Au(I)-catalysed ring-expansion cascade. 

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