Cyclopropane formation abstraction

Monoaryldiazomethanes, readily prepared by a number of methods, are the carbene precursors most frequently used for the synthesis of arylcyclopropanes. When such diazo compounds are decomposed photochemically, thermally, or by using various transition-metal salts in the presence of an alkene, arylcyclopropanes are formed. The yield is often quite high, but in a number of cases cyclopropane formation has been hampered by competing reactions, of which, disregarding intramolecular reactions, azine formation, stilbene formation, and hydrogen abstraction followed by dimerization are the most predominant. Many aspects related to the use of diazomethane derivatives as carbene precursors have been thoroughly discussed by Wentrup. ... 

EPR studies of diphenylmethylene and a number of other arylmethylenes have indicated that these carbenes have triplet ground states.<30) Photolysis of diphenyldiazomethane in olefin matrices results in the formation of triplet diphenylmethylene, which undergoes primarily abstraction reactions with the olefins. Cyclopropanes are produced as minor products. 

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

The alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

Formation of Cyclopropane Ring via Intramolecular 7-Hydrogen Abstraction... 

The cyclopropane synthesis is also suitable for the preparation of highly strained bicyclic hydrocarbons such as [2.1.0]bicyclopentanes (14) and spiropentanes (16) [14a,b]. The formation of the spiropentane 16 is particularly remarkable as it is the result of a homolytic hydrogen abstraction from a cyclopropane ring. Those processes are very rarely observed due to the relatively high C-H-bond energies of cyclopropanes (Sch. 8). 

The easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

The processes are the monomolecular reaction through a protonated cyclopropane produced by the abstraction of H" over Lewis acid sites and the bimolecular mechanism where an olefin takes part in the reaction. The olefin is produced over Bronsted acid sites, in the case of butane in the monomolecular mechanism, isobutane is formed through protonated methylcyclopropane with an activation energy of 8.4kcalmoT followed by the formation of the primary isobutyl cation with high energy [134]. 

The reaction of alkenes with Fischer carbene complexes most typically leads to cyclopropane products however, the formation of a three-membered ring product from a reaction with an alkyne has been observed on only one occasion. The reaction of the cationic iron-carbene complex (199) with 2-butyne presumably leads to the formation of the cyclopropene (200), which was unstable with respect to hydride abstraction by the starting carbene complex and the ultimate product isolated from this reaction was the cyclopropenium salt (201) and the benzyl-iron complex (202). Cyclopropene products have never been observed from Group 6 carbene complexes despite the extensive investigations of these complexes with alkynes that have been carried out since the mid 1970s. 

The reactions of diphenylmethylene and fluorenylidene with olefinic double bonds are not stereospecific. Photochemical or thermal decomposition of diphenyldiazomethane in the presence of alkenes is often accompanied by the formation of a substantial amount of non-cyclic products derived from abstraction-recombination reactions The extent of hydrogen abstraction relative to addition is highly dependent on the substitution pattern of the olefin In contrast, fluorenylidene generated from 9-diazofluorene usually gives cyclopropanes as the major product. Cyclopentadienylidene and its substituted analogues can be generated from the corresponding diazo precursors. They react with olefinic as well as with acetylenic substrates Cycloheptatrienylidene preferentially... 

Due to their good leaving-group properties, all (a-haloalkyl)cyclopropanes except (a-fluoroal-kyl)cyclopropanes are readily substituted by a carbon nucleophile under the correct conditions. Fairly clean reactions have been achieved with sodium cyanide and potassium cy-anide under homogeneous and phase-transfer conditions. Carbon nucleophiles generated by hydrogen abstraction under basic conditions can result in the formation of considerable amounts of byproducts, but successful reactions have been reported, particularly when an intramolecular substitution reaction oc-curs. It is also noteworthy that a phenyl group can be attacked under similar conditions. ... 

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