Carbenes, reaction with alkenes, stereoselectivity

The reactions of these iron carbene reagents with alkenes to give cyclopropanes are stereospecific. They also exhibit high syn stereoselectivity in many cases. Optically active derivatives have been reported that have chiral ligands on iron or chiral alkoxy groups on the prospective caibene center and which have been resolved with the iron itself as a chiral center. Resulting from this work have been some highly enantioselective cyclopropanations. 

The monosubstituted (trifluoromethyl)carbene has only been generated by irradiation of 2,2,2-trifluorodiazocthane, which is prepared by nitrosation of 2,2,2-trifluoroethylamine.1,3 175 The course of reactions with alkenes is dependent on concentration and pressure conditions. Thus, insertion products and pyrazolines may be obtained in competition with [2+1] cycloaddition giving the cyclopropane system. Yields of the latter are often moderate and, depending on singlet or triplet carbene formation, the reaction is not always stereoselective (Table 14). 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

New evidence as to the nature of the intermediates in catalytic diazoalkane decomposition comes from a comparison of olefin cyclopropanation with the electrophilic metal carbene complex (CO)jW—CHPh on one hand and Rh COAc) / NjCHCOOEt or Rh2(OAc)4 /NjCHPh on the other . For the same set of monosubstituted alkenes, a linear log-log relationship between the relative reactivities for the stoichiometric reaction with (CO)5W=CHPh and the catalytic reaction with RhjfOAc) was found (reactivity difference of 2.2 10 in the former case and 14 in the latter). No such correlation holds for di- and trisubstituted olefins, which has been attributed to steric and/or electronic differences in olefin interaction with the reactive electrophile . A linear relationship was also found between the relative reactivities of (CO)jW=CHPh and Rh2(OAc) NjCHPh. These results lead to the conclusion that the intermediates in the Rh(II)-catalyzed reaction are very similar to stable electrophilic carbenes in terms of electron demand. As far as cisjtrans stereoselectivity of cyclopropanation is concerned, no obvious relationship between Rh2(OAc) /N2CHCOOEt and Rh2(OAc),/N2CHPh was found, but the log-log plot displays an excellent linear relationship between (CO)jW=CHPh and Rh2(OAc) / N2CHPh, including mono-, 1,1-di-, 1,2-di- and trisubstituted alkenes In the phenyl-carbene transfer reactions, cis- syn-) cyclopropanes are formed preferentially, whereas trans- anti-) cyclopropanes dominate when the diazoester is involved. 

Until the last decade, product studies formed the main evidence for carbene formation singlet carbenes formed cyclopropanes from alkenes stereospecifically, while triplet carbenes formed cyclopropanes non-stereospecifically. Formation of a cyclopropane (though not by addition to an alkene) via a carbocation route was demonstrated and, more recently, it has been shown that p values for insertion-addition selectivity and for cyclopropanation stereoselectivity vary as to photochemical or thermal generation of the carbene. The authors of this latter study suggest that a ground state diazo compound could be masquerading as a carbene in its thermal reaction with olefins, possibly by electrocyclic... 

Intramolecular reactions of carbenes with alkenes have been exploited in synthesis. The sesquiterpene cycloeudesmol was prepared using, as a key step, the intramolecular cyclopropanation of the diazoketone 122 (4.97). The cyclopropana-tion reaction occurs stereoselectively to give the tricyclic product 123, which was subsequently converted into the natural product. A synthesis of sesquicarene was achieved using the copper(I)-catalysed decomposition of the diazo compound 125, itself prepared by oxidation of the hydrazone 124 (4.98). 

Cyclopropanation Reactions. Davies and Nagashima reported the first example of a catalytic asymmetric cyclopropanation of alkenes on a solid support. Carbene dimerization represents a limitation in solution phase, lowering yields and necessitating additional purification steps. Immobilization of the olefin 97 on a polystyrene diethylsilyl resin followed by reaction with various diazoacetates in the presence of a rhodium catalyst generated the cyclopropanes 98 and 99 in high yield and enabled the removal of dimerization products 102 through a simple wash step (Scheme 6.23). The products 100 and 101 were cleaved as a mixture of diastereomers from the resin under mild conditions. The stereoselectivity of the reaction was not influenced by the solid support, but rather by the catalyst selection most important, >90% ee was observed under these conditions. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Some other electrophiles that convert alkenes to cyclopropanes are not free carbenes but have metals coordinated with their electrophihc site. These are called carbenoids and include the Simmons-Smith reaction and the copper-, rhodium-, or palladium-catalyzed decomposition of diazoketones and esters (Section 7.3). That the metal atom is present in the electrophile is shown by the variation of the stereoselectivity of the reaction with changes in the other ligands on metal. 

Stereoselectivity) is observed however, for ethylidene complexes of Fe(CO)(PR3)Cp (69) the products reflect trans selectivity. This difference in stereoselectivity has been suggested to be dependent upon which conformer is more reactive. The reaction of a chiral-at-iron cationic carbene complex (70) with styrene or vinyl acetate affords optically active cyclopropane products with high enantioselectivity (Scheme 24). h >3 intramolecular cyclopropanation, as in the case of (71), proceeds moderately well for the formation of norcarane-type ring systems however, intramolecular C-H insertion is a competing pathway when the alkene is highly... 

Mechanistic information on the photolytic Bamford-Stevens reaction is provided by the successful isolation of a diazo hydrocarbon as a reaction intermediate from the direct photolysis of a tosylhydrazone sodium salt (90 Scheme 10). This study also clarifies that the carbene derived thermally and that derived photolytically behave differently with respect to the stereoselectivity of the 1,2-hydrogen shift that produces the alkene. 

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