Carbene allyl diazoacetate

The Ru porphyrin complex (8) has also been used as a catalyst for the cyclization of allylic diazoacetates,258 albeit with limited success only the cyclization of F-cinnamyl diazoacetate shows high enantioselectivity (Scheme 81). It is noteworthy that a carbenoid species prepared from allyl o-phenyl-o-diazoacetate and complex (8) has been isolated and subjected to X-ray diffraction analysis, though it does not undergo the desired cyclization. In the structure, the carbene plane lies almost halfway between the two adjacent Ru—N bonds. 

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrroli-done-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene transformations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubsti-tuted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro-gen insertion reactions of diazoacetate esters that result in substituted y-butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabilization of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement. 

In spite of the low reactivity of 1,2-disubstituted and trisubstituted olefins in the intermolecular cyclopropanation, some allyl diazoacetates were easily cyclopropanated to give the corresponding 3-oxabicyclo[3.1.0]hexan-2-one derivatives [44]. The trans isomers gave good results (from 76 to 86% ee),but low enan-tioselectivities were observed for the cis derivatives, Eq. (9). The diazo substrates containing a 2-alkyl substituent did not undergo intramolecular cyclopropanation under a variety of reaction conditions. In these cases, carbene dimers were the only isolated products. 

Occasionally, systems with complexes of ruthenium and cobalt have been reported to catalyze intramolecular cyclopropanation of allylic diazoacetate with high enantiocontrols. For cobalt-salen 39, up to 97% ee was observed (Scheme 30) (128). More recently, methods other than diazo decomposition were applied in intramolecular cyclopropanation. Activated by irradiation, [WlCOle] catalyzed the cyclopropanation of alkynol to give good yield of cyclopropane (Scheme 31) (129). The reaction was proposed via a tungsten—carbene intermediate. 

Photolyses carried out in acetic acid give allylic acetates, apparently via a diazoacetate (Scheme 60)51,57. Carbenes derived from spirofluorene derivatives have been trapped with pyridines and pyridazines to give useful photo-chromic spirodihydroindolizines.163... 

However, the use of Rh2(MPPIM)4 provides enhanced enantiocontrol for cyclopropanation of trans-disubstituted double bonds, up to 96% ee in the cases examined [89]. Trisubstitutec allylic double bonds, even that in famesyl diazoacetate (33, Eq. 5.17) [90], undergo effective, efficient, and highly enantioselective cyclopropanation (Eq. 5.17). Product yields are high except foi those cases in which steric factors appear to limit olefin approach to the metal carbene center. 

In the presence of alkenes, photolysis of alkyl (silyl)diazoacetates leads mainly to the formation of cyclopropanes as diastereomeric mixtures4,111,112. With (Z)- and ( )-but-2-ene, the cyclopropanation is not completely stereospecific with respect to the double bond configuration, but gives a small amount of the wrong isomer these results point to the participation of a triplet carbene in the cyclopropanation reaction. Allylic C,H insertion products are also formed their yield increases in the series 1,1-, 1,2-, tri- and tetrasubstituted C=C bond. With 2,3-dimethyl-but-2-ene, the allylic C,H insertion product is formed at the complete expense of the cyclopropane. 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

The photochemical extrusion of nitrogen from silyl-substituted diazoacetates (hv > 300 nm) in the presence of various alkenes leads mainly to the formation of cyclopropanes (Table 4). Reactions of trimethylsilyl- and triisopropylsilyldiazoacetates with monosubstituted alkenes such as hex-1-ene or styrene (Table 4, entries 1-3) show interesting results. The formation of the thermodynamically less favored Z-isomer increases with growing steric demand of the silyl substituent. The cyclopropanation of ( )- and (Z)-but-2-ene (Table 4, entries 5 and 6) reveals that the addition reaction does not proceed completely stereospecifically. Small amounts of the wrong diastereomer can be detected, which is believed to arise from the triplet spin state of the carbene. Insertion into allylic C-H bonds occurs in the case of di- or trisubstituted alkenes (Table 4, entries 4-7). 

Allyl sulfides and allyl amines. Rhodium-catalyzed decomposition of ethyl diazoacetate in the presence of these allyl compounds generates products 136 and 137, respectively, derived from [2,3] rearrangement of an S- or N-ylide intermediate, besides small amounts of carbene dimers No cyclopropane and no product resulting from the ylide by [1,2] rearrangement were detected. Besides RhjfOAc) and Rhg(CO)i6, the rhodium(I) catalysts [(cod)RhCl]2 and [(CO)2RhCl]2 were found to behave similarly, but yields with the only allyl amine tested, CH =CH—CH NMe, were distinctly lower with the latter two catalysts. Reaction temperatures are higher than usually needed in rhodium-promoted diazoalkane decomposition, which is certainly due to competition between the diazo compound and the allylic hetero-... 

Analysis The carbene synthon is easy it can be ethyl diazoacetate N2CHC02Et. The diene can be made by the Wittig reaction from a familiar allylic bromide (TM 31). 

Further studies of carbene insertion into the C S bond are reported, the authors earlier work on alkyl allyl sulphides, using bis(methoxy-carbonyl)carbene, now being supplemented by a study of the relative tendencies towards C—S insertion and C=C addition for the same substrates with carbethoxycarbene (from ethyl diazoacetate). 1,2-Diphenyl-ethyl phenyl sulphide, PhCH(CHaPh)SPh, is formed by the action of benzyne on dibenzyl sulphide, via the sulphonium ylide. Alkylation at sulphur is one of the best-known reactions of sulphides, and novel syntheses of this type have been reported in which the formation of hydro-quinone sulphonium salts, from quinone and a sulphide in 70% H2SO4 at — 5 to -f 5 and Markownikov addition of a sulphide to an alkene under similar conditions are discussed. 

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