Cyclohexene reaction with carbene

Titanium enolates.1 This Fischer carbene converts epoxides into titanium enolates. In the case of cyclohexene oxide, the product is a titanium enolate of cyclohexanone. But the enolates formed by reaction with 1,2-epoxybutane (equation I) or 2,3-epoxy butane differ from those formed from 2-butanone (Equation II). Apparently the reaction with epoxides does not involve rearrangement to the ketone but complexation of the epoxide oxygen to the metal and transfer of hydrogen from the substrate to the methylene group. 

An exo-type cyclization, proceeding through a cycloalkylidene carbene (49 n = 1, 3, 4), was proposed to explain the formation of enynes (50) and (52) from alkynyl lithium species (48). The proposed carbene (49) could be trapped by addition to cyclohexene and the cycloalkyne intermediate (51) was trapped by Diels-Alder reaction with 1,3-diphenylisobenzofiiran. 

Diazothioxanthene is the usual precursor for the thioxanthylidene carbene, which can be shown to have a nucleophilic character. It adds to fumaric or maleic esters to form cyclopropyl compounds, but does not react with cyclohexene (78JOC3303). Reaction with alkyl phosphites is a very useful means of preparing the phosphonate derivatives (72JIC985). 

These two pieces of evidence show that cyclohexene can add to metal carbene complexes to some extent, but that at low temperature backbiting to give oligomers is preferred to propagation, while at room temperature the product of addition can be trapped, at least for a time, by reaction with norbornene358. 

As anticipated, catalytic cyclopropanation of cyclohexene occurs with a much higher preference for the sterically less hindered diastereomer (anh-20) than in the case of the monosubstituted alkenes (Scheme 7). However, the reaction succeeds only with [Ru2(CO)4(OAc)2] as catalyst, whereas decomposition of 10a-SiMc3 wi CuOTf leads to the carbene dimers 11, and decomposition with Rh2(pfb)4 yields the apparent product of allylic C-H insertion (19) besides some ketazine. 

The mechanism indicates that intermediate formation of a carbene, but tests to confirm the presence of the carbene by possible addition reactions with olefins were negative. The formation of both cyclohexene and cyclohexene are depicted as having the carbene as a common precursor. The experimental data do not, however, rule out the formation of these molecules from diffei ent precursors. For example, cyclohexend could be formed directly from the excited diazirine molecule. Again, further consideration of this point will be deferred until later. 

BICYCLOPROPYLIDENE possesses unique reactivity toward a wide range of electrophiles and nucleophilic carbenes. (E)-I-DIMETH YLAMINO-3-tert-BUTYLDIMETHYL-SILOXY-1,3-BUTADIENE is a highly reactive diene for Diels-Alder reactions, as described in an accompanying procedure for the synthesis of 4-HYDROXYMETHYL-2-CYCLOHEXEN-1-ONE via the Diels-Alder reaction with methyl acrylate. Finally, this section concludes with the preparation of DIETHYL [(PHENYLSULFONYL)METHYL]PHOSPHONATE, a reagent that is very useful for synthesis of [Pg.285]

Reactions with cyclohexene illustrate the simple diastereoselectivity of halogen-substituted carbenes. Carbenoids generated from fluorodiiodomethane or dichloroiodomethane by di-ethylzinc treatment provide erafo-cycloadducts with moderate to good preference4, while chlorocarbene, obtained by thermolysis of (dichlornmethyl)phenylmercury 3, and bromocar-bene, liberated from dibromomethane by base6, are considerably less selective. 

Phenylthiocarbene, generated by phase-transfer catalysis from (phenylthio)chloromethane adds stereospecifically to ( )-1,2-diphenylethene to provide, 2-trans-2,3-cis-, 2-diphenyl-3-(phenylthio)cyclopropane1. The simple diastereoselectivity of reactions with olefins such as cyclohexene and phenylethene is moderate to good, favoring the endo- or cis-products. For further examples of sulfur-substituted carbenes, see Houben-Weyl Vol. E 19b, pp 1682-1745. 

Trimethylsilyl)carbene and (trimethylstannyl)carbene can be generated from the corresponding chloro precursors by a-elimination employing lithium 2,2,6,6-tetramethylpiperidide as base6. Their reactions with cyclohexene proceed with very high exo selectivities giving 7-(trimethylsilyl)bicyclo[4.1.0]heptane (9) and 7-(trimethylstannyl)bicyclo[4.1.0]heptane (10), however, the chemical yields are rather low. 

The reaction of carbenes with aromatic hydrocarbons is related to that with alkenes. Doering and Knox (1950, 1953) investigated this reaction using benzene as substrate even before their work with cyclohexene. They observed, however, ring expansion to give cycloheptatriene besides toluene (8-3, X = H). Norcaradiene as an intermediate was isolated only much later in the addition of dicyanomethylene... 

Bis(dialkylamino)cyclopropanes, 14 in Scheme 6, are potential carbene precursors in what amounts to the reverse of the usual reaction of carbenes with aUcenes. Vilsmaier, Kristen and Tetzlaff reported [41 ] that the flash vacuum pyrolysis of several 7,7-bis(dialkylamino)bicyclo[4.1.0]heptanes gave cyclohexene and amidines resulting from rearrangement of diaminocarbenes, Scheme 8. It... 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Reaction of elemental sulfur or cyclohexene sulfide with the carbene triosmium cluster Os CO)()(f/-rf rf rf-C6H5Ph) gives the corresponding thioketone cluster Os CO)g ii -rf rf rf-SC(,H5Ph)f ... 

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