Halo carbenes

Carboxylic acid, aldehyde, ketone, ether, alcohol, ester, ester-R (the chain attached to the oxygen atom being a generic substituent), anhydride, acetal, amide, epoxide, acid halyde, primary amine, primary imine, cyano, secondary amine, secondary imine, tertiary amine, nitro derivative, metal-1, metal-2, carbene, halo derivative. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

Singlet halo(trifluoromethyl)carbenes [33a]-[33c], which were characterized by IR and UV spectroscopies and chemical trapping with HCl, have been generated from respective 3-halo-3-(trifluoromethyl)diazirines [34a]-[34c] frozen in an argon matrix at 12 K and irradiated with UV light at... 

Nevertheless, acetoxycarbenes are less reactive than analogous chlorocarbenes, as shown by the data in Table 5, where we compare 1,2-H shift rate constants for phenoxymethyl-, cyclobutyl-, and isopropyl halo and acetoxy carbenes. Decreased rates of 1,2-H shifts are observed, due to the influence of the acetoxy spectator substituents, with the kinetic suppression reaching a factor > 900 for the comparison of cyclobutylacetoxycarbene with cyclobutylchlorocar-bene.81... 

The addition of halomethanes across the central bond of [l.l.l]propellane 78 leads to l-halo-7-(n-halomethyl)tricyclo[4.2.0.02,7]octanes 79, which are suitable precursors for the generation of type 80 carbenes.26... 

All synthetic methods described up to now (ligand displacement, acac or halo-methyl precursors, metal-bonded carbene + nucleophile, metal-bonded nucleophile + carbene) result in a metal-bonded ylide through the Ca atom. The reactivity of ylides toward metallic systems is, however, greater than anticipated and other reaction pathways could compete with simple C-bonding. 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

Reactions of [l.l.l]propellane that preserve the tricylic structure are not known. One such reaction was investigated computationally at the MINDO/3 level the generation of the a-halo anions 17 and subsequently the carbene 18 from chloro- and fluoro[l.l.l] propellane (equation 10). The authors concluded that it should be possible to generate the anion 17 under relatively mild phase transfer conditions72. 

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