Photolysis, chloro carbene

In the cyclobutylfluoro system, the excited diazirine, 21-F, is considerably less involved. A parallel analysis indicates that only about 12% of 22-F and 23-F arise from excited diazirine, while 88% of those products descend from carbene 17-F.28 The increased carbene involvement in the photolysis of 21-F presumably reflects the greater stability of fluorocarbene 17-F over its chloro analogue 17-C1 and, consequently, a more favorable partition (ki/k3) of excited diazirine 21-F to the carbene. 

Irradiation of diazo 21d in carbon tetrachloride gave, in addition to 72 (R = Me) and 73, the 3-chloro-3-trichloromethyl-l-methyloxindole (76a) (64JOC3577). Since the photolysis in carbon tetrachloride under comparable conditions produces, in the primary step, CCI3 radicals and chlorine atoms (63JOC3442), in the formation of 76a and probably in the reaction with oxygen to give 74, the carbene 71 shows biradical properties in the... 

In thermolysis and photolysis of 3-chloro-3-benzyldiazirines 23, carbenes 24 are formed as indicated by reaction 2. In both electron-rich (tetramethylethylene) and electron-poor (diethyl fumarate) alkenes (as solvents), cyclopropanes (and chlorostyrenes) are obtained59. The probable reaction pathway is the multi-step model shown in Scheme 9. 

The effect of the halogen substituent (fluoro, chloro, bromo and iodo) on the yield and mechanism of 4-halophenol photolysis was investigated by Durand et al. [24], Transient spectroscopy in aerated aqueous solutions indicated the formation of p-benzoquinone O-oxide from each derivative except 4-iodophenol for which no transients were detected p-benzoquinone and hydroquinone were found as photoproducts for all four compounds. It was concluded that the carbene mechanism was valid for the whole series. Under continuous irradiation, the 4-halophenol degradation quantum yields were determined to be

fluorescence lifetimes decreased in the same order, from 2.1 ns for 4-fluorophenol to 0.4 ns for 4-chlorophenol and < 0.1 ns for 4-bromophenol. 

The pyrolysis of chlorodiazirines appears to parallel that of unsubstituted diazi-rines. For the 3-chloro-3-methyl, 3-chloro-3-ethyl, 3-chloro-3-isopropyl and 3-chloro-3-r-butyI derivatives the decompositions are kinetically of the first order with A factors close to 10 and energies of activation of about 31 kcal.mole . The intermediacy of carbenes is postulated by analogy with the photolysis of 3-chloro-3-methyl-diazirine in the presence of ethylene where the major product has been tentatively identified as 1-chloro-l-methylcyclopropane. 

Photolysis of 5-chloro-2-hydroxybenzonitrile (70) in aqueous solution gives the triplet carbene (71), which can be detected by transient absorption spectroscopy (X,max at 368 and 385 nm). The carbene was recognized by its reactions, e.g. with O2, to produce the corresponding carbonyl O-oxide (A ax at 470 nm), and with propan-2-ol to give 2-cyanophenoxyl radical. In deoxygenated solutions the main stable products are 2,5-dihydroxybenzonitriIe and two substituted biphenyls. 

The photolysis or thermolysis of 3-chloro-3-cyclobutyl- and 3-chloro-3-cyclopropyl-3/7-diazirine " generates chloro(cycloalkyl)carbenes which react with alkenes to form 1-chloro-l-cyclobutyl- or 1-chloro-l-cyclopropylcyclopropanes 1 and 2, respectively (Houben-Weyl, Vol. E19b, pp 1520-1521). 

Addition versus rearrangement of chloro(methyl)-, chloro(ethyl)-, benzyl(chloro)-, chlo-ro(methoxymethyl)- and chloro(isopropyl)carbene generated by photolytic or thermal decomposition of 3-alkyl-3-chloro-3//-diazirines has also been investigated.81 However, it is known that chloro(isopropyl)- or ie/7-butylchlorocarbene does not undergo addition to 2,3-dimethyl-but-2-ene, which is used as a carbene trap. Photolysis or thermolysis of 3-arylmethyl-3-chloro-3//-diazirines in the presence of an alkene has also been studied extensively.84 Addition of the ambiphilic benzylchlorocarbene82 to an alkene to give 1-benzyl-1-chlorocyclopropanes is usually accompanied by its rearrangement to co-chlorostyrene (1,2-hydride shift, for details see ref 83). This isomerization was seriously impeded when the decomposition of 3-benzyl-3-chloro-3//-diazirines was performed in the presence of an alkene,85 with 2,3-dimethylbut-2-ene only traces of co-chlorostyrene were observed.81,86 The results of reactions of benzylchlorocar-benes with some alkenes are collected in Table 6.85... 

Generation of Alkyl and Alicyclic Carbenes. - It has been found that Ceo can act as a mechanistic probe for the formation of carbene (7) (Scheme 2) and diazo compound (8), and for the direct formation of the product (9) from the excited state (6 ), in the photolysis of 3-chloro-3-isopropyldiazirine (6). It seems that the carbene adds to 50 to form a methanofullerene, while the diazo compound adds to form a fulleroid. For photolysis of (6) at — 40°C, the results indicated formation yields of carbene (7), diazo compound (8) and product (9) of... 

A similar preference for cis- or emfo-products is generally observed in the reactions of thermally generated chloro(trichloroethenyl)carbene with a large variety of olefins (see Vol. E 19b, p 740)11. The alkynyl-substituted earbene 4 obtained by photolysis of 5-bromoethynyl-3,3-dimethyl-3//-pyrazole adds exo selectively to 3-cyclohexenone to give an 80 20 mixture of 7-bromo-7-(4-methyl-3-penten-l-ynyl)bicyclo[4.1.0]heptan-3-one (5/6)12. The reaction of chlorophenylcarbene and cyclohexene affords 7-chloro-7-phenylbicyclo[4.1.0]heptane with moderate endo preference (see Vol. E19b, p984), i. 

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