Carbenes formed from photolysis

Carbenes are commonly generated by irradiation or pyrolysis of an appropriate diazo-compound (2). Apart from differences readily traced to the change in temperature, the chemical properties of the carbenes formed from photolysis and from thermolysis are usually quite similar. These observations... 

This reaction is virtually useless for synthetic purposes but illustrates the extreme reactivity of carbene. Treatment in the liquid phase of an alkane such as pentane with carbene formed from the photolysis of diazomethane gives the three possible products in statistical ratios227 demonstrating that carbene is displaying no selectivity. For many years, it was a generally accepted principle that the lower the selectivity the greater the reactivity however, this... 

Similarly, photolysis of 3,3-dimethoxy-5-norbornen-2-one 109 in various solvents (dioxane, acetonitrile, and acetone) gave the 1,3-acyl shift product 110 in minor amounts and the decarbonylated product 111 as the major product (Scheme 25). Irradiation of 109 in methanol gave a complex mixture of products arising through an oxa-carbene formed from the biradical generated after initial a-cleavage. It was also observed that the decarbonylated product 111 was obtained at the expense of the initially formed 1,3-acyl shift product 110. 

As stated previously, the photolysis of cyclobutanone in ethanol solution results in an 8% yield of an acetal, presumably formed from ethanol addition to a carbene intermediate. Alkylation of the a positions of cyclobutanone increases the yield of this rearrangement product,(22)... 

The addition of carbenes is also a stereospecific reaction, each geometrical isomer forms the cis addition product. Thus carbene generated from the photolysis of diazomethane adds in a cis manner to both cis and trans butene-2. 

The reaction of triplet diphenylcarbene with pyridine has been well studied, and a mechanism proposed from kinetic data (90TL953). The carbenes generated from laser flash photolysis of alkylbromo- and alkylfluoro-diazirines were trapped by pyridine to form the pyridinium ylides... 

Cycloaddition of a ketene complex with unsaturated bonds other than alkenes and alkynes is also possible. The ketene 312, formed from 311, adds to imine 313 to give the /1-lactam 314 under sunlight photolysis. The optically active /1-lactam 314 was prepared from the optically active carbene complex 311 with 99% ee, and converted to 315 [95]. Irradiation of carbene complex 316 generates ketene 317, which cyclizes to the o-hydroquinone derivative 318 [96],... 

Scheme 4 Possible reactions of the intermediates formed after photolysis of 3-aryl-3//-diazirines. Inset shows olefin formation from carbene derivative of unsubstituted 3-alkyl-3//-diazirines via a hydride shift...

These results were interpreted with reference to an intramolecular heavy atom effect the authors concluded the carbene is formed from the excited singlet state of the molecule, in competition to intersystem crossing. A direct proof of this statement could, however not be obtained [24], It should be noted here that alternative explanations for the effect of halogen substitution on photolysis quantum yields have been put forward for the 4-haloanilines (see below). 

For easier comparison the result of the thermal reaction is included for compounds 46 and 47. As can be seen in the reaction scheme above direct photolysis of the pyrazolines 46 and 47 proceeds mainly with retention of the original stereochemistry in the cyclopropanes isolated. 48,49 and 50 however lead mainly to the inverted stereochemistry in the cyclopropanes. The singlet biradical 51 formed from 46—49 is therefore clearly not on the same energy surface as a, . possible singlet diradical in the carbene cycloaddition. However one knows today that singlet carbene cycloaddition is a concerted process, so no such diradical should be detectable. 

Carbene reagents also functionalize alkanes. Triplet tCHa adds unselectively to alkane C—bonds. The product mixture obtained from n-pentane was found to be 48% n-hexane, 35% 2-methylpentane and 17% 3-methylpentane, so that addition to a primary C—H bond appears to be favored. Monochloro-methylcarbene, CHCl, is less reactive and more electrophilic and so the normal tertiary > secondary > primary selectivity pattern was observed. Ethoxycarbonylcarbene, formed on photolysis of the corresponding diazo compound, inserts rather unselectively in to alkane C—H bonds to give the ethoxycarbo-nylmethyl derivatives in ca. 50% yield. Transition metals, such as copper(II) or rhodium(I), also usefully catalyze the insertion of caibenes into alkane C—bonds. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

Diazirines were detected when there was broad activity in the carbene field. From their structure, cleavage to nitrogen and carbene was foreseeable, and this was shown to occur on photolysis as well as on thermolysis. As early as 1962, Frey and Stevens in a series of papers reported on photolysis of simple diazirines. According to these authors, diazirines are especially fit for the study of excited intermediates and their stabilization products. Products of isomerization of carbenes, i.e., olefins and cyclopropanes, are formed containing more energy than is necessary for their further decomposition. Their stabilization by loss of energy to partners competes with stabilization by subsequent reactions. 

Evidence for the carbenic nature of nitrile ylides formed by photolysis of azirines has been obtained from detailed mechanistic studies of intramolecular 1,1-cycloaddition reactions.51 It was concluded that these additions (Scheme 8) take place in a stepwise manner via diradical or zwitterionic intermediates. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

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