Photochemical reaction with nitriles

The UV irradiation of the 4,5-dihydro-1,3,5-oxazaphosph(V)olene 39 leads to a cyclic elimination. Analogously to the thermal [5 + 3 + 2] cyclo elimination, the photochemical reaction generated nitrile ylides reacted with alkynes and alkenes to yield the 2/f-pyrrole 40 and the pyrrol-l-ine 41, respectively (equation 17)71. 

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

When compared with the results of Holm et al. the lack of benzonitrile sulfide (35) bands may be explained by a change in the photochemical pathway due to the microscopic environment (argon matrix at 11 K Wentrup et al. versus PVC/EPA Holm et al.) or to rephotolysis of benzonitrile sulfide (35) under the experimental conditions into benzonitrile (28) and sulfur. A dependence of the photochemical pathways due to the microscopic environments are known from other cases <76TL873>. For a further discussion of the generation and reaction of nitrile sulfides see the review by Wentrup and Kambouris <91CRV363>. 

Extensive work has been done to determine and understand the factors controlling diastereoselectivity in the cycloaddition of nitrile oxides to alkenes but very little is known about nitrile ylides in this regard. Work on their reactions with alkenes that are geminally disubstituted with electron-withdrawing groups (e.g., 187) has illustrated some of the difficulties in such studies. When the imidoyl chloride-base route was used to generate the nitrile ylides it was found that the products 188 epimerized under the reaction conditions. When the azirine route was used, the reaction was complicated by the photochemical isomerization of the dipolarophiles (96,97). Thus, in both cases, it proved impossible to determine the kinetic product ratio. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

Phenylthiazirine (9) could be a yet undetected precursor for the nitrile sulfide when generated from the thiatriazole or 8. Its possible formation from the thiatriazole system has attracted other investigators15,29 with regard to certain thermal reactions. However, as a cyclic conjugated 4-n electron-system it should be antiaromatic,30 and therefore thermally unstable. It could possibly be generated in photochemical reactions. 

Cycloaddition reactions with furans remain a source of new compounds. On photochemical excitation of benzofuro-annulated oxanorbomadiene 42 in the presence of dipolarophiles Ill-addition compounds are obtained. The formation of 43 constitutes one of the rare examples of the cycloaddition of a nitrile as a dipolarophile <95LA1503>. 

Atkinson, J. G., D. E. Ayer, G. Btichi, and E. W. Robb Photochemical reactions XII. Addition reactions of olefins and acetylenes with benzo-nitrile. J. Amer. chem. Soc. 85, 2257 (1963). 

For the analogous photochemical reaction of silacyclopropenes with nitriles, see Sakurai, H. Kamiyama, Y. Nakadaira, Y., J. Chem. Soc. Chem. Commun. 1978, 80-81. 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

The substitution of carbonyl ligands in W(CO)6 may be effected either thermally or photochemically. Among the more important reactions from a synthetic standpoint are those of W(CO)6 with nitriles (RCN) to generate yoc-W(C0)3(RCN)3 (3) (equation 1), the nitrile ligands of which may be displaced by other Lewis bases see Lewis Acids Bases) under mild conditions. The reactions of W(CO)6 with the cyclopentadienyl see Cyclo-pentadienyl) reagents MCp (M = Li, Na, K Cp = Cp,... 

The photochemical generation and reactions of nitrile ylides have again been studied. Irradiation of azirine (55) in the presence of 1,4-naphthoquinone (56) gave the isoindoledione (57) via 1,3-dipolar addition of the transient nitrile ylide (SS)."" The kinetics of the reactions of nitrile ylides with electron-poor alkenes have been determined, with evidence being reported for the formation... 

Studies of the reaction of photochemically generated carbenes with nitriles to give nitrile ylides have again attracted attention. Flash photolysis of 1-naphthyldiEizomethane (51) in acetonitrile or of the azirine (52) gave the same transient species to which the nitrile ylide structure (53) was assigned. The first stable nitrile ylide was also obtained in this way irradiation of diazotetrakis(trifluoromethyl)cyclopentadiene (54) in the presence of 1-cyanoadamantane (55) gave the crystalline ylide (56).An X-ray structure determination showed that the 1,3-dipole is essentially linear. 

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