Photochemical transformations of small ring heterocyclic compounds. 75. Photochemistry of arylazirines in hydroxylic media

Abstract

Irradiation of substituted arylazirines produces nitrile ylides which react with methanol to give methoxyimines in excellent yield. Deuterium labeling studies indicate that the electron density is highest on the disubstituted carbon atom of the nitrile ylide formed on irradiation. With this conclusion, all of the regiochemical data found in the photoaddition of arylazirines with electron-deficient dipolarophiles can be explained by use of the frontier orbital method. The addition of alcohol to nitrile ylides was also found to occur intramolecularly. A new synthesis of cycloalkanones was devised and is based upon the photolysis of spiroazirines in alcohol followed by aqueous hydrolysis. Irradiation of 2-phenyl-1 -azaspiro[2.2]pent-1ene in methanol however, resulted in a Griffin-type fragmentation and produces ethylene and 2-phenylazirinylidene. This novel carbene reacts with methanol to produce 2-methoxy-2-phenyl-2//-azirine (33). On further photolysis, azirine 33 undergoes C-C bond cleavage to produce a nitrile ylide which can be trapped by methanol or by an added dipolarophile such as methyl trifluoroacetate. When the irradiation of azaspiro[2.2]pentene is carried out in the presence of oxygen, benzonitrile and carbon monoxide are formed. The formation of these products can best be explained by invoking 2-phenylazirinone as a transient intermediate.

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

“…This procedure has been widely used in the synthesis of a variety of five-membered heterocycles (cf., Scheme 1). [5][6][7][24][25][26][27] Recent quantum-chemical calculations shed some light on the above-mentioned observations. On the basis of complete active space self-consistent field (CASSCF) calculations, Klessinger and Borneman 28,29 demonstrated that the photochemical formation of the nitrile ylide from the respective 2H-azirine (by C-C bond cleavage) occurs in an nπ* singlet excited state and involves a barrierless conical intersection between the S 1 and S 0 potential energy surfaces.…”

[3 + 2]-Cycloadditions of nitrile ylides after photoactivation of vinyl azides under flow conditions

“…Alternatively, the in-situ generated nitrile ylide can be trapped intramolecularly by a nucleophile such as a hydroxy group [25]. This is demonstrated by the photochemical degradation of vinyl azide 1g which yielded 2,5-dihydrooxazole 9 in 76% yield ( c = 0.01 M, flow rate = 0.02 mL/min) under flow conditions (Scheme 7).…”

Reactivities of vinyl azides and their recent applications in nitrogen heterocycle synthesis

“…Under photochemical conditions, the highly strained 2 H -azirines 29 underwent ring-opening to yield the nitrile ylides 42 . 20 These 1,3-dipolar intermediates proved to be excellent [3 + 2] cycloaddition substrates for dipolarophiles 43 . The cyclization provided 3,4-dihydro-2 H -pyrroles 44 in reasonable yields (Scheme 8).…”