Photochemistry of 1,1-dicyano-1-alkenes: The olefin-to-cyclopropane rearrangement

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Vol. 147; no. 3; pp. 177 - 190
• Main Authors: Leitich, Johannes, Ritter-Thomas, Ursula, Heise, Ingeborg
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 26.04.2002; Elsevier Science
• Subjects: 1,2-Shift; Chemistry; Cyclopropane; Electron-deficient olefin; Exact sciences and technology; Excited singlet state; General and physical chemistry; Photochemistry; Photoisomerisation; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Photoisomerisation; Electron-deficient olefin; Excited singlet state; Cyclopropane; 1,2-Shift; Intramolecular reaction; Nucleophilic reaction; Regioselectivity; Isomerization; Electronically excited state; Photochemical reaction; Experimental study; Quantum yield; Cyclopropanation; Near ultraviolet radiation; Reaction mechanism; Wagner Meerwein rearrangement; Ethylenic compound; Dinitrile
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/S1010-6030(01)00604-9

1,1-Dicyano-1-alkenes (DCNA) that lack further unsaturation undergo formation of 1,1-dicyano-cyclopropanes via 1,2-migration of either hydrogen or methyl/alkyl from C-3 to C-2 in their lowest exited singlet state. Quantum yields for this “olefin-to-cyclopropane photorearrangement” (OCPR) were found to span a wide range (≤0.1) and to depend characteristically on alkyl substituents on C-3 and C-2. OCPR occurs preferentially via such 1,2-migration that leaves behind the more alkylated C-3 atom. 1,2-Migration was found to occur suprafacially (i.e. to follow maximum orbital overlap), but to be rather tolerant towards unfavorable orientation of the migrant in the starting DCNA. The ring-closure that completed OCPR was found to be devoid of any intrinsic stereoselectivity; thus, in cyclohexane, each of the two epimeric 2-[(2-methyl-cyclohexyl)-methylene]-malononitriles ( 3 and 4) yielded the same approximately 1:1 mixture of the two epimeric 4-methyl-spiro[2.5]octane-1,1-dicarbonitriles ( 28 and 29). OCPR proceeded via an intermediate that also led to isomeric DCNA and to 1,1-dicyano-3-alkenes as minor by-products. Some deprotonation at C-3 of the photoexcited DCNA was noticeable in methanol, but not in hexane. Supplementary experiments included preparative and kinetic investigations of thermolyses of 1,1-dicyano-cyclopropanes. The combined evidence allowed the deduction of the following reaction path for OCPR. In their lowest excited singlet state, a ππ ∗ state, DCNA exhibit cationic reactivity of their C-2 atoms (presumably in the perpendicular conformation of C-2 relative to C-1, according to Salem’s seminal concept). This reactivity triggers the 1,2 (Wagner–Meerwein type) migration to yield, still on the excited hypersurface, a 1,3 dipole. This 1,3-dipole achieves an almost complete conformational equilibration in cyclohexane (though less so in more polar solvents) before it decays to the electronic ground-state thereby becoming a 1,3-diradical. This 1,3-diradical undergoes three competing terminating reactions: ring closure to cyclopropane (the major path); 1,2-back migration of hydrogen to form starting or isomeric DCNA; 1,4-hydrogen shift to produce 1,1-dicyano-3-alkenes. The 1,3-dipole is too short-lived to undergo a potentially favorable Wagner–Meerwein rearrangement. Like the reactive excited DCNA singlet state, the 1,3-dipole is not trapped to any significant extent by nucleophilic addition of the solvents tert-butanol or methanol to its cationic center. The reason for this failure appears to be the excited-state nature of this species, which bars the formation of a ground-state adduct in an adiabatic reaction.