Cyclobutene, formation

Aliphatic dienes undergo three main photochemical pericyclic processes, whose individual efficiencies depend largely on the torsional angle about the central bond in the specific diene conformer which is excited. These are (a) cyclobutene formation, (b) bicyclof 1.1.0] butane formation and (c) [l,5]-hydrogen migration. A fourth process, methylcyclopropene formation, has also been observed in minor amounts in several cases. 

With acyclic dienes, the quantum yield for cyclobutene formation (4>cb) rarely exceeds ca 0.1, the expected result of the fact that the planar s-trans conformer normally comprises the bulk (96-99%) of the conformer distribution at room temperature. However, 4>cb is often significantly larger than the mole fraction of s-cis form estimated to be present in solution. For example, 1,3-butadiene, whose near-planar (dihedral angle 10-15°105 106) s-cis conformer comprises ca 1% of the mixture at 25 °C, yields cyclobutene with < >cb = 0.04140, along with very small amounts of bicyclo[1.1.0]butane141. A second well-known example is that of 2,3-dimethyl-l,3-butadiene (23 ca 4% gauche s-cis at 25 °C107), which yields 1,2-dimethylcyclobutene (25) with < >cb = 0-12 (equation 16)111. Most likely, these apparent anomalies can be explained as due to selective excitation of the s-cis conformed under the experimental conditions employed, since it is well established that s /raw.v... 

Amino-l-methylimidazole (169) reacted with DMAD145 in dioxane to form 30% of imidazo[l,2-a]pyrimidone (170), identified by the low-field 3-proton, and 5% of the diazepine 171, which was presumably built up by cyclobutene formation across the 4,5-double bond of 169 and ring expansion.148 Both l-methyl-2-methylthioimidazoline and its... 

Photoisomerizations. VI. Cyclobutene formation and diene migration in simple 1.3-dienes. Tetrahedron 21, 1001 (1965). 

Treatment of cyclopropenones with isonitriles [56] [57] and of triafulvenes with isonitriles [58] gives cyclobutenes. Formations of expanded rings are observed by the reactions of enamines with diphenylcyclopropenones [59] [60]. 

At the conical intersection the C1-C2-C3 valence angle <0 is close to 90°, suggesting that in strained systems, where this value is hard to reach, the excited-state barrier on the pathway to the conical intersection or the conical intersection itself may be so high in energy that the photoreaction cannot take place. The quantum yields for cyclobutene formation in dimethylene-cycloalkanes summarized in Scheme 29 (Aue and Reynolds, 1973 Leigh and Zheng, 1991) are in agreement with this expectation. 

Again, for cyclobutene formation, we must focus on the two bonding MOs in it (i.e., ct and n) and then search for their symmetry correspondence with the MOs of 1,3-butadiene. In reference to mirror plane symmetry, which amounts to disrotation,... 

Electrocyclization of conjugated dienes occurs in competition with cis-trans isomerization. The cyclization occurs from the s-cis conformation of the diene. Cyclobutene formation is favored in cyclic dienes and for other dienes where the s-cis diene conformation is dominant. For several dienes, the quantum yield in nonpolar solvents at 257 nm is about 0.1. As the cyclized alkenes do not absorb at this wavelength, the reaction can give substantial preparative yields, despite the competing cis-trans isomerization. 

Vitamin D upon n -> II excitation just follows the now-classlcal pathways of relaxation of a cZt trlene to form photoproducts bicyclo[3.1.0]hexene formation, [1.5]H shift, vinyl-cyclobutene formation. Previtamin D yields a much wider range of products and moreover shows a significant solvent dependence. 

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