Quantum yield cyclobutene formation

A significant amount of blcyclo[3.1.0]hexene (BCH310), 1,2,4-hexatrlene (HT124), and 3-vlnyl-cyclobutene (VCB) can be detected only after six times the time required for the Z E photoequl-llbratlon. Indicating that the quantum yield of formation of these products Is small when compared to those of the Z J E Interconversion. 

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... 

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. 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

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. 

Either direct or triplet-sensitized irradiation of diene ( , )-14 in methanol affords the a s-cyclobutene 15 and the methyl ethers 16 and 17. > Laser flash spectroscopy revealed the formation of a ground-state intermediate having = 360 nm and a lifetime of 0.8 isec at 23°C in methanol. Based on the change of the quantum yields for the formation of ethers 16 and 17 with for a variety of sensitizers, it was concluded that cyclobutene 15 and ether 16 arise from the s-Z-conformation of isomer ( ,Z)-14 and ether 17 from the s- -conformation. In view of large deuterium isotope effects, k /kjy, of 8 1 and 10 2, that were observed for the formation of ethers 16 and 17, respectively, in CH3OD, concerted formation of the CH and CO bonds in the transition states for ether formation was proposed. 

---