The crystal structures of two photodimers of 6,6-dimethyl-2,3-benzo-2,4-cycloheptadienone

Abstract

The structures of two photodimers of 6,6-dimethyl-2,3-benzo-2,4-cycloheptadienone, C26H2802, have been determined from room temperature X-ray diffractometer data. Dimer A crystallizes in the monoclinic space group C2/c with a= 30.141 (16), b= 8-536 (4), c= 15-979 (6) A and fl= 103.1 (2) ° and Z=8. Dimer B crystallizes in the monoclinic space group P2~/c with a = 7.774 (2), b = 15.503 (6), e = 16.752 (5) ,~ and fl= 100.95 (2) ° and Z= 4. Both structures were solved by direct methods and refined by blockdiagonal least-squares techniques to R values of 0.050 (1761 independent reflections) and 0-043 (1172 independent reflections) for dimer A and dimer B respectively. In both molecules the five-to six-membered ring juncture has been found to be cis and the six-to seven-membered ring juncture trans; however, in dimer A the hydrogen atoms H~ and Hc are trans, whereas, in dimer B they are cis. The location of strain in different regions of the two dimers is shown through a comparison of bond lengths and angles and dihedral angles. The conformations of the non-aromatic rings differ in the two molecules, those in dimer A being more flattened than those in dimer B.

3,4;13,14-Dibenzo-6,6,11,11-tetramethyl-1,8,9-cis-tricyclo[7.5.0.02,8]tetradecane-5,12-dione

“…Studies of the photodimerization of 2,3-benzo-6,6-dimethyl-2,4-cycloheptadienone indicate that it yields two crystalline dimers (Biefeld & Barnett, 1974;Hart, Miyashi, Buchanan & Sanson, 1974;Hart & Suzuki, 1975). Through the identity and stereochemistry of the two products, reaction intermediates have been identified and reaction mechanisms have been proposed.…”