The structure of dimesityl sulfine in solution and in the solid the syn mesityl ring perpendicular to the C=S=O plane. From state was determined by NMR spectroscopy and X-ray cry-the coalescence temperature T, at 303 K, the free energy of stallography, respectively. In the crystal the two mesityl activation for the rotation of the mesityl rings was estimated groups are twisted by approximately 60°, while in solution to be 14 k 0.7 kcal/mol. At higher temperatures (140-185°C) an averaged conformation with C, symmetry is observed, dimesityl sulfine yields up to 95% 5,7-dimethyl-l-mesitylwith the anti mesityl ring located in the C=S=O plane and benzo [c] The therrnal[l61 and photo~hemical['~1['~] decomposition of diphenyl sulfine (la) was investigated by Carlsen and Holm. The primary step in these reactions is the ring closure to diphenyl oxathiirane (2a, Scheme I), which at low temperature could be characterized by UV spectroscopy (h,,, = 390 nm).Li5] Oxathiirane 2a was stable in organic glasses, however, melting of the glass leads to quantitative formation of benzophenone (5a). As a result of the thermal lability of 2a, the photolysis of sulfine l a at room temperature, or thermal decomposition at higher temperatures, yields 5a as the sole product, whereas low temperature photolysis of l a produced, in addition, the two isomeric esters ["] A later theoretical (ab initio CASSCF) study on the H2C=S=0 system suggested that this carbonyl sulfide is a secondary product, formed only in the rigid environment of the organic glass by recombination of a ketone (or aldehyde) and sulfur atoms in their S('D) state. Generally, loss of the sulfur atom and formation of a carbonyl compound is the major route for the sulfine decomposition. The fate of the sulfur atom and the capability of the intermediates of type 2a, 6a, or 7a to transfer sulfur atoms has not yet been systematically investigated. Here we report on the structure, conformation, and thermal rearrangement of the sterically congested dimesityl sulfine lb.
Structure of Dimesityl Sulfine (lb) in the Solid State and in SolutionThe sulfine l b was synthesized by oxidation of dimesityl thio~arbonyl[~~I[~~1 with mCPBA and crystallized from pentane. The crystal structure of l b (Figure 1) shows the expected twisting (defined as angle between the planes defined by the C=S=O and the mesityl groups) of both mesityl rings by approximately 60" (58' for ring A and 55" for ring B), similar to dime~itylmethane ['~I and its derivatives. [i91[201 In diphenyl sulfine (la) the syn phenyl ring is twisted by only 25" from the S-C1 -C21 plane, while the anti phenyl ring is twisted by 50".[141 Thus, l a is stabilized by an ap-
