Single-crystal molecular structure determinations and theoretical calculations on alkynyl sulfonate and carboxylate esters

Abstract

The single-crystal X-ray structures of propynyl tosylate, 4, and of ethynyl benzoate, 5, were determined. Both esters are essentially linear acetylenes with m-0 bond angles of 174.7' and 177.6', respectively. The m-0 acetylenimxygen bond length is 1.331 (4) A in 4 and 1.329 (4) A in 5, significantly shorter than the analogous bond length in the corresponding saturated alkyl or vinyl esters. There is a concomitant increase in the S-0 or 0-C(=O) bond lengths from saturated to vinyl (enol) to alkynyl esters. Ab initio molecular orbital calculations (6-31G*) for several smaller model systems are in excellent agreement with the X-ray geometries including the changes that occur as a function of R (R = alkynyl, vinyl, alkyl). HCsOS02R, 7, prefers a gauche conformation (LHSCO = 70° for R = H and 76' for R = Me). The barriers to rotation around the S-0 bond are 2.7 kcal mol-' in 7, R = H, and 4.8 kcal mol-' in 7, R = Me (6-31G*). The energies of several reactions of these esters have been calculated (MP3/6-31G*//6-3 1G*, kcal mol-'). The calculated values below are given in the order R = CeH - R = CH=CH2 - R = CH3. Hydrolysis energies are -7.7, -4.1, and 3.5 for HS020R and -7.2, -2.9, and 3.0 for ROC(=O)H. The hydrogenation energies of the 0-R bonds are -30.7, -17.3, and -21.8 in ROS02H, -30.3, -16.1, and -22.4 in ROC(=O)H, and -23.0, -13.3, and -25.4 in ROH. The hydrogenation energies of the RO- bond are -2.4, 1.1, and 8.8 in RO-S02H and 2.8, 7.2, and 12.9 in RO-C(=O)H. Agreement with experimental data is good. The trends in the above energies as a function of R are discussed. The PM3 and AM1 calculations reproduce. well the geometries of the alkynyl sulfonate and carboxylate esters but fail to reproduce these reaction energies.