Circumvention of orbital symmetry restraints by 1,3-H-shifts of enolic radical cations

Abstract

The reaction coordinates of 1,3-H-shifts across double bonds are traced by theory for three reactions, CH 3 C(OH)CH 2 ϩ⅐ (1) 3 CH 3 C(O ϩ⅐ )CH 3 (2), CH 2 C(OH) 2 ϩ⅐ (3) 3 CH 3 CO 2 H ϩ⅐ (4) and CH 3 C(OH)CH 2 ϩ⅐ (1) 3 CH 2 C(OH)CH 3 ϩ⅐ (1Ј), to explore how the need to conserve orbital symmetry influences the pathways for these reactions. In the first and second reactions, prior to the start of the H-transfer the methylene rotates from being in the skeletal plane to being bisected by it. Thus these reactions are neither antarafacial nor suprafacial, but precisely between those possibilities. This stems from a counterbalancing between the need to conserve orbital symmetry and the large distorting forces required to attain an allowed antarafacial transition state. In contrast to the first two reactions, 1 3 1Ј follows a suprafacial pathway. However, this pathway does not violate conservation of orbital symmetry, as it utilizes lower lying orbitals of appropriate symmetry rather than the antisymmetric uppermost occupied allyl-type orbital. Changes in geometry which presumably produce asymmetric vibrational excitation and the unequal losses of methyl that follow 1 3 2, i.e., nonergodic behavior, are also characterized. (J Am Soc Mass Spectrom 2004, 15, 972-981)

Characterization by theory of H-transfers and onium reactions of CH3CH2CH2N+H=CH2

“…This structure is a transition-state rather than being at a stable energy minimum. H-transfers are substantially lower than those for 4-membered ring H-transfers [5,22]. Consistent with the high critical energy that we find for it, 1 ¡ 3 is not observed experimentally, all fragmentation after Htransfer from C 2 and C 3 being accounted for by the onium reaction (see below).…”

“…The ring-size preference order 6 Ͼ 4 Ͼ 5 for H-transfers predicted in 1 by the critical energies obtained here (173, 241, and 256 kJ mol Ϫ1 , respectively) differs from the order 6 Ͼ 5 Ͼ 4 generally obtained for radical cations [5,8,24]. Critical energies for carbonyl ¡ enol or distonic C n H 2n O ϩ· ions in the order 6-, 5-, 4-membered rings are 15, 72, and 161 kJ mol Ϫ1 (QCISD(T)/6-311G** theory) [5,22], and for aliphatic amine ¡ distonic ions, 18, 68, and 144 kJ mol Ϫ1 (MP2/6-31G* theory) [24]. Thus, corresponding reactions differ substantially in their energy requirements and therefore in preference order between closed and open shell ions.…”

“…However, analogous 4-membered ring H-transfers do occur [11,23], as is described below for CH 3 N ϩ HϭCH 2 . Most 4-membered ring H-transfers across double bonds in both closed shell and radical cations circumvent orbital symmetry restrictions by transferring H in or near a plane containing the -bonded atoms and the atoms attached to them after the destination CH 2 rotates to being bisected by a skeletal plane [11,21,22]. In such reactions, the transition-state is neither suprafacial nor antarafacial, but at the boundary between those two types of reactions, a point at which -bonding does not exist.…”

“…In such reactions, the transition-state is neither suprafacial nor antarafacial, but at the boundary between those two types of reactions, a point at which -bonding does not exist. At the transition-state for 1 ¡ 2, the distances from the hydrogens on C 1 to H t are 1.962 [11,21,22] in which the methylenes and analogous groups rotate largely before and after rather than during H-transfer.…”

“…Four-membered ring isomerizations across -bonds in open and closed shell systems encounter orbital symmetry restraints [11, 19 -23]; avoidance of these by CH 2 twisting gives those reactions high critical energies in both open and closed shell ions, 196 [22], and 265 kJ mol Ϫ1 (6). The four membered-ring H-transfers 1 ¡ 2 and 6 ¡ 6 / are similar to other 4-membered ring H-transfers across double bonds [11,[21][22][23] in that all of the departure and destination ϭCHRs (R ϭ H or ethyl) for H t rotate by about 90°and the H transfer occurs near a plane that bisects the destination CH 2 .…”