A theoretical study on the mechanisms of "Hydride Shift" rearrangements of carbocations

ORGN 683

Tianming Zuo, tianming@vt.edu1, Liaosa Xu, liaosax@vt.edu2, Diego Troya, troya@vt.edu2, and Harry C. Dorn, hdorn@vt.edu2. (1) Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, (2) Department of Chemistry, Virginia Tech, Blacksburg, VA 24061
"Hydride shifts" are very common reactions in organic chemistry. They normally occur at rearrangements of carbocations under the stable-ion conditions. This work focuses on several representative rearrangements of carbocations caused by hydrogen emigration. By analyzing the transition states, their formation processes, and intermediates of these typical reactions, we found that some so called "hydride shift" rearrangements are actually carried out by proton shift. In other words, the mechanism of these formally "hydride shift" is essentially proton shift. However, when we do structural parameters and Mulliken charges analyses of the several typical monocycloalkyl cations (especially medium size rings) in transannular reactions, we found that their most stable structures are u-hydrido-bridged forms. These u-hydrido-bridgeing forms have six-memberred ring by means of the bridged hydride due to the 5,1-hydride shift. The six-membered ring may account for the stability of the u-hydrido-bridged carbocations. Due to the bridged hydride, the mechanism of the rearrangements in monocycloalkyl cations is hydride shift.
 

Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High-Energy Species
1:00 PM-5:00 PM, Wednesday, 16 March 2005 Convention Center -- Room 11A, Oral

Division of Organic Chemistry

The 229th ACS National Meeting, in San Diego, CA, March 13-17, 2005