Shell Analogs of Closed Shell Reaction Paths : The Sw 2 ’ Case

Two major pathways exist in substitution reactions of nucleophiles with allylic substrates (Scheme l).IF3 Attack of the nucleophile at the carbon atom bearing the allylic leaving group leads to direct displacement in a classic sN2 mechanism. Alternatively, attack can also occur at the terminal carbon atom of the allylic double bond, displacing the leaving group in a concerted or consecutive manner. This process is usually referred to as the S N ~ ’ mechanism. We have recently shown that barriers for sN2 displacements are considerably lower in open shell systems compared to the closed shell analogs! In analogy to the closed shell situation one might ask whether a second pathway exists, in which attack occurs at the radical center with concomitant displacement of the leaving group. This process, which appears not to have been considered before in the literature, might be termed the Sm2’ mechanism. We now show on the basis of ab initio calculations that this additional pathway is even more favorable than the direct substitution mechanism. Sm2’ reactions should therefore be considered an additional mechanistic possibility in reactions of open shell species. In a more general sense, this result also provides a first indication of the possibility that the vinylology principle of closed shell chemistry might be complemented by a “methylenology” principle in open shell compounds. To facilitate comparison of the Sm2 and Sw2’ reaction pathways, we have again chosen the C1+ /3-chloroethyl radical ~ y s t e m . ~ Geometry optimizations have been performed at the ab initio UMP2/6-31G* level of theory for various structures along the reaction coordinate, which has been taken as the Cl-C bond distance. Single point energies have been calculated for the UMP2/6-31G* structures at the Mp2, MP3, and QCISD level of theory with the 6-31+G** basis set.5 Spin projected energies have been used for the calculation of relative energies.6 The resulting reaction profiles are shown in Figure 1. The reaction path starts out from ion-dipole complex 1 at R(C-C1) = 3.52 A. In this complex, the chloride anion coordinates to two positively charged hydrogen atoms at the two carbon atoms of the chloroethyl radical. The transition from the side-on to the anti orientatioq causes a small kink in the potential energy curve at around 3.2 A. Further shortening of the C-C1 distance then leads to an energy maximum in the PMP2/6-31+G** curve around 2.65 A. This maximum is followed by a small decline to reach a minimum at the c2k symmetric structure 2, in which both C-Cl bonds have equal lengths of 2.34 A. Frequency analysis shows that this structure is not the expected transition structure for chloride exchange, but a minimum on the UMP2/