This study investigated the nucleophilic substitution reaction mechanisms of 5 oxatriquinane derivatives, namely: oxatriquinane (OTQ), 1,4,7-trimethyloxatriquinane (TMO), 1,4,7-triethyloxatriquinane (TEO), 1,4,7-tri-iso-propyloxatriquinane (TIO) and 1,4,7-tri-tert-butyloxatriquinane (TTO). In addition to the G3 conformation (one with the substituent groups at 1,4 and 7 positions pointing into the plane of the paper) originally proposed by the previous workers, Mascal et al. in 2008 and Gunbas et al. in 2013, one more geometrical isomer was considered again for each of the derivatives, the 2G1 isomer (one in which only 2 of the 3 substituent groups at 1,4 and 7 positions are into the paper plane). Geometry optimization and determination of transition state properties of the conformers corresponding to each molecule (in the presence of azide ion, N3-) provided theoretical evidences on the possible reaction mechanisms. The 2G1 conformer for TTO was found to be unstable. The reactions of OTQ, TMO and TEO with azide ion (N3-) followed SN2 pathway, with SN1mechanism completely lacking. This finding is in agreement with the first set of reports published on this subject in 2008 and 2010 by Mascal’s group. For TIO (in the presence of azide ion), only the presence of SN1 mechanism could be proved without any observation of transition state (TS), even though, it possesses a 2G1 conformer. TTO surprisingly, showed marked evidence of SN1 mechanism also without any evidence of TS. The results obtained showed that OTQ derivatives up to TEO undergo nucleophilic substitution predominantly via SN2, and above which (i.e. for TIO and TTO) the mechanisms predominantly become SN1.
Steric Effects in SN2 Reactions. Determination of Transition State Structures for the Quaternization of 2-Alkylpyridines and -thiazoles by a Combined Experimental and Molecular Mechanics Procedure.
