| Abstract
| - The intrinsic gas-phase reactivity of cyclic N-alkyl- and N-acyliminium ions toward addition ofallyltrimethylsilane (ATMS) has been compared using MS2 and MS3 pentaquadrupole massspectrometric experiments. An order of electrophilic reactivity has been derived and found to agreewith orders of overall reactivity in solution. The prototype five-membered ring N-alkyliminium ion1a and its N-CH3 analogue 1b, as well as their six-membered ring analogues 1c and 1d, lack N-acylactivation and they are, accordingly, inert toward ATMS addition. The five- and six-memberedring N-acyliminium ions with N-COCH3 exocycclic groups, 3a and 3b, respectively, are also notvery reactive. The N-acyliminium ions 2a and 2c, with s-trans locked endocyclic N-carbonyl groups,are the most reactive followed closely by 3c and 3d with exocyclic (and unlocked) N-CO2CH3 groups.The five-membered ring N-acyliminium ions are more reactive than their six-membered ringanalogues, that is: 2a > 2c and 3c > 3d. In contrast with the high reactivity of 2a, its N-CH3analogue 2b is inert toward ATMS addition. For the first time, the transient intermediates of aMannich-type condensation reaction were isolatedthe β-silyl cations formed by ATMS additionto N-acyliminium ionsand their intrinsic gas-phase behavior toward dissociation and reactionwith a nucleophile investigated. When collisionally activated, the β-silyl cations dissociatepreferentially by Grob fragmentation, that is, by retro-addition. With pyridine, they reactcompetitively and to variable extents by proton transfer and by trimethylsilylium ion abstractionthe final and key step postulated for α-amidoalkylation. Becke3LYP/6-311G(d,p) reaction energetics,charge densities on the electrophilic C-2 site, and AM1 LUMO energies have been used to rationalizethe order of intrinsic gas-phase electrophilic reactivity of cyclic iminium and N-acyliminium ions.
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