Nucleofugalities of Neutral Leaving Groups in 80 % Aqueous Acetonitrile

Abstract

Nucleofugalites of tetrahydrothiophene, dimethyl sulfide and differently substituted pyridines in 80 % aqueous acetonitrile have been derived from the SN1 solvolysis rate constants of the corresponding X,Y-substituted benzhydryl derivatives (1-10). In solvolysis of sulfonium ions in 80 % aqueous acetonitrile, where acetonitrile is a good cation solvator, the solvation of the reactant ground state is an important rate determining variable since the positive charge is almost entirely located on the leaving group. As a consequence, reaction rates of sulfonium ions are more sensitive to the substrate structure in 80 % aqueous acetonitrile than in pure and aqueous alcohols, which are less efficient as cation solvators. In solvolysis of pyridinium ions the solvation of the reactant ground state is less important, since the positive charge is considerably distributed between the carbon at the reaction center and the leaving group. In such cases the important rate determining variable is solvation of the transition state. Slower reactions of pyridinium substrates progress over later, carbocation-like transition states in which the solvation is more important, so those substrates solvolyze slightly faster in aqueous acetonitrile than in methanol (k80AN > kM). Faster reactions proceed over earlier TS in which the solvation is diminished, so those substrates solvolyze somewhat faster in methanol than in aqueous acetonitrile (k80AN < kM).

Solvolytic behavior of some ferrocenylphenylmethyl‐4‐methoxypyridinium cations in various solvents

“…Similarly, as in our previous works for solvolyses with positively charged substrates (pyridinium, [ 8 ] dimethylsulfonium, [ 12a ] and tetrahydrothiophenium [ 12b ] salts), a very small variation of the rate of S N 1 reaction with solvent variation was found for solvolysis of 4‐methoxypyridinium ions (Table 1). The solvolysis rates of 4‐methoxypyridinium ions slightly decrease as the solvent polarity increases, which is in line with Hughes–Ingold rule.…”

“…Similarly, we determined nucleofugality in water of 4‐chloropyridine in combination with 4,4′‐bis(morpholino)benzhydrilium ion to be −2.95 and the nucleofugality of pyridine in combination with 4,4′‐bis (methylamino)benzhydrylium ion to be equivalent to −4.00. [ 8a ]…”

“…The values of s f determined for 4‐methoxypyridinium ion are somewhat smaller (average s f = 1.08) than the s f values obtained earlier for 4‐chloropyridinium (average s f = 1.16) and pyridinium (average s f = 1.12) ions in which the values of s f indicate development of a positive charge on the benzhydrylium ion in the TS reaction. [ 8 ] Keeping in mind that ferrocenylphenylmethyl derivatives solvolyze faster than the corresponding benhydryl derivatives, this can be explained due to a somewhat earlier transition state in the heterolytic step of ferrocenylphenylmethylpyridinium salts and delocalization of the positive charge generated on the carbon atom of the reaction center in the transition state to ferrocenyl group. Due to the strong electron‐donating ability of the ferrocenyl group, a reduction of the positive charge on the reaction center in the transition state occurs.…”

“…In our previous works, the leaving group abilities of differently 4‐X‐substituted pyridines (X = Cl, H, CH 3 ) in S N 1 solvolytic reactions have been determined by analyzing the rate constants of X,Y‐substituted benzhydrylpyridinium salts obtained in various solvents. [ 8 ] To obtain a complete profile of uncharged leaving groups (differently substituted pyridines), we investigated the heterolytic bond breaking of ferrocenylphenylmethyl‐4‐methoxypyridinium derivatives and formation of the free 4‐methoxypyridine and differently substituted ferrocenylphenylmethyl ions in the rate‐determining step of the S N 1 reaction (Scheme 1).…”

“…Up to now, the least reactive neutral nucleofuges on the nucleofugality scale developed in line with Equation are 4‐chloropyridine ( N f from −0.95 to −1.37 in various solvents), pyridine ( N f from −2.28 to −3.22 in various solvents), and 4‐methylpyridine ( N f from −3.06 to −4.20 in various solvents). [ 8 ] Nucleofuge‐specific parameters ( N f and s f ) in various solvents for the respective nucleofuges were derived from log k versus E f plots (Equation ) using benzhydryl cations ( E f ≤ 0) as electrofuges. Keeping in mind that the electron‐accepting group on the 4‐position of the pyridine ring increases the rate of the heterolysis step in pyridinium salts, 4‐methoxypyridine of benzhydryl cations with E f ≤ 0 solvolyze too slowly.…”