Introduction
Our interest in the electronic and molecular structures of NSO species (1, 2) is closely related to their reactivity. N-Sulfinylamines (R-N=S=O) were first prepared by Michaelis in 1890 (3) and their reaction with water was one of the first properties to be observed experimentally. While aromatic N-sulfinylamines are insoluble in water and hydrolized very slowly in it as well as in dilute acids, warm alkaline solutions or concentrated acids lead to their rapid hydrolysis (4). Aromatic N-sulfinylamines in particular are widely employed in synthetic organic chemistry, as they readily undergo a variety of cycloaddition reactions to yield N,S-heterocycles (Diels--Alder reactions as both dienes and dienophiles (5-7), 1,2-cycloadditions, and 1,3-dipolar additions (8, 9)). Common to all these reactions is the attack on the sulfur of the NSO group, and hydrolysis can be considered as the prototype reaction. Therefore, an understanding of the initial steps of the hydrolysis reaction mechanism of N-sulfinylaniline is fundamental to its exploitation in similar reactions involving N-sulfinylamines.
The mechanism for hydrolysis of N-sulfinylamines is believed to proceed through nucleophilic addition of a water molecule to the NSO group with the formation of sulfinamic acid, followed by the acid's fast decomposition to sulfur dioxide and a primary amine (Scheme 1) (10). Aromatic N-sulfinylamines are known to be somewhat resistant towards water, whereas aliphatic N-sulfinylamines hydrolyze more readily (4). For N-sulfinylaniline (Ph-N=S=O), kinetics studies using UV spectroscopy showed neutral hydrolysis to be a slow process (11, 12), with an induction period of approximately 4 h (12). The reaction is complicated by autocatalysis from aniline, formed as a product of the reaction, which explains the relatively low activation energy of 9.88 kcal [mol.sup.-1] (1 cal = 4.184 J) in a water--1,4-dioxane (1:1) solution (11). In the presence of pyridine as a base or with a combination of pyridine and a carboxylic acid, the rate of reaction increases significantly (12). General base catalysis was proposed for the hydrolysis in the presence of pyridine, where the formation of a pyridine--water complex increases the nucleophilic properties of water and facilitates its interaction with the electrophilic sulfur atom. This is proposed to be the rate-determining step of hydrolysis. It is a third-order reaction, first-order in each N-sulfinylaniline, water, and pyridine, with an overall rate constant of 2.96 x [10.sup.3] [L.sup.2] [mol.sup.-2] [s.sup.-1] and a low enthalpy of activation of 5.7 kcal [mol.sup.-1] for the 20-40[degrees]C temperature range.
For the combined catalysis by pyridine and a carboxylic acid, initial protonation of either nitrogen or oxygen atoms of the NSO group was predicted (12). This would lead to an increase in the net positive charge on the sulfur atom and promote the addition of a water molecule to the NSO group. This acid catalysis is not part of the discussion in the present paper.
While literature data on the hydrolysis of N-sulfinyl compounds are limited (11, 12), the hydrolysis of their cumulated analogs (isocyanates, R-N=C=O) was intensively studied, both experimentally (13-15) and computationally (16). Based on their structural similarities, one might expect similar reactivities for these two classes of compounds. And while the NCO group is more or less linear, whereas the NSO group is bent with a sulfur bond angle of 120.6[degrees] as determined from X-ray diffraction analysis (17), the similar solvent kinetic isotope effects k([H.sub.2]O)/k([D.sub.2]O) of 1.65 for PhNCO (14) and 1.73 for PhNSO (12) seem to support the idea of similar reactivities and possibly similar mechanisms in the hydrolysis of these compounds.
A second-order dependence on water was found in the neutral hydrolysis of alkyl- and aryl-substituted isocyanates (13, 14, 16), where one molecule acts as a general acid and the other as a general base. This is closely related to the base-catalyzed hydrolysis of N-sulfinylaniline, if one water molecule is considered to take the role of the catalyst (pyridine). For the hydrolysis of 4-chlorophenyl isocyanate, however, a third-order dependence with respect to water concentration was reported (15). We therefore decided to explore the neutral hydrolysis of N-sulfinylaniline computationally to determine its mechanism and the number of water molecules involved.
Computational details
All geometry optimizations were performed with the Becke3 (18)--Lee, Young, and Parr (B3LYP) hybrid density functional (19) with the 6-31+G(2d,2p) basis set, using the GAUSSIAN 98 suite of programs (20). This computational level best reproduces the observed geometry (X-ray analysis) of N-sulfinylaniline (17), and the basis set superposition error (BSSE) (21) consists of less than 0.7 kcal [mol.sup.-1] for the ternary complexes (counterpoise = 3, full geometry optimization). (3) All structures were optimized without constraints. The complexes and their transition states were studied in the gas phase, as it was found in similar studies of the hydrolysis of isocyanates (16) and amides (22) that the inclusion of the solvent as a dielectric continuum only leads to a small decrease in the activation barrier. Vibrational frequencies and zero-point vibrational energies (ZPVE) were obtained at the preceding level of theory. The identity of each transition state was additionally verified using the intrinsic reaction coordinate (IRC) method (23, 24). The total ([E.sub.tot]) and ZPVE-corrected energies ([E.sub.tot] + ZPVE), as well as the enthalpies of the complexes and their transition states, are summarized in Table 1. Throughout the paper, we will report only the enthalpy term at standard state, unless stated otherwise. We chose enthalpies over Gibbs free energies because an enthalpy is available (12) for comparison. Furthermore, the …
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