Mechanism of Reactions of Thiols and Disulfides with Dihaloalkanes: A Quantum-Chemical Study

Abstract

The potential surfaces of the reactions of thiols and disulfides with haloalkanes in the gas phase and with participation of a hydrizine molecule were studied quantum-chemically. The stereoelectronic characteristics of the prereaction complexes and reaction products were studied. The transition states were localized. The four-center reaction mechanism is preferable. The increased activation barrier in exchange of thiols with chloroalkanes is the major factor responsible for formation of unsymmetrical chlorinated sulfides.We have found previously that various thiols I react with dihaloalkanes II in the system hydrazine hydrate3base to form predominantly alkyl chloroalkyl sulfides III with various positions of Cl atoms relative to the S atom [1]: !KYIV R = Alk, Ph, 2-thienyl; X, Y = Cl, Br.Diorganyl disulfides under similar conditions mainly yield bis(organylthio)alkanes IV [2]. The ratio of the products of reactions (2) and (3) depends primarily on particular halogen and on stereoelectronic features of the substituent. The reaction pathway correlates to certain extent with the reactant ratio and amount of alkali taken [1]. Symmetrically substituted dibromoalkanes, irrespective of the particular thiol, yield exclusively bissulfides IV. In this case, either the reaction is concerted or the initially formed unsymmetrical intermediate III transforms with a low activation barrier into bissulfide IV. With mixed dihaloalkanes (X = Cl, Y = Br), reactions (2) and (3) mainly yield chlorinated sulfides III (up to 90%).The most contradictory is the ratio of products III and IV in reactions of thiols with dichloroalkanes. In this case, on the one hand, the structure of R in the thiol RSH is essential because of possible occurrence in III of specific intramolecular interactions stabilizing this product. On the other hand, the ratio of III and IV is influenced by the length of the alkyl chain in II, because of differences in the rotational mobility. In this case, either the contact with the starting reagent can become more probable, or the favorable intramolecular packing can be realized. The yield of III and IV in reactions (1)3(3) will be primarily determined by the ratio of the thermodynamic and kinetic characteristics of steps (2) and (3). It is important to determine what of this steps is limiting.We have made a quantum-chemical study of the critical points of the potential surfaces of the reactions of RSK with X(CH 2 ) n Y (II) in the gas phase in the presence of a hydrazine molecule with the aim to determine the optimal gradient pathway and reveal the factors responsible for variation of the product ratio. The calculations were performed for CH 3 CH 2 SH (Ia), BrCH 2 CH 2 Br (IIa), BrCH 2 CH 2 Cl (IIb), and ClCH 2 . CH 2 Cl (IIc). To predict the topological properties of the systems in the configuration region of the reaction, we used the B3LYP hybrid density functional method with the LANL2DZ basis set, ExtraBasis version [33 7]. Full optimization of the geometry of the molecular systems corresponding to transition struc...

Synthesis of organochalcogen compounds in basic reducing systems

“…97 In the synthesis of bis(ethylseleno)alkanes from di chloro or dibromoethane or dibromopropane, the target products are formed in minor amounts. 97 In the synthesis of bis(ethylseleno)alkanes from di chloro or dibromoethane or dibromopropane, the target products are formed in minor amounts.…”