CH SMe Research Articles

Experimental and Computational Study of the Bonding Properties of Mixed Bis−Ylides of Phosphorus and Sulfur

The reactivity of the known ylide-sulfonium salt [Ph(3)P=CHC(O)CH(2)SMe(2)]Br 1 and the new ylide-sulfide [Ph(3)P=CHC-(O)CH(2)SMe] 2 toward Pd(II) complexes has been studied. Compound 1 reacts with PdCl(2)(NCMe)(2) and NEt(3) to give cis-[PdCl(2)[Ph(3)PCHC(O)CHSMe(2)-kappa-C,C]] 3, which is obtained selectively as the meso diastereoisomer (RS/SR). The reactivity of 3 has been studied, and shows the stability of the bis-ylide unit. However, reflux in NCMe of [PdCl(PPh(3))-[Ph(3)PCHC(O)CHSMe(2)-kappa-C,C]]ClO(4) 6 promotes orthopalladation and affords [PdCl(PPh(3))(C(6)H(4)-2-PPh(2)CHC(O)CH(2)SMe(2)-kappa-C,C)]ClO(4) 12, which is characterized by X-ray methods. Density functional theory (DFT) and Bader's Atoms in Molecules (AIM) studies on S-ylides, mixed P-S bis-ylides, and the corresponding Pd complexes have been performed. Free S-ylides show strong conformational preferences, which lies with the establishment of a set of cooperative intramolecular interactions of weak strength: the 1,4 S...O interactions and the 1,6-C-H...O H-bonds between the protons of the methyl substituents and the carbonyl oxygen was fully characterized for the first time. For free mixed P-S bis-ylides, an additional 1,4-P...O intramolecular interaction of moderate strength was characterized. These interactions play a key role in determining the preferred conformations, which then are transferred to the complexes, explaining the observed diastereoselectivity in complex 3. The ylide-sulfide 2 reacts with PdCl(2)(NCMe)(2) and NEt(3) affording [Pd(Cl)[Ph(3)PCHC(O)CHSMe]](2) 9, which in turn reacts with PPh(3) giving [Pd(Cl)(PPh(3))[Ph(3)PCHC(O)CHSMe-kappa-C,C]] 10. The X-ray structure of 10 shows the anion [Ph(3)PCHC(O)CHSMe](-) acting as a C,C-chelate. The bonding in 10 is produced with complete diastereoselectivity but, instead of the expected meso form, the d,l pair (RR/SS) is formed. This inversion is observed for the first time.

Reactivity of ring systems with two hetero atoms. I. On the reaction of 1,4‐dithiins with n‐butyllithium

AbstractThe reaction of 1,4‐dithiin (1) with n‐butyllithium has been examined. At −110° in tetra‐hydrofuran 2‐lithio‐1,4‐dithiin (2) is sufficiently stable to allow of the preparation of derivatives. Anion (2) undergoes ring opening by a 1,2‐elimination at −60°; subsequent methylation and hydrolytic work‐up gives ZHC≡CS‐CHCHSMe (6), or Z−MeC≡C−S−CHCH−SMe (7). At about −5° butyllithium attacks sulfur in the intermediary Z−LiC≡C−S−CHCH−SLi (4), leading to Z−BuS−CHCH−SLi (8), and LiC≡CLi.These results are not in agreement with the mechanism proposed by Parham et al. for benzo‐1,4‐dithiin and 2,5‐diphenyl‐1,4‐dithiin. We studied also the reactivity of 2,6‐dimethyl‐1,4‐dithiin (13), and 2,6‐ di‐tert‐butyl‐1,4‐dithiin (25), towards butyllithium. Starting from 13 the otherwise, difficultly accessible tri‐ and tetramethyl‐1,4‐dithiins could be prepared in high yield.The ring opening provides a valuable method for the synthesis of stereochemically pure Z‐1‐(alkynylthio)‐2‐(alkylthio)alkenes and Z‐1‐(butylthio)‐2‐(alkylthio)alkenes.