C−H Activation with Elemental Sulfur: Synthesis of Cyclic Thioureas from Formaldehyde Aminals and S8

Abstract

The C-H activation of cyclic formaldehyde aminals LCH2 (L = RN-CH2CH2CH2-NR and RNCH2CH2-NR, R = Me, Et, iPr, tBu, or Ph) with S8 proceeds at unusually low temperatures (T< 160 degrees C) and results in the formation of the respective thioureas LC=S and H2S. The reaction constitutes a new, solvent-free method for the synthesis of thioureas that eliminates the toxic and highly flammable CS2. For R = tBu, the ionic carbenium thiocyanates [LCH]+ SCN- dominate the product spectrum and the respective thioureas are obtained in low yield. The reactivity of the analogous sulfur and oxygen ring systems towards S8 was investigated. 1,3-Dithiolane is cleanly converted into 1,3-dithiolane-2-thione (S8, 14 d, 190 degrees C) and resembles the cyclic formaldehyde aminals in this respect. 1,3-Dioxolane (L = OCH2CH2O) is completely inert towards sulfur even under forceful reaction conditions (190 degrees C, 14 d). The formation of thioureas from aminals was investigated at the CBS-4 and B3LYP/6-31G(d) levels of theory.

Sulfur and SeleniumActivation by Frustrated NHC/B(C6F5)3 Lewis Pairs; Conformational Flexibility of Products

“…The resonances in the 11 B NMR spectra show in comparison to related thioborates (≈ -10 ppm) [12,23] a distinct downfield shift consistent with the known ability of soft donors to form only weak adducts with Lewis acids [24]. The resonances of the former carbene carbon atom in the 13 C NMR spectra appear for 3a at 152.0 ppm and for 3b at 180.9 ppm, and thus are minimally shifted in contrast to the thiones (2a: 160.6 ppm; 2b: 183.6 ppm; [25]). The 19 F NMR spectra of 3a, b exhibit three broad resonances for the ortho-, para-and meta-positions of the C 6 F 5 group.…”

“…The unsaturated compound 3a crystallised with two molecules in the asymmetric unit, while the saturated compound 3b crystallised with one [27]). The C-S distances of 1.7320 (13), 1.7335 (14)Å for 3a and 1.7327 (15)Å for 3b are clearly lengthened compared to related imidazole-2-thiones (1.686 (3)Å; 1.674 (4)Å), because of the loss of the partial double bond character [25]; a search of the Cambridge Database [28] revealed ca. 150 hits for the system C-S with three-coordinate C and one-coordinate S, with bond lengths in the range 1.60 -1.74Å (excluding obvious outliers such as betaines and disordered structures), av.…”

A Ring Expansion Route to Benzofused N‐Heterocycles Through Aryne Insertion into 1,3‐Diaza‐heterocycles

“…In order to test our hypothesis of a formal insertion of benzyne into an aminal C–N σ‐bond, 1,3‐diphenylimidazolidine 9 was exposed to 2‐trimethylsilylphenyl triflate 10a under a variety of aryne‐forming conditions (see Table for selected optimization studies) . Pleasingly, initial attempts using CsF in acetontrile furnished the C–N insertion product, 2,3,4,5‐tetrahydrobenzodiazepine 11a , as the major product in good yield after 4 hours (entry 1).…”

Thiourea and Selenourea and Their Applications

“…Cyclic thioureas have been obtained from formaldehyde aminals and sulfur (S 8 ) (Scheme 5) [22]. Reaction of isothiocyanate with amine to give the corresponding thioureas [1][2][3][4][5][6][7][8][9][10].…”