Early Events in the Photochemistry of 1,2,3-Thiadiazole Studied by Ultrafast Time-Resolved UV–Vis and IR Spectroscopies

Base-promoted anaerobic intramolecular cyclization synthesis of 4,5-disubstituted-1,2,3-thiadiazoles

An alkali-promoted, transition-metal-free and oxidant-free method to construct 4,5-disubstituted-1,2,3-thiadiazoles from N-tosylhydrazone-bearing thiocarbamates by employing a sustainable intramolecular reaction strategy has been developed.

Attempts to Synthesize a Thiirane, Selenirane, and Thiirene by Dealkylation of Chalcogeniranium and Thiirenium Salts

Thiiranium salts [Ad2 SR]+ X- (5, 8, 9, 11, 12; X- =Tf2 N- (Tf=CF3 SO2 ), SbCl6 - ) and seleniranium salts [Ad2 SeR]+ X- (14, 16, 17, 23-25; X- =Tf2 N- , BF4 - , CHB11 Cl11 - , SbCl6 - ) are synthesized from strained alkene bis(adamantylidene) (1). The disulfides and the diselenides (Me3 SiCH2 CH2 E)2 (4, 13), (tBuMe2 SiCH2 CH2 E)2 (7, 22), and (NCCH2 CH2 E)2 (10, 15; E=S, Se) have been used. The thiirenium salts [tBu2 C2 SR]+ X- (34) and [Ad2 C2 SR]+ X- (35, 36) are prepared from the bis-tert-butylacetylene (2) and bis-adamantyl-acetylene (3) with R=Me3 SiCH2 CH2 and tBuMe2 SiCH2 CH2 . Attempts to cleave off the groups Me3 SiCH2 CH2 , tBuMe2 SiCH2 CH2 , and NCCH2 CH2 resulted in thiiranes 27, 30. No selenirane Ad2 Se (33) is formed from seleniranium salts, instead cleavage to the alkene (1) and diselenide (13, 15) occurs. The thiirenium salt [Ad2 C2 SCH2 CH2 SiMe3 ]+ Tf2 N- (35) does not yield the thiirene Ad2 C2 S (37), the three-membered ring is cleaved, forming the alkyne (3) and disulfide (4). All compounds are characterized by ESI mass spectra, NMR spectra, and by quantum chemical calculations. Crystal structures of the salts 8, 12, 25, 17, 26, 36 and the thiiranes 27, 30 are presented.

QM/MM nonadiabatic dynamics simulations on photoinduced Wolff rearrangements of 1,2,3-thiadiazole

The photoinduced rearrangement reaction mechanism of 1,2,3-thiadiazole remains experimentally elusive. Two possible mechanisms have been proposed to date. The first is a stepwise mechanism via a thiocarbene intermediate; the second is an excited-state concerted rearrangement mechanism. Herein we have adopted both the electronic structure calculations and nonadiabatic dynamics simulations to study the photoinduced rearrangement reactions of 1,2,3-thiadiazole in the S₂, S₁, and S₀ states in solution. On the basis of QM(CASPT2)/MM [quantum mechanics(complete active space self-consistent field second-order perturbation theory)/molecular mechanics] calculations, we have found that (1) the thiocarbene intermediate is not stable; thus, the stepwise mechanism should be unfavorable; (2) the excited-state decay from the S₂ via S₁ to S₀ state is ultrafast and completed within ca. 200 fs; therefore, both the S₂ and S₁ states should not have a long enough time for the excited-state rearrangements. Instead, we have computationally proposed a modified photoinduced rearrangement mechanism. Upon irradiation, the S₂ state is first populated (114.0 kcal/mol), followed by an ultrafast S₂ → S₁ → S₀ excited-state decay along the S-N bond fission, which eventually leads to a very "hot" intermediate with the S-N bond broken (18.3 kcal/mol). Then, thermal rearrangements to thioketene, thiirene, and ethynethiol occur in a concerted asynchronous way. This mechanistic scenario has been verified by full-dimensional trajectory-based nonadiabatic dynamics simulations at the QM(CASPT2)/MM level. Finally, our present computational work provides experimentally interesting mechanistic insights into the photoinduced rearrangement reactions of cyclic and acyclic diazo compounds.

Synthesis of Sterically-Crowded Olefins, gem-Dihaloalkenes, Butatrienes, and Thioketenes through the Reaction of Substituted Bornane-2-thiones or Bornane-2-selones with Conventional Carbenes or Metal Carbenoids

Reaction of highly-substituted bornane-2-selones with a diazoalkane in the presence of either Rh2(OAc)4 or CuCl formed sterically-crowded exo-methylenic products, and similar treatment of the thiones or selones with dihalocarbenes or a propadienylidene carbene also formed dihaloalkenes, butatrienes, and thioketenes. All these reactions were assumed to proceed through a pathway involving the in situ formation and subsequent ring closure of chalcogenocarbonyl ylides.