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Fabian J, Krebs A, Schonemann D, Schaefer W: 1,3-Heterocumulene-to-alkyne . J Org Chem. 2000 Dec 29;65(26):8940-7.
Transition structures and energy barriers of the concerted prototypical cycloaddition reaction of 1,3-heterocumulenes (S=C=S, S=C=NR, RN=C=NR, and heteroanalogs) to acetylene resulting in nucleophilic carbenes were calculated by G2 (MP2) and CBS-Q ab initio quantum chemical and by density functional theory (DFT) methods. According to the calculations the activation energies (activation enthalpies) of the homoheteroatomic cumulenes decrease in the order O > S > Se and NH > PH and the reaction energies in the order O > S approximately Se and PH > NH. The reaction of carbon disulfide and acetylene has a lower reaction barrier than that of carbodiimide and acetylene although the first reaction is less exothermic than the second one. The stronger cyclic stabilization of the 1, 3-dithiol-2-ylidene in the transition state is discussed in terms of deformation and stabilization energies and of bond indices. The known reactions of carbon disulfide with ring-strained cycloheptynes were examined by DFT and by DFT:PM3 two-layered hybrid ONIOM methods. In agreement with qualitative experimental findings the activation energy increases and the reaction energy decreases in the sequence S, SO (2), and SiMe (2) if CH (2) in the 5-position of 3,3,7, 7-tetramethyl-1-cycloheptyne is replaced by a heteroatom or heteroatomic group, respectively. The results of these calculations were corroborated by experimental studies with carbon diselenide and isothiocyanates as 1,3-heterocumulenes. The cycloaddition of carbon diselenide to cyclooctyne proceeded faster than with carbon disulfide, the main product being the 1,3-diselenol-2-selone. Under more drastic conditions it was possible to add methyl and phenyl isothiocyanate, respectively, to 3,3,6, 6-tetramethyl-1-thia-4-cycloheptyne. The products are 1:3 adducts (cycloalkyne:isothiocyanate) whose formation is explained by a trapping reaction of the first formed 1,3-thiazol-2-ylidenes. |
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