1,3-heterocumulene-to-alkyne [3+2] cycloaddition reactions: A theoretical and experimental study

Transition structures and energy barriers of the concerted prototypical cyc loaddition reaction of 1,3-heterocumulenes (S=C=S,-S=C=NR, RN=C=NR, and het eroanalogs) 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 ap proximate to Se and PH > NH. The reaction of carbon disulfide and acetylene has a lower reaction barrier than that of carbodiimide and acetylene altho ugh 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 cyclo heptynes were examined by DFT and by DFT:PM3 two-layered hybrid ONIOM metho ds. In agreement with qualitative experimental findings the activation ener gy increases and the reaction energy decreases in the sequence S, SO2, and SiMe2 if CH2 in the Ei-position of 3,3,7,7-tetramethyl-1-cycloheptyne is re placed by a heteroatom or heteroatomic group, respectively. The results of these calculations were corroborated by experimental studies with carbon di selenide and isothiocyanates as 1,3-heterocumulenes. The cycloaddition of c arbon diselenide to cyclooctyne proceeded faster than with carbon disulfide , the main product being the 1,3-diselenol-2-selone. Under more drastic con ditions it was possible to add methyl and phenyl isothiocyanate, respective ly, to 3,3,6,6-tetramethyl-1-thia-4-cycloheptyne. The products are 1:3 addu cts (cyclo alkyne: isothiocyanate) whose formation is explained by a trappi ng reaction of the first formed 1,3-thiazol-2-ylidenes.