2,3,4,5-Tetrahydro-2,4-diimino-1,3,5-triazin-1-ium salts in the reaction of carbodiimides with N -alkylnitrilium salts

2,3,4,5-Tetrahydro-2,4-diimino-1,3,5-triazin-1-ium hexachloroantimonate salts were formed by the addition of carbodiimides to N -alkylnitrilium salts. The compounds were characterized using microanalytical and spectroscopic (IR, MS, and NMR) data.


Results and Discussion
The addition of the carbodiimides 2a-d to a solution of N-alkylnitrilium salts 1a-f in dichloromethane at low temperature afforded the triazinium salts 5a-i in 45-91% yield.It is suggested that the compounds 5a-i are formed through a [2+2+2]-cycloaddition, in analogy to the formation of cycloadducts by the reaction of nitrilium salts with N-substituted imines. 25The reaction involves the formation of intermediate 6 (1:1 adduct) which reacts further with another mole of the carbodiimide 2 (Scheme 3).
The structures of the newly prepared triazinium salts 5a-i are derived from elemental analyses, and IR-and NMR-spectral data that are given in the Experimental Part.It is worth noting that the 1 H-NMR spectrum of 5e in CD 3 CN shows four signals for different isopropyl groups, and only one N-methyl group at δ = 3.01.Considerable line broadening of the 13 C-signal of the N-methyl resonance indicates slow geometrical isomerization of the exocyclic C 2 =N and C 4 =N double bonds.Also, the 13 C-NMR spectrum shows signals at δ =137.4,140.7 and 165.5 ppm which are attributed to C 2 =N, C 4 =N and C 6 =N + , respectively.The IR spectrum of 5e in CH 2 Cl 2 shows stretching absorption peaks at 1690, 1650, 1540 and 1500 cm -1 corresponding to different types of imines and iminium cation.The formation of only one isomer for 5i can be explained by the preferred nucleophilicity of N-methyl-over the N-tert-butyl group as a result of the steric effect of the latter group.0][31][32][33] Geometrical isomerization was also observed in the 13 C-NMR of some compounds such as 5i and 5j.

Mechanism
The nucleophilic addition to nitrilium ions has been reported by Hegarty and his co-workers. 29to be stereo-electronically controlled.The nitrogen lone-pair always develops anti-to the intruding nucleophile (Z-isomer).In the resulting imines, the nucleophile and the N-substituent are synoriented with respect to each other.It is likely, therefore, that the addition of the carbodiimide 2 to a nitrilium ion 1 gives the (Z) adduct as the primary product.Thermal isomerization around the R-C=N-R bond furnishes the (E) form, which may readily undergo ring closure.The formation of 5 by the addition of the carbodiimide 2 to the nitrilium salt 1 is believed to be due to stepwise polar cycloadditions. 29,30This involves the addition of two moles of carbodiimides 2 (added in two successive additions) via the formation of new cyanimidium salts 6 and 7 (Scheme 3).The latter intermediate, 7, isomerizes to the (E)-geometrical isomer 8, followed by ring closure to form the [2+2+2] cycloadduct, salt 5.This pathway is in contrast to the formation of a [2+2] cycloadduct, as in the case of dimerization of carbodiimides, [29][30][31] and the case of the addition of carbodiimides to cyanamidium salts. 25The driving force for the second addition of another molecule of carbodiimide 2 to form 5 (instead of ring closure to diazetidinium salts) is presumably due to the formation of a resonance stabilized [2+2+2] adduct (5 5').

Experimental Section
General Procedures.All experiments were carried out with the exclusion of moisture, in solvents dried by standard methods.Melting points were determined with Electrothermal 9100 apparatus and have not been corrected.The 1 H-and 13 C-NMR spectra were recorded on Bruker AC-250 and Bruker DPX-300 instruments, using TMS as internal standard, and with deuterated chloroform, dichloromethane or acetonitrile as the solvent; chemical shifts (δ) are in ppm and coupling constants (J) in Hz.IR spectra were recorded on a Perkin-Elmer 1600 FT-IR in CH 2 Cl 2 solution; the frequencies are expressed in cm -1 .Elemental microanalyses were obtained on an ARKAT USA, Inc.
General procedure for preparation of substituted 2,3,4,5-tetrahydro-2,4-diimino-1,3,5triazin-1-ium hexachloroantimonate, 5a-i A solution of 10 mmol of the carbodiimide 1a-c dissolved in 20 mL of dichloromethane was added dropwise to a cooled (-20 0 C), stirred solution of 5 mmol of the appropriate nitrilium hexachloroantimonate 2a-f.The solution was stirred at this temperature for 1 h, then allowed to rise to 10 0 C and stirred for an additional 1 h, after which time the IR of the solution showed the disappearance of the absorption of the nitrilium peak of the nitrilium salts (2230 cm -1 ).The reaction solution was reduced to half volume in vacuo, then cooled to -20 0 C, then 100 mL of absolute diethyl ether was added dropwise to give a powder precipitate, which was collected under vacuum and dried.The precipitate was purified by recrystallization from 10 mL/100 mL diethyl ether.

2 Scheme 3 .
Scheme 3. Proposed mechanism for the formation of compounds 5.