The Question of Homoaromaticity in 1,6-Dihydro-1,2,4,5-tetrazines

Article abstract of DOI:10.1021/jo00365a035

1,6-Dihydro-1,2,4,5-tetrazines exist as rapidly interconverting boat conformations in solution.In the 3,6-dimethyl derivatives alkylated on N1 with small or medium-sized groups the equilibrium is strongly biased toward the conformation with C6-Me endo, but in the N-tert-butyl derivative the opposite conformation is completely dominant. 1H NMR spectroscopy shows the C6 endo substituents to be strongly shielded and the exo substituents to be strongly deshielded.The individual conformations thus have dramatically different spectra (e.g., 6 has δ_H, δ_Me 2.08, 1.93; 9 has δ_H, δ_Me 6.00, 0.71).The conformational balance is about equal in the tri-isopropyl compound 30 whose time-averaged C6-H absorption at δ 3.87 (298 K) separates into two absorptions at δ 5.40 and 2.53 below 190 K.If the N1 substituent, regardless of its size, is conjugatively electron withdrawing, only the conformation with C6-H exo is detectable (δ typically 6.2 to 6.7).Since 1-acetyl-1,6-dihydro-1,2,4,5-tetrazine (32), whose two boat conformations must be equally populated, shows no resolution of these down to 173 K the inversion barriers must be very much lower in the N-acyl than in the N-alkyl derivatives.Crystal structure determinations of 1-(hydroxymethyl)- and 1-acetyl-3,6-dimethyl-1,6-dihydro-1,2,4,5-tetrazines (5 and 14) confirm the opposite arrangements of the C6 substituents in the boat conformations.Despite the nonplanarity of the five atoms N1 to N5 and the relatively large seperations between N1 and N5 (2.29 Angstroem in 5 and 2.35 Angstroem in 14), we suggest that a case can be made for the operation of a homoaromatic ring current in these systems to account for the remarkable proton shielding and deshielding observed.