Ambident ethyl N-nitrosocarbamate anion: experimental and computational studies of alkylation and thermal stability

Alkylation of N-nitrosourethane tetrabutylammonium salt (2-Bu(4)N) with four electrophiles (MeI, EtI, i-PrI, and PhCH(2)Br) was studied by (1)H NMR in CD(2)Cl(2) and CD(3)CN solutions. The ratio of the three regioisomers N-alkyl-N-nitrosourethane 3, azoxy 4, and O-alkyldiazotate 5 was practically independent of solvent but dependent on the nature of the electrophile. The anion 2 and O-alkyl derivative 5 are thermally unstable and decompose to ethyl carbonates 9 and 10, respectively, with a first-order rate constant (2-Bu(4)N: k = 18.5 +/- 0.1 x 10(-5) s(-1); 5b (R = Et): k = 1.77 +/- 0.02 x 10(-5) s(-1); 5d (R = PhCH(2)): k = 4.78 +/- 0.08 x 10(-5) s(-1) at 35 degrees C in CD(2)Cl(2)). Further kinetic measurements gave activation parameters for the decomposition of 2 (E(a) = 24.2 +/- 0.3 kcal/mol and ln A = 30.9 +/- 0.1). Gas-phase calculations at the MP2(fc)/6-31+G(d)//MP2(fc)/6-31G(d) level showed that the alkylation of 2 involves the lone electron pairs of the N-N-O atoms, and the calculated activation energies correspond well to the observed ratio of regioisomers 3-5. The theoretical analysis of the decomposition processes supports a concerted mechanism with a four-center transition state in the first step for all four compounds. The calculated activation energy order (2 < 5 < 3 < 4) is consistent with the observed order of stability. Decomposition of 2 and 5 is a unimolecular process, giving carbonates 9 and 10 in a single step. In contrast, rearrangement of 3 and 4 leads to alkyl diazonium ions. A detailed theoretical analysis indicates that the rate-determining step for thermal decomposition of 2 is the loss of molecular nitrogen, while in 5 it is the trans-cis isomerization process. The nonconcerted process involving homolytic cleavage of the O-N bond in 5 was found to be significantly less favorable.