A Comparison of Acetyl- and Methoxycarbonylnitrenes by Computational Methods and a Laser Flash Photolysis Study of Benzoylnitrene

Abstract

Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand the chemical and reactivity differences between acetylnitrene (CH(3)C(=O)N) and methoxycarbonylnitrene (CH(3)OC(=O)N) and related compounds. CBS-QB3 theory alone correctly predicts that acetylnitrene has a singlet ground state. We agree with previous studies that there is a substantial N-O interaction in singlet acetylnitrene and find a corresponding but weaker interaction in methoxycarbonylnitrene. Methoxycarbonylnitrene has a triplet ground state because the oxygen atom stabilizes the triplet state of the carbonyl nitrene more than the corresponding singlet state. The oxygen atom also stabilizes the transition state of the Curtius rearrangement and accelerates the isomerization of methoxycarbonylnitrene relative to acetylnitrene. Acetyl azide is calculated to decompose by concerted migration of the methyl group along with nitrogen extrusion; the free energy of activation for this concerted process is only 27 kcal/mol, and a free nitrene is not produced upon pyrolysis of acetyl azide. Methoxycarbonyl azide, on the other hand, does have a preference for stepwise Curtius rearrangement via the free nitrene. The bimolecular reactions of acetylnitrene and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to have enthalpic barriers that are near zero and free energy barriers that are controlled by entropy. These predictions were tested by laser flash photolysis studies of benzoyl azide. The absolute bimolecular reaction rate constants of benzoylnitrene were measured with the following substrates: acetonitrile (k = 3.4 x 10(5) M(-1) (s-1)), methanol (6.5 x 10(6) M(-1) s(-1)), water (4.0 x 10(6) M(-1) s(-1)), cyclohexane (1.8 x 10(5) M(-1) s(-1)), and several representative alkenes. The activation energy for the reaction of benzoylnitrene with 1-hexene is -0.06 +/- 0.001 kcal/mol. The activation energy for the decay of benzoylnitrene in pentane is -3.20 +/- 0.02 kcal/mol. The latter results indicate that the rates of reactions of benzoylnitrene are controlled by entropic factors in a manner reminiscent of singlet carbene processes.

Computational Rationalization for the Observed Ground-State Multiplicities of Fluorinated Acylnitrenes

“…At the CR-CC(2,3)/ 6-311G(3df,2p)//B3LYP/6-31G(d,p) level of theory, ΔH (298.15 K) is 1.4 kcal/mol for the singlet (indicating a slight disfavoring of having the fluorines adjacent to the CON) and 0.0 kcal/mol for the triplet nitrene. This difference of 1.4 kcal/mol between the singlet and triplet is considerably smaller than the differences obtained by Liu et al 13 for conjugation of the α-carbonyl by an oxygen atom. Importantly, however, it points out that the α-fluorines are essentially inconsequential to the stability of the triplet nitrene.…”

The Missing Link: Triplet Fluorocarbonyl Nitrene FC(O)N

“…The differences in the DE ST values for the alkyl and alkyloxy carbonyl nitrenes RC(O)N and ROC(O)N, respectively, were assumed to be related to an extended p conjugation in the latter ones. [14] Considering the previously reported DE ST values for carbonyl nitrenes XC(O)N (À17 and 24 kJ mol À1 for X = CH 3 and CH 3 O (CBS-QB3) [4] and À3, 16, and 26 kJ mol À1 for formyl and anti-and syn-carboxy nitrenes (X = H, OH; CCSD(T) and CBS extrapolation at the G2 level), [14] respectively), our CBS-QB3 values for X = H (À0.4 kJ mol À1 , ) and fluorocarbonyl nitrene has the largest DE ST . Electronic effects due to strongly electron-withdrawing substituents have been widely reviewed, [15] particularly those of the corresponding structural distortions.…”

“…Much experimental effort has been devoted to the trapping and characterization of such nitrenes formed during azide decomposition, [2,3] and the corresponding decomposition mechanism has been thoroughly explored in recent theoretical studies. [4,5] Of particular interest is the photodecomposition of carbonyl azides, which has been studied by time-resolved infrared spectroscopy. [6][7][8][9] Upon UV irradiation (l 250 nm), excited carbonyl azides may fragment to form two different products, carbonyl nitrenes and the Curtius-rearranged isocyanate (Scheme 1).…”

“…Thus, the singlet-triplet energy splitting (DE ST ) for such carbonyl nitrenes appears to be rather small. [4,13,14] As DE ST for the various carbonyl nitrenes correlates with the stabilization of their lowest singlet states, the bonding interaction between the nitrogen and oxygen atoms depends on the length of the weak NÀO bond as well as on the O-C-N angles. The O-C-N angles are about 908 in closed-shell singlet-state nitrenes, but are much larger (about 1208) in triplet nitrenes.…”

“…The O-C-N angles are about 908 in closed-shell singlet-state nitrenes, but are much larger (about 1208) in triplet nitrenes. [4,14] According to Bents rule, [15] the substitution of R in RC(O)N by a more electron-withdrawing RO group is expected to open the O-C-N angle thereby favoring a triplet ground state. According to recent ab initio calculations, [4] RO groups generally stabilize the carbonyl nitrenes by p conjugation relative to acyl nitrenes (X = H, alkyl).…”