Discrete Existence of Singlet Nitrenium Ions Revisited: Computational Studies of Non-Aryl Nitrenium Ions and Their Rearrangements

Abstract

Nitrenium ion species are examined using computational methods (DFT, MP2, coupled-cluster, and a composite method, CBS-APNO) with a particular emphasis on nonaromatic species (i.e., those lacking an aromatic or heteroaromatic ring in direct conjugation with the formal nitrenium ion center.) The substitution of the N + center with alkyl, alkoxy, vinyl, acyl, and sulfonyl, among others, was evaluated. For these species, three properties are considered. (1) The stability of the nitrenium ions to unimolecular isomerizations such as 1,2 alkyl or H shifts; to the extent that the singlet states could be characterized as discrete minima on the potential energy surface (PES), (2) the effect of the substituents on singlet–triplet energy splitting as well as (3) the relative stabilities of the nitrenium ions as defined by N-hydration enthalpies (RR′N + + H 2 O → RR′NOH 2 + ). Nearly all simple alkyl and di-alkyl nitrenium ion singlet states are predicted to rearrange without detectable barriers, largely through 1,2 H or alkyl shifts. Methyl and N , N -dimethylnitrenium ion singlet states could be characterized as formal minima on the PES. However, these species show small or insignificant barriers to isomerization. Disubstituted nitrenium ions that include an alkyl group and a conjugating substituent such as alkoxyl, vinyl, or phenyl show meaningful barriers to isomerization and are thus predicted to possess nontrivial lifetimes in solution. Alkyl groups substantially stabilize the singlet state relative to the situation in the parent nitrenium ion NH 2 + to the point where the two states are nearly degenerate. Other groups that interact with the nitrenium ion center decrease the Δ E st in the order formoyl < vinyl < phenyl < alkoxy ∼ sulfonyl < cyclopropyl ∼ cyclobutyl. The latter two substituents interact strongly with the (singlet) nitrenium ion center through the formation of a nonclassical bonding reminiscent of the bisected cyclopropylcarbinyl ion case for carbocations. When singlet-state stability is evaluated in the context of N-hydration enthalpies, it is found that the ordering is acyl < vinyl < alkoxyl < phenyl < cyclopropyl and cyclobutyl.

Oxygen Atom Transfer in the Oxidation of Dimethyl Sulfoxide by Oxoammonium Cations

“…In mechanism P1, we postulate the formation of a discrete nitrenium cation C that transforms to final products. The stability of nitrenium cations, in general, depends on their spin state (singlet or triplet) and chemical structure . In order to study the product formation and the corresponding free-energy changes, we have carried out the DFT calculations of selected points along the reaction paths.…”

“…However, full computational exploration of the free-energy surface of the reaction path P2 is out of the scope of this work. In this respect, unsuccessful attempts to model concerted rearrangement of nitrenium cations of even moderate size had been reported elsewhere …”

“…Besides, minimization of the starting structure of singlet Ca led to its barrierless rearrangement also into the six-membered product 2a′ . Such rearrangement is in line with the results of ref . However, the predicted from the computations preference for the formation of the six-membered product 2a′ disagrees with the experimentally observed formation of only the five-membered product 2a .…”

“…The transformation of the bonding pattern from purely aliphatic to partially unsaturated with an additional CN bond accounts for the energy shift of ∼−95 to −97 kcal/mol. These energies are in the same range as the calculated energies of hydrogen migration and hydration of singlet nitrenium ions …”

Targeting Tryptophan for Tagging through Photoinduced Electron Transfer

“…26 Additionally, the predicted transient nature of such species led us to disfavor a nitrenium-based mechanism. 27 NMR characterization of 2a and 3a provided useful mechanistic insight, both indicating that the indole C2-position is the primary site of bond formation (also reinforcing MS-MS characterization) and that the N-methyl group of the carbamate-transferring group is not affected. This confirmed that off-target radical processes, such as 1,2-spin center shifts, are not involved in the labeling process.…”