Nitrenium Ion Analogues of Nonclassical Carbocations: Cyclopropylnitrenium, Allylnitrenium, and Azetidenium Ions and Mechanisms for Their Interconversion

2018

ACS Omega

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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.

Section: Resultssupporting

confidence: 72%

“…This intermediate is connected by a very small barrier [classically 0.19 kcal/mol, negative when zero-point vibrational energy (ZPVE) is considered] to vinylformiminium ion 36-iso . Both 36 and cyclopropylnitrenium ion 15 are part of the interesting and complex C 3 H 6 N + potential energy surface (PES) which is described in detail elsewhere …”

Section: Resultsmentioning

confidence: 99%

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

2018

ACS Omega

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“…Those nitrenium ions that showed potential energy minima for both the singlet state and triplet states are listed Table 1. As noted in previous studies, 3,59,[70][71][72][73][74][75] both singlets and triplets are bent about the R-N-R' bond angle, with the sp 2 singlets preferring bond angles of ca. 105-115 deg, absent sterically demanding substituents.…”

Section: Methodssupporting

confidence: 75%

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

2018

Preprint

Nitrenium ion species are examined using computational methods (DFT, MP2, coupled-cluster, and a composite method, CBS-APNO) with a particular emphasis on non-aromatic species (i.e. those lacking an aromatic or heteroaromatic ring in direct conjugation with the formal nitrenium ion center.) Substitution of the N+ center with alkyl, alkoxy, vinyl, acyl, sulfonyl, among others, were 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. And, to the extent the singlet states could be characterized as discrete minima on the 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¬2O -> 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 non-trivial 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 DEst 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 non-classical 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<br>

“…Geometries of the singlet and triplet nitrenium ions ( 2 ), the corresponding singlet and triplet dications ( 4 ), and the cation radical ( 3 ) were optimized using DFT ((U)M06-2X/6-311G++(d,p)), , and the electronic absorption spectra were calculated using TD-DFT . The M06-2X functional has been shown to be useful in calculating accurate geometries and vibrational frequencies for nitrenium ions , as well as other reactive intermediates such as carbenes and carbocations . The reported structures were all determined to be local minima on the potential energy surface through frequency calculations.…”

Section: Resultsmentioning

confidence: 99%

Generation ofN,N-Di(4-bromophenyl)nitrenium Ion under Acidic Conditions: Search for a Nitrenium Dication

Zeppuhar

2020

J. Org. Chem.

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The behavior of the N, N-di(4-bromophenyl)nitrenium ion under acidic aqueous conditions was examined via laser flash photolysis experiments. A long-lived species forms and can be assigned as the cation radical or the dication. This species is unreactive towards nucleophiles and reactive towards strong electron donors, consistent with a cation radical. Mechanistic analysis indicates its formation is through a separate pathway than that of the nitrenium ion, suggestive of a triplet mechanism.