Time-Resolved Spectroscopic Study of N,N–Di(4–bromo)nitrenium Ions in Acidic Aqueous Solution

Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the N(4,4′–dibromodiphenylamino)–2,4,6–trimethylpyridinium BF4− salt (1) in an acidic aqueous solution. Efficient cleavage of the N–N bond (4 ps) to form the N,N–di(4–bromophenyl)nitrenium ion (DN) was also observed in the acidic aqueous solution. As a result, the dication intermediate 4 appears more likely to be produced after abstracting a proton for the nitrenium ion DN in the acid solution first, followed by an electron abstraction to form the radical cation intermediate 3. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.

Visible-Light-Mediated Amination of π-Nucleophiles with N-Aminopyridinium Salts

Under UV light irradiation <i>N</i>,<i>N</i>-(diphenylamino)pyridinium salts generate nitrenium ions. Herein, we report that in the presence of a photoredox catalyst nitrogen-centered radicals form which then react with enol equivalents to give amino carbonyl compounds in excellent yields.<br>

Visible-Light-Mediated Amination of π-Nucleophiles with N-Aminopyridinium Salts

Under UV light irradiation <i>N</i>,<i>N</i>-(diphenylamino)pyridinium salts generate nitrenium ions. Herein, we report that in the presence of a photoredox catalyst nitrogen-centered radicals form which then react with enol equivalents to give amino carbonyl compounds in excellent yields.<br>

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions

Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved resonance Raman (ns-TR3) experiments were employed to examine the photochemical pathways of N-(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF4− (salt (DN) from just absorption of a photon of light to the production of the important N,N-di(4-bromophenyl)nitrenium ion 2. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion 2 was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion 2 with H2O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate 3 shows that there is an efficient and exclusive reaction pathway for 2 with H2O. The results shown in this paper give new characterization of 2, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies.

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

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>

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

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>