N,N-Di(4-halophenyl)nitrenium Ions:  Nucleophilic Trapping, Aromatic Substitution, and Hydrogen Atom Transfer

Abstract

The reactive intermediates N,N-di(4-chlorophenyl)nitrenium ion and N,N-di(4-bromophenyl)nitrenium ion were generated through photolysis of the corresponding N-amino(2,4,6,-collidinium) ions. The behavior of these diarylnitrenium ions was characterized by laser flash photolysis, analysis of the stable photoproducts, and ab initio calculations with density functional theory. The latter predict these species to have singlet ground states. The halogenated diarylnitrenium ions are significantly longer lived than the unsubstituted diphenylnitrenium ion. Specifically, cyclization to form carbazole derivatives occurs negligibly, if at all, with the halogenated derivatives. They do, however, carry out most of the characteristic reactions of singlet arylnitrenium ions, including combining with nucleophiles on the aryl rings, adding to arenes, and accepting electrons from readily oxidized traps. Interestingly these species also abstract H atoms from 1,4-cyclohexadiene and various phenol derivatives. The implication of the latter process in relation to the computed singlet-triplet energy gaps of ca. -12.5 kcal/mol is discussed.

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

“…In fact, this signalpersists for over 500 μs, and consequently it is not possible to provide a specific lifetime given the constraints of the experimental set up. The 720 nm transient has been observed in other experiments involving LFP of 1 and has been attributed to radical cation 3 10. Indeed, this absorption maximum is consistent with other reports where 3 has been generated by other routes.…”

“…Earlier studies 10,12 demonstrated that N,N-di (4-bromophenyl) for >500 μs in water and does not react at an observable rate with any added nucleophiles. These experiments support the original assignment of the 720 nm signal to the radical cation, rather than the nitrenium dication.…”

Theoretical prediction of linear free energy relationships using proton nucleomers

“…Because singlet arylnitrenium ions have been implicated in carcinogenesis, [34][35][36][37][38][39] the study of the influence of substituents on the relative energies of the singlet and triplet states of such systems has been a topic of considerable theoretical interest. [40][41][42][43][44][45][46] In this context, Sullivan et al [44] have previously computed linear correlations against s and s þ values for several actual functional groups and reported optimal linear correlation with the latter scale at the BPW91/cc-pVDZ level of theory (including thermal free-energy contributions and aqueous solvation effects with the SM5.42R continuum solvation model [47] ). We observed similarly good correlations with s þ values for the levels of theory employed here, with gas-phase r 0 values of about À5.0.…”

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

“…Falvey and coworkers [10,11,12,13] reported several works on the chemical and spectroscopic behavior of the N , N -diphenylnitrenium ion ( 1 ) formed after photoexcitation of the 1-( N , N -dipheylamino)-2,4,6-triphenylpyridinium ion (see Scheme 1), and this aryl nitrenium ion was proven to have a ground state singlet character with some overlap between an unfilled nitrogen based p -orbital with filled π-orbitals on the phenyl groups [12,14]. Furthermore, Thomas and Falvey’s research work showed that the N , N -di(4-bromophenyl)nitrenium ion ( 2 ) formed after photoexcitation of N -(4,4′-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF 4 − salt (DN) had greater stability with a longer lifetime than the unsubstituted nitrenium ion 1 , which was revealed by study of laser flash photolysis to have a lifetime of 124 µs in MeCN, and 6.57 µs in water [10]. Macernis et al, studied the effect of surrounding polar environments on the molecular behavior of the related nitrogen [15].…”