15N nuclear polarisation in nitration and related reactions. Part 5. The borderline between the classical and the electron transfer mechanisms in nitration by the nitronium ion

On the Occurrence of an Electron-Transfer Step in Aromatic Nitration

“…These are only qualitative estimates; specific interactions between charged species and solvent molecules, expected to be important when the solvent is a Lewis base, should be added to these values, making a purely computational prediction extremely difficult. That the dissociation of the contact radical pair is not likely to occur in solution is also confirmed by the absence of the CIDNP effect; however, a slight but significant nuclear polarization has been observed for naphthalene 14b. Further insight could be provided by the standard redox potentials of the couples NO 2 + /NO 2 and ArH + /ArH, but the substantial disagreement between the values reported in the literature and the marked solvent dependence of the NO 2 + /NO 2 potentials 33 do not allow a clear-cut assessment of the problem.…”

“…In fact, even nonconventional nitration techniqueselectrochemical, gas phase, and photoinduced nitration,which have provided important mechanistic indications, have not made it possible to produce decisive evidence in favor of path A or B, since more competing processes may occur; nor have the studies of Ridd and co-workers, based on CIDNP (chemically induced dynamic nuclear polarization), been of help: despite the decisive role of that technique in proving that the reaction of aromatics with the nitrosonium ion occurs via an ET step, it does not permit a definitive assessment of the question, because the absence of nuclear polarization does not necessarily imply the absence of a radical pair on the reaction path, since a significant separation of the radical pair must occur for a CIDNP effect to be observed 10b…”

NO2+ Nitration Mechanism of Aromatic Compounds: Electrophilic vs Charge-Transfer Process

“…The ortho-regioselectivity enhancement in the electrophilic NO 2 + nitration (eq 1) has been suggested to originate via two steps that are the attack of NO 2 + on the ipso-position to form the ipso σ-complex 6∼9 (eq 3) and then the 1,2-shift of the NO 2 group to the ortho-position from the ipso-position of the ipso σ-complex to produce an ortho σ-complex resulting in ortho-regioselectivity (eq 4). , For the 1,2-shift of the NO 2 group, it has been also suggested that a part of the ipso σ-complex undergoes homocleavage of the C−NO 2 bond to produce the radical cation and NO 2 as an alternative charge-transfer process (eq 5) in the NO 2 + nitration. , Para-regioselectivity for the reaction of the radical cation with NO 2 as the reaction intermediates for the direct charge-transfer process (eq 2) and the alternative one (eq 5) in the NO 2 + nitration has been reported . Therefore, we inferred that such a drastic regioselectivity change in the NO 2 + nitration of 1,8-dimethylnaphthalene was caused by a mechanism change between the electrophilic (eq 1) and the direct charge-transfer process (eq 2) or the alternative one (eq 5) …”

“…While the para-regioselectivity was observed in both NO 2 + and NO + nitrations for the lower redox potential substrates than 1,8-dimethylnaphthalene, the ortho-regioselectivity enhancement appeared at lower temperature in the NO 2 + nitration when substrates had a higher redox potential than 1,8-dimethylnaphthalene. As mentioned before, the ortho-regioselectivity enhancement has been considered to be derived from the ipso attack of NO 2 + (eq 3) and then the 1,2-shift of the NO 2 group (eq 4) in the NO 2 + nitration as the electrophilic process (eq 1). , On the other hand, it is clear that formation of the radical cation to give para-regioselectivity via both the direct charge-transfer process (eq 2) and the alternative one (eq 5) is easier with a lower redox potential substrate . Again, it was implied that the NO 2 + nitration had a dual process showing ortho- and para-regioselectivity depending on the redox potential of the substrates.…”

“…However, there has been no clear evidence for the direct charge-transfer process in the NO 2 + nitration, and the direct charge-transfer process between aromatic compounds and NO 2 + has been reported to be unlikely recently because high energy is necessary for the bond coordination change between NO 2 + and NO 2 . On the other hand, the ipso attack of NO 2 + (eq 3) 6-9 has been reported showing ortho-regioselectivity via the 1,2-shift of the NO 2 group to the ortho- from the ipso-position of the ipso σ-complex (eq 4). , During this process, homocleavage of the C−NO 2 bond of the ipso σ-complex sometimes occurs and has been recognized as an alternative charge-transfer process (eq 5) to produce the radical cation with NO 2 . , Despite many recent studies using various nitration reagents such as NO 2 , NO 2 −O 3 , and C(NO 2 ) 4 , the aromatic nitration mechanism with NO 2 + is still ambiguous due to the complexity of the reaction systems.

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