Reactive Intermediate Chemistry

224 ORGANIC RADICAL IONS

Figure 6.11. The 1H CIDNP spectra (cyclopropane resonances only) observed during the photoreaction of chloranil with cis-1,2-diphenylcyclopropane (a) and benzonorcaradiene (b).

The opposite signal directions observed for analogous protons provide evidence that the two radical cations belong to two different structure types.145,146

2 H; aH ¼ 0:5 mT, 4H) suggests an intermediate with C2v symmetry (Fig. 6.12, type D).154 At temperatures >77 K, a nearly isotropic nine-line spectrum (aH ¼ 1:33 mT)

was observed, supporting a dynamic JT effect averaging all eight 1H nuclei. Four distorted minima were probed by calculations (Fig. 6.12)155,156 rectangular

(type A) and rhomboidal structures (type B), resulting from first-order JT distortion; and trapezoidal (type C) and irregular structures (type D) due to second-order JT distortion. A flexible rhombic structure emerged as most stable (QCISD-(T)/ 6-31G*//UMP2/6-31G*).155 A recent assignment that the rhombic structure (4 equiv bonds, 157.3 pm) is a (very low-lying) transition structure between two parallelograms (2 pairs of bonds, 149.5 and 169.5 pm),156 shows poor agreement between

Figure 6.12. Possible structure types of cyclobutane radical cations.

the calculated splitting constants (a ¼ 2:09, 0.29 mT) and the experimental ones (a ¼ 4:9, 1.4 mT). This finding is unsettling, as calculated hyperfine couplings for many strained-ring systems typically agree well with experiment; a closer reproduction of experimental data by calculations appears desirable.

Cyclobutanes disubstituted in the 1,2-positions should favor structure-type C or a related distonic structure with one broken C C bond. Calculations [QCISD-(T)/ 6-31G*//UMP2/6-31G*] suggest a trapezoidal structure for trans-1,2-dimethyl- cyclobutane radical cation.155 This expectation is born out by experimental results

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15þ) and norbornadiene, (16þ); 15 features two

adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene p systems well suited to probe through-space interactions.158,159 Molecular orbital considera-

tions suggest the antisymmetric combination of the ethene p orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries.

226 ORGANIC RADICAL IONS

The structures of the ions rest on CIDNP spectra delineating their hyperfine patterns,160,161 ab initio calculations162–164 ESR and ENDOR data165 for 16þ, and

TR–ESR results for 15þ.166 Ab initio calculations (B3LYP/6-31G*//UMP2/6- 31G*) reproduce positive and negative hyperfine coupling constants164 satisfactorily. Each radical cation is related uniquely to the geometry of one of the precursors (Fig. 6.13).

The unpaired spin density for either species resides on 4 equivalent carbons; the 1H nuclei (Ha) at these centers have large negative hfcs due to p,s spin polarization. The bridgehead protons (Hb) are quite different: a large positive hfc for 15þ is ascribed to hyperconjugation (p,s spin delocalization); the very weak negative hfcs of 16þ is due to ‘‘residual’’ p,s polarization; the hyperconjugative interaction is inefficient because the b protons lie in the nodal plane of the SOMO.166 Large positive hfcs for the bridge protons (g-H) of 16þ and a large negative hfc for the g-H of 15þ are noted; an explanation would go beyond the scope of this chapter.

The unique bonding in bicyclobutane (17) and its unusual chemistry caused an early interest in its radical cation. The structure of 17þ rests on ESR/ENDOR (electron–nuclear double resonance) spectra (aax ¼ 7:7 mT, 2H; aeq ¼ 1:14 mT, 4H).167 The bridgehead carbons bear spin density and the transannular bond is lengthened (178.6 pm, MNDO; 174.3 pm, B3LYP/6-31G*//MP2/6-31G*) and the flap angle is increased (132 , MNDO; 133.6 , B3LYP/6-31G*//MP2/6-31G*).167 The major hyperfine splitting was assigned to the axial protons (Hax); the large difference between aax and aeq supports a puckered geometry; no inversion occurs up to 160 K.

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety; its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (Ha), strong positive hfcs for the non-allylic bridgehead protons (Hg), and negligible hfcs for the allylic bridgehead protons (Hb).168 Accordingly, the spin and charge of 18þ are essentially localized in the alkene moiety, with efficient spin delocalization onto

• +

• +

Figure 6.13. Schematic energy diagram for quadricyclane (15) and norbornadiene (16) and

their radical cations. The respective minima on the two surfaces are in a unique relationship with characteristic changes in bond lengths and angles. Experimental165,166 and calculated

hyperfine coupling constants (in parentheses; G; B3LYP/6-31G*//MP2/6-31G*)164 are shown below.

the non-allylic bridgehead 1H nuclei. The assignment was confirmed by ESR/ ENDOR (a ¼ þ2:79 mT; Ha; a ¼ 0:835 mT; Hg). The splitting of the allylic b protons (ab ¼ 0:158 mT; ENDOR)169 reflects their position in the nodal plane of the p system.

These assignments are consistent with PES data170 and supported by theoretical calculations;164,170 in C2v symmetry, 18þ has two low-lying radical cationic states, 2B1 and 2A1.164 The 2B1 state is the ground state of 18þ; the calculated hyperfine

coupling constants (B3LYP/6-31G*//MP2/6-31G*) are compatible with chemically induced dynamic nuclear polarization (CIDNP) and ESR/ENDOR results. No spin

228 ORGANIC RADICAL IONS

density is found on the g carbons; the transannular C C bond is slightly shortened relative to the parent molecule.

The radical cation (19þ) of the strained bicyclo[2.1.0]pentane also has a puckered conformation, supported by one strongly coupled (flagpole) proton (asyn ¼ 4:49 mT).171 Ab initio calculations indicate that the transannular bond retains some bonding and that the bridgehead carbons remain pyramidal.164

Hsyn

Hanti

+

•

19 • +

4.5. Radical Ions of 1,5-Hexadiene Systems

Radical cations derived from 1,5-hexadiene systems illustrate major differences between the potential surfaces of radical cations and neutral precursors. On the precursor potential surface, the states of intermediate geometry are saddle points (transition structures), but pronounced minima (Fig. 6.14) on the radical cation potential surface.

. 1

1'

2+

4'

24'

2

4'

C2–C4' distance

Figure 6.14. Schematic cross-section through the potential surfaces of dicyclopentadiene (20) and its radical cation (20þ). The energy minimum on the radical cation surface corresponds to a saddle point (or a shallow minimum) on the potential surface of the precursor.

Dicyclopentadiene forms a radical cation (20þ) in which one of the bonds link-

ing the monomer units is cleaved. The species contains two allyl moieties attached to a C4 ‘‘spacer’’. Structure 20þ rests on an unmistakable CIDNP pattern172–174

and is supported by an analysis of the electronic absorption spectrum.175 The large energy gap in the OS of this ion ( E ¼ 1:67 eV) is incompatible with the photoelectron spectrum of the parent molecule ( E ¼ 0:15 eV), but it fits the ringopened structure 20þ.

The third radical cation structure type is the cyclohexane-1,4-diyl radical cation (22þ) derived from 1,5-hexadiene. The free electron spin is shared between two

carbons, which may explain the blue color of the species (‘‘charge’’ resonance). Four axial b and two a protons are strongly coupled (a ¼ 1:19 mT, 6H).179,180

4.6. Bifunctional or Distonic Radical Ions

At least one of the hexadiene radical cations (20þ) can be viewed as a species containing spin and charge in two separate (though equivalent) molecular fragments. This separation can be enforced in systems of lower symmetry; in this context, we mention two radical cations derived from 1,1-diaryl-2-methylenecyclopropane (23), and 6-methoxy-1,2,3,4,6-pentamethyl-5-methylenebicyclo [2.2.0]hex-2-ene (25).

The PET reaction of 23 results in the rapid equilibration of the exo-methylene and the secondary cyclopropane carbons. These findings were explained via a ringopened trimethylenemethane radical cation (24þ). The rearrangement requires that, upon back ET, the diarylmethylene group couple with one of the allyl termini.181 The CIDNP results indicate that the electron spin is localized in the allyl p system; the lack of polarization for the aromatic rings suggests that they are arranged orthogonal to the allyl group.182

Ar = aryl

230 ORGANIC RADICAL IONS

6-Methoxy-1,2,3,4,6-pentamethyl-5-methylenebicyclo[2.2.0]hex-2-ene (25) gives rise to radical cation 26þ, containing a strained cyclobutenyl radical coupled to a methoxyallyl cation.183 Resonance electron donation from the methoxy group stabilizes the charge in the allylic moiety. The significant reorganization upon oxidation is best illustrated by CIDNP effects of two pairs of methyl groups in the 1- and 4- and 2- and 3-positions, magnetically nearly indistinguishable in the parent molecule, but dramatically different in the radical cation.183

The bifunctional radical ions 24þ and 26þ are two examples of a family of species, commonly called ‘‘distonic,’’ which enjoy significant current interest. Much of the research in distonic radical ions is being carried out in the gas phase, typically in tandem mass spectrometers with positive ions as the most frequent

targets. However, distonic radical anions have also been studied, and some species are accessible in fluid solution182,183 or in frozen matrices.

The term distonic was originally coined for species formed by a hydrogen shift in a vertical radical cation,184,185 then redefined to designate radical cations derived

formally by removing an electron from ylids, zwitterions, or diradicals.186 It is important that the unpaired spin and the charge of these radical ions are localized in different functions. This restriction eliminates species, such as semiquinone and semidione radical anions, alkene radical cations, and a series of radical cations derived from strained ring systems, including the cyclohexanediyl radical cation 22þ discussed above. This limitation favors species whose spin or charge are localized in a s orbital. Alternatively, distonic radical ions can be designed with two functions in metadisubstituted benzene systems that also will prevent mutual delocalization.

Not surprisingly, bifunctional species preceded the definition of the term distonic, both in the gas and condensed phases. For example, Hammond et al. studied the cage effect for radical pairs generated by decomposition of azo compounds. Among their targets was the doubly protonated amidine (27), whose decomposition yielded a pair of distonic radical cations (28þ).187 Attachment of the spin-bearing carbon in the 2-position of the diazaallyl function ensures minimal delocalization of unpaired spin into the latter.

Diphenylfulvene is very readily converted to the distonic radical anion (29 ). The aromatic character of the 6-p-cyclopentadienide ring explains the stability of the distonic species.188

Ph

– •

Ph

29 • –

Several gas-phase reactions were known to involve bifunctional radical cations preceding the definition of the term distonic; we mention only the ring-opened oxirane radical cation, which will be discussed presently.189

Among simple hydrocarbon ions, the distonic trimethylene radical cationCH2CH2CH2þ (7þ) is accessible upon internal excitation of the cyclopropane molecular ion 6þ,190 or by loss of formaldehyde from the tetrahydrofuran

(THF) molecular ion.191 The gas-phase reactivity is clearly different from that of its isomers 6þ and 8þ.191

The distonic radical cation (14þ) stabilized by delocalization of spin and charge into one aryl group each, was discussed above as a potential intermediate in the geometric isomerization of 1,2-diaryloxycyclobutane (13).157

Replacing a methylene group of cyclopropane by oxygen changes the system

considerably. The PES data identify an oxygen n orbital (b1), and a ring orbital (a1), as the highest lying MOs.192 The ET from b1 would form species 30 (2B1),

with spin and charge localized on oxygen whereas ET from a1 would generate radical cation 31 (2A1), having spin and charge on the carbon atoms. Ring opening and rotation of one methylene group would form an asymmetric species (32þ) with spin and charge in two separate fragments; rotation of both methylene groups would generate a resonance stabilized oxallyl species (33þ), which has the positive charge on oxygen, where the SOMO has a node.

The radical ion observed in cryogenic matrices has a g-factor (2:0022 2:0024)193 and b-hfcs (a ¼ 1:62 mT) of a magnitude incompatible with an oxygen-

centered radical (30þ), but is compatible with either the ring-closed 31þ or the oxallyl structure 33þ.193,194 A comparison between the electronic absorption195

and ESR spectra196 of the parent oxirane radical cation and those of tetramethyloxirane and 9,10-octalin oxide radical cations leave little doubt that the ions of simple oxiranes have ring-opened oxallyl structures (type 33). Octalin oxide radical cation has a ring-closed structure of type 31, which can be rationalized as a manifestation of Bredt’s rule.197 The subtle question remains whether the ring-opened radical cations have single-minimum or double-minimum potential surfaces.198 The

anion 38 was prepared from benzene and O with loss of H2þ, while the meta and para isomers (e.g., 39) were prepared from 3- and 4-(bis-trimethylsilyl)benzene reacted, successively, with fluoride ion and molecular fluorine. Negative ion PES of these ions showed two series of transitions, characteristic for ionizations forming singlet and triplet benzynes. The differences in the ionization energies reveal the S,T splitting of the corresponding benzynes.205,206

Benzene derivatives such as m-methylanisole (40) can be converted to distonic carbene ions. Reaction of 40 with O occurs with loss of H2þ, generating the ‘‘conventional’’ carbene anion 41 ; this anion reacts with molecular fluorine by dissociative ET, followed by nucleophilic attack of F on the methyl group, forming 42 . In contrast to phenylmethylene, 42 has a singlet ground state; however, upon protonation it gives rise to the triplet state of m-hydroxyphenyl- methylene. This interesting reaction can be viewed as a spin-forbidden protontransfer reaction.207

A distonic diradical cation (44þ) of m-benzyne was investigated as a model for benzyne reactivity. The species was prepared via ipso substitution of one bromine atom in the 1,3,5-tribromobenzene molecular ion (43þ) by 3-fluoropyridine and collision-activated or photoinduced dissociation of the remaining two bromine atoms.208