Donald R. Arnold

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

Abstract The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.

Merostabilization in radical ions, triplets, and biradicals. 5. The thermal cis–trans isomerization of para-substituted tetraphenylethylenes

The cis- and trans-isomers of four para-substituted (three disubstituted and one tetrasubstituted) tetraphenylethylenes have been synthesized, separated, and characterized by either dipole moment or X-ray analysis. The rates of thermal isomerization of the disubstituted derivatives in benzene solution correlate with a substituent parameter reflecting substituent effects on benzylic radical stabilities (σ•), which reflects the biradical nature of the transition state for thermal isomerization in this system. A significant rate enhancement is observed for the tetrasubstituted derivative, in which merostabilization of the transition state is possible.The Arrhenius parameters for thermal isomerization in benzene were determined. From the variation observed for the disubstituted derivatives, an estimate of 35.5u2002kcal mol−1 for the rotational barrier in tetraphenylethylene is obtained. The activation energy for thermal isomerization of the tetrasubstituted derivative is 2u2002kcal mol−1 lower than those of the other...

The Effect of Substituents on Benzylic Radical Esr Hyperfine Coupling Constants. The σ α • Scale Based Upon Spin Delocalization.

The effects of substituents in the meta and para positions of benzylic radicals on the α- and β-hydrogen hyperfine coupling constants (hfc’s) are discussed. The relationship between hfc and radical stabilization energy is also described. The development of the σ•α substituent parameter scale is reviewed; and, examples ofαthe application of the σ•α parameters in Hammett type linear free-energy corre lationsα are given.

NMR study of the plastic crystalline phases of norbornane. An unusual deuterium isotope effect on the hexagonal to cubic phase transition

Abstract Deuterium NMR spectra of solid norbornane- d 4 and norbornane- d 2 indicate that the hexagonal (hcp) ↔ cubic (fcc) phase-transition temperature in these compounds is elevated by 25° and 9° over the value in norbornane. Deuterium and 13 C spin—lattice relaxation data indicate that the rotational correlation times of norbornane- d 4 and norbornane are continuous across the hcp ↔ fcc phase transition, however rotations of norbornane- d 4 appear to be slower than those of norbornane. It is suggested that translational diffusion of norbornane- d 4 is also slower and that the slower motions of norbornane- d 4 are at least in part responsible for the observed isotope effects.

The two-parameter linear free energy treatment of the substituent effects on the half-wave reduction potentials and n,π triplet energies of aromatic ketones. A test of the validity of the approach.

Abstract The half-wave reduction potentials of fluoro- and thiomethyl-substituted benzophenones and the n,π triplet energies of the fluoro-substituted derivatives are found to agree quite closely with values predicted using empirical relations comprised of Hammett σ values and σ· values; the lowest triplet states of the thiomethyl-substituted derivatives are shown to be π,π.

Interconversion and rearrangement of radical cations. Part 2.1 Photoinduced electron transfer and electrochemical oxidation of 1,4-bis(methylene)cyclohexane

The photoinduced electron transfer and electrochemical oxidation of n1,4-bis(methylene)cyclohexane (2) in acetonitrile have been studied in nthe presence and absence of a nucleophile (methanol). The photoinduced nelectron transfer reactions of 2 in acetonitrile–methanol solution nwith 1,4-dicyanobenzene (8) as the electron acceptor gives two products: n4-(methoxymethyl)-1-methylenecyclohexane (16) and n4-(4-cyanophenyl)-4-(methoxymethyl)-1-methylenecyclohexane (17). These nproducts arise from nucleophilic attack on the radical cation followed nby either reduction and protonation or combination with the radical nanion of the electron acceptor, 1,4-dicyanobenzene n(8˙-). These results are in accord with the nproposed mechanism of the photochemical nucleophile–olefin ncombination, aromatic substitution (photo-NOCAS) reaction. In the nabsence of a nucleophile, the photoinduced electron transfer reaction of n2 gives rise to several interesting and unexpected products 18–25 nwhich result from complex reaction mechanisms involving radical, ionic nand radical ion intermediates.The electrooxidation of 2 in acetonitrile in the presence of nmethanol leads to products 27–35. Under these conditions the nradical cation 2˙+ reacts with the nucleophile followed nby a second oxidation and subsequent reactions leading to products n(electrochemical–chemical–electrochemical, ECE). One of the nproducts (28) is the result of protonation of 2 followed by nucleophilic nattack. The electrochemical oxidation of 2 in acetonitrile (no methanol) nyields 2˙+ which is deprotonated and then further noxidized to give 39–43. These products arise from ionic nintermediates (ECE); oxidation of 2 all the way to aromatic compounds nwas observed in 39–41.In none of these experiments was there any evidence for the nformation of cyclized products, nor was there any indication of ncarbon–carbon bond cleavage in 2˙+. The products nare consistent with the initial formation of the intermediate radical ncation. The products as well as the possible mechanisms of formation of nthese species are discussed.

Substituent effect on singlet–triplet splitting: diarylcarbene–diarylmethylene; electron spin resonance study. Merostabilization in diarylmethylenes

An analysis of the e.s.r. spectra of unsymmetrically substituted diphenylmethylenes (e.g. p,p′-methoxy, cyano, dimethylamino, or nitro) indicate these species have a triplet ground state which is delocalized by merostabilization.

The electron transfer photochemistry of allenes with cyanoarenes. Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) and related reactions

The photochemical reactions of 1,2,4,5-tetracyanobenzene (1), 1,4-dicyanobenzene (2) and 1,4-dicyanonaphthalene (3) with tetramethylallene (4) and 1,1-dimethylallene (5) in acetonitrile–methanol solution have been investigated. Both 1 and 2 give 1∶1∶1 arene–allene–methanol products (7–12) in which addition of methanol occurs exclusively at the central allene carbon with aromatic substitution at the termini. The formation of these products is rationalised on the basis of a photoinduced electron transfer mechanism described as the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Cyanoarene 3 undergoes photochemical reactions initially similar to the photo-NOCAS reaction, but with addition to the arene occurring instead of substitution. The primary product (example, 22) was not isolated since it undergoes further photochemical reactions. Compound 22 and its dimethyl analogues react via a photoinduced intramolecular [2πxa0+xa02π] cycloaddition across the 3,4-double bond to give tricyclo 1∶1∶1 arene–allene–methanol adducts (14, 17 and 18) or add methanol and cyclise to bicyclo 1∶1∶2 arene–allene–methanol adducts (15, 19 and 20). The use of biphenyl (6) as a co-donor enhances the efficiency of all the photoinduced electron transfer reactions studied. In fact, removing 6 from the reaction mixture containing 3 and 5 diverts the reaction from the electron transfer pathway and instead gives an exciplex-mediated [4πxa0+xa02π] cycloadduct, 21. The mechanisms, with particular reference to the regiochemical selectivities observed in the photoreactions involving 5, are discussed.

Thermal rearrangement of 1,1,2,2-tetraphenylcyclopropane

The thermal rearrangement of 1,1,2,2-tetraphenylcyclopropane (1) to 1,1,3-triphenylindan (2) involves formation of an intermediate triene (3) which has been detected by u.v. spectroscopy and trapped by reaction with enophiles.

Electronic excited states of small ring compounds. Substituent effects on the photochemical ring cleavage of 3,3-dimethyl-1,2-diarylcyclopropenes

The mixture of dienes formed upon direct irradiation of 3,3-dimethyl-1,2-diarylcyclopropenes [1-phenyl-2-p-methoxyphenyl (Ib) and 1-phenyl-2-p-cyano-phenyl (Ic)] indicates regioselective cleavage of the cyclopropene bond bearing the electron donating aryl ring.