The importance of conformational effects on the carbon–carbon bond cleavage of β-phenethyl ether radical cations

Abstract

The photosensitized (electron transfer) bond cleavage of some β-phenylethyl ether radical cations has been investigated. In previous studies the feasibility of the bond cleavage was thought to depend on the bond dissociation energy (BDE). However, this simple hypothesis led to several incorrect predictions and therefore additional criteria, conformational effects, were added to the hypothesis. This study has now been extended and additional examples of the importance of the conformation on the carbon–carbon bond cleavage of radical cations are provided. The four β-phenylethyl ethers studied are 2-methoxy-3-phenylbutane (9, both diastereomers), and cis- and trans-2-methyl-3-phenyltetrahydropyran (10c, 10t). Generally, bond cleavage will occur if the (calculated) BDE in the radical cation is less than 55 kJ/mol, and if there is significant overlap between the singly occupied molecular orbital (SOMO) and the vulnerable (C—C or C—H) bond. In the case of a β-phenylethyl ether radical cation, the alkoxy group must also be oriented so that an oxygen lone pair of electrons can overlap with the C—C sigma antibonding (σ*) orbital. The calculated BDE values of the vulnerable C—C bond in the radical cations of the four ethers studied here are well below the threshold value, 55 kJ/mol, and C—C cleavage will therefore be governed by conformational effects. Molecular mechanics (MM3) calculations were used to identify the most stable conformers of the neutral molecules. Based on the calculated angles and overlap between orbitals it was predicted that the global-minimum conformers of the ethers 9 and 10c would not give C—C bond cleavage products or deprotonation to any significant extent. The global minimum of ether 10t is well oriented for C—C cleavage but not for deprotonation. Irradiation of an electron-accepting photosensitizer, 1,4-dicyanobenzene (2), in the presence of the ethers showed that the ethers 9 did not cleave efficiently; no deprotonation or isomerization was observed. This is in good agreement with the predictions based on the MM3 calculations. Both ethers 10c and 10t gave reasonable yields of the C—C cleavage products; in fact, ether 10c cleaved more efficiently than 10t. This can be explained by the fact that a conformer of 10c, only 4.35 kJ/mol higher in energy than the global minimum, is perfectly aligned for cleavage. Ether 10t did not show any evidence for deprotonation whereas 10c did. This is also in good agreement with the calculations. Keywords: photoinduced electron transfer, radical cations, bond cleavage, conformation, molecular mechanics calculations.

Radical Ions in Photochemistry. 44. The Photo-NOCAS Reaction with Acetonitrile as the Nucleophile

“…It has been well established that alkene radical cations undergo rapid nucelophilic addition when generated in the presence of a nucleophile. − A commonly used nucleophile in this reaction is an alcohol (e.g., methanol). Several examples are provided by the extensive studies on the photochemical nucleophile−olefin combination, aromatic substitution (photo-NOCAS) reactions of a wide variety of alkenes and dienes .…”

“…It has been well established that alkene radical cations undergo rapid nucelophilic addition when generated in the presence of a nucleophile. − A commonly used nucleophile in this reaction is an alcohol (e.g., methanol). Several examples are provided by the extensive studies on the photochemical nucleophile−olefin combination, aromatic substitution (photo-NOCAS) reactions of a wide variety of alkenes and dienes . For example, irradiation of an acetonitrile−methanol (3:1) solution of isobutylene (2-methylpropene, 1 ), 1,4-dicyanobenzene ( 2 ), and biphenyl ( 3 ), serving as a codonor, leads to the formation of the two 1:1:1 (nucleophile−alkene−aromatic) substitution products (reaction [1]) 1b.…”

Catalytic Aerobic Generation of Acyliminium Ions through Electron‐Transfer‐Mediated Carbon‐Carbon Bond Activation

“…[19] Although both substrates can formally be considered to be tertiary carbamates with an adjacent ether substituent, the oxazolidine reacts smoothly whereas the acyclic analogue decomposes. We postulate that this result arises from preferential oxidation of the tertiary carbamate over the arene, as expected on the basis of an analysis of the relative oxidation potentials of these groups.…”

Diastereoselective Tetrahydropyrone Synthesis through Transition‐Metal‐Free Oxidative Carbon–Hydrogen Bond Activation

“…This useful result can be attributed to the steric interactions that limit access to radical-cation conformer 5 in which the benzylic hydrogen atom is aligned appropriately with the p system of the arene to effect efficient benzylic oxidation. [12] The scope of the method is shown in Table 1. Remarkably, although it reacts relatively slowly, a nonsubstituted benzyl ether ( Table 1, entry 1) proved to be a suitable substrate for the cyclization despite the observation that electron-donating groups are required for related bimolecular oxidative alkylation reactions.…”