Persilylated phosphoranyl radicals: the first persistent noncyclic phosphoranyl radicals

Persistent noncyclic phosphoranyl radicals have been prepared and observed by electron paramagnetic resonance (EPR) for the first time. They were obtained by UV-photolysis of a solution containing a bis(trialkylsilyl) peroxide (R = Me, Et) and a tris(trialkylsilyl) phosphite (R = Me, Et, iPr). EPR parameters (a(P) approximately 100 mT) are typical of phosphoranyl radicals exhibiting a trigonal-bipyramidal structure, with the odd electron in an equatorial site. Analysis of the pseudo-first-order decay shows that these phosphoranyl radicals decay by S(H)2 homolytic substitution on the bis(trialkylsilyl) peroxide and by loss of a trialkylsilyloxyl radical (alpha-scission reaction). Both the S(H)2 and alpha-scission reactions depend on the steric bulk of the alkyl groups, that is, the bulkier the alkyl group, the slower the S(H)2 and alpha-scission reactions.     

The structure of phosphino and phosphoranyl radicals

Equilibrium geometries and isotropic nuclear hyperfine coupling constants have been calculated for the radicals PX₂ and PX₄ (X = H, F, Cl) using the INDO approximations; PH₂ and PF₂ have also been investigated by ab initio methods.     

31P CIDNP in the reactions of phosphoranyl radicals

The polarisation of phosphorus nuclei (³¹P CIDNP) during the free radical reactions of triakyl and triaryl phosphites proceeding via intermediate phosphoranyl radicals is discussed.     

Effects of substituents on the stability of phosphoranyl radicals

The effect of substituents on the geometries, apicophilicities, radical stabilization energies, and bond dissociation energies of (*)P(CH(3))(3)X (X = CH(3), SCH(3), OCH(3), OH, CN, CF(3), Ph) were studied via high-level ab initio molecular orbital calculations. Two alternative definitions for the radical stabilization energy (RSE) were considered: the standard RSE, in which radical stability is measured relative to H-P(CH(3))(3)X, and a new definition, the alpha-RSE, which measures stability relative to P(CH(3))(2)X. We show that these alternative definitions yield almost diametrically opposed trends; we argue that alpha-RSE provides a reasonable qualitative measure of relative radical stability, while the standard RSE qualitatively reflects the relative strength of the P-H bonds in the corresponding H-P(CH(3))(3)X phosphines. The (*)P(CH(3))(3)X radicals assume a trigonal-bipyramidal structure, with the X-group occupying an axial position, and the unpaired electron distributed between a 3p(sigma)-type orbital (that occupies the position of the "fifth ligand"), and the sigma orbitals of the axial bonds. Consistent with this picture, the radical is stabilized by resonance (along the axial bonds) with configurations such as X(-) P(*+)(CH(3))(3) and X(*) P(CH(3))(3). As a result, substituents that are strong sigma-acceptors (such as F, OH, or OCH(3)) or have weak P-X bonds (such as SCH(3)) stabilize these configurations, resulting in the largest apicophilicities and alpha-RSEs. Unsaturated pi-acceptor substituents (such as phenyl or CN) are weakly stabilizing and interact with the 3p(sigma)-type orbital via a through-space effect. As part of this work, we challenge the notion that phosphorus-centered radicals are more stable than carbon-centered radicals.     

Study of destruction rate of some phosphoranyl radicals

Conclusions The difference in the reactivity of the trialkoxy-tert-butoxy- and dialkyl-di-tertbutoxyphosphoranyl radicals were characterized quantitatively.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2148–2150, September, 1977.     

The structure and hyperfine splittings of simple phosphoranyl radicals

The ab initio UHF method has been employed to calculate equilibrium geometries and isotopic hyperfine coupling constants for the radicals PH₂, PF₂, PH₄ and PF₄.     

Highly stereoselective and easy synthesis of enantiopure phosphoranyl oxiranes

The highly stereoselective synthesis of enantiopure phosphoranyl alkyl oxiranes has been accomplished by the addition of a nucleophilic stable phosphino(silyl)carbene to chiral aldehydes prepared from (−)-verbenone and d-mannitol, respectively. In the process, two new stereogenic centers are created one of them being on an oxirane quaternary carbon. The excellent π-facial diastereoselection of the aldehydes combined with the diastereoselectivity of the carbene addition allowed us to synthesize the title compounds as single stereoisomers with well defined absolute configuration.     

Novel phosphoranyl radicals Ph3·PS− and PH3·PSR

ESR spectra obtained from γ-irradiated triphenylphosphine sulphide in CD₃OD are shown to be in agreement with novel tetrahedral phosphoranyl radicals (Ph₃·PS^? and Ph₃·PSR).     

Electron spin resonance of a stereochemically non-rigid penta-coordinate phosphoranyl radical

Evidence based on temperature dependent ESR line shape changes is presented for the mobility of the ligands in the phosphorus-centered radical (CH ₃)₃COPH₃ resulting by addition of a tert-butoxy radical to phosphine. CNDO calculations favor for the slow exchange limit a trigonal bipyramidal structure with an axial oxygen atom and with the unpaired electron in an equatorial position. A density matrix analysis gives an activation energy of 5.3 kcal/mole for the exchange process which permutes the hydrogen atoms.     

Proof for the existence of an early endo transition state in the Diels-Adler reactions of 2-substituted 1,3-butadienes

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1196–1197, May, 1988.     

Stereoselective synthesis of phosphoranyl aryloxiranes through the addition of a nucleophilic stable carbene to aromatic aldehydes

The [2+1] addition of the stable (phosphanyl)(silyl)carbene 1 to aromatic aldehydes affords phosphoranyl aryloxiranes, a new class of polyfunctional epoxides, in high yields and excellent diastereoselectivity. No reaction is observed for aldehydes bearing strongly electron-donating groups. Theoretical calculations show a good correlation between Gibbs activation energy and the electronic nature of the substituent on the phenyl ring.     

Activation barriers in the homolytic cleavage of radicals and ion radicals

As revealed by several experimental examples, radicals and ion radicals may, in contrast with closed-shell molecules, undergo exothermic homolytic cleavages (.A..B --> A: +.B) with substantial activation barriers. A two-state semiclassical model is proposed for explaining the existence of the barrier and estimating its magnitude. It is based on the intersection of the potential energy surfaces characterizing the dissociation of a bonding state, .A..B -->.A. +.B, on one hand, and the approach to bonding distance of a repulsive state, A: +.B --> A therefore B, on the other. After inclusion of the bond cleavage and formation as Morse curves in the normal-mode analysis, a simple activation driving force relationship is obtained, the two main ingredients of the intrinsic barrier being the triplet excitation energy of the A moiety and the pi*--> sigma* excitation energy in .A-B. The model is then tested by quantum chemical calculations, first on a simplified system to evaluate the calculation techniques and then on a real system. A comparison of the model predictions with experiment is finally performed using the rate data recently gathered for the cleavage of 4-cyanophenyl alkyl ether anion radicals, which cover a respectable range of driving forces, showing satisfactory agreement between theoretical predictions and experimental data.     

Free radicals in the perimidine series. 2-Tert-butylperimidyl radicals

Electrochemical and EPR methods have been applied to a study of perimidyl free radicals containing a bulky tert-butyl substituent in the 2-position. A stepwise mechanism is proposed for the dehydrogenation of 1,3-dimethyl-2-tert-butyl-2,3-dihydroperimidine; the mechanism involves formation of a cation-radical from the substrate in the first step. Shielding of the nitrogen atoms seems to be an important factor in increasing the stability of free perimidyl radicals.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 970–972, July, 1986.