Tert-butyl-diphenylmethylnitroxyl radical formation with the reaction of triphenylbismuth dicrotonate and C-phenyl-N-tert-butylnitrone in benzene

Decomposition of triphenylbismuth dicrotonate in diffused light in bensene in the presence of spin trap of C-phenyl-N- tert -butylnitrone leads to formation of phenyl radicals which are registered as the adduct PhCH(Ph)N(O•)Bu-t. Triphenylbismuth dicrotonate Ph 3 Bi(O 2 CCH=CHCH 3 ) 2 in benzene solition in the presence of C-phenyl-N- tert -butylnitrone PhCH=N(O)Bu-t decomposes in light. An adduct of phenyl radical and spin trap PhCH(Ph)N(O•)Bu-t was registered by ESR method.


Introduction
Polymethylmethacrylate with organobithmuth compounds additives was known to be obtained with the radical polymerization in the presence of peroxide initiators [1,2].It was established that Bi(V) acrylates accelerated the polymerization reaction.It was suggested that the acceleration was caused by decomposition of organometallic compounds on exposure by diffused light, as the polymerization slowed in the dark.The character of the process was not studied.When various diacyl derivatives of triphenylbismuth were used, no significant change in polymerization rate or molar mass was observed, indicating the predominant role of the Ph 3 Bi(V) fragment.Besides, the previous study of the photo-induced cation polymerization of oxiranes and vinyl monomers was carried out with triaryl(1-pyrenyl)bismuth salts as initiators [3,4].When such compounds were irradiated by visible light, the homolysis of Bi-C(pyrene) bond occured with the formation of pyrenyl radical and cation radical of triarylbismuth, which subsequently initiated polymerization.It is also known that organometallic compounds of bismuth(III) can cause the controlled living radical polymerization [5,6].Recently a phenyl-tert-butylnitroxyl stable radical was fixed in the triphenylbismuth dicro-tonate reaction with 2-methyl-2-nitrosopropane [7].That result isn't clear proof of the phenyl radical formation by light discharge of organometallic compounds, because of spin trap 2-methyl-2-nitrosopropane in diffused light can be cleavaged to give initial tert-butyl radicals.So, phenyl radical could be a secondary product of homolytic substitution reaction: That is why the aim of present work was investigation of triphenylbismuth dicrotonate decomposition reaction in light in the presence of another spin trap C-phenyl-N-tert-butylnitrone which is stable for sun light.
The triphenylbismuth dicrotonate was chosen as organobismuth(V) derivative with good solubility in benzene, hydrolitical and oxidative stability on air.The structure of this compound has been studied [8].The thermal stability of this compound was investigated by thermogravimetric analysis.The experimental data were used to calculate the standard thermodynamic functions: heat capacity, enthalpy, entropy and Gibbs energy over the range from T = 0 to 400-420 K.For the compounds under study the standard entropy of formation was calculated at T = 298.15K [9].

Experimental
ESR spectra were registered on the Bruker ER200D9.5 GHz in degassed tubes.
Benzene, Et 2 O and THF were dried over anhydrous calcium chloride, then distilled and kept over sodium wire.Chloroform was dried over anhydrous calcium chloride and distilled.Hexane was used without previous purification.Anhydrous BiCl 3 was purified by sublimation (350 °C, 0.5 torr).
Triphenylbismuth was synthesized according to conventional procedure [10] from BiCl 3 and PhMgBr with the use of the benzene and THF (1:2) mixture as the solvent.
The synthesis of triphenylbismuth dicrotonate was carried out according to conventional procedure [11] by the oxidative addition reaction at room tempera-ture in Et 2 O from triphenylbismuth, crotonic acid and tert-butylhydroperoxide (reagent ratio 1:2:1).The product was purified by recrystallization from the medium hexane -chloroform (4:1).The yield of the purified reaction product was 73 %, melting point 153 °C.
Triphenylbismuth dicrotonate decomposition in diffused light was prepared as follows: in one elbow of tube a sample of PBN (0.2 mmol) was placed, while the other elbow was filled by the benzene solution of triphenylbismuth dicrotonate (1 mL, 0.2 mol/L concentration).The ampoule was degassed, sealed and after mixing the ESR spectrum was registered.

Results and Discussion
We have investigated the decomposition of triphenylbismuth dicrotonate in benzene in the presence of PBN PhCH=N(O)Bu-t by ESR method in a sealed and degassed ampoule in diffuse light.The choice of the solvent is due to its low activity in radical reactions at the chosen conditions.
We recorded ESR spectrum of the mixture and found an adduct of spin trap PhCH(Ph)N(O•)Bu-t with phenyl radical, formed by decomposition reaction: The ESR spectrum of triphenylbismuth dicrotonate in benzene (with PBN as the spin trap) is show in Fig. 1.
For the adduct PhCH(Ph)N(O•)Bu-t the values of hyperfine interaction constants have been determined: a N = 14.4 Oe, a H = 2.2 Oe, g = 2.0060, which are in agreement with the literature data for the addition compound generated in benzene by photolysis of Ph 3 Bi (a N = 14.5 Oe, a Н = 2.3 Oe), Ph 3 Sb (a N = 13.2Oe, a Н = 2.2 Oe), Ph 3 As (a N = 13.4Oe, a Н = 2.2 Oe) [13].

Fig. 1 .
Fig. 1.The ESR spectrum of triphenylbismuth dicrotonate in benzene, the spin trap is PBN