Persilylated Phosphoranyl Radicals: The First Persistent Noncyclic Phosphoranyl Radicals

Abstract

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.

1,4‐Addition of TMSCCl3 to Nitroalkenes: Efficient Reaction Conditions and Mechanistic Understanding

“…One explanation for the formation of 2 r is reaction of TMSCCl 3 (in the presence of TBAT) with O 2 , leading to TMS 2 O 2 and CCl 3 radicals that cascade to 2 r , via 3 and 4 . Although we could not detect any TMS 2 O 2 peroxide ( δ Si ≈−27 ppm12) in air or in O 2 ‐exposed samples of TMSCCl 3 , in the presence or absence of TBAT, the latter was rapidly converted to TMS 2 O. It is likely that any peroxide would be both generated and decomposed as shown in Scheme .…”

“…[11] we assigned this new species as TMS 2 O. One explanation for the formation of 2r is reaction of TMSCCl 3 (in the presenceo fT BAT) with O 2 ,l eading to TMS 2 O 2 and CCl 3 radicals that cascade to 2r,v ia 3 and 4.A lthough we could not detect any TMS 2 O 2 peroxide (d Si %À27 ppm [12] )i na ir or in O 2 -exposeds amples of TMSCCl 3 ,i nt he presence or absence of TBAT, the latter was rapidlyc onverted to TMS 2 O. It is likely that any peroxide would be both generated and decomposed as shown in Scheme4.T he siloxane can also be generated from TMSOH generated by elimination from 4.L iterature bicyclesrelated to 2r have been generated, either by oxidation of nitronate anions [13] or through nitrile oxide formation and Scheme3.Isolated addition products from TBAT-catalysed (5 mol %) additionso fTMSCCl 3 to nitroalkenes 1. subsequent[ 2+ 3] cycloaddition chemistry, [13] from the expected 1,4-addition product 5.A lthough we cannot completely exclude such possibilities, such approaches normally require stronger oxidants than molecular oxygen or prolonged heating at 60 8C.…”