Intramolecular Carbozincation of Unactivated Alkenes Occurs through a Zinc Radical Transfer Mechanism

Abstract

The cyclizations of a number of terminally unsaturated alkenyl zinc iodides to cyclopentylmethylzinc iodides, formerly believed to be nonradical in nature, have been revealed as radical chain cyclizations initiated by adventitious oxygen. Five cases are presented in which the published carbozincation cis/trans selectivities are essentially the same as those found for the cyclizations of the unsaturated alkyl iodide precursors of the alkylzinc iodides by the iodine atom transfer method at approximately the same temperatures. In addition, it has been found that one of the organozinc cyclizations does not occur in a system in which oxygen has been rigorously excluded. The combined findings strongly suggest that these organozinc cyclizations occur by a zinc radical transfer mechanism rather than by a conventional carbometallation that is thought to occur with the analogous organolithium and organomagnesium cyclizations.

Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O2

“…Benzene, toluene, pentane, diethyl ether, and tetrahydrofuran were dried and deoxygenated using an IT PureSolv system. Benzene-d 6 Caution! High-pressure glass apparatuses, reactions of oxygen and reduced compounds, and peroxide-containing materials must be handled with care.…”

“…Due to the potentially pyrophoric and explosive nature of silylalkylperoxides, 26 the BnMe 2 SiOOR (R = Et, n-C 3 H 7 , i-C 3 H 7 and t-Bu) species were not isolated; these compounds were characterized in solution and compared with the corresponding BnMe 2 SiOR using 1 H, 13 (6). To M ZnCl (0.206 g, 0.426 mmol) and PhLi (0.036 g, 0.430 mmol) were dissolved in THF (12 mL) and stirred for 8 h at ambient temperature.…”

“…Compounds 2−5 react with O 2 (1 atm) in benzene-d 6 over 3 days at room temperature, 6−8 h at 60°C, or 30 min at 120°C to form To M ZnOOR [R = Et (8), n-C 3 H 7 (9), i-C 3 H 7 (10), tBu (11)] as the only species detected (eq 1). These synthetic conditions highlight the remarkable thermal stability of 8−11 and strongly contrast the low-temperature preparation and kinetic lability typically associated with 3d transition-metal and many main-group alkylperoxides.…”

“…The postulated radical chain mechanism includes a bimolecular homolytic substitution process (S H 2) at zinc that likely involves an electron transfer from the HOMO (the Zn− C bond) to 6 ) gives 14 (8 h, 120°C). Conversion of 15 to 14 is faster than the overall rate of alkylperoxide decomposition, making that step kinetically competent for the overall conversion.…”

“…5 Oxygen initiates carbozincations, which is taken as evidence for a radical chain pathway. 6,7 However, direct kinetic support for a radical chain reaction is limited, 4 and a rate law for the reaction of alkylzinc compounds and O 2 has not even been reported.…”