eq. 3). Allylation with ethyl 2-((phenylsulfonyl)methyl)acrylate8 was also investigated. Despite the use of 1.6 equivalents of the trap and initiation by di-tert-butyl hyponitrite at 45 °C, the yield for the formation of allylated compound 10b remained modest (Scheme 7, eq. 4). Other radical traps such as phenyl phenylethynyl sulfone23 and cyclohexenenone26 were also tested but did not provide the desired products.
6bPh6bPh1) 2 (1.3 eq), HSiEt3 (2.6 eq) benzene, rt, 2 h1) 2 (1.3 eq), HSiEt3 (2.6 eq) benzene, rt, 2 hPhCl9b (dr 4:5, 94%) 10b (dr 4:5, 38%) PhCO2Et(3) (4) 2) PhSO2Cl (1.1 eq) rt, 18 h2) PhSO2R (1.6 eq) t-BuON=NOt-Bu 45 °C, 36 hR = CH2C(CO2Et)=CH2
Scheme 7. Hydroboration-radical chlorination and allylation of alkenes.
Reproducibility of the results. The reactions reported here were shown to be difficult to reproduce. All reactions were repeated several times and significant variations of conversion and yield were observed. All our efforts to identify the factors responsible for these variations were inconclusive. However, experimental observations indicate that the problem is related to the formation of the 9-alkyl-9-borafluorene and not to the radical reaction. Indeed, oxidative treatment of the organoborane 7b obtained by hydroboration of 6b with freshly distilled 9- chloro-9-borafluorene 3 was investigated (Scheme 8). The alcohol 11b was obtained in only 45% yield together with 42% of starting alkene 6b. The possibility of a reversible hydroboration step was investigated but could not be clearly demonstrated. The same batch of 7b afforded the sulfide 8b in 30–45% yield upon reaction with PhSO2SPh (the reaction was repeated 3–4 times).
