Allylstannylation of Carbon−Carbon and Carbon−Oxygen Unsaturated Bonds via a Radical Chain Process¹

Abstract

In the presence of a radical initiator, allyltributylstannanes bearing an electron-withdrawing group at the beta-position smoothly reacted with electron-deficient terminal alkenes to give allylstannylated products in good yields. The stannyl group was introduced into the terminal carbon with high regioselectivity. The allylstannylation of homochiral 8-phenylmenthyl acrylate proceeded with moderate to good diastereoselectivity. Terminal and electron-deficient internal alkynes as well efficiently underwent the radical-initiated allylstannylation in an anti addition mode. The reaction of terminal alkynes showed the same regioselectivity as that of terminal alkenes. The present radical reaction was applicable to allylation of aromatic aldehydes and ketones.

Mechanism of Z-Selective Hydroalkylation of Terminal Alkynes

“…In a separate competition experiment, we also measured the intrinsic kinetic isotope effect for protodeargentation of an alkenyl silver complex and obtained KIE of 5.3 (Scheme 10c). 31 The high intrinsic KIEs associated with protodemetalation steps (KIE = ∼5) and the fact that these KIEs do not affect the overall rate of the reaction, further strengthened the conclusion that the protodemetalation steps were not turnover limiting in the catalytic reaction.…”

“…The unimolecular nature of the rearrangement makes it insensitive to the concentration of any component of the reaction other than the concentration of the borate complex, which in turn is limited by the amount of the available silver catalyst. In this scenario, the thermoneutral 1,2metalate shift is rendered irreversible by the fast protodeargentation of the product of the 1,2-metalate shift (31), consistent with the results of stoichiometric protodeargentation experiments (Scheme 9). An important implication is that the 1,2-metalate shift would also be a diastereodeterming step of the reaction.…”

Carbostannolysis Mediated by Bis(pentamethylcyclopentadienyl)lanthanide Catalysts. Utility in Accessing Organotin Synthons

“…The data in Table indicate that the kinetic product CC orientation is predominantly E , and only subsequent heating yields appreciable quantities of the less hindered Z isomers, ( Z )-8 and ( Z )-9 . In addition to photoisomerization, vinylstannanes are known to undergo isomerization by thermally or chemically induced radical pathways. , In the present study, bond connectivity is established by HSQC and HMBC NMR, and double-bond orientation is assigned by NOESY NMR.…”

“…When the cyclohexylacetylene + 1 reaction is examined in situ by 1 H NMR, the E : Z isomer ratio is 95:5 after heating for 12 h at 60 °C. However, after heating overnight at 120 °C, the E : Z ratio falls to 11:89, arguing that the catalytic cycle first produces E CC stereochemistry, which then undergoes isomerization by a known thermal/radical process (vide infra). , The reaction of 1 + cyclohexylacetylene without an organolanthanide catalyst was also investigated under identical conditions. After heating at 60 °C for 30 h, negligible reaction is observed by 1 H NMR, confirming that these processes are mediated by the organolanthanide catalyst.…”

“…11 Catalytic carbostannylation is achieved using transition-metal agents such as palladium, 5d−g,i,j,7b gold, 12 nickel, 5h or ruthenium 5b or by radical processes. 13 Frequently these catalysts are limited to alkyne substrates having electronwithdrawing substituents, and for this reason there is a need for new approaches to the carbostannylation of unfunctionalized alkynes. Additionally, in these reported transformations, the product alkene stereochemistry can be highly dependent on the nature of the substrate.…”