Tri-2-furanylgermane in the presence of a catalytic amount of Et(3)B adds to internal alkenes as well as terminal alkenes effectively to give the corresponding adducts in good to excellent yields. The addition of tri-2-furanylgermane to silyl enol ethers followed by elimination of germyl and siloxy moieties provides a new route for the conversion of ketones into alkenes.
Treatment of (2,3-epoxy-4-pentenyloxy)trialkylsilane with radical precursors such as triphenylgermane and a-halo carbonyl compounds in the presence of Et 3 B yields a,b-unsaturated aldehydes. The reaction involves b-scission of a secondary alkoxy radical that releases a siloxymethyl radical.
Radical addition of triphenylgermane to vinyloxiranes proceeded in the presence of triethylborane to yield 4-triphenylgermyl-2-buten-1-ol derivatives in good yields. Iodo acetals, prepared by iodoetherification of vinyl ethers with the obtained allylic alcohol, underwent radical cyclization to give 2-alkoxy-4-vinyltetrahydrofurans, which were converted into 4-vinyltetrahydro-2-furanones by Jones oxidation. The cyclization involved fragmentation of b-triphenylgermyl radical.Vinyloxiranes are useful synthetic intermediates that undergo many transformations including ring opening and rearrangement. 1 Radical addition to vinyloxirane also attracts much attention. 2 We have been interested in radical addition of germanium hydride reagents to alkenes and alkynes. 3 Here we wish to report reaction of triphenylgermane with vinyloxiranes. The utility of the adducts, 4-triphenylgermyl-2-buten-1-ol derivatives, was also examined, and synthesis of vinyl-substituted cyclic compounds was established. This synthesis involves b-germyl radical fragmentation, which is more difficult compared to the case of b-stannyl radical. 4In our initial experiment, 3,4-epoxy-1-dodecene (0.50 mmol) 5 was treated with a slight excess of triphenylgermane (0.55 mmol) in hexane (3 mL) in the presence of triethylborane (0.60 mmol) 6 at 25 °C for 4 h. Concentration followed by silica gel column purification provided (E)-1-triphenylgermyl-2-dodecen-4-ol (2a) in 63% yield (Table 1, entry 1). Reaction conditions were further surveyed to improve the yield. The use of two equimolar amount of vinyloxirane to triphenylgermane was found to give 2a in 96% yield based on triphenylgermane (entry 4). 7 Reaction with a catalytic amount of triethylborane (10 mol%) resulted in poor conversion (entry 6).Triphenylgermane added to various vinyloxiranes as shown in Table 2. Addition to butadiene monoxide gave rise to a mixture of (E)-and (Z)-4-triphenylgermyl-2-buten-1-ol (2b, E/Z = 79/21) in 84% yield. In contrast to ionic ring opening reaction, the reaction of vinyloxirane 1d proceeded without affecting the carbonyl group to yield (E)-allylic keto alcohol 2d. 8 A reasonable reaction mechanism for the chain reaction is depicted in Scheme 1. Addition of germyl radical to vinyloxirane gives 3, which is transformed into 4-Z or 4-E. These rearrangements are reversible and would be quite rapid. 9 4-E would be more stable than 4-Z. Additionally, oxygen-centered radical of 4-E is likely to be trapped by germanium hydride more easily than 4-Z because of steric hindrance around the oxygen atom. Therefore, 4-E would mainly abstract hydrogen from triphenylgermane to give 2 with concomitant reproduction of germyl radical. 10
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