Silver-Catalyzed Diallylation and Dibenzylation of gem-Dibromoalkanes with Grignard Reagents

Abstract

Treatment of gem-dibromoalkanes with allylic Grignard reagents in the presence of a catalytic amount of silver triflate in diethyl ether leads to diallylation to afford 1,6-heptadiene derivatives. Benzylic Grignard reagents also undergo a similar transformation to yield 1,3-diphenylpropanes. Controlled sequential treatment of gem-dibromoalkane with two different Grignard reagents results in selective displacement of the two bromine atoms with allylic and benzylic moieties.

Key words

magnesium - silver - dibromoalkanes - allylation - benzylation

References and Notes

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• 7 Typical procedure for diallylation: The reaction of 1c is representative. Silver trifluoromethanesulfonate (3.2 mg, 0.0125 mmol) was placed in a 20-mL reaction flask. Et₂O (2 mL) and substrate 1c (146 mg, 0.50 mmol) were added under argon at -10 ˚C. Allylmagnesium bromide (0.79 M in Et₂O, 1.90 mL, 1.50 mmol) was then added to the reaction mixture at -10 ˚C. After stirring for 15 h at -10 ˚C, the reaction mixture was poured into saturated ammonium chloride (20 mL). The mixture was extracted with hexane (3 × 10 mL) and the combined organic layer was dried over Na₂SO₄ and concentrated. Silica gel column purification(hexane) of the crude product provided the corresponding diallylated product 2c (83%, 89.2 mg, 0.42 mmol). 4-Methyl-4-(2-phenylethyl)-1,6-heptadiene (2c): IR (neat): 3074, 2920, 1638, 1498, 1454, 996, 913, 743, 698 cm^-¹; ^¹H NMR (CDCl₃) δ = 0.93 (s, 3 H), 1.47-1.52 (m, 2 H), 2.02-2.10 (m, 4 H), 2.56 (dt, J = 4.5, 4.5 Hz, 2 H), 5.04-5.10 (m, 4 H), 5.79-5.90 (m, 2 H), 7.14-7.23 (m, 3 H), 7.24-7.32 (m, 2 H); ^¹³C NMR (CDCl₃): δ = 24.79, 30.35, 36.23, 41.76, 44.02, 117.43, 125.80, 128.53, 128.54, 135.29, 143.53; Anal. Calcd for C₁₆H₂₂: C, 89.65; H, 10.35. Found: C, 89.51; H, 10.07
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10

Lithium dimethylcuprate reacted with gem-dihaloalkanes to yield the corresponding dimethylated compounds. However, the reactions proceeded through halogen-copper exchange, not through sequential nucleophilic methylation reactions. See reference 2.

12

Procedure for Ag-catalyzed sequential allylation/benzylation of gem- dibromoalkane: Silver trifluoromethanesulfonate (3.2 mg, 0.0125 mmol) was placed in a 20-mL reaction flask. Et₂O (2 mL) and substrate 1d (143 mg, 0.50 mmol) were added under argon at -30 ˚C. Allylmagnesium bromide (0.76 M in Et₂O, 0.99 mL, 0.75 mmol) was then added to the reaction mixture at -30 ˚C. After stirring for 1 h at -30 ˚C, benzylmagnesium bromide (1.02 M in Et₂O, 0.74 mL, 0.75 mmol) was added to the reaction mixture at -30 ˚C. After stirring for 2 h at room temperature, the reaction mixture was poured into a saturated ammonium chloride (20 mL) and extracted with hexane (3 × 10 mL). The combined organic layer was dried over Na₂SO₄ and concentrated. Silica gel column purification(hexane) of the crude product provided the corresponding product 6 (66%, 85.0 mg, 0.33 mmol).
4-Methyl-4-(phenylmethyl)-1-undecene (6): IR (neat): 3029, 2929, 2855, 1638, 1466, 1377, 911, 734, 701 cm^-¹; ^¹H NMR (CDCl₃): δ = 0.81 (s, 3 H), 0.89 (t, J = 7.0 Hz, 3 H), 1.10-1.38 (m, 12 H), 1.98 (dddt, J = 7.5, 7.0, 3.5, 1.5 Hz, 2 H), 2.49-2.56 (m, 2 H), 5.00-5.09 (m, 2 H), 5.86 (ddt, J = 8.5, 5.0, 7.5 Hz, 1 H), 7.10-7.16 (m, 2 H), 7.17-7.22 (m, 1 H), 7.23-7.28 (m, 2 H); ^¹³C NMR (CDCl₃): δ = 14.34, 22.91, 23.87, 24.79, 29.64, 30.66, 32.15, 37.09, 39.06, 43.85, 46.09, 117.24, 125.96, 127.86, 130.91, 135.78, 139.44; Anal. Calcd for C₁₉H₃₀: C, 88.30; H, 11.70. Found: C, 88.19; H, 11.95.