Synthesis of Allylic Alcohols

The carbene complex 2 (X=O) reacts with triphenylphosphane - just like its linearly tethered analogue 1 (X=O) (see path B)- with concomitant loss of carbon monoxide and methanol to give the substituted trimethylenemethane complex 29.

With carbon nucleophiles like lithium enolates, complex 2 reacts to give rather unstable intermediates which we have not been able to isolate and characterize so far and only tentatively ascribe the structure 27. When immediately treated with H₂O₂ / NaOH, these intermediates are oxidized to leave the corresponding allylic alcohols 28 in good yields. As the starting complex 2 itself derives from isobutene diol, this sequence makes for a convenient 3-step procedure for the monosubstitution of one hydroxy group by a beta-oxo-alkyl residue ^[3].

Experimental data

Tricarbonyl[eta⁴-(triphenylphosphonio)trimethylenemethane]iron(0) tetrafluoro borat (29): A solution of carbene complex 2 (3.40 g; 10.0 mmol) in CH₃CN (50 ml) was treated with triphenylphosphane (2.62 g; 10.0 mmol) at room temperature and then stirred for 12 h. All volatile components were removed in vacuo and the residue thus obtained was purified by CC over silica gel. Pure 29 could be eluted with CH₃CN / CH₂Cl₂ 1:1, giving yellow crystals upon evaporation of the eluate; m.p. 178°C (decomp.); yield 4.44 g (8.2 mmol; 82%). Ð IR(KBr): 3075, 2970, 2060, 1990, 1975, 1430, 1045 cm^-1. - ¹H NMR (CD₃CN, 400 MHz): 1.83 [dd, ⁴J (3-H^b/4-H^a) = 4.3, ²J (4-H^a/4-H^b) = 1.22 Hz, 1 H, 4-Ha], 2.31 [dd, ²J (4-H^a/4-H^b) = 1.22, ⁴J (1-H/4-H^b) = 2.44 Hz, 1 H, 4-H^b], 2.59 [d, ⁴J (3-H^a/P) = 10.7 Hz, 1 H, 3-H^a], 2.87 [d, ⁴J (3-H^b/4-H^a) = 4.3 Hz, 1 H, 3-H^b], 3.21 [dd, ²J (1-H/P) = 5.8, ⁴J (1-H/4-H^b) = 2.44 Hz, 1 H, 1-H], 7.72-7.88 (m, 15H, H^ar).Ê- ¹³C NMRÊ(CD₃CN, 100.5 MHz): 47.60 [d, ¹J (P/C-1) = 74.80 Hz, C-1], 57.22 (C-4), 60.24 [d, ³J (P/C-3) = 18.30 Hz, C-3], 110.68 (C-2), 121.40 [d, ¹J (P/C^ipso) = 88.50 Hz, C^ipso], 131.20 [d, ²J (P/C^ortho) = 12.20 Hz, C^ortho], 135.00 [d, ³J (P/C^meta) = 10.7 Hz, C^meta], 136.20 [d, ⁴J (P/C^para) = 3.0 Hz, C^para], 208.55 and 208.75 and 209.9 (CO). - ³¹P NMR (CD₃CN, 162 MHz, H₃PO₄ extern): 22.2. - MS (70 eV); m/z (%): 317 (63) [M⁺ - Fe(CO)₃], 262 (100) [Ph₃P⁺], 181 (78), 124 (58), 44 (69), 28 (36). Ð C₂₅H₂₀BF₄FeO₅P (542.1): calcd. C 55.39, H 3.72; found C 55.81, H 3.90. - Crystal structure: Clear, bright yellow single crystals were obtained by slowly cooling a solution of 29 in dichloromethane/diethyl ether 1:1 to 0°C; formula C₂₅H₂₀BClF₄FeO₃P, molar mass 584.00 g mol^-1 (including CH₂Cl₂), crystal size 0.30 x 0.20 x 0.10 mm, a = 32.781(11), b = 7.928(2), c = 20.434(6) Å, a = 90°, b = 92.08(2)°, g = 90°, V = 5307(3) ų, T = 293(2) K, d_calc= 1.462 g cm^-3, Z = 8, monoclinic, space group C2/c.

6. Synthesis of carbamates 28 (General Procedure): Solutions of the required lithium enolates in THF were prepared from 1.1 mmol of the carbonyl compound and then added via a cannula into a slurry of the carbene complex 2 (340 mg; 1.0 mmol) in THF (5 ml) kept at -78°C. The entire mixture was stirred for 2 h at -78°C, then allowed to warm to room temperature and all volatile components were finally removed in vacuo. The resulting residue was quickly extracted with diethyl ether, the extracts were concentrated on an oil pump and then swiftly chromatographed over a short plug of silica (diethyl ether/petrol ether 1:2). The crude products 27 thus obtained were weighed and immediately redissolved in methanol (20 ml) and chilled with ice. H₂O₂ (6 ml of a 30% aqueous solution; 6.0 mmol) was added, whereupon the solutions turned brownish. Then solid sodium hydroxide (240 mg; 6.0 mmol) was added within 2 h and stirring was continued for another 2 h. Finally, the mixtures were extracted thrice with diethyl ether (100 ml) and the combined extracts were washed with saturated aqueous NH₄Cl solution and dried over MgSO₄. After removal of the solvent on a rotary evaporator, the crude products were purified by CC (silica gel; diethyl ether/petrol ether 1:2).
6-Methylene-3-oxo-2,4,4-trimethyl-heptan-7-ol (28a):
IR(neat):3400, 3060, 2940, 2910, 2850, 1690, 1460, 1370, 1250, 1080 cm^-1. - ¹H NMR (CDCl₃, 400 MHz): 1.06 [d, ³J (2-H/CH₃) = 6.60 Hz, 6 H, 1-H, 2-CH₃], 1.18 (s, 6 H, 4-CH₃), 2.35 (s, 2 H, 5-H), 2.79 (s, 1 H, OH), 3.13 [sep, ³J (2-H/CH₃) = 6.60 Hz, 1 H, 2-H], 3.98 (s, 2 H, 7-H), 4.81 and 5.09 (s each, 2 H, =CH₂).-¹³C NMR (CDCl₃, 100.5 MHz): 20.15 (C-1 and 2-CH₃), 24.88 (4-CH₃), 34.55 (C-2), 41.22 (C-5), 47.92 (C-4), 66.14 (C-7), 114.06 (=CH₂), 145.85 (C-6), 219.13 (C-3). Ð MS (70 eV); m/z (%): 184 (3) [M⁺], 141 (12) [M⁺ Ð ⁱPr], 111 (22), 95 (94), 71 (62), 43 (100) [ⁱPr⁺].Ê- C₁₂H₂₀O₂ (184.3): calcd. C 71.70, H 10.94; found C 71.63, H 10.87.