Generation of a Geminal Dicarbenoid of Chromium: Formation of Allenes from Terminal Alkenes in One-Pot

Abstract

Treatment of CCl₄ with 4 equiv of CrCl₂ in THF generates a geminal dicarbenoid of chromium by reduction of two of the chlorine atoms of CCl₄, and the dicarbenoid species selectively reacts with terminal olefins to give cyclopropylidene carbenoids, which readily decompose to allenes.

Key words

chromium(II) - carbenoids - allenes - cyclopropanes

References

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8

Treatment of 1-dodecene with CBr₄ or CI₄ instead of CCl₄ at 0 °C for 24 h gave less than 5% of the corresponding allene, and most of the alkene was recovered.

10

When 5 equiv. of the alkenes 3, 5, and 7 were treated with 1 equiv of CCl₄ and 4 equiv of CrCl₂, the yields of allenes based on CCl₄ were 75%, 68%, and 70%, respectively.

14

3-Phenylpropyl trans -2-Chlorocyclopropanecarboxylate (21, Equation 5): To a suspension of CrCl₂ (0.98 g, 8.0 mmol) in THF (14 mL) was added a solution of 3-phenylpropyl acrylate (20, 0.19 g, 1.0 mmol) in THF (2 mL) at 0 °C. Carbon tetrachloride (0.19 mL, 2.0 mmol) was added to the suspension at 0 °C over several minutes, and the resulting mixture was stirred at 0 °C for 24 h. The mixture was poured into a saturated NaF solution (10 mL) and stirred at 25 °C for 2.5 h. This mixture was extracted with Et₂O (3 × 10 mL), and the organic extracts were dried over anhydrous MgSO₄ and concentrated. Purification by column chromatography on silica gel (hexane-EtOAc, 100:1) gave the carboxylate 21 (0.14 g, 0.59 mmol) in 59% yield as a colorless oil: IR(neat): 3064, 3027, 2957, 2926, 2858, 1728, 1454, 1399, 1371, 1268, 1209, 1176, 747, 700, 666 cm^-1; ¹H NMR (CDCl₃): δ 1.32-1.37 (m, 1 H), 1.51-1.59 (m, 1 H), 1.94-2.03 (m, 3 H), 2.70 (t, J = 7.2 Hz, 2 H), 3.35-3.38 (m, 1 H), 4.11 (t, J = 6.4 Hz, 2 H), 7.18-7.33 (m, 5 H); ¹³C NMR (CDCl₃): δ 18.4, 23.7, 30.0, 32.1, 33.3, 64.5, 126.0, 128.4, 128.5, 141.0, 171.5; MS m/z (%): 238 (M⁺, 0.5), 118(100), 117(56), 91(31), 77(7). HRMS m/z calcd for C₁₃H₁₅ClO₂ (M⁺): 238.0761, found 238.0750.

16

Because chromium carbenoids shows both electrophilic [3a] and nucleophilic characters, [5a] the reactivity of the species generated by the reduction of CCl₄ with CrCl₂ was examined. When the reduction of carbon tetrachloride with chromium(II) (6 equiv) in THF was conducted in the presence of 3-phenylpropanal (4 equiv), (Z)-2-chloroallylic alcohol 24 was obtained in 56% yield based on CCl₄, stereoselectively (Equation 6). The product 24 was the 1:2 adduct of CCl₄ and the aldehyde, and this new species acts as a chloromethane trianion equivalent. [18]

Equation 6

19

Treatment of the allene 4 with the CCl₄-CrCl₂ reagent resulted in 91% recovery under the same reaction conditions as in Equation 2.