Biomimetic rearrangement of the 2-cyclopentyl-2-propyl cation

Alkylation of Alkenes: Ethylaluminum Sesquichloride-Mediated Hydro-Alkyl Additions with Alkyl Chloroformates and Di-tert-butylpyrocarbonate

“…Indeed, reaction of oleic acid (2) under standard reaction conditions with chloroformate 12e afforded the regioisomeric mixture of 9-and 10sec-butyloctadecanoic acid (32,33) in 74% yield. Analogously, reaction of alkene 9 with chloroformate 12e in the presence of equimolar amounts of triethylsilane showed the addition of the 2-butyl cation, yielding 72% of 12-methyltetradecanoic acid (34). Cyclohexene (5) yielded only 30% of sec-butylcyclohexane (35).…”

“…A 1,2-H shift followed by a concerted 1,2-CH 3 shift and a ring enlargement gives the most stable endocyclic tert-cyclohexyl cation and, finally, by hydride transfer, product 55. Ring enlargement of 1-ethylcyclopentyl to 1-methylcyclohexyl cation 33 and of 2-(2-cyclopentyl)propyl cation to 1,2-dimethylcyclohexyl cation 34 is well known and studied by quantum mechanical calculations. 35 The respective addition of tert-pentyl cation by reaction with neopentyl chloroformate (12i) analogously gave the isomeric cyclohexane derivatives 50 and 51 because of a competitive 1,2-shift of methyl and ethyl in the adduct cation (Table 3, entry 6).…”

“…The reaction of oleic acid (2) (1.42 g, 4.2 mmol) and 1-butyl chloroformate (12e) (0.69 g, 5 mmol) was carried out as described (procedure 1) and gave, after purification of the crude product by column chromatography, 1.06 g (74%) of a mixture of 32 and 33 (≈1:1) as a colorless oil; n D 20 ) 1.4580; 1 H NMR (300.1 MHz) δ ) 0.78 (d, J ) 6.7 Hz, 3H, CH3CH), 0.88 (2× t, J ) 6.7 Hz, 6H, CH3CH2CH and 18-H), 1.15-1.45 (m, 30H), 1.63 (tt, J ) 7.4, 7.1 Hz, 2H, 3-H), 2.35 (t, J ) 7.5 Hz, 2H, 2-H); 13 C NMR (75.8 MHz) δ ) 12.3 (CH3CH2CH), 14.0 (C18) 15.0 (CH3CH), 34.0 (C2), 36.3 (CH3CH), 42.0 (C9(10)), 180.0 (C1); MS (EI), m/z (%) 340(4) [M + ], 311(5), 283(100), 265(37); C22H44O2 (340.59) calcd, C 77.58, H 13.02; found, C 77.52, H 12.98. 15. 12-Methyltetradecanoic Acid (34). The reaction of 10-undecenoic acid (9) (0.92 g, 5 mmol) and 1-butyl chloroformate (12e) (0.68 g, 5 mmol) was carried out as described (procedure 2) and gave, after purification of the crude product by column chromatography, 0.87 g (72%) of 34 as a colorless liquid; n D 20 ) 1.4528; 13 C NMR (125.8 MHz) δ ) 11.38 (C14), 19.20 (CH3CH), 31.97 (C13), 34.13 (C12, C2), 36.64 (C11), 180.52 (C1); GC/MS (EI) of the methyl ester of 34, m/z (%) 256 (17) [M + ], 227(5), 225(5), 199(10), 74(100).…”

Rearrangement Pathways of Five-Membered Ring Enlargement in Carbocations: Quantum Chemical Calculations and Deuterium Kinetic Isotope Effects

“…We showed in our preliminary investigation that at about −100 °C a ring-enlargement process and formation of the 1,2-dimethyl-1-cyclohexyl cation ( 2 ) occurred . The rearranged cation 2 undergoes a rapid degenerate hydride shift over a low barrier (Δ G ⧧ = 3.7 kcal/mol at −136 °C) .…”