Dehydrohalogenation of haloalkylarenes in the synthesis of alkenylaromatic hydrocarbons

Abstract

The alkylation of aromatic hydrocarbons with haloalkenes in the presence of acid catalysts makes it possible to obtain haloalkylaromatic compounds with a halogen atom in a strictly defi ned position in the side chain [1][2][3]. The products of this reaction are promising raw material for the preparation of diverse diffi cultly available organic substances. In this connection in extension of study [4] we investigated the dehydrohalogenation of haloalkylarenes with KOH solution in ethylene glycol in order to evaluate the opportunity of preparation of alkenylaromatic hydrocarbons of a defi nite structure.The dehydrohalogenation of [2-chloro(bromo)propan-1-yl]benzene afforded allylbenzene, (1Z)-and (1Е)-prop-1-en-1-ylbenzenes in the ratio 1 : 6 : 93.The dehydrochlorination of (1-chloropropan-2-yl) benzene led to the formation of a chromatographically pure prop-1-en-2-ylbenzene, and at the dehydrohalogenation of (1-bromo-and 1-iodopropan-2-yl)benzenes alongside the prop-1-en-2-ylbenzene the (1Е)-prop-1-en-1-ylbenzene was obtained (Scheme 1).Expectedly the (2-methyl-1-chloropropan-2-yl) benzene did not suffer the dehydrochlorination. Unexpectedly dehydroiodination also failed with the 1-(1-iodopropan-2-yl)naphthalene. We succeeded to obtain the 1-(1-prop-1-en-2-yl)naphthalene from the 1-(1-iodopropan-2-yl)naphthalene only by pyrolysis of the 1-(1-acetoxypropan-2-yl)naphthalene that was the product of iodine substitution by an acetoxy group (Scheme 2). The dehydrohalogenation of 1,4-dimethyl-2-[2-chloro(bromo)propan-1-yl]benzenes furnished the 1,4-dimethyl-2-[(1Е)-prop-1-en-1-yl]benzene in a nearly quantitative yield.For the dehydrohalogenation of the other haloalkylarenes we used the mixtures of the alkylation products of methyl-, ethyl-, and isopropylbenzenes with allyl chloride and bromide in the presence of Lewis acids. These products were mixtures of o-, m-, and p-(2-halopropan-1-yl) derivatives of alkylarenes.Dehydrohalogenation procedure. To a solution of 0.12 mol of KОН in 25 ml of di(ethylene glycol) was added 0.1 mol of haloalkylarene, and the mixture was boiled for 1.5 h. The reaction products were extracted into benzene. The extract was washed with water, benzene was distilled off, the residue was subjected to rectifi cation.From (1-chloropropan-2-yl)benzene we obtained 11.4 g (97%) of a mixture of bp 193-196°С, d 4 20 0.9068, n D 20 1.5498. (1Е)-Prop-1-en-1-ylbenzene was isolated of bp 73.5°С (20 mm Hg), d 4 20 0.9083, n D 20 1.5470. МR D 41.20, calc. 39.69. IR spectrum, ν, cm -1 : 965 s (trans-С=С), 770, 730 s (monosubstitution in benzene). The physicochemical constants were consistent with the published data [5].The dehydrohalogenation of the mixture of (2-chloropropan-1-yl)toluenes gave 12.8 g (97%) of a mixture of 1-methyl-2-[(1Е)-prop-1-en-1-yl]benzene (43-47%) and 1-methyl-3-and 1-methyl-4-[(1Е)-prop-1-

Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1′-Lithio-1′-(2,6-dimethylphenyl)propenes

“…A sample (183 mg) distilled at 70–88 °C (bath temp. )/13 Torr (ref : 75 °C/4 Torr) gave a colorless mixture (103 mg) of ( Z )- 7 and ( E )- 7 (63:37). 1 H NMR of ( Z )- 7 (CDCl 3 , 400 MHz) δ 1.45 (dd, 3 J = 6.8 Hz, 4 J = 1.7 Hz, 3H, 2′-CH 3 cis to aryl), 2.19 (s, 6H, 2-/6-CH 3 ), 5.83 (dq, 3 J = 11.3 Hz, 3 J = 6.8 Hz, 1H, 2′-H), 6.30 (dm, 3 J = 11.3 Hz, 1H, 1′-H), 7.02 and 7.06 (A 2 B system, 3 J = 7.4 Hz, 2 + 1H, 3-/5-H and 4-H) ppm (the literature values are too large by ca.…”