シクロペンタジエンとブタジエンのDiels-Alder反応から得られる三量体とその13GNMRスペクトル

中井 成人, 岩瀬 省治, 石井 康敬, 小川 雅弥

doi:10.4011/shikizai1937.51.132

Diels-Alder reactions between cyclopentadiene (CP) and butadiene (BD) give a variety of dimers having known structures such as 4-vinyl-cyclohexene (VCH), 5-vinyl-2-norbornene (VNB), cis-3a, 4, 7, 7a-tetrahydroindene (THI) and dicyclopentadiene (DCP) and many unknown trimers. In the present investigation, reactions of each dimer with BD or CP were carried out, and the structures of the trimers formed were determined by means of ¹³C-NMR spectrometry. VCH was allowed to react with BD at 230°C for 3hr in an autoclave. The BD combined with the double bond of the vinyl group to yield two stereoisomers (I-A) and (I-B) while practically no addition products of BD and cyclohexene ring were formed. Conformational equilibria for compounds (I-A) and (I-B) are shown in Fig. 1, and these compounds may be the most stable in the conformation with an equatorial-cyclohexene ring with each other at C₁ and C₁' carbons. The hydrogenation of these isomers gave the same product (II). Reactions of VCH with CP were carried out at 200°C for 3hr, and a 15.8% yield of trimers were obtained. These products were adducts (III-A) and (III-B) which CP reacted with the double bond of the vinyl group, whereas the adducts of CP to the cyclohexene ring could not be isolated due to much lower yields. Isomers (III-A) and (III-B) were also formed by Deils-Alder reaction between VNB and BD, and the hydrogenation of these isomers yielded the same product (IV). The endo-structures of compounds (III-A) and (III-B) were assigned by comparing the chemical shifts of the carbon atoms in the C-6 and C-7 positions of endo- and exo-VNB, respectively (Fig. 2). Compound (V) was formed by the reaction of VNB with CP at 170°C for 3hr. The assigned endo-structure of compound (V) was due to the observed ¹³C-NMR spectral data of high field shift at the C-6 position by steric compression (Fig. 3). In the reaction of VNB with BD or CP, the addition of the diene occurred more readily of the double bond of norbornene than of the vinyl group, and formed only the endo-adducts, (III-A), (III-B), and (V). In the reaction of THI with BD, the adduct (VIII) of BD to the cyclopentene ring and two additional adducts, (VIII-A) and (VIII-B), to the cyclohexene ring were formed as major and minor products, respectively (Fig. 4). The addition of CP to THI was similar to that of BD to THI (Fig. 5), that is, the adduct (IX) of CP to the cyclopentene ring and two adducts, (X-A) and (X-B), to the cyclohexene ring. The ¹³C-chemical shifts for the compounds, (VIII-A) and (VIII-B), could not be assigned owing to the difficulty in separating these isomers. Compounds, (XI-A) and (XI-B), were identified as the trimers of CP from the results of ¹³C-NMR data reported previously (Fig. 6). In conclusion, the stereochemistry of adducts can be accounted for by the reactivity of a double bond and the steric hindrance involved in the addition.

シクロペンタジエンとブタジエンのDiels-Alder反応から得られる三量体とその<SUP>13</SUP>GNMRスペクトル

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