Ando Takashi

Fujita Mitsue

Kimura Takahide

宗宮 創

Departmental Bulletin Paperdepartmental bulletin pape

Kimura, Takahide

Fujita, Mitsue

宗宮, 創

Ando, Takashi

Shiga University of Medical Science Repository BI WAKO / 滋賀医科大学機関リポジトリ

Isomerization mechanism of α-phenethylthiocyanate in the presence of a supportedreagent著者 Kimura Takahide, Fujita Mitsue, SohmiyaHajime, Ando Takashijournal orpublication title滋賀医科大学基礎学研究volume 6page range 1-5year 1995-03その他の言語のタイトル担持試薬存在下におけるフェネチルチオシアナートの異性化機構タンジ シヤク ソンザイカ ニ オケル フェネチル チオシアナート ノ イセイカ キコウURL http://hdl.handle.net/10422/1215Bulletin of Shiga University of Medical Science (General Education)6: 1-5 (1995Isomerization Mechanism of α-Phenethyl Thiocyanatein the Presence of a Supported ReagentTakahide Kimura, Mitsue Fujita, Hajime Sohmiya, and Takashi AndoDepartment of Chemistry, Shiga University of Medical Science, Otsu, Shiga 520-21Received March 8, 1995The isomerization of α-phenethyl thiocyanate to the corresponding isothiocyanate was ob-served in the course of the isothiocyanation reaction of α-phenethyl bromide with Ba(NCS)2 supported on CaF2 in cyclohexane. The most probable reaction mechanism is sug-gested as an SN2-like push-pull mechanism with the assistance of α-phenethyl bromide andBaBr,/CaF2Interest in using supported reagents for organic synthesis has developed tremendouslybecause of easy product isolation, mild reaction conditions, and rapid and selective courseof reaction. We recently reported the dramatic change in the sulfur - nitrogen selectivity ofthe SCN substitution of t - butyl bromide by simply supporting various alkaline earth metalthiocyanates on various inorganic solids.1 For example, Ba(NCS)2 supported on CaF2 witht-butyl bromide gave a 79% yield of a mixture of t -butyl thiocyanate and isothiocyanatein the ratio of 1 : 99 in cyclohexane after 3 h at 50-C, while Ba(NCS)2 supported onBaF2 gave a 26% yield of the products in the ratio of 82 : 18 under the same conditions.The solid supports have the dramatic effect not only on the reactivity but also on theselectivity, therefore salt - support interactions must be important in changing the relativenucleophilicity of the S and N atoms of the SCN~ anion.During the extension of this study, summarized in Table 1, we observed an isomeriza-tion of products, α-phenethyl thiocyanate and isothiocyanate in the reaction of α-pheneth-yl bromide with Ba(NCS)2 supported on CaF2 (Ba(NCS);/CaF2).PhCH(CH3)BrBa(NCS)2/CaF2Cyclohexane, 50 C,3h　　　　　　　　　　9GPhCH(CH3)-SCN+PhCH(CH3) -NCSBa (NCS)2/CaF2 gave a 61% yield of a mixture of α-phenethyl thiocyanate and isothio-cyanate in the ratio of 4 : 96 under the same conditions as for the reaction of t-butyl bro-mide, while unsupported Ba(NCS)2 gave a 14% yield in the ratio of 93 : 7. Although theisomer ratio obtained with Ba(NCS);/CaF2 was similar for α - phenethyl bromide and t -butyl bromide, a distinct difference appeared in the course of the reaction, as shown inFigs, la and b. In the reaction of t-butyl bromide with Ba(NCS)2/CaF2, the formation oft-butyl isothiocyanate is a result of the kinetic control, and there is minimal isomenzation- 1 -Takahide Kimura, Mitsue Fujita, Hajime Sohmiya, and Takasm AndoTable 1. Reaction ofAlkyl Halide with unsupported Ba(NCS)2and Ba(NCS)2 supported on CaF2 a)Productssubstrate Reagent 'Isomer ratioc)Thiocyanate : IsothiocyanateI-Bu-BrPhCH(CH3)-Br1 -Adamantyl-Brd)PhCH2-BrPhCH=CHCH2-CI???????????????????????????????????????91 : 91 :9993: 74:9678:2282:18100: 0100: 043:5779:21a) Alkyl halide and Ba(NCS)2, each of 0.5 mmol. Cyclohexane as a solvent (5ml-) Reaction timeand temperature, 3 h and 50-C, respectively, b) Reagent a: unsupported Ba(NCS)2; reagentb: Ba(NCS)2 (0.5 mmol) supported on CaF2 (1 g). c) Determined by GC. d) Reaction time, 20 h.40　　　80　　120　　160　　　200　　　　　　　　　　40　　　80　　120Time (min)Figure 1. Time dependent profile of the production of thiocyanate (○) and isothiocyanate(・) in the reaction of (a)t-butyl bromide and (b) α -phenethyl bromide with Ba(NCS);supported on CaF2. (Total yield二 □　Alkyl bromide 0-5 mmol; Ba(NCS)2/CaF2 0-5 mmol/g; cyclohexane (5 mL) as a solvent at 50-C.of t -butyl thiocyanate to t - butyl isothiocyanate, which is thermally more stable thanthiocyanate. For α -phenethyl bromide, on the other hand, the system first proceedstoward the formation of the kinetic product, thiocyanate. This derivative appears only atthe beginning of the reaction, allowing the isothiocyanate isomer, a thermodynamic prod--　2　-Isomerization Mechanism of a-Phenethyl Thiocyanate in the Presence of a Supported Reagentuct, to accumulate.Isomerization of t - butyl thiocyanate and α - phenethyl thiocyanate to the correspondingisothiocyanates was not catalyzed by the supported reagent, Ba(NCS) :/CaF2. Other mate-rials that catalyze the isomerization of α - phenethyl thiocyanate must be present in thereaction medium. Therefore, the catalytic ability of materials that might be present in thereaction medium were examined for the isomerization. The results are illustrated in Fig.2. When a mixture of α -phenethyl thiocyanate and isothiocyanate was heated at 50 C for20 h in cyclohexane in the presence of CaF2, BaBr2　α -phenethyl bromide, Ba(NCS);/CaF2, (CaF2 plus BaBr2), or (α -phenethyl bromide plus BaBr2) , the isomer ratio of themixture was almost unchanged. On the other hand, BaBr2/CaF2> (α - phenethyl bromideplus CaF2) , or (α -phenethyl bromide plus BaBr2/CaF2) catalyzed the isomerization. Par-ticularly, the isomerization was very fast in the presence of both α-phenethyl bromide andBaBr,/CaF2.Obviously, interaction between CaF2 and Br of α - phenethyl bromide and/or BaBr2 isimportant for the occurrence of the isomerization.Several investigations have、 reported on the isomerization of alkyl thiocyanates to thecorresponding isothiocyanates.2-4 It is known that t - butyl bromide reacts with Hg(SCN);to give t - butyl isothiocyanate, but that t -butyl thiocyanate does not isomerize to t - butylisothiocyanate with Hg(SCN)2 or HgBr2.2 Nevertheless, we found that t-butyl thiocyanatedoes isomerize in the presence of both HgBr2 and t -butyl bromide under the present reac-tion conditions; the ratio of thiocyanate and isothiocyanate changed from 81 : 19 to 27 : 73in 2 h in cyclohexane at 50C.The isomerization of α - phenethyl thiocyanate to the corresponding isotmocyanate occur-red readily in the system composed of KSCN in DMF.2 This is natural, as the free SCNion should be formed in the dipolar aprotic solvent and an SNl mechanism occurs. In anon- polar solvent, however, it is difficult to postulate the SN1- like mechanism, that is,an alkyl thiocyanate dissociates to a carbocation and a thiocyanate ion, and recombines togive a thermodynamically stable alkyl isothiocyanate. The acceleration of the isomeriza-tion by α -phenethyl bromide strongly suggests that a push-pull process which involves thepush by　~NCS-R'+-Br-Ba-Br-CaF2 and the pull by R'+-Br　蝣・・BaBr2-CaF2 may oper-ate. In a similar reaction of alkyl halides with silver nitrate, the contribution of a push -pull mechanism has already been established.3In order to study the interaction of alkyl bromide with alkyl thiocyanate, the isomeriza-tion of benzyl thiocyanate was followed in the presence of α -phenethyl bromide andBaBr,/CaF2 0r of benzyl bromide and BaBr2/CaF2.PhCH2-SCN+PhCH (CH3) BrlO0%PhCH,-SCN+PhCH,Br100%BaBr ,/CaF2phCH2Br+phCH (CH3) -NCSCyclohexane,50。C,20h　85%　　　　85%BaBr>/ CaF,PhCH,SCN+PhCH,-NCSCyclohexane,50。C,20h　97%-　3　-Takahide Kimura, Mitsue Fujita, Hajime Sohmiya, and Takashi Ando?????????+ CaF2十α -PhenethyトBr+ BaBr2+ Ba(NCS)2,/CaF3+ CaF2 + BaBr,+ α-Phen-Br + BaBr2Time(hour)Figure 2. Catalytic abilities of various materials to isomerize α - phenethyl thiocyanate. Amixture of α - phenethyl thiocyanate and isothiocyanate 0.5 mmol; catalysts: α - phenethylbromide, Ba(NCS)2, and BaBr2, each of 0.5 mmol or CaF2 1g; cyclohexane (5 mL) as asolvent at 50 C.Only α - phenethyl bromide was substituted with SCN of benzyl thiocyanate to give α -phenethyl isothiocyanate, while benzyl bromide seemed to be unchanged. Thus, it is rea-sonable that Br of secondary alkyl bromide is pulled by BaBr2/CaF2 to a greater extentthan Br of primary alkyl bromide, making it more carbocationic in character. In a non -polar solvent, the nature of the substitution reaction should still be SN2-like. Therefore, t- butyl thiocyanate and 1- adamantyl thiocyanate did not isomerize as presented in Table 1because of their steric hindrance. A presumed mechanism for the isomerization of α -phenethyl thiocyanate in the presence of BaBr2/CaF2 and α-phenethyl bromide can be sum-marized in the following scheme.R-Br+ BaBr2-CaF2　　　> [RJ+[Br-BaBr2-CaF2J-〔F,Ca-Br Ba-Br〕--R-SCN- 〔R〕+　　　>〔F,Ca-Br Ba-Br〕~〔R〕 +SCN-RREFERENCES1. T. Kimura, M. Fujita, and T. Ando, /. Chem. Soc, Chem. Commun., 1213 (1990).2. a) A. Iliceto, A. Fava, U. Mazzucato, and 0. Rosseto, /. Am. Chem. Soc., 83,2729 (1961); b) A. Fava, A. Iliceto, and S. Bresadola, J. Am. Chem. Soc., 87, 4791(1965); c) A. Ceccon, A. Fava, and I. Papa, J. Am. Chem. Soc, 91, 5547 (1969).-　4　-Isomenzation Mechanism of a-Phenethyl Thiocyanate in the Presence of a Supported Reagent3. N. Watanabe, M. Okano, and S. Uemura, Bull. Chem. Soc. Jpn., 47, 2745, (1974).4. R. Catusse, R. EI Gharbi, and A. Gaset, Reactive Polymers, 8, 79 (1988).5. N. Kornblum, R. A. Smiley, R. K. Blackwood, and D. C. Iffland, J. Am. Chem.Soc, 77, 6269 (1955).-　5　-

Isomerization mechanism of α-phenethyl thiocyanate in the presence of a supported reagent

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タンジシヤクソンザイカニオケルフェネチルチオシアナートノイセイカキコウ

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タンジ シヤク ソンザイカ ニ オケル フェネチル チオシアナート ノ イセイカ キコウ

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タンジシヤクソンザイカニオケルフェネチルチオシアナートノイセイカキコウ