Thomas F. Mich Everen J. Nienhouse Thomas E. Farina and Joseph J. Tufariello' State University of N e w Yort a t Buffalo Buffalo, N e w York 14214
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The Generation of Benzyne -A Warning
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T h e reactions of benzyne with a variety of organic substrates provide excellent undergraduate laboratory experiments. Wittig and Knauss2 have reported the generation of benzyne from o-bromofluorobenzene and magnesium. However, the formation of benzyne by the diazotization of anthranilic acid3is more economical and the generation of benzyne by this method has been incorporated into our undergraduate laboratory program. We were recently engaged in the synthesis of benznorbornadiene(I1) from benzyne and cyclopentadiene. During the course of the preparation of benzyne by the
diazotization procedure, we experienced a violent explosion. Thus, when a solution of anthranilic acid in T H F was added to a solution of cyclopentadiene, iso-amylnitrite, and T H F a t room temperature,' a precipitate formed. The mixture was stirred at room temperature for 16 hr, and the solid which remained was isolated by filtration. The material was air-dried on the funnel for 45 min and examined. Upon being touched with a metal spatula, the solid detonated with great violence and shattered the Buchner funnel. The solid was presumably some of the diazonium salt (I)=which had not decomposed in the reaction mixture. Thus, ambient temperature was insufficient to promote the complete decomposition of the diazonium salt as it was generated within the reaction mixture. Moreover, it appears that this salt may he less soluble in T H F than in methylene chloride, for example.
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Journal of Chemical Edumtion
When a solution of iso-amylnitrite in methylene chloride is heated to reflux (-4O0C), the decomposition of the diazonium salt can be monitored by observing gas evolution as the solution of acetone, anthranilic acid, and cyclopentadiene is added. Once the reaction has started, as signaled by the gas evolution, external heating can be discontinued since the heat of reaction is sufficient to maintain the methylene chloride a t reflux. After the addition is complete, the entire mixture is refluxed until gas evolution ceases. This usually takes 2-5 hr. This method is a modification of that employed by Friedman and Logullo for t.he synthesis of triptycenes and has been utilized on numerous occasions without incident. A typical run illustrating the improved procedure is described below. The product was identified by comparison of its bp and spectral properties with those reported by Wittig and Knauss. Prepamlion of Benznorhomadime. I n a 2-1 three-necked flask equipped with stirrer, condenser, and addibion funnel was placed a solution of 64.35 g (0.55 mole) i-amylnitrite and 800 ml of methylene chloride. A solntion of 6 8 3 g (0.50 mole) anthranilic acid, 33.0 g (0.50 mole) freshly cracked cyclopentadiene, and 300 ml of acetone was added to the stirred solution over a 1-hr period. The reaction was heated a t the start until the methylene chloride started to reflnx and gas evolution was observed. (CAUTION: a white solid began to separate on one occasion when the solrttion was not heated initially.) As the reaction progressed, sufficient heat was evolved to maintain gentle reflux. After the addit,ion was complete, the reaction was refluxed for 5 hr and cooled by permitting it to stand overnight a t room temperature. The solvents were removed (reduced pressure) and the black oil diluted with 750 ml of ether and 500 ml of saturated NaHCOr solution. After considerable CO1 evolution, the layers were separated and the eqneaos layer extracted with ether. The comhined ether layers were washed four times with 150 ml portions of saturated NaHCO, solution, t,wice with satwsted salt and dried over anhydrous N&O4. Removal of the ether a t reduced pressure and distillation through a 10-in. Vigreux column afforded i-amyl alcohol, bp 4 5 T / 1 0 mm, and 28.50 g bensnorhornadiene, bp ?2D-RI'T/10 mm, 40Y0 yield. Reported (Friedmsn and Logullo) bp 82.5"-83T/12 mm. The nmr spectrum showed a complex pattern a t 6 = 6.63-7.16 ppm (6 H) due to aromatic and olefinic protons, narrow m~lltipletsa t 6 = 3.65 ppm (2 H ) due to the bridgehead protons, and a t 6 = 2.20 ppm (2 H) due to the bridging protons. 1 To whom inquiries should be addressed. WITTIG,G., AND KNAUSS,E., Ber., 91, 895 (1958). 3 F~rannrm.L.. AND LOGULLO.F. M..J . Am. Chern. Sac., 85, 1549 (196i). ' 6 NIENHOUSE, E. J., Ph.D. Dissertation, State University of New York a t Buffalo, Febmary, 1967. q e e footnote 3.
