The effect of pressure on the trimerization and Diels‐Alder reaction of cyanoacetylene. Synthesis and reactivity of 2,3,5‐tricyanobicyclo[2.2.0]hexa‐2,5‐diene („tricyano Dewar benzene”︁)

Abstract

The trimerization of cyanoacetylene (la) and the Diels-Alder reaction of l a with 1,3-cyclohexadiene (2) show a powerful pressure-induced acceleration which allows the reaction temperature to be reduced from 1 6 O O C at 1 bar to 40°C at 12 kbar or from 100°C at 1 bar to room temperature at 7 kbar.At high pressure thermally unstable intermediates like the tricyano Dewar benzene 12 generated in the trimerization of l a or the primary adduct 3a formed in the Diels-Alder reaction of l a with 2 were isolated.Pressure in the range from 5 to 20 kbar strongly influences the rate and position of equilibrium of many chemical reactions. The quantities characteristic of the effect of pressure -the volume of reaction and activation A P and A P -can be determined from the pressure-dependence of equilibrium and rate constants, respectively. Processes accompanied by a decrease of volume ( A V < 0, e.g. bond formation) are accelerated by the use of high pressure while those accompanied by an increase of volume ( A P > 0, e.g. bond dissociation) are retarded"]. Therefore, the application of high pressure seems to be particularly useful in controlling the course of competitive and consecutive reactions and can lead to an improvement of chemo-, regio-, and stereoselectivity[21. Here we report on the effect of pressure on the Diels-Alder reaction of cyanoacetylene (1 a) with 1,3-~yclohexadiene (2) and the trimerization of l a . In these cases the use of high pressure allows us to reduce the reaction temperature and thus to isolate thermally unstable transient intermediated3].At atmospheric pressure l a is only a moderate dienophile in Diels-Alder reactions in contrast to dicyanoacetylene (lb). For example, l b reacts with 1,3-cyclohexadiene (2) readily at 0°C within 1 hour or spontaneously at room temperature leading to the dihydrobarrelene derivative 3br4], whereas the corresponding reaction of l a with 2 requires a temperature of ca. 100°C at which the primary Diels-Alder adduct 3a is thermally not stable and undergoes a retroDiels-Alder reaction producing benzonitrile and etheneL31. Heating of equimolar amounts of l a and 2 dissolved in CDC13 at 100°C for 24 h afforded only 6% of the primary adduct 3a and 68% of benzonitrile besides recovered starting materials (8% of l a and 18% of 2)r51. Recently, it has been shown that the reaction of l a with [2,2]paracyclophane (4) (proceeding only at a temperature 2160°C) does not lead to the Diels-Alder adduct 5 expected by analogy with the addition of l b to paracyclophane 4c6]. Surprisingly, four regioisomeric (2: 1) adducts of type 6 were formedt71. On heating l a in the absence of 4 at 160°C either in solution or in the gas phase 1,2,4-(7) and 1,2,3-tricyanobenzene (8) and ortho-and para-dicyanobenzene (9,lO) were produced in a 20:3: 1 : 1 ratio[7b]. In no case 1,3,5-tricyano-or 1,3-dicyanobenzene was observed. These results may be rationalized if one assumes that 1 a initially dimerizes to 1,2-dicyanocyclobutadiene (11) certainly in a stepwise [2 + 21 cycloaddition comparable to the d...

On the Trimerization of Cyanoacetylene: Mechanism of Formation of Tricyanobenzene Isomers and Laboratory Detection of Their Radio Spectra

“…Synthesis of 1,2,3‐ and 1,2,4‐tricyanobenzene (13 and 14) : Although both compounds could be prepared by trimerization of cyanoacetylene ( 1 , see above), this route is rather involved and requires extensive separation work 8a,b. e Fortunately, both isomers have been prepared previously by the simple routes summarized in Scheme , both of them having been reported in the literature 11…”

“…The computed relative energies of the five Dewar benzenes span a rather wide range, from 74 to 84 kcal mol −1 . The tricyano‐Dewar benzene derivative that is computed to be the lowest in energy, isomer 10 , has been experimentally isolated as one of the trimerization products of cyanoacetylene 8e…”

Modeling Molecules Under Pressure with Gaussian Potentials

“…85,86 Another illustrative example of the capabilities of GOSTSHYP in the simulation of pressure-induced chemical reactions is the cyclotrimerization of acetylene under pressure yielding benzene. 87,88 Three acetylene molecules were placed in the same plane to mimic the configuration on the surface of a heterogeneous catalyst. Geometry optimizations of this arrangement of molecules at different pressures were carried out at the B3LYP 76-78 -D3BJ 89 /6-31G(d) 79 level of theory.…”

Modeling Molecules Under Pressure with Gaussian Potentials

“…85,86 Another illustrative example of the capabilities of GOSTSHYP in the simulation of pressure-induced chemical reactions is the cyclotrimerization of acetylene under pressure yielding benzene. 87,88 Three acetylene molecules were placed in the same plane to mimic the configuration on the surface of a heterogeneous catalyst. Geometry optimizations of this arrangement of molecules at different pressures were carried out at the B3LYP 76-78 -…”