The activation and reaction volumes (∆V /∆V) were showing negative volumes of activation (∆V Ͻ 0) whereas the others are slightly retarded (∆V Ͼ 0). From the analysis determined for the retro Diels-Alder reactions of the parent dihydrobarrelene 1a, its 2-cyano derivative 1b, the exo and of the volume data including the van der Waals volumes (V w ) one can conclude that the packing coefficients of the endo Diels-Alder adducts of maleic anhydride to naphthalene exo-, endo-4, and the exo and endo Diels-Alder pericyclic transition states are equal to or even larger than those of the corresponding Diels-Alder adducts. This finding adducts of N-phenylmaleic imide to 6,6-dimethylfulvene exo-, endo-8 from the pressure dependence of the rate may be explained with the restriction of the degrees of freedom in the transition states leading to a contraction of constants and the partial molar volumes (V) of reactants and products at various temperatures. The cleavage of exo-4, the expansion volume. endo-4, and exo-8 are slightly accelerated by pressure Many DielsϪAlder reactions show a powerful pressure-The intrinsic molar volumes, the so-called van der Waals volumes, V W , calculated for the pericyclic transition strucinduced acceleration which is often turned to good synthetic purposes.[1][2] The activation volumes ∆V resulting tures of DielsϪAlder reactions by the use of the structural parameters obtained from quantum-mechanical calcufrom the pressure dependence of the rate constants are usually highly negative (∆V ഠ Ϫ25 to Ϫ45 cm 3 mol Ϫ1 ), some-lations [6] and the van der Waals radii of the different types of atoms derived from X-ray data are generally larger than times even more negative than the corresponding reaction volume ∆V so that the ∆V /∆V ratio is close to or even those calculated analogously for the corresponding cycloadducts. [3g,7] Grieger and Eckert [8] considered two explalarger than unity (Θ ϭ ∆V /∆V Ն 1). [3] Within the scope of the transition-state theory, activation nations of the ratio Θ > 1 in the DielsϪAlder reaction of isoprene as diene with maleic anhydride as dienophile: a volumes can be considered to be a measure of the relative partial molar volume of the transition states [∆V ϭ larger dipole moment of the transition state or secondary orbital interactions [9] from which the secondary orbital in-V (TS) Ϫ ΣV(reactants)]. Accordingly, the molar volumes of transition states of many DielsϪAlder reactions are ap-teractions can only occur in endo DielsϪAlder reactions.The finding that the difference between the activation volproximately equal to or even smaller than those of the corresponding cycloadducts. These surprising results could be umes of many endo and exo DielsϪAlder reactions is small (∆∆V > 1Ϫ2 cm 3 mol Ϫ1 ), [1e] seems to rule out that secondconfirmed by two independent studies. In the DielsϪAlder reactions of furan with acrylonitrile [4] and in that of N-ary orbital interactions are important and induce a larger contraction of the volume of the endo transition state. benzoylpyrro...

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...

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Keywords: [4 + 21 Cycloadditions / [2 + 2 1 Cycloadditions / Cyclobutadienes / Cyanoacetylene / High-pressure reactions At 1 bar and 160°C the reaction of cyanoacetylene (1) with (2.2](2,5)furanoparacyclophane (3) produced the unexpected "ring-enlarged" ketones 6-11. In the reaction of 1 with [8](2,5)furanophane (4) comparable products 21 and 22 were observed, in addition to the products 19 and 20 expected from a consecutive Diels-Alder addition, Alder-Rickert cleavage process and the Diels-Alder addition of 1,4-dicyano-1,3-cyclobutadiene (2a) to 4, respectively. In the reaction of 1 with the parent furan 5 only the (2:l) and (1:2) DielsAlder adducts 23, 25, 26, and 27 were found. High-pressure experiments and the reactivity of 2-cyano-7-oxabicyclo-[2.2.l]hepta-2,5-diene (24), which was prepared independently by flow-thermolysis of the (1 :2) Diels-Alder adducts 26 and 27, provide evidence that the (2:l) adducts 20, 23, and 13 are probably formed by a sequence of Diels-Alder and [2 + 21 cycloadditions rather than by the reverse sequence starting with [2 + 21 cyclodimerization of 1 followed by Diels-Alder reaction with cyclobutadiene 2a as postulated by the analogy to the trimerization of 1 and the cycloaddition of 1 to paracyclophane. The high-pressure experiments led us to propose a new mechanism of formation of the "ringenlarged" ketones 6-11.Recently, it has been shown that cyanoacetylene (1) can undergo two types of cycloadditions['l. 1 is either a moderate dienophile in Diels-Alder reactions or it dimerizes in the fashion of a [2 + 21 cycloaddition. The latter process affords the antiaromatic l+dicyano-1,3-cyclobutadiene (2a) and its tautomer 2b as highly reactive intermediates that can react further as dienophiles or dienes in Diels-Alder reactions. All these processes involving 1 show a marked pressure-induced acceleration thus allowing the reaction temperature to be decreased and thermally labile reactive intermediates to be isolated[ld1. Here we report in detail on the cycloadditions of 1 with the heterophanes 3[21, 4L31, and the parent furan 5. In particular the reaction of 1 with [2.2](2,5)furanoparacyclophane (3) leads to entirely unexpected 2: 1 adducts in which the original furan subunit of 3 has been completely disassembled. The investigation of strain, temperature, and pressure effects provides further insight into the mechanisms of their formation.The reaction of cyanoacetylene 1 with furanoparacyclophane 3 at atmospheric pressure (1 bar) and 160 "C (22 h) resulted in a complex mixture of surprising products from which the adducts shown in Scheme 2 could be isolated by means of chromatographic methods and repeated recrystallization.

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