The set of 1,3,4,6-tetraphenylhexa-1,5-dienes (1) represents a perturbation of Cope's rearrangement by four radical-stabilizing phenyl groups all positioned to drive the transition region toward the homolytic−colligative end of the mechanistic spectrum. The appearance of (Z)-isomers being suppressed thermodynamically by a steric interaction of +2.6 kcal mol-1 per cis double bond, an equilibration that is stereochemically not of any Cope type, emerges as the predominant reaction. It is an interconversion of rac -( E , E )-1 and meso -( E , E )-1 (48:52; 77.3−115.3 °C) with the following values of the enthalpy, entropy, and volume of activation:  ΔH ⧧ = 30.7 ± 0.2 kcal mol-1, ΔS ⧧ = +2.1 ± 0.4 cal mol-1 K-1, and ΔV ⧧ = +13.5 ± 0.1 cm-3 mol-1, respectively. Structures have been established by X-ray crystallographic analysis; a possible relationship between dihedral angle and bond lengths in the styrene portions is proposed. The entropy of activation is incompatible with a chair or boat Cope rearrangement; the volume of activation is neither low enough for a pericyclic Cope (“concerted”) mechanism nor high enough for a homolytic−colligative mechanism involving full dissociation as the rate-determining step. Trapping and a crossover experiment give some but only partial support to the intermediacy of free radicals. At higher temperatures, however, electron spin resonance experiments demonstrate an equilibrium with kinetically free (E,E)-1,3-diphenylallyl radicals. These observations are rationalized in terms of geometric reorganization within the confines of a ‘cage'. Resolution by chiral chromatography of rac -( E , E )-1 allows recognition of a fast racemization (40−65 °C), of which ΔH ⧧ (21.3 ± 0.1 kcal mol-1), ΔS ⧧ (−13.2 ± 0.3 cal mol-1 K-1), and ΔV ⧧ (−7.4 ± 0.4 cm-3 mol-1) are consistent with a pericyclic Cope rearrangement. Enriched (Z)-isomers undergo Cope rearrangements in accord with the known influence of axiality and the chair/boat alternative on the energy of the transition region.

Many Diels-Alder reactions are accelerated under high pressure. and this effect is frequently employed in synthetic work. The activation volumes, which can be determined from the dependence of the reaction rate on pressure, are usually very small (AV* in the range of -25 to -45 cm3mol-'). Sometimes they are even smaller (i.e.. more negative) than the reaction volumes (AVR in the range -30 to -40 cm3mol-'; (AV*/AV,>I).['l Accordingly. the transition states"] have volumes similar to, or in some cases smaller than, the products. Competing one-step, pericyclic cycloadditions and multistep cycloadditions that take place via diradical intermediates show relatively large differences in their activation volumes (AA V * % 10 cm3 mol-'), which. at high pressure, has the effect of increased selectivity in Favor of pericyclic reaction^.'^] Examples are the dimerization of ~hloroprene.["~ 1,3-~ycIohexadiene, [~~ and 1,3-b~tadiene.[~'% 61 The preference for pericyclic reactions under high pressure is because the cyclic transition states-like the ground states-exhibit larger packing coefficients than the transition states for the corresponding multistep reactions. The packing coefficients of transition states and ground states are defined as the ratio of the van der Waals volumes[71 to the partial molar volumesIn the reorganization processes mentioned in the title, new interactions occur in the pericyclic transition states. The investigation of the pressure dependence reported here demonstrates a simple relationship between the number of new interactions in the transition state and the size of the activation volume.On the basis of stereochemical and kinetic investigations, most Cope rearrangements of 1,Shexadiene derivatives are regarded a s pericyclic processes.I8] The van der Waals volumes m / c i i l~i i d for the parent 1,Shexadiene (1) and the pericyclic transition state 2* (Scheme 1) are approximately the same. This is understandable since in the symmetrical transition state 2' the bond breaking and making have proceeded to the same extent so that the effects of the two processes on the van der Wads volumes compensate, and no great overall effect of pressure o n the Cope rearrangement is to be expected. If it is assumed that. by analogy with the one-and multistep cycloaddition reactions already mentioned, the transition states here also exhibit larger packing coefficients because of their cyclic geometry, the activation volumes ought to be negative, and the Cope (rl = V G / V * ) . Telcl'ix: Int code + (201)183-3082 [**I This work was supported by the Deutschen Forschungsgemeinschaft. the Minijterium fur Wissenschaft und Forschung des Landes Nordrhein-Westfillen. and the Fonds der Chemischen Industrie. rearrangement should be accelerated under pressure. The activation volume can be estimated a t approximately -12 cm3 m o l~ if the packing coefficient for the transition state is re-1 2+ 1 V, 63.9 62.8 A$ = -1.1 107.7 AV* = -11.7 V 119.4 Scheme 1 . Allvolumesaregivenincm'mol~'.Thestructuralparan~eters necessary for the ca...

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...

Viele Diels-Alder-Reaktionen laufen unter hohem Druck erheblich schneller ab, wobei diese Beschleunigung oftmals auch fur Synthesezwecke genutzt wird. Die aus der Druckabhangigkeit der Reaktionsgeschwindigkeit bestimmbaren Aktivierungsvolumina sind in der Regel sehr klein (A Y' im Bereich von -25 bis -45 cm3mol-'), manchmal sogar kleiner (d. h. im Betrag groDer) als die Reaktionsvolumina (A V, im Bereich von -30 bis -40 cm3mol-'; AV*/AV, 2 I) [']. Demnach sind die Ubergangszustande['I in ihrem Volumen produktihnlich und in manchen Fallen sogar kleiner als die der Produkte. Konkurrierende einstufige: pericyclische sowie mehrstufige, uber Diradikal-Zwischenstufen verlaufende Cycloadditionen zeigen eine relativ gro8e Differenz in ihren Aktivierungsvolumina (AAV* z 10 cm3mol-'), die sich bei hohem Druck in einer Selektivitatssteigerung zugunsten der pericyclischen Reaktionen a~s w i r k t [~~. Beispiele hierfur sind die Dimerisierungen von Chloroprenr4], 1,3-Cy~lohexadien[~~ und 1 , 3 -B~t a d i e n [~~% ' I. Die Bevorzugung der pericyclischen Reaktionen unter hohem Druck ist damit zu erklaren, da8 cyclische Ubergangszustande -analog den Grundzustanden -gr6Bere Packungskoeffizienten q aufweisen als die Ubergangszustande der entsprechenden mehrstufigen Reaktionen. Dabei sind die Packungskoeffizienten von Ubergangszustanden und die von Grundzustanden definiert als das Verhaltnis von van-der-Waals-V~lumenBei den im Titel genannten intramolekularen Reorganisationsprozessen treten in den pericyclischen ijbergdngszustanden neue Wechselwirkungen auf. Die hier berichtete Untersuchung der Druckabhangigkeit dieser Reaktionen 1aDt eine einfache GesetzmaDigkeit zwischen der Zahl der neuen Wechselwirkungen im Ubergangszustand und der GroDe des Aktivierungsvolumens erkennen.Aufgrund stereochemischer und kinetischer Untersuchungen werden heute die meisten Cope-Umlagerungen von 1 ,S-Hexadienderivaten als pericyclische Prozesse betrachtettS1. Die fur das Stammsystem -1,5-Hexadien 1 und den pericyclischen Ubergangszustand 2 * -berechnetcrn van-der-Waals-Volumi-

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.