Pericyclynes

Chem Int Ed Engl 33 869. de Meijere A, Kozhusov S, Haumann T, Boese R, Puls C, Cooney MJ, Scott LT (1995) Chemistry 1 124. Review Scott LT, Cooney MJ (1995) Pericyclynes. In Diederich F, Stang PJ (eds) Modern Acetylene Chemistry. Verlag Chemie, New York... 

The pericyclynes constitute a series of hydrocarbons composed only of CH2 and -C=C- units. The prefix [jV] indicates the number of CH2 groups and... 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

There are several potential examples of bond homoconjugation in the literature, of which the 3,5-dehydrophenyl cation, 14, is probably best known (see also Figure 2)33. If the sp2 hybrid orbitals at position 1, 3 and 5 point toward the centre of the ring, they can overlap and form a homoconjugative 2-electron-3-centre system. In plane overlap between 71 orbitals, cr-type overlap can also be expected for tetracyclo-[8.2.2.225.269]-l,5,9-octadecatriene, 15, in which three double bonds are kept face to face by frames made out of cyclohexane rings34. Finally, [3]pericyclyne and 1,5,9-cyclododeca-triyne could be considered to be better examples for [Pg.353]

Density functional theory (DFT) studies on [A]chalcogena[iV]pericyclynes (n = 0-3, 5) demonstrate their relative stability and hence their possible existence as stable species. By minimizing repulsive interactions between the chalcogens lone pairs, the molecules adopt structures that resemble, in shape, cycloalkanes or elemental chalcogens. [3]Chalcogena[3]pericyclynes may be interconverted with their valence tautomers, benzene derivatives with three fused three-membered rings <20040L589>. 

Three-membered rings with one selenium or tellurium atom have attracted some theoretical attention since the last review <1996CHEC-II(1)259>. Of particular interest are the comparison of theoretical methods, computations on a reaction course (that of selenirane with silylene), and the valence tautomerization of [3]chalcogena[3]pericyclynes. 

In cases where stable carbocations can be generated, e.g., tertiary propargylic cations, the rather low nucleophilicity of silylated terminal alkynes is still sufHcient for a coupling reaction. Ring closures have been carried out in this fashion to give the permethylated pericyclynes, which are discussed by L. T. Scott and M. J. Cooney in Chapter 9 of this monograph [8]. 

Our story begins with a family of compounds we refer to as the [iVJpericyclynes [7, 8] a name intended to capture the structural essence of molecules composed of N alkyne units distributed symmetrically around the perimeter of a cycle with TV vertices (Fig. 9-2). Hydrocarbon 1 would be called [3]pericyclyne and represents the smallest member of the family. To preclude experimental complications arising from labile hydrogens on the carbon atoms between acetylenes in the parent molecules, we have generally appended methyl groups at the saturated centers. 

Section 9.3 moves next to exploded pericyclynes, in which each side of the polygon consists of a longer 1,3-diyne unit. This is followed by hybrid systems wherein some sides are alkynes and others are diynes, and we conclude with a return to [AG pericyclynes, introducing heteroatoms at one or more of their vertices. 

Treatment of pentayne 11 with acetyl chloride under the same conditions likewise produced propargylic chloride 14, and this could be cyclized to decamethyl[5]pericyclyne, 3, in 35%... 

Figure 9-3 Homologation sequence for the construction of synthetic precursors to permethylated pericyclynes (a) 2 equiv n-BuLi, then 2.5 equiv TMSCl, then cone. HCl, Cu powder, CaClj, 71% overall (b) NaH, Me2S04, 80%, (c) EtMgBr, then CuCl, then 8, 83% for 9 - 10, 62-83% for each coupling in 10 - 11 (d) KOH, MeOH, 76-80% for each desilylation in 10 11.

Figure 9-5 The first synthesis of a pericyclyne and the likely intermediates involved in the cydization step (a) CH3COCI, SnCl4, 82% (b) AlCl, CS, 35%.

Macrocycles 3, 4, 5 and 6 are all colorless, crystalline solids that are stable in the air and light indeilnitely at room temperatiue. None has ever exhibited any shock sensitivity. X-ray crystal structures of 3, [9] 4, [27] and 5 [27] have all been determined, and the structural resemblance between these macrocycles and the corresponding cycloalkanes with the same number of vertices is striking the [5]pericyclyne, 3, adopts an envelope conformation, the [6]pericyclyne, 4, adopts a chair conformation and the [7]pericyclyne 5, adopts a tub conformation. Unfortunately, the [8]pericyclyne, 6, crystallizes as fine hairs that are unsuitable for X-ray diffraction analysis. 

Of particular interest to us was the question of cyclic homoconjugation in the pericyclynes. The existence of strong orbital interactions among the acetylenic units in such molecules is... 

Electron transmission spectroscopy of 3 and 4, which measures gas-phase electron affinities, reveals an even larger splitting (ca. 1.6 eV) of the nonbonding orbitals in these pericyclynes [9]. The lowest unoccupied molecular orbitals (LUMOs) are stabilized by 0.4-0.7 eV relative to the LUMO of acetylene. 

Figure 9-7 The first syntheses of a [4]pericyclyne and a quinone of (8)pericyclyne (a) EtMgBr, then HCOOEt, then NH4CI, 10% 22, 6% 23 (b) Jones Reagent, 54% for 22 - 24, 32% for 23 25.

The symmetrical acyclic tetrayne 21 used as a precursor to these new pericyclynes was easily assembled by two different methods (Fig. 9-8) [22]. the lengthier of the two started with the familiar building block 8, whereas the shorter route relied on the more difficultly-accessible 3,3-dimethylpenta-l,4-diyne, 26. 

An analogous cyclization of 31 gave the [5]pericyclyne ring system by this new route and also yielded small quantities of the first 30-membered ring in this family, 33 [22]. Oxidation of the cyclic alcohols as before gave the [5]pericyclynone, 34, and the quinone of the [10]peri-cyclyne, 35. An X-ray crystal structure of octamethyl[5]pericyclynone, 34, showed the ring to be perfectly planar [22, 27]. 

TWo methods were devised for construction of the symmetrical acyclic pentayne 31 (Fig. 9-10) [22]. One started at the center and built outward, adding two acetylene imits at a time, while the other took advantage of the availability of tetrayne 38, an intermediate from Fig. 9-3 used in our first synthesis of pericyclynes. 

As an alternative route to [5]pericyclynone 34, we also prepared octamethyl[5]pericyclyne, 41 [8], and oxidized it to the corresponding ketone (Fig. 9-11) [22]. This approach followed the earlier route outlined in Fig. 9-3, deviating only by the omission of methyl groups on the final propargylic alkylating agent. 

As expected, all the pericyclynes with one or more hydrogen atoms at a doubly-propargylic position (22, 23, 32, 33, and 41) exhibited marked sensitivity to air and base, and this prevented detailed examination of their electronic properties by UV or photoelectron spectroscopy. The ketones and quinones (24, 25, 34, and 35), however, proved relatively easy to... 

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