Small-ring propellanes

In certain small-ring systems, including small propellanes, the geometry of one or more carbon atoms is so constrained that all four of their valences are directed to the same side of a plane (inverted tetrahedron), as in 98. An example is 1,3-... 

At the time our work was done (1968) very few small-ring propellanes were known5). Nevertheless it was clear that these molecules would be strained and that... 

Once we leave the realm of small-ring propellanes5 and post-1975 reports41,42 , it appears that it should be possible to convert suitably disposed sets of dispiran moieties into their respective propellane counterparts 39,40) because of the relative stabilities of the two families. By the same token, there are plenty of examples of acid-catalyzed rearrangement of propellanes, through which systems of yet greater stability may be obtained 5 41,42). 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

On the other hand, alkali metal atoms can be obtained by simple evaporation at moderate temperatures, and these may serve to abstract bromine or iodine from organic halides, thus providing access to radicals, biradicals,or highly strained hydrocarbons, such as small-ring propellanes. The technique has been described in some detail by Otteson and Michl. ... 

The Diels-Alder reaction of 108 and its analogs with other dienes such as furan and benzindole derivatives has been demonstrated as a valuable route to small-ring propellanes containing the bicyclobutane moiety. The interesting chemistry of these propellanes was also investigated " but it is beyond the scope of this chapter. 

Although it is not surprising that Wiberg extended to tricyclic systems (propellanes) his studies on hybridization of the bridgehead carbons in small-ring bicyclic compounds and the chemistry resulting therefrom, we shall perhaps extend somewhat the realm of the propellane substrates. This is done pragmatically (every definition is, by definition, arbitrary) in order to permit inclusion of certain piquant results. 

But before we turn to this aspect of the structure of not only [l.l.l]propellane but certainly of other [m.n.ljpropellanes, let us consider how these and other small-ring [m.n.ljpropellanes are prepared, in more or less chronological order. 

The ultimate in small-ring propellanes (C2 exists but I don t consider it to be [O.O.OJpropellane), [I.l.ljpropellane (96), was finally synthesized from 2,3-dibromo-bicyclo[l.l.l]pentane (95) with t-BuLi . It undergoes thermal rearrangement to 97 with a half-life of 5 min at 114°C. Thus, Wiberg s prediction that this compound ought to be relatively stable as compared to other small-ring propellanes is amply borne out. In acetic acid it affords l-acetoxy-3-methylenecyclobutane (98). 

In this paperthe relative stabilities of various small-ring propellanes are discussed in terms of enthalpies of hydrogenolysis of the conjoining bond and dissociation energies of this bond in the various substrates. This is perhaps the place to state that the mechanism of addition of bromine, in the dark, to the conjoining bond of several [m.n.l]propellanes, has been discussed in generaF. It is concluded that thermally initiated low temperature radical chain addition to the cyclopropane rings is involved. 

We turn now to theoretical studies carried out on small-ring [m.n.l]propellanes (which often include other small-ring propellanes which do not contain a cyclopropane ring). 

The Majerskis have commented on the nature of the conjoining bond in small-ring propellanes " . 

Top. Curr. Chem. 1999, 201, 43-79. (d) Bunz, U.H.F. Rubin, Y. Tobe Y. Polyethynylated cyclic 7t-systems scaffoldings for novel two and three-dimensional carbon networks. Chem. Soc. Rev. 1999, 28(2., 107-119. (e) Tobe, Y. All-carbon molecules from small-ring propellanes. In Advances in Strained and Interesting Organic Molecules. Halton, B., Ed., JAI Press Stamford, 1999 Vol. 7, 153-184. (f) Tobe, Y. Wakabayashi, T. Carbon-rich compounds acetylene-based carbon allotropes. In Acetylene Chemistry Chemistry, Biology and Material Science. Stang, P.J. Diederich, F. Tykwinski, R., Eds. VCH Weinheim, 2005 387 426. 

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