Propellanes, strained

The coupling of 1,8-diiodonaphthalene (25) with acenaphthylene (26) affords acenaphth[l,2-a]acenaphthylene (27). It should be noted that the reaction involves unusual trans elimination of H—Pd—1[32], This tetrasubstituted double bond in 11 reacts further with iodobenzene to give the [4, 3, 3]propellane 28 in 72%. This unusual reaction may be accelerated by strain activation, although it took 14 days[33]. 

The corresponding ethano and propano compounds only exist in their propellane forms. Apparently, the smaller bridge leads to higher strain in the azocine tautomer, so that the triene structure predominates. Longer chains (> hexano), however, exhibit the normal behavior of tetraenic compounds. 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

Two illustrations that show the power of this reaction for the preparation of strained cycloalkenes are the contractions of 102 to the propellane 103 , an application that has been reviewed , and of 104 to the bicyclo[2.1.1]hexene 105 . The utility of the Ramberg-Backlund rearrangement in the preparation of various natural products such as steroids , terpenoids and pheromones has been demonstrated. In addition to the synthetic applications mentioned in the previous subsection, several selected examples taken from the recent literature are given in equations 66-69. These examples further demonstrate the potential of this method for alkene synthesis in general. 

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... 

The next four procedures describe the preparation of strained ring systems. Preparation of 3-CHLORO-2-(CHLOROMETHYL)-l-PROPENE provides a facile approach to the olefin required for the synthesis of [1.1.1 [PROPELLANE, one of the most strained hydrocarbons prepared to date. The ready availability of this hydrocarbon should prove particularly useful to those interested in the development of the chemistry of this fascinating compound. Preparation of N-BENZYL-2,3-AZETIDINEDIONE provides an efficient approach to the unadorned a-keto-/3-lactam, a potential... 

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]. 

There often is a correlation between the strain relief in a reaction and the rate of thermolysis, but other factors may also be of importance. The Cg propellanes 16 and 17 have quite different reactivities. Whereas 16 undergoes thermal cleavage at 360 17 undergoes cleavage at 25 °C. ° Aside from the difference in... 

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. ... 

Bicyclo[2.2.0]hex-l(4)-ene (8) is one of the most highly strained of the alkenes (SE = 87, OS = 35)50 that can be observed at room temperature.51 In dilute solution, it undergoes moderately rapid dimerization at room temperature, presumably giving the pentacyclic propellane 9 as an intermediate in the formation of the diene 10. 

The Wolff rearrangement has been very successfully used in the synthesis of propellanes.40 The highly strained 3-diazo[3,2.2]propellan-2-one was photolyzed in dichloromethane at — 70 °C in the presence of dimethylamine to give A W-dimethyl tricyclo[2.2.2.0 u4]octane-2-carboxamide (5), a very highly strained interesting compound with inverted geometry at the bridgehead carbons,42 in 40% yield.41... 

More recently, the doubly lithiated derivatives of [4.1.1]- and [3.1.1]propellasilanes underwent bridging metathesis at the bicyclobutane ends with dihalides of germanium, tin and transition metal titanium, to give the corresponding highly strained metallacyclic propellanes (equation 7)29. 

The failure of the empirical NMR coupling constant correlation is actually not very surprising. These correlations have a reputation of breaking down in highly strained hydrocarbons24, of which [l.l.l]propellane surely is a prime example, and do not do particularly well even in bicyclo[l. 1.0]butane. 

Considering their high expected strain energy, [1.1.1 [propellane and its derivatives possess substantial thermal stability. The neat parent hydrocarbon la is moderately stable at room temperature, particularly in the absence of air, but after a few hours, a considerable amount of polymer builds up36 37. A 3-5% solution can be stored in a refrigerator for weeks without noticeable losses. 

Angle strain is severe. Accordingly, [3.2.1]propellane reacts rapidly with bromine at —60° and with hydrogen over palladium at room temperature ... 

The l,6-disila[4,4,4]-propellane formed is stable in air and the small batho-chromic shift relative to hexamethyldisilane can be explained by ring straining. 

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