Pyrolysis 4.3.1 propellanes

Pyridines. Azuleno[l,2-b]pyridone (214 Scheme 49) forms in the vacuum pyrolysis of 3-ethynyl-2-phenylpyridine (213) as a by-product (02EJO254 . Azuleno[l,8-cd]pyridinedione 216 similarly occurs as a minor by-product on irradiating the bridged dihydronaphthalene 215 (a tetraenic propellane) to form tetracyclodecadienes by an electrocyclic reaction (69ISJ435). The formation of azulene 216 involves dehydrogenation. 

The parent [4.1.1]propellane (125) is one of the products of pyrolysis of the sodium salt of tosylhydrazone 126 . This carbene insertion gives a mixture of methylenecyclo-heptenes and the desired propellane. While 125 has been characterized spectroscopically, it is quite unstable. Its solutions are stabilized by addition of a radical inhibitor. 

A second intramolecular carbene addition similar to that used for 125 has provided another type of complex [4.1.1]propellane. Pyrolysis of the dry sodium salt of 135 furnished the rather stable 136 in 75% yield. This hydrocarbon was recovered... 

Vincovic and Majerski prepared a second [3.1.1]propellane 165 from pyrolysis of 166 This system is more strained than the parent 158, decomposing in 30 minutes at room temperature in benzene. It reacts with HCl at — 80°C to yield only the nortricyclylmethyl chloride 167. Although this appears to involve external opening, rather than cleavage of the central propellane bond, protonation of the central bond to give 168 followed by cyclobutyl to cyclopropylcarbinyl rearrangement can also provide 167. 

Several papers have appeared dealing with the synthesis of strained bridgehead olefins (anti-Bredt olefins). Conditions are described whereby a 10 1 mixture of the olefins (25) and (26) is formed by the vacuum pyrolysis of the bridgehead chloro-compound (24). ° The olefin (25) can form a reversible stabilized complex with [Pt°(PPh3)2], and this same catalyst can also effect irreversible isomerization to (27). The lead tetra-acetate-induced oxidative decarboxylation of the propellane carboxylic acid (28) produces the stable olefin (29) in good yield. ... 

In the course of these mechanistic studies a number of propellanes have been prepared. Paquette has used the route via a-chlorosulphones described last year to prepare a series of substituted [4,4,2]propellanes. Pyrolysis of triene (267) affords the cyclo-octatetraene (268) and similarly (269) gives (270). Labelling studies show that rearrangement proceeds via an intramolecular +, 2J cycloaddition followed by valence isomerization. Further Diels-Alder additions to (271) and related propellanes are described, and the e.s.r. spectrum of the radical obtained by electrolytic reduction of (272) is recorded. Other important synthetic studies concern [4,2,l]propellanes, [4,3,1]-propellanes, and [4,2,2]propellanes. ... 

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