The products and the kinetics of the thermolysis of the title compounds 3 and 5 were investigated. Like the trans-isomers, the cis-azo compounds also undergo homolytic decomposition without accompanying cis-trans-isomerization. The observed structure-reactivity relationships are discussed. On irradiation of trans-tetra-tert-butylazomethane (trans-5) at 60 -120°C in benzene or chlorobenzene an almost quantitative yield of 1,1,2,2-tetra-tertbutylethane is obtained. Aliphntische Amverbindungen, XV ' ) cis-und trtuts-Tetracyclopropyl-und Tetra-tert-butylazomethanDie Produkte und die Kinetik der Thermolysen der Titelverbindungen 3 und 5 wurden untersucht. Wie die trans-Isomeren, so zerfallen auch die cis-Azoverbindungen in Radikale ohne begleitende cis-trans-Isomerisierung. Der EinfluD der Strukturvariation auf die Reaktivitat wird diskutiert. Durch Bestrahlen von trans-Tetra-tert-butylazomethan (trans-5) bei 60-120°C in Benzol oder Chlorbenzol la& sich fast quantitativ 1,1,2,2-Tetra-tert-butylethan darstellen.The cis-trans-isomerizations and the thermolysis reactions of tertiary azoakanes have been investigated much more thoroughly than those of secondary ones 1*). This is due to an additional complicating side reaction of the latter, the tautomerization to hydrazones 2 3). RzCH-N=N-CHRzR~CH-NH-N=CRZ 1 2Through our attempts to prepare 1,1,2,2-tetra-tert-butylethaneg and 1,1,2,2-tetracyclopropylethane ' z 6 ) the title azo compounds became available. The observation that they were not very sensitive to tautomerization stimulated our interest in the investigation of their thermal decomposition reactions. SynthesisA mixture of cis-and trans-l,1,1',1'-tetracyclopropylazomethane (3) was obtained6) from dicyclopropyl ketone via the azine, followed by reduction to the hydrazine and subsequent air oxidation.

Three diastereomeric hydrocarbons 7 were formed on reaction of 1 -adamantyl-1 ,1 -dibromo-2.2-dimethylpropane with magnesium. They were separated by fractional crystallization and identified by 'H NMR spectroscopy and X-ray crystallography as meso-7 and the two rotamers of D , L -~ mentioned in the title. They are the first examples of simple aliphatic hydrocarbon rotamers, which do not interconvert at room temperature and above. Analogous 1-norbornyl derivatives were also obtained but could not be separated. The kinetics and the products of thermolysis of these hydrocarbons were investigated.Thermolabile Kohlenwasserstoffe, XXVIII 1.2) Trennnng, Struktnranalyse und Thermolyse eines stabilen Rotamerenpaares; P-(R*,R*)-und M-(R*,R*)-o.~-3,4-Di-l-adamantyl-2,2,5,5-tetramethylhexan Aus 1-Adamantyl-1 ,l-dibrom-2,2-dimethylpropan erhielt man bei der Umsetzung mit Magnesium drei diastereomere Kohlenwasserstoffe 7, die durch fraktionierte Kristallisation getrennt werden konnten. Sie wurden durch NMR-Spektroskopie und Kristallstrukturanalyse als meso-7 und die rotameren Titelverbindungen 7 erkannt. Es handelt sich um das erste Beispiel bei Raumtemperatur und dariiber nicht interkonvertierender, einfacher aliphatischer KohlenwasserstoffRotamere. Die analog hergestellten 1-Norbornylderivate konnten nicht getrennt werden. Die Thermolyse der Verbindungen wurde kinetisch und praparativ studiert.For a bond connecting two tertiary carbons (C, -C, systemsz)) in aliphatic hydrocarbons gauche-conformations 2 are preferred over anti-conformations 12). 3--

The title compound 2 was prepared by a Wurtz procedure. Its highly deformed structure was determined by X-ray analysis and matched with high precision by MM2 force field calculations. The three neighbouring CqCq bonds are longer than 164 pm and the C,C,C, bond angles are 118". According to MM2 calculations and a kinetic investigation 2 is the most strained and least stable member of the CqCq alkane series investigated previously*).Thermolabile Kohlenwasserstoffe, XXVII') 2,3-Di-l-adamantyl-2,3-dimethylbutan; lange Bindungen und geringe thermische Stabilitat Die Titelverbindung 2 wurde durch Wurtz-Synthese dargestellt und eine Kristallstrukturanalyse durchgefiihrt. Die Strukturdaten des sehr stark deformierten Molekulgeriistesdrei benachbarte C,Cq-Bindungen sind langer als 164 pm, CqCqCq-Bindungswinkel sind bis 11 8 aufgeweitetwerden durch Rechnungen mit dem MM 2-Kraftfeld sehr gut wiedergegeben. Nach den Kraftfeldrechnungen ist 2 der am starksten gespannte, nach kinetischen Messungen der thermisch labilste Vertreter der bereits friiher bearbeiteten C,Cq-AlkaneZ).Previously we discussed the factors which influence the strengths and the lengths of CC bonds between two quaternary carbons in open-chain structures2-6). Substituents stabilizing the radicals, which are formed on bond homolysis, decrease the bond strength but apparently have little effect on the bond Increasing ground state strain H , on the other hand results in a decrease in bond strength and also in an increase in bondThe most highly strained and thermally most labile known member of the unsubstituted aliphatic C,C, series is octamethylhexane 12), the structure and thermochemistry of which were so far investigated only by molecular mechanics and NMR2*'). A crystal structure analysis was prevented by the low melting point of 1, the experimental determination of its heat of formation by the poor yield of its preparation. It was an unfortunate situation in the context of our search for relationships between structure, strain, and thermal stability of highly strained corn pound^^-^) that the MM 2 force field")

Wurtz coupling of two molecules (III) gives the title compound (IV).

The yellow title compounds (II) and (III) or (VII) (yield given in g) and (VIII) (no yield given) can be prepared as shown in the scheme.