Preparation and reactivity of [D3d]-octahedrane:: The most stable (CH)12 hydrocarbon

The synthesis of the (CH),, hydrocarbon [D-3d]-octahedrane (heptacyclo[6.4.0.0(2,4). 0(3,7).0(5,12).0(6,10).0(9,11)]dodecane) 1 and its selective functionalization retaining the hydrocarbon cage is described. The B3LYP/6-311+G* strain energy of 1 is 83.7 kcal mol(-1) (4.7 kcal mol(-1) per C-C bond) which is significantly higher than that of the structurally related (CH)16 [D-4d]-decahedrane 2 (75.4 kcal mol(-1); 3.1 kcal mol(-1`) per C-C bond) and (CH)(20) [I-h]-dodecahedrane 3 (51.5 kcal mol(-1); 1.7 kcal mol(-1) per C-C bond); the heats of formation for 1-3 computed according to homodesmotic equations are 52, 35, and 4 kcal mol(-1). Catalytic hydrogenation of 1 leads to consecutive opening of the two cyclopropane rings to give C-2-bisseco-octa-hedrane (pentacyclo[6.4. 0.0(2.6).0(3.11).0(4.9)]- dodecane) 16 as the major product. Although 1 is highly strained, its carbon skeleton is kinetically quite stable: Upon heating, 1 does not decompose until above 180 degrees C. The B3LYP/6-31G* barriers for the S,2 attack of the tBuO(.) and Br3C. radicals on a carbon atom of one of the cyclopropane fragments (Delta G(298)*, = 27-28 kcal mol-1) are higher 29 than those for hydrogen atom abstraction. The latter barriers are virtually identical for the abstraction from the C-1-H and C-2-H positions with the tBuO(.) radical (Delta G(298)' = 17.4 and 298 17.9 kcal mol(-1), respectively), but significantly different for the reaction at these positions with the Br3C. radical (Delta G(298)* = 18.8 and 21.0 kcal mol(-1)). 298 These computational results agree well with experiments, in which the chlorination of 1 with tert-butyl hypochlorite gave a mixture of 1- and 2-chlorooctahedranes (ratio 3:2). The bromination with carbon tetrabromide under phase-transfer catalytic (PTC) conditions (nBu(4)NBr/NaOH) selectively gave 1-bromooctahedrane in 43% isolated yield. For comparison, the PTC bromination was also applied to 2,4-dehydroadamantane yielding 54% 7-bromo2,4-dehydroadamantane.