Formation of dehydrogenated derivatives upon the oxidative dehydrogenation of adamantanes

Dehydroadamantanes whose framework includes a three membered cyclic fragment are usually obtained from the corresponding dihalides.1 We demonstrated for the first time with adamantane (1), 1,3 dimethyladamantane (2), and 3,5 dimethyladamantan 1 ol (3) as examples that three membered rings can close upon direct oxida tive dehydrogenation of these compounds on poly oxometallates (POMs) or under the action of iodine. The presence of strained three membered rings makes dehydroadamantanes convenient starting reagents for the preparation of difficultly accessible adamantane deriva tives.2—4 Compounds 1 and 2 were dehydrogenated in the presence of the POMs K5FeSi(H2O)W11O39 and Na6MnSi(H2O)W11O39 applied to Al2O3 at 150—250 °C in the pulse regime in a flow of helium (with H2O2 as an oxidant or without any oxidant) or air. Actually, the de hydrogenation occurs only with the iron containing POM. Starting from adamantane (1), we obtained (250 °C, air, contact time 6 s) two compounds with [M]+ 134 in the ratio 3 : 1 (yield 0.8%). Apparently, these are 2,4 dehydro adamantane5 and 1,3 dehydroadamantane (4), respec tively. When oxidized with H2O2 (150—250 °C, con tact time 2 to 6 s), hydrocarbon 2 gave 1,3 dime thyl 4,8 (5), 4,6 (6), and 4,9 (7) dehydroada mantanes (1.6 : 1 : 1.6) in 2.0—2.6% yield (Scheme 1). With atmospheric oxygen as an oxidant, compounds 5 and 6 were formed in the ratio 1 : 2.3 (yield 0.1—3.0%). In a flow of helium, without any oxidant except for the oxygen contained in POM, the sole product was dehydro adamantane 6 (250 °C, contact time 6 s, yield 0.2%). No oxygen containing products were detected. Under the action of iodine, adamantane (1) (250 °C, the molar ratio 2 : 1, 2 h) decomposes into HI and a number of destruction products (yield 84%). The reaction of 1,3 dimethyladamantane (2) with I2 (200 °C, 1 : 1, 2 h) yields compound 6 only in trace amounts (<0.1%). The presence of the OH group significantly increases the reac tivity of the H atoms of the adamantane framework. De hydrogenation of alcohol 3 with iodine (200 °C, 1 : 1, 2 h) gives only dehydrogenated products, namely, 3,5 di methyl 6,8 (8), 6,10 (9), and 2,10 (10) dehydro adamantan 1 ols (2 : 7 : 1, yield 40%, see Scheme 1).