Dehydrogenation of cyclooctane

The turnover rate of the dehydrogenation of cyclooctane at room temperature is about ten times that of the carbonylation of benzene. Heating further accelerates the dehydrogenation the quantum yield reached 0.2 at 96 °C (Scheme 8). The catalytic system has a long life a total turnover over 1000 was readily achieved in the dehydrogenation of cyclooctane. When acyclic alkanes were dehydrogenated,... 

Scheme 8. Dehydrogenation of cyclooctane by the RhCI(CO)(PR3)2 system — high quantum yield.

Scheme 10. Visible light-promoted dehydrogenation of cyclooctane.

Apart from the well-known oxidative additions to Ir1, hydrogen can also be added to Ir111. In the case of (21-VIII) the reaction proceeds at room temperature the resulting Irvtetrahydride reductively eliminates H2 only on heating above 130°C and is a highly active catalyst for the transfer dehydrogenation of cyclooctane.28... 

Phosphino enolate complexes such as (21-XXIII) catalyze the transfer dehydrogenation of cyclooctane with norbornene at 60-90°C under H2 pressure.136 Iridi-um(III) dihydrides with P—C—P chelate ligands of type (21-VIII) are thermally stable at 150-200°C and catalyze the dehydrogenation of cyclooctane at rates of up to 12 turnovers min-1 (200°C). The transfer-dehydrogenation of ethylcyclohexane with this catalyst gives ethylcyclohexenes, EtPh, and styrene.137 Mechanistic studies of the transfer dehydrogenation of cyclooctane with CH2=CHBu in the presence of IrH2ClL2 indicate that cyclooctane coordination is required for the reductive... 

The chief advantages to using dense CO2 as the reaction medium were claimed to be its inertness, its general ability to dissolve the reactants, and its easy separation from the reaction mixture. No mechanistic work was reported, but the same mechanism as previously proposed in alkane solvents was suggested, that is, the key step was oxidative addition to the excited state of the metal complex. The C02/Rh complex system was also reported to be an effective photocatalytic system for the dehydrogenation of cyclooctane. 

The transfer dehydrogenation of w-octane was tested 2 years later including complexes bearing N-tert-hutyl (47e) and Af-adamantyl (47f) substituents. However, only complexes 47a and 47d showed catalytic activity in this reaction with small TONs of 12 and 10 under the same conditions used for the transfer dehydrogenation of cyclooctane [16b]. As already known from the reactivity of PCP Ir pin-cer complexes [43], Chianese observed only internal isomers of octene and therefore investigated the activity of complex 47a in the isomerization of 1-hexene. Already after 15 min at 150 "C, 1-hexene was isomerized with a TON of 420 to a mixture of tr ws-2-hexene, cis-2-hexene, and 3-hexenes in a 67 29 4 ratio and after 60 min (TON 730) in a 65 26 8 ratio. Therefore the isomerization of terminal olefins is much faster than the transfer dehydrogenation. It was also concluded that the isomerization of terminal olefins is much faster than that of 2-hexenes to 3-hexenes. The isomerization of 1-octene was shown to proceed already at 100 °C with identical TON of almost 500 and nearly complete consumption of 1-octene after 24 h for 47a, 47e, and 47f. In this case, the addition of NaO Bu is required (Figure 9.14). 

Wavelength dependence of two photoreactions catalyzed by RhCl(CO)(PMe3)2, the carbonylation of benzene and the dehydrogenation of cyclooctane. Scheme 21, was studied using laser light of 308 (XeCl) and 351 (XeF) nm [65]. For the dehydrogenation both wavelengths were about equally effective (O = 0.02) but the carbonylation is much more effective at... 

A year later, the dehydrogenation of cyclooctane to cyclooctene was made catalytic by Felkin s group with the complex [ReH7(PPh3)2], (equation below) and Crabtree showed that alkanes can also be catal)itically dehydrogenated using the complex [Ir (CF3)2CO)2 H2(PPh3)2]+. 

Zuo W, Braunstein P. N-Heterocyclic dicarbene iridium(III) pincer complexes featuring mixed NHC/abnormal NHC ligands and their application in the transfer dehydrogenation of cyclooctane. Organometallics. 2012 31 2606-2615. 

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