Cyclooctane, transfer dehydrogenation

Scheme 2.14 Cyclooctane transfer dehydrogenation using tbe as a sacrificial hydrogen acceptor.

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 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). 

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. 

The iridium(l) PCP pincer complexes 1 exhibit remarkable activity in the catalytic dehydrogenation of unfunctionalized alkanes (Scheme 12.1). The H2, which is formally produced during this process, may be transferred to either tert-butyleth-ylene (TBE) or norbomene (NBE) as a sacrificial hydrogen acceptor. For example, complex la converts cyclooctane (COA) to cyclooctene (COE) in the presence of TBE, which in turn is reduced to tert-butylethane (TBA ueo-hexane) [6]. 

Alkane dehydrogenation took a step forward in 1996 with the report of rhodium and iridium pincer complexes that could catalyze transfer hydrogenation. While the rhodium complex was found to be active but unstable, the iridium complex was stable even after a week at 200 °C. This permitted it to efficiendy catalyze the transfer hydrogenation of cyclooctane to cyclooctene (12 t.o./min, Scheme The reaction is inhibited by high concentrations of olefin, either the... 

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