Cyclooctane dehydrogenation

If alkyl groups having (3-hydrogens are present on platinum cis to an open site, (3-H-elimination will indeed occur, reversibly sometimes, and it can occur both from Pt(II) and Pt(IV) (52,97,213-219). Catalytic dehydrogenation of an alkane using a soluble platinum complex has been reported in an early study on acceptorless thermal dehydrogenation. At 151 °C, cyclooctane was catalytically dehydrogenated (up to 10 turnovers)... 

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]. 

I object to the use of the term alicyclicity in this connection. The methods used by Peover, Wender, and Fuks are selective for that group of alicyclic substances capable of yielding aromatic structures on dehydrogenation—i.e., for hydroaromatic rings. If the sulfur method really dehydrogenates any alicyclic structures (e.g. cyclooctane or camphene), then it would yield olefins rather than aromatics and could probably also convert saturated chains to olefins. On the other hand, if it attacks only hydroaromatic structures, then alicyclicity is an incorrect and misleading expression, and hydroaromaticity should be used. 

Dehydrocyclizatlon Pt(0) can be used for dehydrogenation of alkanes to alkenes. Ti(0) is known to absorb H2 to form a dihydride. Paquette et al.1 reasoned that a combination of the two metals could in principal effect dehydrocyclization. Indeed, cyclooctane when heated with Pt(0) and Ti(0) (1 1) adsorbed in A1203 is converted into bicyclo[3.3.0]octane (equation I). 

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.

Russian workers have demonstrated that cyclooctane can be dehydrogenated to crs-bicyclo[3.3.0]octane when passed in the gas phase over a heated catalyst such as platinum on carbon, platinum and iron on carbon, or nickel on Kieselguhr.93-98 Unfortunately, the yields are highly variable (0.5—70 %) and details of these processes are scanty. Perhaps more promising is their discovery that n-octane and its cyclodehydrogenation product n-propylcyclopentane can be converted to the bi-cyclic hydrocarbon under comparable conditions. 

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... 

Hydride complexes have been important precursors in the study of Alkane Activation. For example, alkanes can be catalytically dehydrogenated by ReH7(PR3)2 or [hH2(OH2)2(PR3)2] or (PCP)hH2 thermally or photo-chemically. Cyclooctane is the best snbstrate, presumably because it has the least unfavorable heat of dehydrogenation of all common alkanes (equation 27). 

Cycloheptane and cyclooctane gave r/ -cycloheptadienyl and cyclooc-tadiene complexes, respectively, but other alkanes, e.g., methylcyclopen-tane, adamantane, or bicyclooctane, give as yet uncharacterized products which probably contain the dehydrogenated alkane. 

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. 

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