Alkanes transfer dehydrogenation

Another inhibitor of this catalytic cycle was the a-olefin product resulting from K-alkane transfer dehydrogenation, which can ji-bond with the 14e complex, Ir-2. By density functional theory (DFT) calculations, the easier transfer dehydrogenation of COA relative to -alkanes was attributed to the lesser amount of Jt-bonding of the COE product to Ir-2 [112]. 

Using I-H2 or 4a-H2 as the transfer-dehydrogenation catalysts, 1.25 and 2.05 M total product concentrations, respectively, were obtained from 7.6 M -hexane after 1 day at 125°C. In accord with the proposal illustrated in Scheme 2, exclusively linear -alkanes were formed (in contrast with the Basset systems). But in contrast with the idealized cycle depicted in Scheme 2, the mixtures were not restricted to C2n-2 alkane and ethane as products. 

The high activity of iridium PCP pincer complexes in transfer dehydrogenation has been applied in a very elegant approach to devise the first homogeneous alkane metathesis process (Equation 12.5) [3]. 

Alkane/NBE transfer dehydrogenation experiments were typically conducted as follows. In an argon-atmosphere glovebox, 0.5 mL alkane solution (15 mM catalyst, and NBE) was placed in a 5-mL reactor. The reactor was fitted with a Kontes high-vacuum stopcock, which enables freeze-pump-thaw cycles and addition of argon,... 

Rhodium and iridium complexes of basic trialkylphosphines catalyze the dehydrogenation of alkanes to alkenes and H2, or the transfer dehydrogenation of alkanes in the presence of H2 receptors such as CH2=CHBu . For example, RhCl(CO)(PMe3)2 becomes active after photolytic CO dissociation, while the compounds RhClLy (L = PMe3, PPr 3 or PCy3) are thermally active. The initial step is the formation of a Rh111 dihydride 135... 

Transfer Dehydrogenations of Alkanes and Related Reactions Using Iridium Pincer Complexes... 

A major breakthrough in transfer dehydrogenation of alkanes was achieved in 1996 by Jensen, Kaska, and coworkers [16, 17]. They reported that the iridium pincer complex ( " PCP)IrH2, la, was highly reactive and exceptionally thermally stable for transfer dehydrogenation of COA employing TBE as the acceptor [Eq. (3)]. For example, at 200°C the turnover frequency was reported to be 12/min with no noticeable catalyst decomposition over 7 days. 

Other n(Mi-phosphine-based catalysts active for alkane dehydrogenation were developed by the Jensen group [53]. The complex ( " AsOCOAs)IrHCl, 13 (Fig. 9), combined with NaO Bu catalyzed the transfer dehydrogenation of COA with TBE (1 1) giving TONs of up to 930 after 24 h with initial rates of 600 TO h at 200°C. The leveling off in catalytic activity was explained by inhibition by the COE product, as weU as thermal decomposition of the catalyst over time. 

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