Transfer dehydrogenations

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

Johnson and Backvall reported that the bimetallic Shvo catalyst can also catalyze the transfer dehydrogenation of alcohols (Eq. (48)) [83]. 

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]. 

The reaction conditions applied for transfer dehydrogenation using complex 5a are compatible with the conditions required for olefin metathesis with Schrock s molybdenum alkyhdene complex 10 [21]. 

At this point it became clear that, although a wide variety of complexes could be employed for both transfer dehydrogenation and photochemical dehydrogena-... 

Transfer Dehydrogenative Coupling of Trialkylsilanes in the Presence of Olefins... 

This reaction is also a transfer dehydrogenative reaction, as two reactant hydrogen atoms are not incorporated into the enol silyl ether product but instead serve to hydrogenate another molecule of starting alkene. For example, in the reaction of vinylcyclohexane, ethylcyclohexane is obtained in equal amounts to the silylated product. Both iridium complexes effectively catalyze the reaction. Various silanes can be used, including di-ethylmethyl-, triethyl-, and dimethylphenylsilane. The reaction is successful for a range of terminal alkenes, even those bearing cyano, acetal, and epoxide functionalities. The E isomer of the product is predominantly formed. 

Dehydrogenation of 1,4- or 1,5-diols to lactones. This Ru-catalyzed transfer dehydrogenation can be effected with high selectivity when the 2-position of 1 is... 

The use of anaerobic conditions would overcome safety concerns relating to oxygen. Homogeneous Ru and Ir complexes have recently been reported to be efficient systems for acceptor-free dehydrogenation of alcohols [50-52], while very few heterogeneous catalysts are reported to be active in transfer dehydrogenation, the most recent being Pd [53-55] and Ru [56] systems, active only with benzylic or allylic alcohols. 

Table 13.3 Transfer dehydrogenation of different substrates using Cu/Al203.a...

Although a suitable acceptor for the transfer dehydrogenation of benzylic alcohols has not yet been found, under the present conditions the low conversion of benzylic alcohols is only an apparent drawback. Indeed, it has a positive side as it allows us to fine-tune the system s selectivity. This makes the catalytic system unique among all the others known, operating both under aerobic and anaerobic conditions, that preferentially oxidize benzylic alcohols with respect to nonacti-vated secondary ones. 

It is also worth noting that the use of transfer dehydrogenation allows operation under anaerobic atmosphere, thus avoiding the formation of overoxidation prod-... 

To better appreciate the flexibility of these catalytic systems it is worth mentioning that under transfer dehydrogenation conditions, namely by adding styrene to the reaction mixture, oxidation of (R)-1-phenylethanol proceeds without racemization up to 80% conversion [58]. This proves that it is possible to maximize and exploit the selectivity of the catalyst by tuning reaction conditions and by choosing the proper donor-acceptor couple. 

Also dihydrocarvone 11 can be obtained in moderate yield from carveol 1 using Cu/A1203. The comparison with transfer dehydrogenation reaction carried out over the same substrate (Table 13.3 entry 10), shows once more the pivotal role and unique properties of the hydrogen acceptor (Scheme 13.7). 

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,... 

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... 

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... 

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... 

Grubbs reported tandem olefin metathesis/hydrogenation as a means to make satnrated cyclic prodncts and a one-pot RCM/transfer dehydrogenation/hydrogenation ronte to (R)-(-)-Mnscone utilizing (4a) (Scheme 19). Snapper and coworkers took advantage of the isomerization side reaction... 

Cyclopentadimylidene (2). 2 was very easily obtained by photolysis or thermolysis of the corresponding diazo-cyclopentadienes (72). These were prepared by straightforward procedures (diazo-group transfer, dehydrogenation of hydrazones, or Bamford-Stevens reaction of tosyl-hydrazones) 26-80) from either cyclopentadienes (77) or substituted cyclopentadienones (73). 

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