1,10-Phenanthroline metal hydrides

As well as the above addition and elimination sequences, metal-catalyzed nucleophilic addition can occur. Examples include the zinc(II)-catalyzed hydride ion addition in the reduction of the formyl group of the phenanthroline aldehyde (49)91 and the copper(II)-catalyzed ring opening of 2-pyridyloxirane (50)92 (equations 22 and 23). 

The NAD+-dependent alcohol dehydrogenase from horse liver contains one catalytically essential zinc ion at each of its two active sites. An essential feature of the enzymic catalysis appears to involve direct coordination of the enzyme-bound zinc by the carbonyl and hydroxyl groups of the aldehyde and alcohol substrates. Polarization of the carbonyl group by the metal ion should assist nucleophilic attack by hydride ion. A number of studies have confirmed this view. Zinc(II) catalyzes the reduction of l,10-phenanthroline-2-carbaldehyde by lV-propyl-l,4-dihy-dronicotinamide in acetonitrile,526 and provides an interesting model reaction for alcohol dehydrogenase (Scheme 45). The model reaction proceeds by direct hydrogen transfer and is absolutely dependent on the presence of zinc(II). The zinc(II) ion also catalyzes the reduction of 2- and 4-pyridinecarbaldehyde by Et4N BH4-.526 The zinc complex of the 2-aldehyde is reduced at least 7 x 105 times faster than the free aldehyde, whereas the zinc complex of the 4-aldehyde is reduced only 102 times faster than the free aldehyde. A direct interaction of zinc(II) with the carbonyl function is clearly required for marked catalytic effects to be observed. 

Theoretically, in a simple kinetic resolution the ee value should not exceed 32 % at this specific conversion. In addition to the rhodium complex, this reaction requires acetophenone as stoichiometric hydride acceptor, phenanthroline as coligand and potassium hydroxide as base. An ee value of 98 % at 60 % conversion (theoretical value 67 %)is achieved with [Rh2(OAc)4] without an added base after 3 days. Surprisingly, the enzyme tolerates potassium hydroxide in amounts up to 20 mol% at elevated temperatures however, the enantiomeric excesses are somewhat lower than those obtained in an ordinary kinetic resolution. Unselective, base- or metal-catalyzed acylation might be the reason for the somewhat lower ee value. 

An early claim 332, 334) to have prepared V(bipy)3+ has been revised 431) the compound V(bipy)3l is considered to be a mixture of V(bipy)3 and V(bipy)3l2. The reaction of V(bipy) + or of Cr(bipy) + with aluminum hydride gives the neutral tris-bipyridyl complexes, but with other dipositive metals Al(bipy)3 is the major product 357). Attempts to prepare bipyridyl and phenanthroline derivatives of hexacarbonylvanadium led to the disproportionation of vanadium(O) 376). 

A bio-inspired catalyst comprising a combined organic hydride donor with metal centre has been reported for TH of imines. A previously used catalyst relied on a Hantzsch ester, but the newer derivative benefits from easier preparation and greater accessibility. Yields are high for the Rh complex containing all the components, and more than for the Ir complex or the complex containing the phenanthroline ligand alone (Fig. 38) [122, 123]. 

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