Hydrogenation with achiral reactants

Acetylpyridine, cathodic pinacolisation, 40 165 Achiral reactants hydrogenation with, 42 489-498 Acid-base catalysis... 

A nice analysis of non-linear effects in Rh-chiral diamine-catalysed transfer hydrogenation has been performed that reinforces the need to consider the kinetics of all of the steps in reaction manifolds (e.g. reversible formation of diastereomeric precursors and their subsequent interaction with achiral reactants). ... 

On many occasions, a reaction carried out with achiral reactants results in the formation of a chiral product. In the absence of any chiral influence, the outcome of such reactions is the formation of a racemic form. For example, hydrogenation of ethylmethylketone yields a racemic mixture of 2-hydro-xybutane. 

How important is stereochemistry in chemical reactions The answer depends on the nature of the reactants. First, consider the formation of a chiral product from achiral reactants for example, the addition of hydrogen bromide to 1-butene to give 2-bromobutane in accord with Markovnikov s Rule. 

When a reactant is chiral but optically inactive because it is racemic any products derived from its reactions with optically inactive reagents will be optically inactive For example 2 butanol is chiral and may be converted with hydrogen bromide to 2 bromo butane which is also chiral If racemic 2 butanol is used each enantiomer will react at the same rate with the achiral reagent Whatever happens to (/ ) (—) 2 butanol is mir rored m a corresponding reaction of (5) (+) 2 butanol and a racemic optically inactive product results... 

Diastereoselective addition has been carried out with achiral reagents and chiral substrates, similar to the reduction shown on page. 1201, but because the attacking atom in this case is carbon, not hydrogen, it is also possible to get diastereoselective addition with an achiral substrate and an optically active reagent. Use of suitable reactants creates, in the most general case, two new chiral centers, so the product can exist as two pairs of enantiomers ... 

Substituted acrylates (which reseitible the enamide substrates employed 1n asymmetric hydrogenation) may be deracemized by reduction with an optically active catalyst, especially DIPAMPRh . Selectivity ratios of 12 1 to 22 1 have been obtained for a variety of reactants with compounds of reasonable volatility, separation of starting material and product may be effected by preparative GLC. Recovered starting material can then be reduced with an achiral catalyst to give the optically pure anti product. Examples of kinetic resolutions by this method are given in Table II. More recently very successful kinetic resolutions of allylic alcohols have been carried out with Ru(BINAP) catalysts. 

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