Water is cheap, generally available, non-toxic and non-flammable and its application as a benign and sustainable alternative to organic solvents has become a highly active research field. Chemically, water is unique as it combines high nucleophilicity and donor-strength with a large dielectric constant, excellent hydrogen-bonding ability and the intrinsic availability of protons. While most of these properties commonly limit applications of aqueous reactions due to the low stability and solubility of reactants as well as catalysts, we recently established cross-coupling protocols utilizing the high polarity of water for straightforward product isolation by basic filtration since lipophilic coupling products separate from the aqueous phase under optimized conditions.<br/>In this presentation we detail our studies on enantioselective allylic alkylations. Our initial results indicated that under otherwise similar conditions, chiral induction is much lower in water than in organic solvents. Spectroscopic and kinetic studies revealed significant mechanistic differences for aqueous media, which we attribute to the solvent’s high polarity. Based on these results, we were able to develop a catalytic protocol, which achieves enantioselectivities of up to 98%. We also used our results to develop artificial metalloenzymes for the allylic alkylation reactions.<br/>Our results show that water is a particularly versatile solvent for palladium catalysed cross-coupling reactions, which contributes to outstanding catalyst activities under optimized conditions and enables the combination of biology and organometallic catalysis. However, the high polarity introduces changes in mechanism, which need to be considered to achieve high asymmetric induction.<br/>

Highly enantioselective allylic alkylations in water

251st National Meeting (2016)

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