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to conclude , stereo- and chemodivergent asymmetric reaction pathways are observed upon treatment of alkylarylketenes and chloral with chiral nhcs , giving selectively either -lactones ( up to 88:12 dr , up to 94 % ee ) or -chloroesters ( up to 94 % ee ) , with 2-arylsubstitution or -branching within the alkyl chain of the ketene unit leading to the -chlorination pathway .
computational studies on a model system have allowed the structural parameters that lead to selectivity in these reaction processes to be analysed .
current research from this laboratory is directed toward developing alternative uses of nhcs and other lewis bases in asymmetric catalysis .
for general experimental details , full characterisation data , nmr spectra and hplc traces , see the supporting information . to a flame dried schlenk flask under an argon atmosphere
was added nhc precatalyst ( 0.10 mmol ) , base ( 0.09 mmol ) and toluene ( 6 ml ) and the mixture stirred for 15 min . the mixture
was then cooled to 0 c in an ice / h2o bath followed by addition of a 0 c solution of the requisite ketene ( 1.00 mmol ) in toluene ( 12 ml ) , immediately followed by chloral ( 1.00 mmol ) .
toluene ( 2 ml ) was added to wash residual reactants into solution and the reaction was stirred for the stated time at 0 c before opening the flask to the air for 30 min and concentration in vacuo .
the resulting crude residue was purified by flash silica chromatography ( ether : petrol ) to provide either the isolated lactone or chlorinated ester as stated . in instances
where ketene dimerization was competitive with lactonisation or chlorination the ketene was added dropwise . to a flame dried schlenk flask under an argon atmosphere
was added nhc precatalyst ( 0.10 mmol ) , base ( 0.09 mmol ) and toluene ( 6 ml ) and the mixture stirred for 15 min . the mixture
was then cooled to 0 c in an ice / h2o bath followed by addition of chloral ( 1.00 mmol ) .
a 0 c solution of the requisite ketene ( 1.00 mmol ) in toluene ( 12 ml ) was subsequently added over 0.5 h. the reaction was stirred for an additional 3 h at 0 c before opening the flask to the air for 0.5 h and concentration in vacuo .
the resulting crude residue with the stated diastereomeric ratio was purified by flash silica chromatography ( ether : petrol ) to provide either the isolated lactone or chlorinated ester .
to a flame dried schlenk flask under an argon atmosphere was added nhc precatalyst ( 0.10 mmol ) , base ( 0.09 mmol ) and toluene ( 6 ml ) and the mixture stirred for 15 min . the mixture
was then cooled to 0 c in an ice / h2o bath followed by addition of a 0 c solution of the requisite ketene ( 1.00 mmol ) in toluene ( 12 ml ) , immediately followed by chloral ( 1.00 mmol ) .
toluene ( 2 ml ) was added to wash residual reactants into solution and the reaction was stirred for the stated time at 0 c before opening the flask to the air for 30 min and concentration in vacuo .
the resulting crude residue was purified by flash silica chromatography ( ether : petrol ) to provide either the isolated lactone or chlorinated ester as stated .
in instances where ketene dimerization was competitive with lactonisation or chlorination the ketene was added dropwise . to a flame dried schlenk flask under an argon atmosphere
was added nhc precatalyst ( 0.10 mmol ) , base ( 0.09 mmol ) and toluene ( 6 ml ) and the mixture stirred for 15 min . the mixture
was then cooled to 0 c in an ice / h2o bath followed by addition of chloral ( 1.00 mmol ) .
a 0 c solution of the requisite ketene ( 1.00 mmol ) in toluene ( 12 ml ) was subsequently added over 0.5 h. the reaction was stirred for an additional 3 h at 0 c before opening the flask to the air for 0.5 h and concentration in vacuo .
the resulting crude residue with the stated diastereomeric ratio was purified by flash silica chromatography ( ether : petrol ) to provide either the isolated lactone or chlorinated ester .
as a service to our authors and readers , this journal provides supporting information supplied by the authors .
such materials are peer reviewed and may be re - organized for online delivery , but are not copy - edited or typeset . technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors
stereo- and chemodivergent enantioselective reaction pathways are observed upon treatment of alkylarylketenes and trichloroacetaldehyde ( chloral ) with n - heterocyclic carbenes , giving selectively either -lactones ( up to 88:12 dr , up to 94 % ee ) or -chloroesters ( up to 94 % ee ) . 
 either 2-arylsubstitution or an -branched ipr alkyl substituent within the ketene favours the chlorination pathway , allowing chloral to be used as an electrophilic chlorinating reagent in asymmetric catalysis .