A structurally characterized Rh(I) iminocarbene complex (N,C)Rh(CO)Cl is activated with AgOTf to act as a highly cis-selective catalyst for the cyclopropanation of substituted styrenes and other alkenes with ethyl diazoacetate (11 examples, 10-99% yield, up to >99% cis-selectivity).
The performance of recently reported highly cis-diastereoselective Rh(I) cyclopropanation catalysts has been significantly improved by a systematic study of different reaction parameters (catalyst activation, solvent, temperature, stoichiometry). The catalyst efficiency and diastereoselectivity were enhanced by changing the activating agent from AgOTf to NaBArf. With this new system, the Rh(I) catalyst was shown to be a highly efficient and cis-diastereoselective cyclopropanation catalyst in reactions between α-diazoacetates and a range of different alkenes and substituted derivatives. Particularly noteworthy is the remarkable reactivity and cis-diastereoselectivity displayed in the reactions between ethyl diazoacetate and cyclopentene, 2,5-dihydrofuran, and benzofuran, with yields up to 99% and cis-selectivities greater than 99%.
A computational study at the DFT level using BP86 and
dispersion-corrected
D-BP86 methods has been performed on the mechanism of a highly cis-diastereoselective cyclopropanation reaction between
ethyl diazoacetate and styrene, catalyzed by a Rh(I) complex bearing
a chelating imine-functionalized NHC ligand. The key steps in the
mechanism have been elucidated. The favored mechanistic pathway was
found to be a stepwise mechanism involving the formation of a Rh metallacyclobutane
intermediate. The results from the theoretical study indicate that
the diastereoselectivity is determined in the step where styrene coordinates
to the Rh(I) carbenoid and that the high cis-diastereoselectivity
can be attributed to an unfavorable steric interaction between styrene
and the substituents on the N-aryl ring in the ligand
system, which disfavors the formation of the trans cyclopropanation product.
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