The benzene ring of indolin-2-ones (2-oxindoles) and 3,4-dihydroquinol-2-ones was converted to a saturated cyclohexane ring by hydrogenation in the presence of the rhodium complex Cy (CAAC)Rh(cod)Cl. The stereoselectivity of the process was found to be high with respect to both external substituent R 1 within the saturated part of the heterocyclic ring and substituent X on the benzene ring. Twenty-one hexahydroindolin-2(3H)-ones (70−99% yield, dr = 83/17 to >99/1) and twelve octahydro-2(1H)-quinolinones (87−96% yield, dr = 64/36 to >99/1) were obtained with the major diastereoisomer exhibiting the hydrogen atoms in an all-cis arrangement. The high tolerance toward functional groups and the compatibility with existing stereogenic centers are key features of the hydrogenation protocol presented here.Letter pubs.acs.org/OrgLett
The direct hydrogenation
of aromatic systems is a compelling tool
to access saturated carbo- and heterocycles, an omnipresent motif
in natural products, pharmaceuticals, and agrochemicals. Although
a variety of methods have been established for simple arenes, the
selective hydrogenation of aromatic 2,5-diketopiperazines (2,5-DKPs)
has not been explored. We herein report a modular approach to perform
a highly diastereoselective arene hydrogenation of symmetric 2,5-DKPs
catalyzed by a rhodium complex. Twenty-eight saturated pentacyclic
2,5-DKPs were obtained in high yields with exquisite diastereoselectivities,
exhibiting the hydrogen atoms in an all-cis arrangement. The high
tolerance toward functional groups and the compatibility with existing
stereogenic centers are key features of the transformation.
Bis(oxazolinyldimethylmethyl)pyrrol (PdmBox) stereodirecting ligands provided the key to the chromium(ii)-catalysed highly enantioselective hydrosilylation of ketones. A rare square planar, chiral chromium(ii) alkyl complex was found to serve as a potent precatalyst for the reduction of a broad range of aryl alkyl and dialkyl ketone derivatives. The stereoelectronic preference of the open d4 shell of chromium(ii) firmly locks the molecular catalyst in a square planar geometry giving rise to two blocked quadrants of the coordination sphere. This earth-abundant base metal catalytic platform produces the corresponding chiral alcohols in excellent isolated yields with up to 98 %ee under mild reaction conditions (-40 °C to rt) and at low catalyst loadings (as low as 0.5 mol%).
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