Rhodium-catalyzed
asymmetric anti-Markovnikov hydroformylation
of α-substituted acrylates/acrylamides has been developed. By
employing the Rh/(S,S)-DTBM-YanPhos
complex, a series of β-chiral linear aldehydes were obtained
in high yields (up to 94% yield) and high enantioselectivities (up
to 96% ee). The utility of this methodology is demonstrated by a gram-scale
reaction and a concise synthetic route to chiral γ-butyrolactone.
Strem Chemicals, Inc., established in 1964, is a privately held manufacturer & marketer of high purity specialty chemicals. Strem's products are used for R&D and commercial scale applications, especially in the pharmaceutical, microelectronics, chemicals & petrochemicals industries. Some key products include catalysts, ligands, organometallics, CVD/ALD precursors and nanomaterials. For more information, N N Cu CF 3
A deeply
ingrained assumption in the conventional understanding
and practice of organometallic chemistry is that an unactivated aliphatic
C(sp3)–H bond is less reactive than an aromatic
C(sp2)–H bond within the same molecule given that
they are at positions unbiasedly accessible for activation. Herein,
we demonstrate that a pincer-ligated iridium complex catalyzes intramolecular
dehydrogenative silylation of the unactivated δ-C(sp3)–H (δ to the Si atom) with exclusive site selectivity
over typically more reactive ortho δ-C(sp2)–H bonds. A variety of tertiary hydrosilanes undergo
δ-C(sp3)–H silylation to form 5-membered silolanes,
including chiral silolanes, which can undergo further oxidation to
produce enantiopure β-aryl-substituted 1,4-diols. Combined computational
and experimental studies reveal that the silylation occurs via the
Si–H addition to a 14-electron Ir(I) fragment to give an Ir(III)
silyl hydride complex, which then activates the C(sp3)–H
bond to form a 7-coordinate, 18-electron Ir(V) dihydride silyl intermediate,
followed by sequential reductive elimination of H2 and
silolane. The unprecedented site selectivity is governed by the distortion
energy difference between the rate-determining δ-C(sp3)–H and δ-C(sp2)–H activation, although
the activation at sp2 sites is much more favorable than
sp3 sites by the interaction energy.
Chiral bridged [2,2,1] bicyclic lactones are privileged structural units in pharmaceutics and bioactive nature products. However, the synthetic methods for these compounds are rare. Here we report an efficient method for enantioselective construction of bridged [2,2,1] bicyclic lactones bearing a quaternary stereocenter via Rh-catalyzed asymmetric hydroformylation/intramolecular cyclization/pyridium chlorochromate (PCC) oxidation. By employing a hybrid phosphine-phosphite chiral ligand, a series of cyclopent-3-en-1-ols are transformed into corresponding γ-hydroxyl aldehydes with specific syn-selectivity. Then, hemiacetals form in situ and oxidation with PCC in one-pot affords bridged [2,2,1] bicyclic lactones in high yields and excellent enantiomeric excess. Replacing the hydroxyl group by an ester group, cyclopentanecarbaldehydes with a chiral all-carbon quaternary stereocenter in the γ-position can be generated efficiently.
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