We describe a photoinduced copper‐catalyzed asymmetric radical decarboxylative alkynylation of bench‐stable N‐hydroxyphthalimide(NHP)‐type esters of racemic alkyl carboxylic acids with terminal alkynes, which provides a flexible platform for the construction of chiral C(sp3)−C(sp) bonds. Critical to the success of this process are not only the use of the copper catalyst as a dual photo‐ and cross‐coupling catalyst but also tuning of the NHP‐type esters to inhibit the facile homodimerization of the alkyl radical and terminal alkyne, respectively. Owing to the use of stable and easily available NHP‐type esters, the reaction features a broader substrate scope compared with reactions using the alkyl halide counterparts, covering (hetero)benzyl‐, allyl‐, and aminocarbonyl‐substituted carboxylic acid derivatives, and (hetero)aryl and alkyl as well as silyl alkynes, thus providing a vital complementary approach to the previously reported method.
Transition metal-catalyzed enantioselective Sonogashira-type oxidative C(sp3)—C(sp) coupling of unactivated C(sp3)−H bonds with terminal alkynes has remained a prominent challenge. The difficulties mainly stem from the regiocontrol in unactivated C(sp3)—H bond functionalization and the inhibition of readily occurring Glaser homocoupling of terminal alkynes. Here, we report a copper/chiral cinchona alkaloid-based N,N,P-ligand catalyst for asymmetric oxidative cross-coupling of unactivated C(sp3)—H bonds with terminal alkynes in a highly regio-, chemo-, and enantioselective manner. The use of N-fluoroamide as a mild oxidant is essential to site-selectively generate alkyl radical species while efficiently avoiding Glaser homocoupling. This reaction accommodates a range of (hetero)aryl and alkyl alkynes; (hetero)benzylic and propargylic C(sp3)−H bonds are all applicable. This process allows expedient access to chiral alkynyl amides/aldehydes. More importantly, it also provides a versatile tool for the construction of chiral C(sp3)—C(sp), C(sp3)—C(sp2), and C(sp3)—C(sp3) bonds when allied with follow-up transformations.
The
enantioconvergent C(sp3)–N cross-coupling
of racemic alkyl halides with (hetero)aromatic amines represents an
ideal means to afford enantioenriched N-alkyl (hetero)aromatic
amines yet has remained unexplored due to the catalyst poisoning specifically
for strong-coordinating heteroaromatic amines. Here, we demonstrate
a copper-catalyzed enantioconvergent radical C(sp3)–N
cross-coupling of activated racemic alkyl halides with (hetero)aromatic
amines under ambient conditions. The key to success is the judicious
selection of appropriate multidentate anionic ligands through readily
fine-tuning both electronic and steric properties for the formation
of a stable and rigid chelating Cu complex. Thus, this kind of ligand
could not only enhance the reducing capability of a copper catalyst
to provide an enantioconvergent radical pathway but also avoid the
coordination with other coordinating heteroatoms, thereby overcoming
catalyst poisoning and/or chiral ligand displacement. This protocol
covers a wide range of coupling partners (89 examples for activated
racemic secondary/tertiary alkyl bromides/chlorides and (hetero)aromatic
amines) with high functional group compatibility. When allied with
follow-up transformations, it provides a highly flexible platform
to access synthetically useful enantioenriched amine building blocks.
We describe a photoinduced copper‐catalyzed asymmetric radical decarboxylative alkynylation of bench‐stable N‐hydroxyphthalimide(NHP)‐type esters of racemic alkyl carboxylic acids with terminal alkynes, which provides a flexible platform for the construction of chiral C(sp3)−C(sp) bonds. Critical to the success of this process are not only the use of the copper catalyst as a dual photo‐ and cross‐coupling catalyst but also tuning of the NHP‐type esters to inhibit the facile homodimerization of the alkyl radical and terminal alkyne, respectively. Owing to the use of stable and easily available NHP‐type esters, the reaction features a broader substrate scope compared with reactions using the alkyl halide counterparts, covering (hetero)benzyl‐, allyl‐, and aminocarbonyl‐substituted carboxylic acid derivatives, and (hetero)aryl and alkyl as well as silyl alkynes, thus providing a vital complementary approach to the previously reported method.
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