One of the most challenging transformations in current organic chemistry is the catalytic carboxylation of a C(sp(3))-H bond using CO(2) gas, an inexpensive and ubiquitous C1 source. A sequential protocol for C(sp(3))-H carboxylation by employing a nitrogen-directed, metal-assisted, C-H activation/catalytic silylation reaction in conjunction with fluoride-mediated carboxylation with CO(2) was established. The carboxylation proceeded only at the benzylic C(sp(3))-Si bond, not at the aromatic C(sp(2))-Si, which is advantageous for further manipulations of the products.
Catalytic carboxylation of the allylic C(sp)-H bond of terminal alkenes with CO was developed with the aid of a Co/Xantphos complex. A wide range of allylarenes and 1,4-dienes were successfully transformed into the linear styrylacetic acid and hexa-3,5-dienoic acid derivatives in moderate to high yields, with excellent regioselectivity. The carboxylation showed remarkable functional group tolerability, so that selective addition to CO occurred in the presence of other carbonyl groups such as amide, ester, and ketone. Since styrylacetic acid derivatives can be readily converted into optically active γ-butyrolactones through Sharpless asymmetric dihydroxylation, this allylic C(sp)-H carboxylation showcases a facile synthesis of γ-butyrolactones from simple allylarenes via short steps.
Now that is just silylated: In the presence of iridium or rhodium catalysts, C(sp3)H bonds adjacent to a nitrogen atom were silylated by the aid of a pyridine‐directing group. In iridium catalysis, a hydrogen‐trapping reagent such as norbornene or tert‐butylethylene, which is usually required in late transition‐metal‐catalyzed dehydrogenative coupling reactions, was not required. In rhodium catalysis, however, 1 equivalent of COD (1,5‐cyclooctadiene) was necessary to induce higher conversion.
Alkane-gem-diboronic acids have emerged
as versatile
organoboron catalysts for dehydrative amidation of α-amino acids.
A phenol-substituted multiboron catalyst with a B–C–B
structure outperformed simple arylboronic acids in the condensation
of α-amino acids with suppressed epimerization of electrophiles. gem-diboronic acid catalysis were compatible with various O, N, and S-functionalized
α-amino acids bearing N-protecting groups including
common carbamates used in peptide synthesis (Boc, Cbz, Fmoc). N-trifluoroacetyl protection enabled an unprecedented catalytic
dehydrative peptide synthesis at room temperature. Preliminary mechanistic
studies revealed carboxylate-binding nature of gem-diboronic acids, orthogonal to the activation of carboxylic acids
by arylboronic acids. The distinctive reactivity of the gem-diboronic acids would open prospects for mild catalytic peptide
condensation.
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