Understanding Halide Counterion Effects in Enantioselective Ruthenium-Catalyzed Carbonyl (α-Aryl)allylation: Alkynes as Latent Allenes and Trifluoroethanol-Enhanced Turnover in The Conversion of Ethanol to Higher Alcohols via Hydrogen Auto-transfer

Abstract

Crystallographic characterization of RuX­(CO)­(η3-C3H5)­(JOSIPHOS), where X = Cl, Br, or I, reveals a halide-dependent diastereomeric preference that defines metal-centered stereogenicity and, therefrom, the enantioselectivity of C−C coupling in ruthenium-catalyzed anti-diastereo- and enantioselective C−C couplings of primary alcohols with 1-aryl-1-propynes to form products of carbonyl anti-(α-aryl)­allylation. Computational studies reveal that a non-classical hydrogen bond between iodide and the aldehyde formyl CH bond stabilizes the favored transition state for carbonyl addition. An improved catalytic system enabling previously unattainable transformations was developed that employs an iodide-containing precatalyst, RuI­(CO)3(η3-C3H5), in combination with trifluoroethanol, as illustrated by the first enantioselective ruthenium-catalyzed C−C couplings of ethanol to form higher alcohols.

Stereo- and Site-Selective Conversion of Primary Alcohols to Allylic Alcohols via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by 2-Butyne

“…In recent work, ruthenium complexes modified by JOSIPHOS ligands were found to catalyze redox-neutral couplings of sec -alkyl- or aryl-substituted propynes with primary alcohols to form branched secondary homoallylic alcohols with excellent control of anti -diastereo- and enantioselectivity . These processes occur via tandem alkyne-to-allene redox isomerization and allene-aldehyde reductive coupling via hydrogen autotransfer.…”

“…Regarding the scope of the alkyne, as previously described, aryl or sec -alkyl propynes provide branched products of allylation due to competing alkyne-to-allene isomerization (eq ). Higher linear alkynes, such as 3-hexyne, provide mixtures of vinylation and allylation products. However, as demonstrated by the formation of 3n (eq ), 3o (eq ), and epi - 3o (eq ), 2-pentyne 1b participates in stereo- and site-selective carbonyl vinylation with good levels of regiocontrol.…”

“…Thus, to account for contrasteric regioselectivity in the reaction of 2-pentyne, we propose that rapid and reversible alkyne hydroruthenation enables a Curtin–Hammett scenario in which contrasteric regioselectivity arises in response to nonbonded interactions associated with the indicated 4-centered transition state for carbonyl vinylation (eq ). Another mechanistic possibility that was considered involves alkyne-to-allene isomerization to form products of crotylation (as described in allylation eq ), which may then be followed by redox isomerization to generate the product of vinylation . To probe this possibility, the terminal olefin iso - 3a was subjected to standard reaction conditions in the presence of 2-propanol.…”

Enantioselective Metal-Catalyzed Reductive Coupling of Alkynes with Carbonyl Compounds and Imines: Convergent Construction of Allylic Alcohols and Amines

“…Deuterium labeling experiments corroborate a catalytic cycle in which alcohol dehydrogenation forms an aldehyde and a ruthenium hydride, which upon alkyne hydrometalation delivers a nucleophilic vinylruthenium species that participates in carbonyl addition. Beyond interactions with the chiral ligand, the absolute stereochemical course of vinylation is influenced by formyl CH•••I[Ru] and CH•••OC[Ru] hydrogen bonding 26,27 and iodide-dependent stereogenicity of the ruthenium center. 28 3.4.…”

Computational study of Cu-catalyzed 1,2-hydrocarboxylation of 1,3-dienes with CO2: Pauli repulsion-controlled regioselectivity of Cu–Bpin additions

“…, II and III ) could react with CO 2 through 6-membered transition states. 2 c ,6 These further increase the complexity of positions for incorporating CO 2 . The protonation of the C–Cu bonds with MeOH can also compete with CO 2 insertion.…”