Catalytic enantioselective Grignard Nozaki-Hiyama methallylation from the alcohol oxidation level: chloride compensates for π-complex instability

Methallyl chloride serves as an efficient allyl donor in highly enantioselective Grignard Nozaki-Hiyama methallylations from the alcohol or aldehyde oxidation level via iridium catalyzed transfer hydrogenation. Under identical conditions, methallyl acetate does not react efficiently. Double methallylation of 1,3-propanediol provides the C(2)-symmetric adduct as a single enantiomer, as determined by HPLC analysis.     

Catalyst‐Directed Diastereo‐ and Site‐Selectivity in Successive Nucleophilic and Electrophilic Allylations of Chiral 1,3‐Diols: Protecting‐Group‐Free Synthesis of Substituted Pyrans

The iridium‐catalyzed, protecting group‐free synthesis of 4‐hydroxy‐2,6‐cis‐ or trans‐pyrans through successive nucleophilic and electrophilic allylations of chiral 1,3‐diols occurs with complete levels of catalyst‐directed diastereoselectivity in the absence of protecting groups, premetallated reagents, or discrete alcohol‐to‐aldehyde redox reactions.     

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol or aldehyde oxidation level via transfer hydrogenative coupling of allyl acetate: departure from chirally modified allyl metal reagents in carbonyl addition

Under the conditions of transfer hydrogenation employing an iridium catalyst generated in situ from [Ir(cod)Cl]2, chiral phosphine ligand (R)-BINAP or (R)-Cl,MeO-BIPHEP, and m-nitrobenzoic acid, allyl acetate couples to allylic alcohols 1a-c, aliphatic alcohols 1d-l, and benzylic alcohols 1m-u to furnish products of carbonyl allylation 3a-u with exceptional levels of asymmetric induction. The very same set of optically enriched carbonyl allylation products 3a-u are accessible from enals 2a-c, aliphatic aldehydes 2d-l, and aryl aldehydes 2m-u, using iridium catalysts ligated by (-)-TMBTP or (R)-Cl,MeO-BIPHEP under identical conditions, but employing isopropanol as a hydrogen donor. A catalytically active cyclometallated complex V, which arises upon ortho-C-H insertion of iridium onto m-nitrobenzoic acid, was characterized by single-crystal X-ray diffraction. The results of isotopic labeling are consistent with intervention of symmetric iridium pi-allyl intermediates or rapid interconversion of sigma-allyl haptomers through the agency of a symmetric pi-allyl. Competition experiments demonstrate rapid and reversible hydrogenation-dehydrogenation of the carbonyl partner in advance of C-C coupling. However, the coupling products, which are homoallylic alcohols, experience very little erosion of optical purity by way of redox equilibration under the coupling conditions, although isopropanol, a secondary alcohol, may serve as terminal reductant. A plausible catalytic mechanism accounting for these observations is proposed, along with a stereochemical model that accounts for the observed sense of absolute stereoinduction. This protocol for asymmetric carbonyl allylation transcends the barriers imposed by oxidation level and the use of preformed allyl metal reagents.     

Ruthenium-catalyzed C-C bond forming transfer hydrogenation: carbonyl allylation from the alcohol or aldehyde oxidation level employing acyclic 1,3-dienes as surrogates to preformed allyl metal reagents

Under the conditions of ruthenium-catalyzed transfer hydrogenation, commercially available acyclic 1,3-dienes, butadiene, isoprene, and 2,3-dimethylbutadiene, couple to benzylic alcohols 1a-6a to furnish products of carbonyl crotylation 1b-6b, carbonyl isoprenylation 1c-6c, and carbonyl reverse 2-methyl prenylation 1d-6d. Under related transfer hydrogenation conditions employing isopropanol as terminal reductant, isoprene couples to aldehydes 7a-9a to furnish identical products of carbonyl isoprenylation 1c-3c. Thus, carbonyl allylation is achieved from the alcohol or the aldehyde oxidation level in the absence of preformed allyl metal reagents. Coupling to aliphatic alcohols (isoprene to 1-nonanol, 65% isolated yield) and allylic alcohols (isoprene to geraniol, 75% isolated yield) also is demonstrated. Isotopic labeling studies corroborate a mechanism involving hydrogen donation from the reactant alcohol or sacrificial alcohol (i-PrOH).     

Diastereo- and Enantioselective Reductive Aldol Addition of Vinyl Ketones via Catalytic Hydrogenation

An overview of studies on hydrogenative reductive aldol addition is presented. By simply hydrogenating enones in the presence of aldehydes at ambient temperature and pressure, aldol adducts are generated under neutral conditions in the absence of any stoichiometric byproducts. Using cationic rhodium complexes modified by tri(2-furyl)phosphine, highly syn-diastereoselective reductive aldol additions of vinyl ketones are achieved. Finally, using novel monodentate TADDOL-like phosphonite ligands, the first highly diastereo- and enantioselective reductive aldol couplings of vinyl ketones were devised. These studies, along with other works from our laboratory, demonstrate that organometallics arising transiently in the course of catalytic hydrogenation offer byproduct-free alternatives to preformed organometallic reagents employed in classical carbonyl addition processes.     

Reductive aldol coupling of divinyl ketones via rhodium-catalyzed hydrogenation: syn-diastereoselective construction of beta-hydroxyenones

Catalytic hydrogenation of divinyl ketones 1a and 1e in the presence of diverse aldehydes 2a-e at ambient temperature and pressure using cationic rhodium catalysts ligated by tri-2-furyl phosphine enables formation of aldol products 3a-e and 5a-e, respectively, with high levels of syn diastereoselection. Through an assay of counterions (Rh(COD)2X), Rh(COD)2SbF6 is identified as the optimum precatalyst for reductive aldol couplings of this type. For para-substituted styryl vinyl ketones 1b-e, a progressive increase in isolated yield is observed for electron-releasing para substituents. [reaction: see text].     

Enantioselective reductive coupling of 1,3-enynes to heterocyclic aromatic aldehydes and ketones via rhodium-catalyzed asymmetric hydrogenation: mechanistic insight into the role of Brønsted acid additives

Hydrogenation of 1,3-enynes 1a-e in the presence of heterocyclic aromatic aldehydes and ketones using chirally modified cationic rhodium precatalysts results in reductive coupling to afford dienylated alpha-hydroxy heteroarenes 2-23 with exceptional levels of regio- and enantiocontrol. Coupling of enyne 1a to 2-pyridinecarboxaldehyde using an achiral rhodium catalyst in the presence of a chiral Akiyama-Terada-type phosphoric acid derived from BINOL as the Br?nsted acid co-catalyst provides the coupling product 2 with substantial levels of optical enrichment (82% ee). This result suggests that substrate protonation and/or formation of a strong hydrogen bond occurs in advance of the stereogenic C-C bond forming event. Further, the high levels of asymmetric induction demonstrate that interaction of the aldehyde with the Br?nsted acid activates the system toward C-C coupling. Reductive coupling of enyne 1a and 2-pyridinecarboxaldehyde under an atmosphere of elemental deuterium provides the monodeuterated product deuterio-2, consistent with a catalytic mechanism involving alkyne-carbonyl oxidative coupling followed by hydrogenolytic cleavage of the resulting oxametallacycle. The diene side chain of the coupling products is subject to diverse selective transformations, as demonstrated by the conversion of coupling products 2 and 8 to compounds 24-26 and 27-29, respectively.     

Molecular-Recognition-Directed Self-Assembly of Pleated Sheets from 2-Aminopyrimidine Hydrogen-Bonding Motifs

The self-assembly of the 2,2′-diamino-5,5′-(dialkylmethylidene)methylenedipyrimidine derivatives 3a – c allows for the molecular-recognition-directed generation of pleated sheets as revealed by X-ray crystallographic analysis. The data provide insight into the interplay of the different structural and interactional features of the molecular components in the generation of the supramolecular assembly. The introduction of the adamantylidene group as in 3c leads to the dominance of the H-bonding factor and the resulting formation of a regular, fully interconnected array of pleated sheet type. The results suggest further manipulation of the interplay of the different factors to induce the self-assembly of other supramolecular architectures.