Trisubstituted Substrates Research Articles

NHC-Copper-Catalyzed Tandem Hydrocupration and Allylation of Alkenyl Boronates

A tandem hydrocupration/allylation of alkenyl boronates efficiently proceeds with NHC-copper catalysts using hydrosilane and allyl phosphate as reagents. In the presence of IMes–Cu catalyst, the allylation of in situ generated Β-α-copper intermediates smoothly occurs to give homoallylic boronates in high yields, despite competitive side reaction pathways. Mono- and disubstituted alkenyl boronates were effective in the reaction with terminal allyl phosphates, but trisubstituted substrates showed limited reactivity.

Transfer Hydrogenation of Nitriles, Olefins, and N-Heterocycles Catalyzed by an N-Heterocyclic Carbene-Supported Half-Sandwich Complex of Ruthenium

In the presence of KOBut, N-heterocyclic carbene-supported half-sandwich complex [Cp(IPr)Ru(pyr)2][PF6] (3) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) catalyzes transfer hydrogenation (TH) of nitriles, activated N-heterocycles, olefins, and conjugated olefins in isopropanol at the catalyst loading of 0.5%. The TH of nitriles leads to imines, produced as a result of coupling of the initially formed amines with acetone (produced from isopropanol), and showed good chemoselectivity. Reduction of N-heterocycles occurs for activated polycyclic substrates (e.g., quinoline) and takes place exclusively in the heterocycle. The TH also works well for linear and cyclic olefins but fails for trisubstituted substrates. However, the C═C bond of α,β-unsaturated esters, amides, and acids is easily reduced even for trisubstituted species, such as isovaleriates. Mechanistic studies suggest that the active species in these catalytic reactions is the trihydride Cp(IPr)RuH3 (5), which can catalyze these reactions...

ChemInform Abstract: A New Modular Phosphite‐Pyridine Ligand Library for Asymmetric Pd‐Catalyzed Allylic Substitution Reactions: A Study of the Key Pd‐π‐Allyl Intermediates.

A library of phosphite-pyridine ligands L1–L12 a–g has been successfully applied for the first time in the Pd-catalyzed allylic substitution reactions of several di- and trisubstituted substrates by using a wide range of C, N and O nucleophiles, among which are the little studied α-substituted malonates, β-diketones, and alkyl alcohols. The highly modular nature of this ligand library enables the substituents/configuration at the ligand backbone, and the substituents/configurations at the biaryl phosphite moiety to be easily and systematically varied. We found that the introduction of an enantiopure biaryl phosphite moiety played an essential role in increasing the versatility of the Pd-catalytic systems. Enantioselectivities were therefore high for several hindered and unhindered di- and trisubstituted substrates by using a wide range of C, N and O nucleophiles. Of particular note were the high enantioselectivities (up to>99 % ee) and high activities obtained for the trisubstituted substrates S6 and S7, which compare favorably with the best that have been reported in the literature. We have also extended the use of these new catalytic systems in alternative environmentally friendly solvents such as propylene carbonate and ionic liquids. Studies on the Pd-π-allyl intermediates provide a deeper understanding of the effect of ligand parameters on the origin of enantioselectivity.

Phosphite-Thiazoline versus Phosphite-Oxazoline for Pd-Catalyzed Allylic Substitution Reactions: A Case for Comparison

Come together, right now, over ee: Through the combination of substrates with phosphite-oxazoline or phosphite-thiazoline ligands, we identify one of the few catalytic systems that provide high regio- and enantioselectivities in both enantiomers of the alkylation product for a wide range of hindered and unhindered mono-, di-, and trisubstituted substrates with several carbon nucleophiles.

A New Modular Phosphite‐Pyridine Ligand Library for Asymmetric Pd‐Catalyzed Allylic Substitution Reactions: A Study of the Key Pd‐π‐Allyl Intermediates

A library of phosphite-pyridine ligands L1-L12 a-g has been successfully applied for the first time in the Pd-catalyzed allylic substitution reactions of several di- and trisubstituted substrates by using a wide range of C, N and O nucleophiles, among which are the little studied α-substituted malonates, β-diketones, and alkyl alcohols. The highly modular nature of this ligand library enables the substituents/configuration at the ligand backbone, and the substituents/configurations at the biaryl phosphite moiety to be easily and systematically varied. We found that the introduction of an enantiopure biaryl phosphite moiety played an essential role in increasing the versatility of the Pd-catalytic systems. Enantioselectivities were therefore high for several hindered and unhindered di- and trisubstituted substrates by using a wide range of C, N and O nucleophiles. Of particular note were the high enantioselectivities (up to>99% ee) and high activities obtained for the trisubstituted substrates S6 and S7, which compare favorably with the best that have been reported in the literature. We have also extended the use of these new catalytic systems in alternative environmentally friendly solvents such as propylene carbonate and ionic liquids. Studies on the Pd-π-allyl intermediates provide a deeper understanding of the effect of ligand parameters on the origin of enantioselectivity.

Highly Enantioselective Epoxidation of α,β‐Unsaturated Esters by Chiral Dioxirane.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly enantioselective epoxidation of alpha, beta-unsaturated esters by chiral dioxirane.

This paper describes a highly enantioselective epoxidation of alpha,beta-unsaturated esters using the fructose-derived ketone 2 as catalyst and Oxone as oxidant. High ee's have been obtained for a number of trans and trisubstituted substrates (82-98% ee). The results described show that it is feasible for dioxiranes to effectively epoxidize electron-deficient olefins with high ee's.

Synthesis and enzyme-catalysed reductions of 2-oxo acids with oxygen containing side-chains

A series of novel 2-oxo esters with protected alcohols at the 3-, 4-, 5-, 6- and 7-positions has been prepared either via coupling of an aldehyde with an organometallic reagent (Zn, In or Cr) or via a one-carbon homologation of the precursor acid. A one-pot dual enzyme system was used to convert the simpler 2-oxo acids (with a single MOM ether at either C-3 or C-4) into enantiopure protected 2,3- and 2,4-dihydroxy acids in good yields, but in the cases of the more complex trisubstituted substrates, significant decomposition occurred. Biotransformations have proved valuable for the enantioselective synthesis of protected 2,6,7-trihydroxyhept-3-enoic acids.