Iterative asymmetric allylic substitutions: syn- and anti-1,2-diols through catalyst control

A copper-catalyzed asymmetric allylic boronation (AAB) gives access to syn- and anti-1,2-diols. The method facilitates an iterative strategy for the preparation of polyols, such as the fully differentiated L-ribo-tetrol and protected D-arabino-tetrol. P=protecting group.     

Screening of a phosphite-phosphoramidite ligand library for palladium-catalysed asymmetric allylic substitution reactions: the origin of enantioselectivity

We have designed a new library of readily available, highly modular phosphite-phosphoramidite ligands for asymmetric allylic substitution reactions. They are easily prepared in one step from commercially available chiral 1,2-amino alcohols. The introduction of a phosphoramidite moiety into the ligand design is highly advantageous for the product outcome. This ligand library affords high reaction rates (TOFs of up to 800 mol (mol h)(-1)) and enantioselectivities (ees of up to 99%) and, at the same time, contains a broad range of disubstituted hindered and unhindered substrate types. NMR study of the Pd-pi-allyl intermediates provide a deeper understanding of the effect of the ligand parameters on the origin of enantioselectivity.     

Ir‐Catalysed Asymmetric Allylic Substitutions with Cyclometalated (Phosphoramidite)Ir Complexes—Resting States,Catalytically Active (π‐Allyl)Ir Complexes and Computational Exploration

Mechanistic aspects of allylic substitutions with iridium catalysts derived from phosphoramidites by cyclometalation were investigated. The determination of resting states by ³¹P NMR spectroscopy led to the conclusion that the cyclometalation process is reversible. A novel, one‐pot procedure for the preparation of (π‐ allyl)Ir complexes was developed, and these complexes were characterised by X‐ray crystal structure analyses and spectral data. They are fully active catalysts of the allylic substitution reaction. DFT calculations on the allyl complexes, transition states of the allylic substitution and product olefin complexes gave further mechanistic insight.     

A theoretical study of S(N)2' reactions of allylic halides: role of ion pairs

Various disparate experimental results are explained by the hypothesis that reactions of anionic nucleophiles with allylic halides are generally S(N)2. The S(N)2' reactions that do occur proceed generally with anti stereochemistry. Reactions with ion pair nucleophiles occur preferentially as S(N)2' reactions with syn stereochemistry. This hypothesis is consistent with a variety of computations at the HF, B3LYP, mPW1PW91 and MP2 levels with the 6-31+G(d) basis set of reactions of Li and Na fluoride and chloride with allyl halides and 4-halo-2-pentenes. Solvation is considered by a combination of coordination of dimethyl ether to the lithium and sodium cations and "dielectric solvation" with a polarized continuum model.     

The rates of S(N)2 reactions and their relation to molecular and solvent properties

The energy barriers of symmetrical methyl exchanges in the gas phase have been calculated with the reaction path of the intersecting/interacting-state model (ISM). Reactive bond lengths increase down a column of the Periodic Table and compensate for the decrease in the force constants, which explains the near constancy of the intrinsic barriers in the following series of nucleophiles: F(-) approximately Cl(-) approximately Br(-) approximately I(-). This compensation is absent along the rows of the Periodic Table and the trend in the reactivity is dominated by the increase in the electrophilicity index of the nucleophile in the series C相似文献   

Ligand bite governs enantioselectivity: electronic and steric control in Pd-catalyzed allylic alkylations by modular fenchyl phosphinites (FENOPs)

Modular fenchyl phosphinites (FENOPs) containing different aryl units-phenyl (1), 2-anisyl (2), or 2-pyridyl (3)-are efficiently accessible from (-)-fenchone. For comparison of the influence of the different aryl units on enantioselectivities and reactivities, these FENOPs were employed in Pd-catalyzed allylic alkylations. The strongly chelating character of P,N-bidentate 3 is apparent from X-ray structures with PdCl2 ([Pd3Cl2]), and with allyl-Pd units in ([Pd3(eta1-allyl)] and [Pd3(eta3-allyl)]). FENOP3 gives rise to a PdL* catalyst of moderate enantioselectivity (42 % ee, R product). Surprisingly, higher enantioselectivities are found for the hemilabile, monodentate FENOPs 1 (83 % ee, S enantiomer) and 2 (69 % ee, S enantiomer). Only small amounts of 1 or 2 generate selective PdL* catalysts, while complete abolition of enantioselectivity appears with unselective PdL*2 species with higher FENOP concentrations in the cases of 1 or 2. Computational transition structure analyses reveal steric and electronic origins of enantioselectivities. The nucleophile is electronically guided trans to phosphorus. endo-Allyl arrangements are favored over exo-allyl orientations for 1 and 2 due to Pd-pi-pyridyl interactions with short "side-on" Pd-aryl interactions. More remote "edge-on" Pd-pi-aryl interactions in 3 with Pd-N(lp) coordination favor endo-allyl units slightly more and explain the switch of enantioselectivity from 1 (S) and 2 (S) to 3 (R).     

Synthetic scope of Ru(OH)x/Al2O3-catalyzed hydrogen-transfer reactions: an application to reduction of allylic alcohols by a sequential process of isomerization/Meerwein-Ponndorf-Verley-type reduction

Reduction of allylic alcohols can be promoted efficiently by the supported ruthenium catalyst Ru(OH)x/Al2O3. Various allylic alcohols were converted to saturated alcohols in excellent yields by using 2-propanol without any additives. This Ru(OH)x/Al2O3-catalyzed reduction of a dienol proceeds only at the allylic double bond to afford the corresponding enol, and chemoselective isomerization and reduction can be realized under similar conditions. The catalysis is truly heterogeneous and the high catalytic performance can be maintained during at least three recycles of the Ru(OH)x/Al2O3 catalyst. The transformation of allylic alcohols to saturated alcohols consists of three sequential reactions: oxidation of allylic alcohols to alpha,beta-unsaturated carbonyl compounds; reduction of alpha,beta-unsaturated carbonyl compounds to saturated carbonyl compounds; and reduction of saturated carbonyl compounds to saturated alcohols.     

New dihydroxy bis(oxazoline) ligands for the palladium-catalyzed asymmetric allylic alkylation: experimental investigations of the origin of the reversal of the enantioselectivity

The origin of the reversal of the enantioselectivity in the palladium-catalyzed allylic alkylation of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate anion using chiral dihydroxy bis(oxazoline) "BO" ligands derived from (1S,2S)-(+)-2-amino-1-phenyl-1,3-propanediol was investigated. To determine the structural effects of the dihydroxy BO ligand on this unique phenomenon, new homochiral dihydroxy BO ligands were prepared from L-threonine and L-serine and were assessed in the transformation. The results obtained with these novel BO ligands, compared with the one obtained by using the dihydroxy BO ligands derived from (1S,2S)-(+)-2-amino-1-phenyl-1,3-propanediol, reveal that the reversal in the enantioselectivity observed with the dihydroxy BO ligand depends on the structure of the ligand. The effect of different bases used to generate the dimethyl malonate anion was also examined. The results are discussed in terms of the interaction of one hydroxy group in the intermediate pi-allyl palladium complex with the dimethyl malonate anion.     

Iridium‐Catalyzed Allylic Substitutions with Cyclometalated Phosphoramidite Complexes Bearing a Dibenzocyclooctatetraene Ligand: Preparation of (π‐Allyl)Ir Complexes and Computational and NMR Spectroscopic Studies

(π‐Allyl)Ir complexes derived from dibenzocyclooctatetraene and phosphoramidites by cyclometalation are effective catalysts for allylic substitution reactions of linear monosubstituted allylic carbonates. These catalysts provide exceptionally high degrees of regioselectivity and allow the reactions to be run under aerobic conditions. A series of (π‐allyl)Ir complexes were prepared and characterized by X‐ray crystal structure analyses. An allylic amination with aniline displayed different resting states depending on the presence of a strong base. DFT calculations were carried out on the mechanistic aspects of these reactions. The results suggest that for the (π‐allyl)Ir complexes, the formation and reactions with nucleophiles proceed with comparable rates.