Rhodium-Catalyzed Asymmetric Conjugate Additions of Boronic Acids to enones using DIPHONANE: a novel chiral bisphosphine ligand

[reaction: see text] The synthesis of a novel enantiopure C2-symmetric bisphosphine, DIPHONANE, was accomplished starting from 2,5-norbornadione, utilizing (R,R)- and/or (S,S)-(2,3-O-di[(phenylamino)carbonyl]tartaric acid for the resolution of an intermediate phosphineoxide. The application of this ligand in the rhodium-catalyzed asymmetric conjugate addition of boronic acids to cyclic enones provides the 1,4-addition products in good yields (69-98%) and high ee's (78-95% ee). A byproduct arising from a consecutive 1,4-addition and 1,2-addition was also observed.     

Synthesis of novel diphosphite ligands derived from d-mannitol and their application in Cu-catalyzed enantioselective conjugate addition of organozinc to enones

A series of novel chiral diphosphite ligands have been synthesized from d-mannitol derivatives and chlorophosphoric acid diary ester, and were successfully employed in the copper catalyzed enantioselective conjugate addition of organozinc reagents diethylzinc and dimethylzinc to cyclic and acyclic enones. The stereochemically matched combination of d-mannitol and (R)-H₈-binaphthyl in ligand 1,2:5,6-di-O-isopropylidene-3,4-bis[(R)-1,1′-H₈-binaphthyl-2,2′-diyl] phosphite-d-mannitol was essential to afford 93% ee for 3-ethylcyclohexanone, 92% ee for 3-ethylcyclopentanone, and 90% ee for 3-ethylcycloheptanone in toluene, using Cu(OTf)₂ as a catalytic precursor. The results clearly indicated that the chiral organocopper reagent exhibited high enantioselectivies for cyclic enones bearing different ring sizes. As for the backbone of this type of ligand, it has been demonstrated that 1,2:5,6-di-O-isopropylidene-d-mannitol was more efficient than 1,2:5,6-di-O-cyclohexylidene-d-mannitol. The sense of the enantiodiscrimination was mainly determined by the configuration of the diaryl phosphite moieties in the 1,4-addition of cyclic enones.     

Highly practical and enantioselective cu-catalyzed conjugate addition of alkylzinc reagents to cyclic enones at ambient temperature

[reaction: see text] A new ligand for Cu-catalyzed enantioselective additions of dialkylzincs to cyclic enones has been developed. In addition to providing good to excellent enantioselectivities with a range of cyclic enones and dialkylzincs, the ligand has several notworthy features: it can be readily prepared in just two steps, is an air-stable crystalline solid, and provides optimal performance at ambient temperature.     

Screening of a modular sugar-based phosphite ligand library in the Cu-catalyzed asymmetric 1,4-addition reactions

A sugar-based monophosphite ligand library L1–L5 was screened in the Cu-catalyzed asymmetric 1,4-addition to cyclic and aliphatic linear enones. These ligands are derived from d-glucose, d-galactose and d-fructose, which lead to a wide range of sugar backbones, and contain several substituents/configurations in the biaryl moiety, with different steric and electronic properties. Systematic variation of the ligand parameters indicates that the catalytic performance (activities and enantioselectivities) is highly affected by the configuration of C-4 of the carbohydrate backbone, the size of the ring of the sugar backbone and the cooperative effect between configurations of C-3 and of the binaphthyl phosphite moiety. Good activities and enantioselectivities up to 57% and 51% were achieved for cyclic and aliphatic linear enones, respectively.     

Catalytic asymmetric allylation of ketones and a tandem asymmetric allylation/diastereoselective epoxidation of cyclic enones

A simple procedure is reported for the catalytic asymmetric allylation of ketones, utilizing titanium tetraisopropoxide, BINOL, 2-propanol additive, and tetraallylstannane as allylating agent. A variety of ketone substrates, including acetophenone derivatives and alpha,beta-unsaturated cyclic enones, reacted to form tertiary homoallylic alcohols in good yields (67-99%) and with high levels of enantioselectivity (generally >80%). A novel one-pot enantioselective allylation/diastereoselective epoxidation has also been introduced. Thus, upon completion of the allyl addition to conjugated cyclic enones, 1 equiv of tert-butyl hydroperoxide is added and the directed epoxidation of the allylic double bond ensues to afford the epoxy alcohol with high diastereoselectivity.     

Copper-catalyzed asymmetric conjugate addition of Grignard reagents to trisubstituted enones. Construction of all-carbon quaternary chiral centers

The copper-catalyzed asymmetric conjugate addition of Grignard reagents to trisubstituted cyclic enones affords enantioenriched all-carbon quaternary centers with up to 96% ee. The chiral ligand is a diaminocarbene, directly generated in situ. The combination of Grignard reagent and diaminocarbene is unprecedented in conjugate addition, and the additon of the phenyl group, on such enones, cannot be done by other conjugate addition methods.     

Palladium-catalyzed asymmetric conjugate addition of arylboronic acids to five-, six-, and seven-membered β-substituted cyclic enones: enantioselective construction of all-carbon quaternary stereocenters

The first enantioselective Pd-catalyzed construction of all-carbon quaternary stereocenters via 1,4-addition of arylboronic acids to β-substituted cyclic enones is reported. Reaction of a wide range of arylboronic acids and cyclic enones using a catalyst prepared from Pd(OCOCF(3))(2) and a chiral pyridinooxazoline ligand yields enantioenriched products bearing benzylic stereocenters. Notably, this transformation is tolerant to air and moisture, providing a practical and operationally simple method of synthesizing enantioenriched all-carbon quaternary stereocenters.     

Aluminum oxide catalyzed isomerization of acylated cycloalkenes

A homologous series of cyclic olefins and their corresponding 1-methyl derivatives were acylated using acetic anhydride/zinc chloride system. The resulting mixtures were, after appropriate analysis, isomerized quantitatively to the conjugated enones by adsorption on neutral alumina. A detailed, general experimental procedure is given.     

Stereospecific Conversion of Boronic Esters into Enones using Methoxyallene: Application in the Total Synthesis of 10-Deoxymethynolide

Enones are widely utilized linchpin functional groups in chemical synthesis and molecular biology. We herein report the direct conversion of boronic esters into enones using commercially available methoxyallene as a three-carbon building block. Following boronate complex formation by reaction of the boronic ester with lithiated-methoxyallene, protonation triggers a stereospecific 1,2-migration before oxidation generates the enone. The protocol shows broad substrate scope and complete enantiospecificity is observed with chiral migrating groups. In addition, various electrophiles could be used to induce 1,2-migration and give a much broader range of α-functionalized enones. Finally, the methodology was applied to a 14-step synthesis of the enone-containing polyketide 10-deoxymethynolide.     

Heterogeneous Pd/C-catalyzed ligand-free Suzuki-Miyaura coupling reaction using aryl boronic esters

Heterogeneous Pd/C-catalyzed Suzuki-Miyaura cross-coupling reaction of aryl boronic esters with aryl bromides was successfully carried out in aqueous media at room temperature without the use of a ligand such as phosphine derivatives.     

Study of chemoselective asymmetric hydrogenation of (1-bromo-1-alkenyl)boronic esters with iridium–PˆN complexes

In this paper we report the first chemoselective asymmetric hydrogenation of (1-bromo-1-alkenyl)boronic esters, which constitutes a new route to (α-bromoalkyl)boronic esters. The study demonstrates that excellent chemoselectivities along with full conversions can be obtained for hydrogenation of alkyl substituted derivatives with iridium–PˆN complexes. Moreover, acyclic alkyl derivatives afford (α-bromoalkyl)boronic esters in good enantioselectivities ranging from 64 to 73% ee. A cyclic alkyl derivative was obtained only in a nearly racemic form. The (1-bromo-1-alkenyl)boronic esters appear to be less reactive towards homogenous hydrogenation conditions than their chloro analogues as demonstrated by the higher catalyst loadings required to achieve full conversions for alkyl derivatives and lower conversions observed for the aryl substituted derivatives.     

Synthesis of diphosphite ligands derived from glucopyranoside and their application in the Cu-catalyzed asymmetric 1,4-addition of organozinc to enones

Novel chiral diphosphite ligands derived from glucopyranoside and H₈-binaphthol were synthesized, and successfully employed in the Cu-catalyzed asymmetric 1,4-addition of organozinc reagents dimethylzinc, diethylzinc, and diphenylzinc to cyclic and acyclic enones with up to 96% ee. The stereochemically matched combination of d-glucopyranoside backbone and (R)-H₈-binaphthyl in the ligand 2,4-bis{[(R)-1,1′-H₈-binaphthyl-2,2′-diyl] phosphite}-phenyl 3,6-anhydro-β-d-glucopyranoside was essential for inducing high enantioselectivity. A significant dependence of stereoselectivity on the type of enones and the ring size of cyclic enones was observed. Moreover, the sense of the enantiodiscrimination of the products was mainly determined by the configuration of the H₈-binaphthyl moieties.     

Enantioselective conjugate addition of dialkylzinc and diphenylzinc to enones catalyzed by a chiral copper(I) binaphthylthiophosphoramide or binaphthylselenophosphoramide ligand system

The enantioselective conjugate addition of dialkylzinc or diphenylzinc to enones was catalyzed by a copper(I)-axially chiral binaphthylthiophosphoramide or binaphthylselenophosphoramide ligand system at room temperature (20 degrees C) or 0 degrees C, affording the Michael adducts in high yields with excellent ee for cyclic and acyclic enones. The enantioselectivity and reaction rate achieved here are one of the best results yet for the Cu-catalyzed conjugate addition to enones. It was revealed that this series of chiral phosphoramides was a novel type of S,N-bidentate ligands on the basis of (31)P NMR and (13)C NMR spectroscopic investigations. The mechanism of this asymmetric conjugate addition system has been investigated as well. We found that the acidic proton of phosphoramide in these chiral ligands play a significant role in the formation of the active species. A bimetallic catalytic process has been proposed on the basis of previous literature. The linear effect of product ee and ligand ee further revealed that the active species is a monomeric Cu(I) complex bearing a single ligand [Cu(I):ligand 1:1].     

Enantioselective rhodium-catalysed addition of boronic acids using C2-symmetric aryl diphosphite ligands

The enantioselective rhodium-catalysed conjugate addition of aryl boronic acids to dehydroalanine derivatives has been successfully carried out in the presence of C₂-symmetric aryl diphosphite ligand (R,R,R-4) to prepare unnatural amino acid esters. The products have been obtained in up to 72% ee and with good isolated yield.     

First example of electrophile induced Baylis-Hillman reaction: a novel facile one-pot synthesis of indolizine derivatives

Treatment of pyridine-2-carboxaldehyde with activated alkenes such as alkyl vinyl ketones and cyclic enones in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) provides a novel facile one-pot synthesis of indolizine derivatives, thus for the first time describing an electrophile induced Baylis-Hillman reaction.     

Novel N,N,P-tridentate ligands for the highly enantioselective copper-catalyzed 1,4-addition of dialkylzincs to enones

Use of 0.25 mol % of the N, N, P-tridentate ligand containing the 2-quinolyl moiety ( 1 and 2) and 0.1 mol % of Cu(OTf) 2 enabled the enantioselective 1,4-addition of dialkylzincs to cyclic enones to produce 1,4-adducts in up to 99% ee.     

Chiral dihydrobenzofuran-based diphosphine (BICMAP): optical resolution and application to rhodium(I)-catalyzed asymmetric 1,4-addition of aryl- and alkenylboronic acids to cyclic enones

Chiral dihydrobenzofuran-based diphosphine ligand (BICMAP) 1 was used as a ligand for the rhodium(I)-catalyzed asymmetric 1,4-addition of arylboronic acids to cyclic enones up to 99% ee. We also found that the BICMAP-rhodium system was an efficient catalyst for the 1,4-addition of alkenylboronic acids to 2-cyclohexenone in good enantioselectivities.     

Asymmetric 1,4-reductions of hindered beta-substituted cycloalkenones using catalytic SEGPHOS-ligated CuH

The reagent combination of catalytic amounts of copper hydride ligated by a nonracemic SEGPHOS ligand leads in situ to an extremely reactive species capable of effecting asymmetric hydrosilylations of conjugated cyclic enones in very high ees. An unprecedented substrate-to-ligand ratio as high as 275 000:1 for this transformation has been documented. [reaction: see text]     

Metal-catalyzed release of supported boronic acids for C-C bond formation

The viability of solid-supported boronic acids as reagents for Suzuki couplings and nucleophilic additions to aldehydes and enones was successfully demonstrated. This metal-catalyzed cleavage strategy allows the synthesis of a series of functionalized biphenyl products, benzylic alcohols, and beta-substituted ketones.     

An electrophilic cleavage procedure for the asymmetric dihydroxylation: direct enantioselective synthesis of cyclic boronic esters from olefins

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of olefins is described that yields cyclic boronic esters from olefins in a straight-forward manner. This process represents the first real product alteration in asymmetric dihydroxylation, since all previous protocols lead to free diols exclusively. A protocol based on the Sharpless AD conditions (for enantioselective oxidation of prochiral olefins) was developed that gives cyclic boronic esters with excellent enantiomeric excesses (ee's). Some of the ee's are higher than those reported for conventional AD. The unprecedented role of phenyl boronic acid on the course of the AD reaction was investigated in detail. PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of olefin oxidation intact. The main role of the boronic acids-apart from protecting the diol products against potential overoxidation-relies on removing the diol entity in an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols. Thus, a mechanistically new cleavage for enantioselective dihydroxylation reactions is introduced within the present work.