Highly Regioselective Gold-Catalyzed Ring-Opening Allylation and Azidation of Dihydrofurans

Gold(I)-Catalyzed Nucleophilic Allylation of Azinium Ions with Allylboronates

Gold(I)-catalyzed nucleophilic allylations of pyridinium and quinolinium ions with various allyl pinacolboronates are reported. The reactions are completely selective with respect to the site of the azinium ion that is attacked, to give various functionalized 1,4-dihydropyridines and 1,4-dihydroquinolines. Evidence suggests that the reactions proceed through nucleophilic allylgold(I) intermediates formed by transmetalation from allylboronates. Density functional theory (DFT) calculations provided mechanistic insight.

Transition Metal Catalyzed Azidation Reactions

A wide range of methodologies for the preparation of organic azides has been reported in the literature for many decades, due to their interest as building blocks for different transformations and their applications in biology as well as in materials science. More recently, with the spread of the use of transition metal-catalyzed reactions, new perspectives have also materialized in azidation processes, especially concerning the azidation of C–H bonds and direct difunctionalization of multiple carbon-carbon bonds. In this review, special emphasis will be placed on reactions involving substrates bearing a leaving group, hydroazidation reactions and azidation reactions that proceed with the formation of more than one bond. Further reactions for the preparation of allyl and vinyl azides as well as for azidations involving the opening of a ring complete the classification of the material.

Gold(I)-Catalyzed One-Pot and Diastereoselective Synthesis of trans-2-Silyl-4,5-dihydrofurans from Propargylsilanes and Aldehydes

A diastereoselective and high-yielding gold-catalyzed synthesis of trans-2-silyl-4,5-dihydrofurans is described. In addition to a sequential manner, this reaction could be performed in a one-pot procedure from propargylsilanes and aldehydes. A mechanistic proposal for the cis-trans isomerization step is formulated. To provide experimental support for this proposal, which involves ring opening/ring closing steps of the dihydrofuran, several isotopically labeled experiments, intramolecular capture of a proposed intermediate, and construction of a Hammett plot have been performed.

Allylic azides: synthesis, reactivity, and the Winstein rearrangement

Organic azides are useful synthetic intermediates, which demonstrate broad reactivity. Unlike most organic azides, allylic azides can spontaneously rearrange to form a mixture of isomers. This rearrangement has been named the Winstein rearrangement. Using allylic azides can result in low yields and azide racemization in some synthetic contexts due to the Winstein rearrangement. Effort has been made to understand the mechanism of the Winstein rearrangement and to take advantage of this process. Several guiding principles can be used to identify which azides will produce a mixture of isomers and which will resist rearrangement. Selective reaction conditions can be used to differentiate the azide isomers in a dynamic manner. This review covers all aspects of allylic azides including their synthesis, their reactivity, the mechanism of the Winstein rearrangement, and reactions that can selectively elaborate an azide isomer. This review covers the literature from Winstein's initial report to early 2019.

Study on the mechanism of platinum(ii)-catalyzed asymmetric ring-opening addition of oxabicyclic alkenes with arylboronic acids

The mechanism of an asymmetric ring-opening (ARO) addition of oxabicyclic alkenes catalyzed by a platinum(ii) catalyst was investigated by M06-2X/6-311G(d,p) using density functional theory (DFT). All the structures were optimized in the solvent model density (SMD) solvation model (solvation = the mixture of H2O/CH2Cl2 1 : 10, v/v) for consistence with experimental conditions. The overall mechanism is considered as a four-step reaction including transmetalation, carboplatinum, β-oxygen elimination, and hydrolysis. The transmetalation and carboplatinum steps are multi-step processes, and both the regioselectivity and the enantioselectivity lie in the carboplatinum process. Based on the natural population analysis (NPA) and the orbital composition analysis of oxabicyclic alkenes, the preferable coordination site with a platinum(ii) center is considered as the bridging oxygen atom by exo-coordination because of the less steric hindrance and the stronger electronic effect. This coordination is thought of as origin of the regioselectivity and the enantioselectivity, which is different from that proposed previously. The Gibbs free energy profiles show that the rate-determining step involves the migration of an aryl group from the platinum(ii) center to one of the closer enantiotopic carbon atoms in an alkene of the oxabicyclic alkenes. The theoretically predicted enantiomeric excess (ee) value of 82% for this reaction is very close to the experimental ee value of 80%. It was found that the hydrogen bonds between the oxabicyclic alkenes and water molecules promotes the platinum(ii) catalyst leaving the reaction system effortlessly and entering the next catalysis recycle. In the overall catalytic cycle, the highest free energy barrier is 30.1 kcal mol-1 and the process releases an energy of 26.3 kcal mol-1. The results confirm that the Pt(ii)-catalyzed ARO reactions take place at mild experimental conditions, which is consistent with the experiment observations. Thus, this study is important for understanding the catalytic behavior of the transition metal platinum(ii) in an asymmetric ring-opening reaction.

Various difunctionalizations of acrylamide: an efficient approach to synthesize oxindoles

In this review, we summarize the advancement and applications in the difunctionalization of acrylamide to synthesize oxindoles based on the mechanism.

Gold-Catalyzed Benzylic Azidation of Phthalans and Isochromans and Subsequent FeCl3-Catalyzed Nucleophilic Substitutions

The benzylic positions of the phthalan and isochroman derivatives (1) as benzene-fused cyclic ethers effectively underwent gold-catalyzed direct azidation using trimethylsilylazide (TMSN3) to give the corresponding 1-azidated products (2) possessing the N,O-acetal partial structure. The azido group of the N,O-acetal behaved as a leaving group in the presence of catalytic iron(III) chloride, and 1-aryl or allyl phthalan and isochroman derivatives were obtained by nucleophilic arylation or allylation, respectively. Meanwhile, a double nucleophilic substitution toward the 1-azidated products (2) occurred at the 1-position using indole derivatives as a nucleophile accompanied by elimination of the azido group and subsequent ring opening of the cyclic ether nucleus produced the bisindolylarylmethane derivatives.

Biarylmethane and Fused Heterocyclic Arene Synthesis via in Situ Generated o- and/or p-Naphthoquinone Methides

o- and/or p-naphthoquinone methides (NQMs) can be selectively prepared by the ring opening of 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene derivatives based on a substituent effect at the 4 position of the substrates. The 4-alkyl- or silyl-substituted 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene was transformed to o-NQM (1-naphthoquinone-2-methide), which underwent Friedel-Crafts 1,4-addition of the α,β-unsaturated carbonyl moiety to provide the 2-benzyl-1-naphthol as the biarylmethane and [4 + 2]-cycloaddition with a dienophile to give the fused heterocyclic arene. Meanwhile, the 4-unsubstituted 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene could be converted to the corresponding 4-benzyl-1-naphthol by the Friedel-Crafts 1,6-addition of p-NQM (1-naphthoquinone-4-methide) generated by the site-selective ring opening of the 1,4-epoxy moiety. Furthermore, the 4-(siloxymethyl)-(1,4-bis(siloxymethyl))-1,4-epoxy-1,4-dihydronaphthalene was transformed into a 2,4-bisbenzyl-1-naphthol or pentacyclic derivative via both the o- and p-NQM intermediates.

Stereospecific cross-coupling reactions of aryl-substituted tetrahydrofurans, tetrahydropyrans, and lactones

The stereospecific ring-opening of O-heterocycles to provide acyclic alcohols and carboxylic acids with controlled formation of a new C-C bond is reported. These reactions provide new methods for synthesis of acyclic polyketide analogs with complex stereochemical arrays. Stereoselective synthesis of the cyclic template is utilized to control relative configuration; subsequent stereospecific nickel-catalyzed ring-opening affords the acyclic product. Aryl-substituted tetrahydrofurans and tetrahydropyrans undergo nickel-catalyzed Kumada-type coupling with a range of Grignard reagents to furnish acyclic alcohols with high diastereoselectivity. Enantioenriched lactones undergo Negishi-type cross-coupling with dimethylzinc to afford enantioenriched carboxylic acids. Application in a two-step enantioselective synthesis of an anti-dyslipidemia agent is demonstrated.