Arylative Ring Expansion of 3-Vinylazetidin-3-Ols and 3-Vinyloxetan-3-Ols to Dihydrofurans by Dual Palladium and Acid Catalysis

In contrast to the well-studied 1-vinylcyclobutanols, the reactivity of 3-vinylazetidin-3-ols 1 and 3-vinyloxetan-3-ols 2 under transition metal catalysis remains largely unexplored. We report herein their unique reactivity under dual palladium and acid catalysis. In the presence of a catalytic amount of Pd(OAc)2(PPh3)2, AgTFA and triflic acid, the reaction of 1 or 2 with aryl iodides affords 2,3,4-trisubstituted dihydrofurans, which are valuable heterocycles in organic synthesis. Mechanistic studies reveal that this arylative ring-expansion reaction proceeds via a domino process involving Heck arylation of alkene, acid-catalyzed transposition of allylic alcohol and ring opening of the azetidine/oxetane by an internal hydroxyl group.

Iridium-Catalyzed 1,3-Rearrangement of Allylic Ethers

The 1,3-rearrangement of allylic derivatives has rarely been reported, except for allylic alcohols. Herein, we describe an iridium-catalyzed 1,3-rearrangement of readily available allylic ethers to access the difficultly prepared allylic ethers with a large steric hindrance. The developed method shows a broad substrate scope and could be used in the late-stage modification of several natural products. In addition, a possible reaction pathway is also provided on the basis of the control experiments.

Ferrocenium Boronic Acid Catalyzed Deoxygenative Coupling of Alcohols with Carbon- and Nitrogen-Based Borate and Silane Nucleophiles

A boronic acid catalyzed carbon-carbon and carbon-nitrogen bond-forming reaction for the functionalization of various π-activated alcohols has been developed. Ferrocenium boronic acid hexafluoroantimonate salt was identified as an effective catalyst in the direct deoxygenative coupling of alcohols with a variety of potassium trifluoroborate and organosilane nucleophiles. In a comparison between these two classes of nucleophiles, the use of organosilanes leads to higher reaction yields, increased diversity of the alcohol substrate scope, and high E/Z selectivity. Furthermore, the reaction proceeds under mild conditions and yields up to 98%. Computational studies provide a rationalization for a mechanistic pathway for the retention of E/Z stereochemistry when E or Z alkenyl silanes are used as nucleophiles. This methodology is complementary to existing methodologies for deoxygenative coupling reactions involving organosilanes, and it is effective with a variety of organosilane nucleophile sub-types, including allylic, vinylic, and propargylic trimethylsilanes.

Iridium-Catalyzed 1,3-Rearrangement of Allylic Alcohols

The allylic alcohol structural motif is prevalent in many important molecules and valuable building blocks. The rearrangement reaction is one of the most important transformations, however there are only a few reports for the 1,3-rearrangement of allylic alcohols. Herein, a 1,3-rearrangement of allylic alcohols catalyzed by an Ir(III) dihydride complex is described. This reaction could provide the corresponding less accessible allylic alcohols regio- and stereoselectively from readily available E/Z mixtures of the substrates. Furthermore, a tandem alkene isomerization followed by 1,3-rearrangement of homoallylic alcohols was also realized. In addition, this rearrangement reaction could be used to synthesize the natural product Navenone B. Mechanistic investigation indicated that the reaction pathway involved a π-allyl-Ir(V) intermediate and that the dihydride in the iridium catalyst acts as a hydrogen switch to modulate the valence of the iridium center.

Iridium-Catalyzed Double Convergent 1,3-Rearrangement/Hydrogenation of Allylic Alcohols

Enantioconvergent catalysis has the potential to convert different isomers of a starting material to a single highly enantioenriched product. Here we report a novel enantioselective double convergent 1,3-rearrangement/hydrogenation of allylic alcohols using an Ir-N,P catalyst. A variety of allylic alcohols, each consisting of a 1:1:1:1 mixture of four isomers, were converted to the corresponding tertiary alcohols with two contiguous stereogenic centers, in up to 99% ee and 99:1 d.r. DFT calculations, and control experiments suggest that the 1,3-rearrangement is the crucial stereodetermining element of the reaction.

Synergistic Organoboron/Palladium Catalysis for Regioselective N-Allylations of Azoles with Allylic Alcohols

A method for regioselective palladium-catalyzed allylic alkylation of ambident nitrogen heterocycles, employing simple allylic alcohols as electrophile precursors, is described. An organoboron co-catalyst serves both to activate the azole-type nucleophile toward selective N-functionalization and to accelerate the formation of a π-allylpalladium complex from the allylic alcohol. The method can be applied to various heterocycle types, including 1,2,3- and 1,2,4-triazoles, tetrazoles, pyrazoles, and purines, and can be extended to substituted allylic alcohol partners.

Arylboronic Acid-Catalyzed Racemization of Secondary and Tertiary Alcohols

The use of 2-carboxyphenylboronic acid (5 mol %) and oxalic acid (10 mol %) with 2-butanone as a solvent for the racemization of a range of enantiomerically pure secondary and tertiary alcohols is demonstrated. The process is postulated to proceed via reversible Brønsted acid-catalyzed C–O bond cleavage through an achiral carbocation intermediate.

Regioselective Cycloaddition and Substitution Reaction of Tertiary Propargylic Alcohols and Heteroareneboronic Acids via Acid Catalysis

We report an acid-catalyzed formal cycloaddition and dehydrative substitution reaction of tertiary propargylic alcohols and heteroareneboronic acids. The properties of the substituents on the alkynyl moiety determines the regioselectivity of the reaction, which could selectively construct fused heterocycles, tetrasubstituted allenes, or 1,3-dienes. This reaction proceeds efficiently with a wide array of substrate scope in up to 89% yield. A significant advantage of this protocol is the transition-metal-free and mild conditions needed.

Meyer-Schuster Rearrangement of Propargylic Alcohols Mediated by Phosphorus-Containing Brønsted Acid Catalysts

Commercially available (aqueous) hypophosphorus acid is an efficient catalyst for the synthesis of α,β-unsaturated carbonyl compounds from their corresponding propargylic alcohols. Reactions were carried out in technical toluene in the...

A Formal Rearrangement of Allylic Silanols

We show that 1M aqueous HCl/THF or NaBH₄/DMF allows for demercurative ring-opening of cyclic organomercurial synthons into secondary silanol products bearing terminal alkenes. We had previously demonstrated that primary allylic silanols are readily transformed into cyclic organomercurials using Hg(OTf)₂/NaHCO₃ in THF. Overall, this amounts to a facile two-step protocol for the rearrangement of primary allylic silanol substrates. Computational investigations suggest that this rearrangement is under thermodynamic control and that the di-tert-butylsilanol protecting group is essential for product selectivity.

Meyer–Schuster-type rearrangement for the synthesis of α-selanyl-α,β-unsaturated thioesters

A new approach to prepare α-selanyl-α,β-unsaturated thioesters from propargylthioalkynes and an electrophilic selenium species is reported.

Site-selective redox isomerizations of furanosides using a combined arylboronic acid/photoredox catalyst system

In the presence of an arylboronic acid and a hydrogen atom transfer mediator under photoredox conditions, furanoside derivatives undergo site-selective redox isomerizations to 2-keto-3-deoxyfuranosides. Experimental evidence and computational modeling suggest that the transformation takes place by abstraction of the hydrogen atom from the 2-position of the furanoside-derived arylboronic ester, followed by C3-O bond cleavage via spin-center shift. This mechanism is reminiscent of the currently accepted pathway for the formation of 3'-ketodeoxynucleotides by ribonucleotide reductase enzymes.

Boronic acid-catalysed C-3 selective ring opening of 3,4-epoxy alcohols with thiophenols and thiols

In this protocol we described a boronic acid-catalysed C-3 selective ring opening of 3,4-epoxy alcohols with thiophenols and thiols as nucleophiles. This diastereo- and enantiospecific reaction provides an efficient entry to prepare a variety of hydroxyl sulfides. Through the directing effect of the hydroxyl group, nucleophilic attack on the C-3 position of the epoxide moiety is favoured. It can be rationalized in a proposed transition state, in which the boronic acid catalyst tethers both epoxides and S-nucleophiles.

Two-component boronic acid catalysis for increased reactivity in challenging Friedel–Crafts alkylations with deactivated benzylic alcohols

The combination of a boronic acid catalyst with perfluoropinacol as a co-catalyst improves the scope of Friedel–Crafts benzylations of arenes with electronically deactivated primary and secondary benzylic alcohols.

Boronic acid catalysis

Although boronic acids are recognized primarily for their utility as reagents in transition metal-catalyzed transformations, other applications are emerging, including their use as reaction catalysts.

Boronic acid/Brønsted acid co-catalyst systems for the synthesis of 2H-chromenes from phenols and α,β-unsaturated carbonyls

Protocols for the synthesis of substituted 2H-chromenes from α,β-unsaturated carbonyls and phenols are described. Optimal combinations of arylboronic acids and Brønsted acids have been identified, such that both can be employed in catalytic quantities to accelerate these condensations. The method has been used to synthesize a variety of substituted 2H-chromenes, as well as photochromic naphthopyrans. The use of pentafluorophenylboronic acid and diphenylphosphinic acid enabled an expansion of the electrophile scope to include α,β-unsaturated ketones. Hall's 'phase-switching' of boronic acids has been exploited to achieve the separation of the two co-catalysts from unpurified reaction mixtures by a simple liquid-liquid extraction.

Brønsted Acid-Mediated Cycloaromatization of 1H-Indol-2-yl Propargyl Benzoates to 7H-Benzo[c]carbazoles

A synthetic method for the efficient assembly of benzo[c]carbazole derivatives that relies on silica gel-activated benzoic acid-mediated cycloaromatization of 1H-indol-2-yl propargyl benzoates under atmospheric conditions is described. Robust with a variety of substitution patterns tolerated, the reaction provides a one-step strategy to construct a member of the N-heterocycles family in good to excellent yields. A tentative mechanism is proposed in which the cycloaromatization process is thought to involve a Brønsted acid-mediated formal 1,3-acyloxy migration/6π-electrocyclization pathway.

A coherent study on the Z-enoate assisted Meyer–Schuster rearrangement

The impact of temperature, solvent, concentration of the counter ion and the nature of the arene nucleophile on the Z-enoate assisted Meyer–Schuster rearrangement of propargylic alcohols was studied.

BF3·OEt2-mediated syn-selective Meyer-Schuster rearrangement of phenoxy propargyl alcohols for Z-β-aryl-α,β-unsaturated esters

Synthesis of Z-β-aryl-α,β-unsaturated esters from readily available 1-aryl-3-phenoxy propargyl alcohols is achieved via a BF3-mediated syn-selective Meyer-Schuster rearrangement under ambient conditions. The reaction mechanism is postulated to involve an electrophilic borylation of an allene intermediate as the key step to kinetically control the stereoselectivity.

Borinic acid catalysed peptide synthesis

The catalytic synthesis of peptides is a major challenge in the modern organic chemistry hindered by the well-established use of stoichiometric coupling reagents. Herein, we describe for the first time that borinic acid is able to catalyse this reaction under mild conditions with an improved activity compared to our recently developed thiophene-based boronic acid. This catalyst is particularly efficient for peptide bond synthesis affording dipeptides in good yields without detectable racemization.

The Z-enoate assisted, Meyer–Schuster rearrangement cascade: unconventional synthesis of α-arylenone esters

Brønsted acid promoted nucleophilation of propargylic alcohols during the Meyer–Schuster rearrangement (M–S) has been introduced. The reverse polarization of the M–S intermediate allenyl cation has been realized by employing a novel concept of cis-enoate assistance strategy.