Brønsted Acid-Catalyzed Three-Component Hosomi–Sakurai Reactions

Pre-electrolysis of LiClO4 in Acetonitrile: Electrochemically Induced Protolytic Carbon-Carbon Bond Formation of Benzylic Ethers and Acetals with Allyl Trimethylsilane and Other Carbon Nucleophiles

The pre-electrolysis of LiClO4 in acetonitrile in an undivided cell applying only "catalytic" amounts of current (e.g., 0.05 F) led to the formation of a strong acidic medium for the activation of benzylic ethers and acetals. The activated primary and secondary benzylic ethers and acetals could be converted with a range of carbon nucleophiles, such as allyl trimethylsilane, silyl enol ethers, and enol acetates, for the formation of new carbon-carbon bonds. A chemoselective reaction was observed when electron-deficient benzylic acetals were converted with allyl trimethylsilane to the monoallylated products, whereas an electron-rich benzylic acetal led to the double allylated product under activation of both ether groups.

Unified Approach to Imidodiphosphate-Type Brønsted Acids with Tunable Confinement and Acidity

We have designed and realized an efficient and operationally simple single-flask synthesis of imidodiphosphate-based Brønsted acids. The methodology proceeds via consecutive chloride substitutions of hexachlorobisphosphazonium salts, providing rapid access to imidodiphosphates (IDP), iminoimidodiphosphates (iIDP), and imidodiphosphorimidates (IDPi). These privileged acid catalysts feature a broad acidity range (pK_a from ∼11 to <2 in MeCN) and a readily tunable confined active site. Our approach enables access to previously elusive catalyst scaffolds with particularly high structural confinement, one of which catalyzes the first highly enantioselective (>95:5 er) sulfoxidation of methyl n-propyl sulfide. Furthermore, the methodology delivers a novel, rationally designed super acidic catalyst motif, imidodiphosphorbis(iminosulfonylimino)imidate (IDPii), the extreme reactivity of which exceeds commonly employed super-Brønsted acids, such as trifluoromethanesulfonic acid. The unique reactivity of one such IDPii catalyst has been demonstrated in the first α-methylation of a silyl ketene acetal with methanol as the electrophilic alkylating reagent.

Quaternary Phosphonium Salts as Active Brønsted Acid Catalysts for Friedel-Crafts Reactions

A readily available quaternary phosphonium salt containing a trifluoroacetonyl group and a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr^F₄^-) counterion was demonstrated to be a highly active Brønsted acid catalyst for Friedel-Crafts-type reactions of an array of electron-rich heteroarenes and aniline derivatives with isatin-derived ketimines, even at 0.1 mol % catalyst loadings.

Organocatalytic Mukaiyama Mannich Reactions of 2,5-Bis(trimethylsilyloxy)furan

The organocatalytic synthesis of densely substituted mono- and bis-γ-lactams involving the Mukaiyama Mannich addition of 2,5-bis(trimethylsilyloxy)furan to imines is described. Use of a ditoluenesulfonylimide catalyst produces γ-lactams from monoaddition, whereas a more acidic catalyst (triflic acid) produces fused bis-lactams from double addition. Optimized organocatalytic conditions allow for the selective synthesis of either desired core as well as the one-pot, multicomponent assembly of the trisubstituted monolactams from aldehydes, amines, and bis-trimethylsilyloxyfuran. An examination of chiral acids found these organocatalysts to be highly active and diastereoselective in the monoaddition reaction, albeit with no enantioselectivity.

A Stereoselective Reductive Hosomi-Sakurai Reaction

A novel reductive variant of the classical Hosomi-Sakurai reaction is reported. This transformation hinges on a redox-neutral, stereoselective internal reduction event under mild conditions. This operationally simple reaction relies on readily available starting materials and leads to useful products in diastereoselectivities of up to 7:1. The versatility of this new method is demonstrated through the stereoselective one-step synthesis of an AChE inhibitor.

Ir-Catalyzed Asymmetric and Regioselective Hydrogenation of Cyclic Allylsilanes and Generation of Quaternary Stereocenters via the Hosomi-Sakurai Allylation

A number of cyclic dienes containing the allylsilane moiety were prepared by a Birch reduction and subjected to iridium-catalyzed regioselective and asymmetric hydrogenation, which provided chiral allylsilanes in high conversion and enantiomeric excess (up to 99 % ee). The compounds were successively used in the Hosomi-Sakurai allylation with various aldehydes employing TiCl₄ as Lewis acid, providing adducts with two additional stereogenic centers in excellent diastereoselectivity.

MCM-41-supported phosphotungstic acid-catalyzed cleavage of C–O bond in allyl aryl ethers

A solid acid (PWA/MCM-41) efficiently catalyzed the deprotection of allyl group from the allyl aryl ethers of various types of substrates, in the presence of other oxygen protecting groups.

The remarkable journey of catalysts from stoichiometric to catalytic quantity for allyltrimethylsilane inspired allylation of acetals, ketals, aldehydes and ketones

Allyltrimethylsilane inspired allylation of acetals, ketals, aldehydes, and ketones comprises one of the most viable, significant and graceful methods for the formation of carbon–carbon bonds leading to stereochemical transformation.

A Disulfonimide Catalyst for Highly Enantioselective Mukaiyama-Mannich Reaction

A new BINOL-derived chiral disulfonimide has been developed by introducing 4-methyl-3,5-dinitrophenyl substituents at its 3- and 3'-positions. This chiral disulfonimide catalyst displays high catalytic efficacy toward the asymmetric Mukaiyama-Mannich reaction of imines with ketene silyl acetals leading to β-amino acid esters in good yields (up to 99%) with high diastereoselectivities (syn/anti up to 97:3) and enantioselectivities (up to 98% ee). The long-standing problem of the chiral phosphoric acid-catalyzed asymmetric Mukaiyama-Mannich reaction that requires a 2-hydroxyphenyl moiety was solved by this disulfonimide catalyst.

2,4-Dinitrobenzenesulfonic acid in an efficient Brønsted acid-catalyzed controlled/living ring-opening polymerization of ε-caprolactone

2,4-Dinitrobenzenesulfonic acid as an efficient Brønsted acidic catalyst has been evaluated for the controlled/living ring-opening polymerization of ε-caprolactone end-functionalized, α,ω-dihydroxy telechelic poly(ε-caprolactone), and diblock copolymers were also synthesised successfully.

Boron-based pronucleophiles in catalytic (asymmetric) C(sp3)–allyl cross-couplings

Allylic and allenyl boronates or boranes were uncovered as suitable pronucleophiles in catalytic C–C bond formations with C(sp3) electrophiles such as O,O-acetals and N,O-aminals or ethers and carbohydrates. These transformations were most efficiently catalyzed by In(I) triflate. Importantly, chiral counteranion-directed, catalytic asymmetric allylation and allenylation of N,O-aminals was developed by employing a catalyst system composed of In(I) chloride and a chiral silver 2,2'-dihydroxy-1,1'-binaphthalene (BINOL)-phosphate.

Low-oxidation state indium-catalyzed C-C bond formation

The development of innovative metal catalysis for selective bond formation is an important task in organic chemistry. The group 13 metal indium is appealing for catalysis because indium-based reagents are minimally toxic, selective, and tolerant toward various functional groups. Among elements in this group, the most stable oxidation state is typically +3, but in molecules with larger group 13 atoms, the chemistry of the +1 oxidation state is also important. The use of indium(III) compounds in organic synthesis has been well-established as Lewis acid catalysts including asymmetric versions thereof. In contrast, only sporadic examples of the use of indium(I) as a stoichiometric reagent have been reported: to the best of our knowledge, our investigations represent the first synthetic method that uses a catalytic amount of indium(I). Depending on the nature of the ligand or the counteranion to which it is coordinated, indium(I) can act as both a Lewis acid and a Lewis base because it has both vacant p orbitals and a lone pair of electrons. This potential ambiphilicity may offer unique reactivity and unusual selectivity in synthesis and may have significant implications for catalysis, particularly for dual catalytic processes. We envisioned that indium(I) could be employed as a metallic Lewis base catalyst to activate Lewis acidic boron-based pronucleophiles for selective bond formation with suitable electrophiles. Alternatively, indium(I) could serve as an ambiphilic catalyst that activates both reagents at a single center. In this Account, we describe the development of low-oxidation state indium catalysts for carbon-carbon bond formation between boron-based pronucleophiles and various electrophiles. We discovered that indium(I) iodide was an excellent catalyst for α-selective allylations of C(sp(2)) electrophiles such as ketones and hydrazones. Using a combination of this low-oxidation state indium compound and a chiral semicorrin ligand, we developed catalytic highly enantioselective allylation, crotylation, and α-chloroallylation reactions of hydrazones. These transformations proceeded with rare constitutional selectivities and remarkable diastereoselectivities. Furthermore, indium(I) triflate served as the most effective catalyst for allylations and propargylations of C(sp(3)) electrophiles such as O,O-acetals, N,O-aminals, and ethers, and we applied this methodology to carbohydrate chemistry. In addition, a catalyst system composed of indium(I) chloride and a chiral silver BINOL-phosphate facilitated the highly enantioselective allylation and allenylation of N,O-aminals. Overall, these discoveries demonstrate the versatility, efficiency, and sensitivity of low-oxidation state indium catalysts in organic synthesis.