Catalytic Asymmetric Hydrophosphination of ortho-Quinone Methides

Brønsted acid-mediated tandem cyclization of triarylphosphines and in situ generated ortho-alkynyl quinone methides: access to heterocyclic quaternary phosphonium salts

Heterocyclic Quaternary Phosphonium Salts (HQPS) have emerged as promising chemicals for organic synthesis and medicinal chemistry. However, the present synthetic methodology of this type of compound is still limited. Here, we report a deconstructive reorganization strategy based on Brønsted acid-mediated tandem 1,4 addition/intramolecular cyclization of triphenylphosphine derivatives and in situ generated o-AQMs for the first time. This protocol provides a novel approach to heterocyclic quaternary phosphonium salts. The method also features a non-metal catalyst, mild reaction conditions, high efficiency and wide substrate scope. Moreover, a series of obtained heterocyclic phosphonium salts can be converted to isotopically labelled 2-benzofuran compounds directly by simple deuteration reactions.

Catalyst-free thiophosphorylation of in situ formed ortho-quinone methides

A metal-, chloride reagent and base-free thiophosphorylation reaction of in situ formed ortho-quinone methide (o-QM) to synthesize functionalized thiophosphates has been developed. The reaction is an atom-economical process, producing water as the sole byproduct. (EtO)₂P(O)SH functions as both a Brønsted acid and nucleophilic thiolate to produce the o-QM intermediate and the thiophosphate product, respectively. The aza o-QMs were also successfully thiophosphorylated in the presence of catalytic TsOH to form sulfonamido thiophosphates.

Phenolic 3° Phosphine Oxides as a Class of Metal-Free Catalysts for the Activation of C–O Bonds in Aliphatic Alcohols: Direct Synthesis of Catalyst Candidates, and Kinetic Studies

It was recently reported that a (2-hydroxybenzyl)phosphine oxide (2-HOBPO) can serve as a phosphorus-centered catalyst for the stereo-invertive coupling of aliphatic alcohols and acidic pronucleophiles (akin to a Mitsunobu reaction, but without additional reagents). Herein, we report an improved synthesis, which provides direct access to systematically varied 2-HOBPOs in a single step from commercially available precursors (salicylaldehydes and secondary phosphines). The efficiency and generality of the synthetic method enabled limited structure–activity relationship (SAR) studies, from which it was determined that substituents on both the phenolic and phosphine oxide portions can exert significant influence on the turnover frequency (TOF) of each catalyst. Importantly, for all catalytically active 2-HOBPOs examined, the molecularity of catalyst in the rate law of the alcohol coupling was determined to be <1. Thus, for high catalyst loadings, differences in catalytic activity between 2-HOBPOs appear to be dominated by differences in catalytic auto-inhibition, while for low catalyst loadings, differences are attributed to inherent differences in the energetic span of the catalytic cycle, ignoring off-cycle species, in good agreement with density functional theory (DFT) modeling at the ωB97X-D/6-311G(d,p) level.

A formal intermolecular [4 + 1] cycloaddition reaction of 3-chlorooxindole and o-quinone methides: a facile synthesis of spirocyclic oxindole scaffolds

Herein, we developed an efficient and straightforward method for the rapid synthesis of spirocyclic oxindole scaffolds via the [4 + 1] cyclization reaction of 3-chlorooxindole with o-quinone methides (o-QMs), which were generated under mild conditions. The products could be obtained in excellent yields with numerous types of 3-chlorooxindole. This methodology features mild reaction conditions, high atom-economy and broad substrate scope.

Herein, we developed an efficient and straightforward method for the rapid synthesis of spirocyclic oxindole scaffolds via the [4 + 1] cyclization reaction of 3-chlorooxindole with o-quinone methides (o-QMs), which were generated under mild conditions.

Hydroquinine-catalyzed asymmetric 1,4-hydrophosphination of in situ generated aza-o-quinone methides with H-phosphine oxides

An organocatalytic enantioselective 1,4-addition of H-phosphine oxides to in situ generated aza-o-quinone methides has been successfully established using hydroquinine.

Organocatalytic 1,6-hydrophosphination of para-quinone methides: enantioselective access to chiral 3-phosphoxindoles bearing phosphorus-substituted quaternary carbon stereocenters

An efficient organocatalytic enantioselective 1,6-hydrophosphonylation of para-quinone methides has been achieved for the construction of phosphorus-substituted quaternary carbon stereocenters.

Enantioselective one-pot synthesis of 4H-chromene derivatives catalyzed by a chiral Ni(ii) complex

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee. The method is compatible with various β-ketoester substrates, and the products obtained could be converted into biologically active 4H-chromene derivatives.

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee.

Lewis acid-catalyzed tandem cyclization of in situ generated o-quinone methides and arylsulfonyl hydrazides for a one-pot entry to 3-sulfonylbenzofurans

Lewis acid-mediated one-pot tandem cyclization of o-QMs with arylsulfonyl hydrazides was described for the first time and the corresponding 3-sulfonylbenzofuran products were obtained in moderate to good yields.