Stereoselectivity Inversion by Water Addition in the −SO₃H‐catalyzed Tandem Prins‐Ritter Reaction for Synthesis of 4‐amidotetrahydropyran Derivatives

Abstract

A range of heterogeneous -SO 3 H functionalized catalysts including carbon and halloysite nanotubes, commercial K10 clay, Amberlyst-15 etc. was investigated for the first time using as a model the Prins-Ritter reaction of (À )-isopulegol with benzaldehyde and acetonitrile producing 4-amido derivatives of octahydro-2H-chromenes (as (S)-and (R)-diastereomers). A strong effect of water addition prior the reaction on the overall selectivity and the ratio of isomers in the case of heterogeneous and homogeneous (p-toluenesulfonic acid) catalysis was found for the first time. The yield of the (R)-diastereomer sharply increased with increasing amount of added water, while the S-isomer prevailed with a minimum amount of added water. Experimental results and DFT calculations clearly indicate a kinetic control for R-amide formation. Typically synthesis of 4amidooctahydro-2H-chromenes requires subzero temperatures and toxic catalysts, which were avoided in the current work. Nevertheless, the yield of the desired products (up to 83 %) at 30°C after water addition exceeded the values reported previously. Thus, adding water is a simple and a very effective method for controlling both the yield and stereoselectivity of the Prins-Ritter reaction products under mild conditions.

Sulfonic Acid‐Functionalized (Bio)Materials as Catalysts for Efficient Amide Bond Synthesis

“…The catalysts were previously characterized from different Techniques, [21–23] and the main physical‐chemical properties are presented in SI, Table S1 (adapted [25] ). The catalysts K10‐CSP and MWCNTs‐CSP present the highest specific surface area (S BET ), 155 and 181 m 2 /g, respectively.…”

“…The MWCNT functionalization was performed according to Peixoto et al [21] . in the previously oxidized multi‐walled carbon nanotubes ( o ‐MWCNTs); MWCNT‐CSP: 3.0 g of o‐MWCNT were suspended in 200 mL of dry toluene and CSPTMS (1.38 mL).…”

“…All the sulfonic acid‐catalysts were prepared according to the procedures described by Peixoto et al [19–22] . Reagents: Montmorillonite K10 (K10), halloysite nanotubes (HNT), p‐toluenesulfonic acid monohydrate (≥98 %) were purchased from Sigma Aldrich, chlorosulfonic acid (≥98 %) from Fluka; 2‐(4‐chlorosulphonylphenyl)ethyltrimetoxysilane (CSPTMS, 50 % in methylene chloride) from ABCR GmbH; Cloisite‐Na + (CLOI) was kindly provided by Southern Clay.…”

“…have developed new sulfonic acid‐(nano)materials based on natural clays, [19,20] multiwalled carbon nanotubes (MWCNTs) [21 ] and bio‐based carbons [22] (prepared from vineyard pruning waste). The catalysts have been used in biodiesel production, [19,20] Prins‐Ritter reaction for selective synthesis of terpenoid‐derived 4‐amidotetrahydropyran compounds, [21,23] and production of fuel bioadditives as ethyl levulinate [20] and 5‐( t ‐butoxymethyl)furfural [24] (continuous flow) from 5‐hydroxymethylfurfural. Due to the excellent performance of these sulfonic acid catalysts under homogeneous conditions in the above mentioned reactions, we decided to expand our studies for the production of amides via the Ritter reaction using several substrates including the derived platform molecules, such as 5‐HMF and furfuryl alcohol.…”