A Regio‐ and Diastereoselective Platinum‐Catalyzed Tandem [2+1]/[3+2] Cycloaddition Sequence

Abstract

Platinkomplexe von phosphinigen Säuren vermitteln eine effiziente intermolekulare Tandemsequenz aus [2+1]‐ und [3+2]‐Cycloadditionen. Diese Prozesse öffnen den Zugang zu neuartigen tricyclischen Systemen, und sie verlaufen zudem regio‐ und diastereoselektiv (siehe Schema; Cy=Cyclohexyl). Für die [3+2]‐Cycloaddition wurden weiterhin zwei verschiedene Alkinpartner eingesetzt.

[2+1] Cycloaddition Affording Methylene- and Vinylidenecyclopropane Derivatives: A Journey around the Reactivity of Metal-Phosphinito-Phosphinous Acid Complexes

“…A slight yield improvement and a cleaner reaction could even be obtained upon reversing the ratio of norbornadiene/phenylacetylene (Table , entry 7). In this case, with the most efficient catalyst 21d (possessing phenyl and cyclohexyl substituents on the phosphorus atoms), we observed the formation of dicycloadduct 22a in a 1:1.2 diastereomeric ratio along with the expected cycloadduct 4a (Table , entry 8) ,. Importantly, dicycloadducts 22 were never detected in the case of palladium‐mediated [2+1] cycloadditions.…”

“…During the investigation of the scope of the platinum‐catalyzed [2+1] cycloaddition, we observed the formation of an unexpected product when propargyl acetate was used as an alkyne partner . As shown in Table , entry 1, only small quantities of ACP 4w were isolated, whereas tricyclic compound 23w was the major product (62 %).…”

“…Nonetheless, after a brief optimization (4 equiv of phenylacetylene added dropwise for 16 h), the expected tetrahydroindene 27w was obtained as a single diastereomer in good yield. A mechanism was proposed to rationalize both regio‐ and diastereoselectivities (Scheme ) . We assumed that the cationic platinum species F coordinated both the alkyne and carbon–carbon double bond of the ACP on its exo face and oxidative coupling would form platinacyclopentene H .…”

“…In this case, with the most efficient catalyst 21d (possessing phenyl and cyclohexyl substituents on the phosphorus atoms), we observed the formation of dicycloadduct 22a in a 1:1.2 diastereomeric ratio along with the expected cycloadduct 4a ( Table 8, entry 8). [27,28] Importantly, dicycloadducts 22 were never detected in the case of palladium-mediated [2 + 1] cycloadditions. Similarly to palladium, the platinum-catalyzed formation of ACPs was successfully widened to norbornene, benzonorbornene, or diazabicyclic alkene (Scheme 22).…”

“…A mechanism was proposed to rationalize both regio-and diastereoselectivities (Scheme 26). [28] We assumed that the cationic platinum species F coordinated both the alkyne and carbon-carbon double bond of the ACP on its exo face and oxidative coupling would form platinacyclopentene H. Due to steric interactions between substrates and ligands, compound H would be obtained as a single regioisomer. For more clarity and to explain the diastereoselectivity, compound H has been drawn as a single enantiomer, I. Cyclopropane fragmentation would occur by a 1,2-shift of the platinum assisted and oriented by the oxygen atom to generate the platinacyclohexene J.…”