Palladium‐Catalyzed Allylic Substitution at Four‐Membered‐Ring Systems: Formation of η¹‐Allyl Complexes and Electrocyclic Ring Opening

Abstract

Palladium allyl complexes have been extensively studied over the past decades owing to their relevance in homogeneous catalysis. They represent key intermediates in catalytic allylic substitution reactions (Tsuji-Trost reactions) and considerable efforts have been made to elucidate their solution structures and mechanistic behavior in detail. [1] 1,3-Diphenylpropenyl acetates and carbonates are the benchmark linear systems employed for the study of allyl palladium intermediates. [2] When cyclic substrates are concerned, cycloalk-2-enyl esters are often biased by undesired b-hydride elimination events, which critically affect the stability of the resulting metal complexes as they rapidly lead to the corresponding cycloalkyldiene byproducts. [3,4] To date, palladium allyl complexes derived from five-and four-memberedring systems have been virtually unexplored. [4] We recently reported a diastereodivergent synthesis of 3,4-disubstituted cyclobutenes through a deracemizing palladium-catalyzed asymmetric allylic alkylation. [5] In those studies, we uncovered unprecedented behavior of the allylic electrophiles (rac)-1 and (rac)-cis-2 in the presence of phosphino-oxazoline ligands L (Scheme 1): nucleophilic attack by stabilized carbanions (i.e. malonates, azlactones, b-ketoesters) at the putative h 3 -allyl intermediate took place to afford the trans-cyclobutene products in excellent levels of diastereo-and enantioselectivity.This unusual reactivity of four-membered cyclic systems in the presence of ligands L along with the lack of literature precedent in the field warranted further investigation. Herein we present our preliminary findings on the unique structure and reactivity of cyclobuten-3-yl palladium complexes as well as the unprecedented electrocyclic ring opening of an organopalladium derivative.The addition of stoichiometric amounts of L-Ph and [Pd(dba) 2 ] (dba = trans,trans-dibenzylideneacetone) to the substrate (rac)-cis-2 proceeded instantaneously at À30 8C (Scheme 2). The thus formed intermediate 3 is unstable at room temperature. Nevertheless, it could be extensively characterized by multinuclear NMR spectroscopy at 10 8C as a h 1 -allyl complex (Scheme 2). The 13 C NMR spectrum was diagnostic for a h 1 -allyl ligand, with characteristic chemical shifts at 129.1, 147.9, and 46.0 ppm for C2, C3 and C4 (bearing Pd), respectively. [6] Consistent with literature precedence for the arrangement of ligands around square-planar Pd II centers, the h 1 -allyl moiety is located trans to the nitrogen donor of L-Ph, a ligand of weaker trans influence than phosphorus. [7] This configuration was determined by NMR analysis: the small coupling constant 2 J CP = 4.3 Hz indicates a cis relationship across the Pd center, [8] while the experimental NOE data for the 1 H atoms at positions 1 and 4 indicates an anti configuration of the substituents on the cyclobutene.Although h 3 /h 1 slippage of allyl ligands plays a major role in the Tsuji-Trost reaction and constitutes one of the critical issues in the control of regio-and ...

Dynamic behaviour of monohaptoallylpalladium species: internal coordination as a driving force in allylic alkylation chemistry

“…The internal distances of the four-membered ring have typical lengths for localized C–C single (1.621 to 1.660 Å) and C C double (1.365 Å) bonds. In contrast to our prior findings, 4 the internal disubstituted C–C single bond is shorter than the other two, due to the chelation from the amide carbonyl. The short distance between the Pd-centre and the amide carbonyl oxygen (bond distance 2.08 Å) further validates this assumption.…”

“…21 A key interaction common to both structures is a hydrogen bond between the chlorine and the hydrogen atoms of the amide moiety. 4 , 22 Additionally, two CH/π interactions between the naphthyl groups of both ligands are apparent in both structures, with H-arene distances within the purview of what has been observed experimentally and computationally for this type of interaction. 23 A structural feature distinguishing between the two diastereomers is the positioning of the dimethyl amino group that is located in 5b under and in close proximity to the cyclobutene ring.…”

“…The stability of these complexes at room temperature stands in contrast to the temperature-sensitive nature of the analogous complexes of bidentate ligand L1a . 4 …”

“…In close analogy with amide 3 , the ( rac )- cis -chloroacid 2 also leads to the formation of a single anti -η 1 -allyl palladium complex 8 upon stoichiometric combination with Pd(dba) 2 and L2a ( Scheme 4 ). As the acid 2 had proved to be an ideal electrophile for catalytic deracemization in our previous work, 4 we investigated the reaction of the monohaptoallylpalladium complex 8 with a suitable nucleophile. As shown, treatment of complex 8 with sodium (2-methyl)dimethylmalonate at 0 °C yielded the cis -disubstituted cyclobutene 9 in 74% ee.…”

“…Employing the bidentate ligand L1a , we eventually identified a series of η 1 -allyl palladium complexes, prone to very facile electrocyclic ring opening at temperatures close to r.t., as key species in the deracemization process. 4 We subsequently became interested in the case of monodentate ligands such as L2a–c . Herein we present our findings on the structure of internally chelated η 1 -allyl palladium complexes containing those ligands, their kinetic study enabling a direct insight into metallotropic equilibria, as well as a rare showcase of the value of chelation in allylic substitution chemistry.…”