Cracking Cavitands: Metal‐Directed Scission of Phosphinyl‐Substituted Resorcinarenes

Abstract

Resorcinarene-derived tetramethylene cavitands bearing a diphenylphosphino group grafted to their wider rim undergo facile, directed C-O bond breaking upon reaction with transition-metal ions in the presence of nucleophiles. One possible reaction mechanism involves formation of a P,O-chelate complex, which weakens the adjacent O-CH2 bond, leading to the formation of an oxacarbenium intermediate.

Cavitand Scission by Transition‐Metal Centres – Cleaved Cavitand Chirality and Its Consequences

“…[7,9] We have previously shown that the treatment of 1 with the neutral complex [RuCl 2 (p-cymene)] 2 leads quantitatively to a single form of the chelate complex [RuCl(p-cymene)(P,O)] (3). [2] By extending this work to an investigation of the reaction of 1 with the bromido complex [RuBr 2 (p-cymene)] 2 under conditions similar to those applied for the preparation of 3, we have now found that two isomeric products result and can be distinguished readily by their 31 P NMR chemical shifts. Chromatographic separation gave the bromido complex 4, the crystalstructure determination of which (see below) showed it to be the analogue of 3, in 38 % yield and a minor product (5, 15 % yield, Scheme 3) with very similar spectroscopic properties and composition.…”

“…These results define a rather rare system in which the solubility difference between the "racemic compound" and "racemic conglomerate" crystal forms of a chiral molecule is so slight that cocrystallisation is possible; [11] furthermore, this occurs through the formation of different solvates. 2 ], in which the two ligand units can adopt a rather remote disposition, may be indicative of the relative facility with which an intermediate Ni(P,O) species could accept a second cavitand ligand. A significant feature of the cavitand cleavage reaction is that it results in the conversion of the molecule from an achiral to a chiral form.…”

“…However, in a recent study, we found that the phosphine cavitand 1 (Figure 1) reacted with ruthenium and palladium complexes with a readily available coordination site to form phosphino-phenolato chelate complexes in which the macrocyclic core had undergone breaking of one of the two C-OCH 2 bonds adjacent to the P-substituted resorcinol unit. [2] The observed C-O bond cleavage was attributed to the transient formation of a "neutral" phosphine-ether chelate, which increases the strain within the eight-membered ring containing the coordinated OCH 2 fragment and, therefore, facilitates the nucleophilic attack of endogenous nucleophiles, in our case water, at the corresponding methylenic carbon atom. Theoretical calculations showed that C-O bond breaking may occur either the primary coordination sphere has a cis-NiO 2 P 2 geometry.…”

“…After anisotropic refinement, most of the hydrogen atoms were found by Fourier difference. The structures were then refined with SHELXL97 [15] by using the full-matrix least-squares technique {use of F 2 ; x, y, z, ij for C, O, Br, Cl, P and Ni or Ru atoms; x, y, z in the riding mode for H atoms; for 2·CH 2 2 )/3}. The crystal and refinement data are given in Table 1, and selected bond lengths and angles are listed in Tables 2, 3, and 4.…”

“…Theoretical calculations showed that C-O bond breaking may occur either the primary coordination sphere has a cis-NiO 2 P 2 geometry. In contrast, C-O bond breaking occurred only once per metal centre when 1 was treated with [RuBr 2 (p-cymene)] 2 . This led to a crystallographically characterised complex [RuBr(P,O)(p-cymene)] (4) along with a minor amount of a species (5) that is assumed to be a stereoisomer.…”