Attack of molecular iodine to novel palladacyclopentadienyl complexes bearing isocyanides as spectator ligands. A computational and mechanistic study
Graphical abstract
We have carried out some oxidative addition of I2 on palladacyclopentadienyl complexes bearing isocyanides ligands. On the basis of a kinetic study we have proposed a plausible mechanism for the formation of Pd(II) complex bearing iodine, the open butadienyl fragment σ-coordinated and the isocyanides trans to each other.
Introduction
Many natural and bioactive compounds enclose in their structures highly conjugated poly-unsaturated units. Consequently, the stereospecific synthesis of conjugated dienes became an important field of study and several catalytic approaches based on different metals [1] such as the diyne reduction with zinc/copper or sodium/mercury amalgam [2] and synthetic processes through the metallacyclopentadienyl derivatives of titanium, zirconium, iridium [3] and palladium [4] were proposed.
In particular we have investigated the mechanism of formation of the palladacyclopentadienyl compounds when activated alkynes such as dimethyl-2-butynedioate (dmdb) and methyl(4-nitrophenyl)propynoate (pna) attack Palladium(0) alkene complexes bearing bidentate heteroditopic and symmetric spectator ligands [5].
Eventually, addition of halogens (X2) or organic halides (RX) to the palladacyclopentadienyl complexes A induces the formation of the (σ-butadienyl) Pd(II) species B or B′, respectively. Further addition of halogens liberates the 1,3-dienes unit (C) and the di-halo palladium(II) complex (D), respectively (Scheme 1) [4g]. Alternatively, transmetalation between B′ and tin reagents can also yield the unsaturated organic moieties C′ and the olefin derivative E [4](d), [4](f).
We have now prepared some new cyclopentadienyl palladium derivatives bearing isocyanides as spectator ligands with the aim of study the propensity of such complexes characterized by the presence of four carbon atoms bound to the metal centre toward the addition of molecular halogens (Scheme 2). Moreover, we hoped to shed light on the reaction mechanism thanks to the retarding effect that the electron-withdrawing isocyanide could have on the rate of the oxidative process. As a matter of fact, in almost all the cases the complexes react immediately and quantitatively with a stoichiometric amount of I2 to give the σ-butadienyl derivative, apart from the case of complex 1a.
Since in this latter case the reaction went to completion in a reasonable time interval, we had the opportunity to carry out a kinetic study which will be described in this paper and was never dealt with at the best of our knowledge. In addition to the starting complexes, in the following Scheme 2 are reported the reaction products and the related numbering scheme.
Section snippets
Synthesis of the palladacyclopentadienyl complexes [Pd(CNR)2C4(COOR′)4]
The polymer [PdC4(COOR′)4]n (R′ = Me, t-Bu) prepared according to published procedure [4a] reacts in anhydrous acetone under inert atmosphere with a stoichiometric amount of CNR isocyanides yielding the Pd(II) complexes 1a–f as easily separable and stable whitish or yellowish solids. The symmetry of the ensuing complexes (C2v) induces a remarkable simplification in the 1H and 13C NMR spectra. The two coordinated isocyanides resonate as a unique signal and at a markedly different field from that
Conclusion
We have synthesized some novel palladacyclobutadienyl complexes bearing the isocyanides DIC, TOSMIC and TIC as spectator ligands. The reaction of the cis-diisocyanide tetramethyl palladacyclo-1,3-diene-1,2,3,4-tetracarboxylate complexes with iodine yields without exception the trans-diisocyanide-tetramethyl pallada 1-iodobuta-1,3-diene-1,2,3,4-tetracarboxylate. In the favourable case of the DIC derivative (1a) we were able to measure the reaction rate of the intramolecular conversion of the
Experimental section
All solvents were purified by standard procedures and distilled under argon immediately before use [11]. 1D- and 2D-NMR spectra were recorded using a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given relative to TMS (1H and 13C NMR).
Peaks are labelled as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br). The proton and carbon assignment was performed by 1H-2D COSY, 1H-2D NOESY, 1H–13C HMQC and HMBC experiments.
IR spectra were recorded on a
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