Transmetalation reactions. The role of the stabilizing olefin in determining the overall reaction rate

doi:10.1016/j.jorganchem.2008.07.028

Journal of Organometallic Chemistry

Volume 693, Issues 21–22, 15 October 2008, Pages 3324-3330

https://doi.org/10.1016/j.jorganchem.2008.07.028 Get rights and content

Abstract

A systematic study concerning the transmetalation reaction between the palladium butadienyl complexes [PdCl((ZC double bond CZ)₂Me)(L-L′)] (Z = COOMe; L-L′ = MeN-SPh (1A), N-SPh (1B), DPPQ-Me (1C), BiPy (1D), DPPE (1E)) and tributyl-phenylethynyl-stannane in the presence of some stabilizing olefins (ma, fn, nq, dmfu, and tmetc) was undertaken. The dependence of the reaction rate on the nature of the ancillary ligand was discussed in terms of the donor capability and steric characteristics of the ligand. It has been noticed that, other things being equal, the joined distorted MeN-SPh ligand imparts the highest reactivity to its derivative (complex 1A). The most surprising issue was however represented by the olefin which seems to affect heavily the reactivity of the starting substrate thereby increasing the overall reaction rate. The most active olefins were ma and fn. In the case of the reaction between the complex 1A and tributyl-phenylethynyl-stannane in the presence of fn an exhaustive kinetic study was carried out and a mechanistic hypothesis was advanced.

Graphical abstract

Introduction

The Stille reaction consisting in the palladium catalyzed cross coupling between carbon electrophiles and organo-stannanes is an important and attractive method in modern synthetic organic chemistry [1]. It is usually represented by a catalytic cycle in which the Pd(0) catalyst is firstly oxidized by the organic electrophile to the corresponding Pd(II) derivative which undergoes the transmetalation reaction followed by the reductive elimination of the coupled organic derivative and the subsequent restoration of the Pd(0) catalyst [1](f), [2]. The complexity of the entire mechanism is testified by the number of papers dealing with the different steps of the cycle and by the fact that the nature of the solvent, the electronic and steric properties of the ancillary ligand, the stannane and the organic electrophile heavily affect the reactivity of every single step [3a]. The rate-determining step of the whole process can be therefore represented by the oxidative addition, by the transmetalation and even by the reductive elimination [1](a), [1](i), [2], [4].

However, the transmetalation reaction seems to be the most complex and therefore the less granted process even though its associative character (apart from few exceptions [4g]) is recognized on the basis of the work of Espinet [3](a), [3](b), [3](c). In particular, two different associative pathways which are strongly dependent on the nature of the solvent and the reactant are possible. Thus, poorly coordinating solvents with low dielectric constant and strongly coordinating ligands favour the S_E2 cyclic step whereas the S_E2 open path is preferred when the solvents display remarkable polarity and coordinating capability and the ligands are weak as coordinating and bridging species [5]. Consequently, the general catalytic cycle for the Stille reaction can be represented by Scheme 1.

In order to optimize the efficiency of the transmetalation step two different approaches are available i.e. the enhancement of the nucleophilic character of the organo-stannanes and the enhancement of the electrophilic character of the palladium complex. The former can be achieved by the use of suitable additives as fluoride [6] and hydroxide ions [7] whereas the electrophilicity of the palladium complex can be generally obtained by decreasing the basicity of the ancillary ligands (i.e. arsine complexes are more reactive than their phosphine analogues [1](e), [4](e), [4](f) and the reactivity order induced by coordinated halide is I < Br < Cl [3a, p. 4720]). In this respect, it is well known that coordinated electron-poor olefins are effective in removing electron density from the metal centre. Such an effect was widely utilized to facilitate reductive elimination in palladium(II) substrates [8], and for that reason when the reductive elimination step becomes the rate determining step in a palladium catalyzed C–C coupling the addition of an exogenous olefin was particularly advantageous. Thus, Schwartz and co-workers have shown that the coupling of allyl halides with allyl tin [9a] reagents (and with other allyl organometallic substrates [9](b), [9](c)) does not proceed unless an electron-withdrawing olefin such as maleic anhydride was added. This is confirmed by the detailed kinetics and theoretical studies of Kurosawa et al. on allyl aryl palladium complex which showed that the reductive elimination occurs from an η³-allyl palladium complexes with a coordinated olefin [10].

In a preliminary study concerning the synthesis of some poly-unsaturated compounds by transmetalation we noticed that the nature and the concentration of the olefin employed heavily interferes with the overall rate of the stoichiometric reaction, thereby showing that its role was not simply confined to the stabilisation of the palladium(0) reaction product [11]. Moreover, in this case the rate determining step was better represented by the transmetalation step than by of the reductive elimination which was proved to be fast [12]. Since the mechanism governing the palladium catalyzed synthesis of poly-unsaturated compounds is a widely investigated topics [13] and therefore its comprehension would represent a very important task, we decided to undertake an exhaustive study of the kinetics of the reactions between some butadienyl palladium(II) complexes bearing differently designed bidentate ancillary ligands (prepared according to published methods [14]) and tributyl-phenylethynyl-stannane.

The reaction and the compounds involved in the present paper are reported in Scheme 2.

Section snippets

Synthesis of palladium butadienyl and palladium(0) olefin derivatives

The complex [PdCl((MeOOC–C double bond C–COOMe)₂Me)(MeN-SPh)] (1A) was obtained by reacting an excess of dmbd (dimethyl-2-butynedioate) with the complex [PdCl(Me)(MeN-SPh)] (dmbd: complex = 3:1) for 22 h in CH₂Cl₂ at RT [14a]. The palladium butadienyl derivatives [PdCl((ZCCZ)₂Me)(L-L′)] (Z = COOMe; L-L′ = N-SPh (1B), DPPQ-Me (1C), BiPy (1D), DPPE (1E)) were obtained according to published methods [14] by adding an equimolar amount of the appropriate ligand (B, C, D, E) to a solution of the complex (1A) in freshly

Conclusions

From the experimental results of the present work it is possible to conclude that:

(a)
The olefin heavily interferes in the mechanism of transmetalation step in the C–C coupling between the palladium-butadienyl complexes and alkynyl-stannane and its nature is of primary importance in determining the overall reaction rate.
(b)
The most efficient olefin is maleic anhydride, probably thanks to its high electron-withdrawing character and reduced steric requirements which favour its pre-coordination.
(c)
In the

Solvents and reagents

Acetone and CH₂Cl₂ were distilled over 4 Å molecular sieves and CaH₂, respectively. CHCl₃ was distilled over silver foil under inert atmosphere. All the other chemicals were commercially available grade products and were used as purchased.

Data analysis

Mathematical and statistical analysis of data was carried out by locally adapted non linear regression algorithms written under SCIENTIST™ environment.

IR, NMR, and UV–Vis measurements

The IR and the ¹H and ¹³C NMR spectra were recorded on a Perkin–Elmer Spectrum One spectrophotometer and on a

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Transmetalation reactions. The role of the stabilizing olefin in determining the overall reaction rate

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of palladium butadienyl and palladium(0) olefin derivatives

Conclusions

Solvents and reagents

Data analysis

IR, NMR, and UV–Vis measurements

Bull. Chem. Soc. Jpn.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Organometallics

J. Am. Chem. Soc.

J. Org. Chem.

J. Am. Chem. Soc.

Chem. Eur. J.

J. Am. Chem. Soc.

Angew. Chem., Int. Ed.

Eur. J. Inorg. Chem.

Organometallics

Angew. Chem., Int. Ed. Engl.

Synthesis

Tetrahedron Lett.

Adv. Metalorg. Chem.

Chem. Eur. J.

J. Org. Chem.

Tetrahedron

The Stille Reaction

J. Am. Chem. Soc.

Organometallics

Angew. Chem., Int. Ed.

J. Am. Chem. Soc.

Angew. Chem., Int. Ed.

J. Am. Chem. Soc.

Chem. Eur. J.

J. Am. Chem. Soc.

Syn. Lett.

Org. Lett.

J. Org. Chem.

Syn. Lett.

Tetrahedron Lett.

Tetrahedron Lett.

Organometallics

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Chem. Soc., Dalton Trans.

Chem. Rev.

Chem. Commun.

J. Organomet. Chem.

J. Am. Chem. Soc.

J. Org. Chem.

Tetrahedron Lett.