Article pubs.acs.org/Organometallics
Cationic Gold(I) π-Complexes of Terminal Alkynes and Their Conversion to Dinuclear σ,π-Acetylide Complexes Timothy J. Brown and Ross A. Widenhoefer* Department of Chemistry, French Family Science Center, Duke University, Durham, North Carolina 27708-0346, United States S Supporting Information *
ABSTRACT: Treatment of a suspension of (IPr)AuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] and AgSbF 6 with terminal arylacetylenes led to the formation of thermally unstable gold π-alkyne complexes of the form [(IPr)Au(η 2-HC CAr)]+SbF6− in ≥86 ± 5% yield, which were characterized by spectroscopy without isolation. Warming these complexes to 0 °C led to C(sp)−H bond cleavage and formation of dinuclear gold(I) σ,π-acetylide complexes of the form {[(IPr)Au]2(η 1,η 2-C CAr)}+SbF6−, three of which were isolated in 99% yield and one of which was characterized by X-ray crystallography. 1H NMR analysis of the conversion of gold π-arylacetylene complexes to σ,π-acetylide complexes established protonation of free arylacetylene, indicating the generation of a strong Brønsted acid under reaction conditions.
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transformations represent only a small subset of the gold(I)catalyzed transformations that employ terminal alkynes, and the potential involvement of gold acetylide intermediates in this larger collection of transformations remains largely unexplored.8 Noteworthy in this regard is the combined experimental and computational analysis of the cycloaddition of 1,5-allenynes that invoked dual activation of the terminal CC moiety through formation of a dinuclear gold σ,π-acetylide species.11 Despite the widespread employment of terminal alkynes in gold(I)-catalyzed transformations and the potential involvement of gold acetylide complexes in these transformations, little experimental information is available regarding the conversion of gold π-(terminal alkyne) complexes to gold σ-acetylide complexes. This deficiency is due in large part to the scarcity of well-defined gold π-(terminal alkyne) complexes, which contrasts with the large number of known gold σ-acetylide complexes,12 including those containing N-heterocyclic carbene ligands,13 and the growing number of gold(I) π-(internal alkyne) complexes.14,15 In a singular and particularly relevant example, Bertrand has described the isolation of a gold(I) π-phenylacetylene complex containing a cyclic alkyl amino carbene (CAAC) ligand and the conversion of this complex to the corresponding σ-acetylide complex upon treatment with diethylamine.16 Here we report the in situ generation and spectroscopic characterization of thermally unstable gold π-arylacetylene
INTRODUCTION The formation of coinage metal acetylide complexes from terminal alkynes under basic conditions is a process central to a number of important synthetic procedures.1 Notable among these are the palladium/copper-catalyzed coupling of terminal alkynes with aryl or alkenyl halides (Sonogashira reaction)2 and the copper-catalyzed 1,3-dipolar cycloaddition of azides and terminal alkynes.3 A diverse range of transformations likewise exploit the reactivity of silver acetylide complexes generated in situ from terminal alkynes.4 The generally accepted mechanism for conversion of a terminal alkyne to a copper or silver acetylide involves initial formation of a transient metal π-alkyne complex, which activates the acetylenic proton toward deprotonation by external base.1−5 DFT analysis of the copper-catalyzed 1,3-dipolar cycloaddition of azides and terminal alkynes suggests that the pKa of an alkynyl proton decreases by 10 pKa units upon coordination to Cu(I).6 Gold(I) complexes are particularly active catalysts for the functionalization and cycloaddition of alkynes,7,8 reactivity that is presumably initiated by π-complexation of the alkyne to the electrophilic gold(I) center, which renders the alkyne susceptible toward nucleophilic attack. The depletion of π-electron density from the alkyne upon coordination to the electrophilic gold(I) center should also enhance the acidity of the acetylenic proton, facilitating acetylide formation. Indeed, there is a growing body of gold-catalyzed transformations of terminal alkynes in which the involvement of gold(I) acetylides is implicit, 9 notably the coupling of terminal alkynes with secondary amines and aldehydes to form propargylic amines.10 However, these © 2011 American Chemical Society
Received: September 6, 2011 Published: October 19, 2011 6003
dx.doi.org/10.1021/om200840g | Organometallics 2011, 30, 6003−6009
Organometallics
Article
complexes containing an N-heterocyclic carbene ligand and the facile conversion of these complexes to dinuclear gold η 1,η 2acetylide complexes in the absence of external base.
Table 2. Equilibrium Constants for Displacement of NCArF [NCArF = NC-3,5-C6H3(CF3)2] from [(IPr)Au(NCAr F )] + SbF 6 − (2) with para-Substituted Arylacetylenes in CD2Cl2 at −60 °C
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RESULTS AND DISCUSSION Synthesis of Gold π-Complexes of Terminal Alkynes. Treatment of a suspension consisting of a 1:1 mixture of the gold(I) N-heterocyclic carbene complex (IPr)AuCl [IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene] and AgSbF6 with phenylacetylene (1.0 equiv) in CD2Cl2 at −78 °C for 10 min led to formation of the cationic gold π-alkyne complex [(IPr)Au(η 2-HCCC6H5)]+SbF6− (1a) in 92 ± 5% yield by 1 H NMR analysis. Complex 1a was thermally unstable (see below) and was characterized in solution by 1H and 13C NMR spectroscopy at −60 °C. Coordination of phenylacetylene to gold was established by the downfield shift of the acetylenic proton of 1a (δ 4.56) relative to that of free phenylacetylene (δ 3.06) in the 1H NMR spectrum and by the difference in the 13 C NMR shifts of the acetylenic carbon atoms of 1a [δ 72.7 (d), 93.5 (s)] relative to free [δ 77.3 (d), 83.7 (s)] phenylacetylene. The 1JCC coupling constant of the phenylacetylene ligand of the 13 C-isotopomer [(IPr)Au(η 2 -H 13 CCC 6 H 5 )] + SbF 6 − (1a-13C1) (1JCC = 133.8 Hz) was diminished significantly relative to free phenylacetylene (1JCC = 175.9 Hz). This behavior contrasts the minimal diminution (∼6 Hz) of the 1JCC of the isobutylene ligand of [(IPr)Au(η 2-H213CCMe2)]+SbF6− relative to free isobutylene17 and points to a more significant π-back-bonding component to the gold π-alkyne bond than to the gold π-alkene bond. In addition to 1a, thermally unstable gold π-alkyne complexes [(IPr)Au(η 2-HCCAr)]+SbF6− [Ar = 4-C6H4Me (1b), 4-C6H4CF3 (1c), 4-C6H4OMe (1d), and 4-C6H4Br (1e)] were generated and characterized in situ by NMR at −60 °C (Table 1).
Ar
Keq
4-C6H4OMe 4-C6H4Me C6H5 4-C6H4Br 4-C6H4CF3
1.25 0.52 0.21 0.065 0.025
± ± ± ± ±
0.07 0.03 0.02 0.006 0.003
alkynes to the 12-electron gold fragment (IPr)Au+, we likewise determined equilibrium constants for the displacement of NCArF from 2 with para-substituted arylacetylenes HCC-4C6H4X (X = OMe, Me, Br, CF3).17,18 The relative binding affinities of the arylacetylenes decreased with decreasing electron density by a factor of ∼50 (Table 2), and a plot of log(KX/KH) versus the Hammett σ-parameter displayed a slope of ρ = −2.1 ± 0.3 (Figure 1). While the magnitude of the binding affinities of
Table 1. Synthesis of Gold π-Complexes of Arylacetylenes
Figure 1. Plot of log KX/KH versus the Hammett σ-parameter for the equilibrium displacement of HCArF from 2 with arylacetylenes in CD2Cl2 at −60 °C (ρ = −2.1 ± 0.2). Ar′
cmpd
yield
Ph 4-C6H4Me 4-C6H4CF3 4-C6H4OMe 4-C6H4Br
1a 1b 1c 1d 1e
92 ± 5% 97 ± 5% 99 ± 5% 86 ± 5% 91 ± 5%
arylacetylenes to (IPr)Au+ are similar to vinyl arenes, the electronic dependence of the binding affinity of arylacetylenes is somewhat diminished relative to vinyl arenes (ρ = −2.4),17 suggesting a greater π-back-bonding contribution to the Au− (π-alkyne) bond relative to the Au−(π-alkene) bond. Nevertheless, the large negative ρ value points to a gold−alkyne interaction dominated by σ-donation from the alkyne CC bond to gold.19 Formation of Dinuclear Gold σ,π-Acetylide Complexes. Addition of 2,6-di-tert-butylpyridine (DTBP; 1 equiv) to a solution of 1a (51 mM) at −60 °C led to immediate (
