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Organometallics 2010, 29, 1712–1716 DOI: 10.1021/om901101f
Relative Kinetic Basicities of Organogold Compounds Katrina E. Roth and Suzanne A. Blum* Department of Chemistry, University of California, Irvine, California 92697-2025 Received December 21, 2009
The relative kinetic basicities of a series of differentially substituted and hybridized neutral organogold compounds were examined through competitive protodeauration experiments and were found to span 2.2 orders of magnitude. The effect of electron-withdrawing and electron-donating substituents on the rate of protodeauration of alkenylgold and arylgold compounds was explored. An acid counterion effect indicated the presence of a gold-mediated substrate preequilibrium before protodemetalation, and hybridization effects and a Hammett correlation with Fþ = -0.41 indicated the involvement of the C-C π system in the protodeauration of vinylgold, alkynylgold, and arylgold complexes.
Introduction
Results and Discussion
In the last seven years, the gold-catalyzed rearrangement of alkenes, allenes, and alkynes has become a broadly applicable method in organic synthesis.1-3 The majority of these reactions feature protodeauration of an organogold intermediate as the key step for catalyst regeneration. Recently, strategies to outcompete protodeauration by trapping the organogold intermediate with carbon,4-6 silicon,7 and sulfonyl8 electrophiles or via palladium-catalyzed crosscoupling9 have been developed. Despite the key role of protodeauration in competitive catalyst regeneration, to date there has not been a comprehensive and quantitative study of the relative basicities of organogold compounds. An understanding of the relative basicites of organogold compounds would be useful for expanding the substrate scope of reactions that proceed through turnover-limiting protodemetalation and for the design of new reactions that outcompete protodemetalation with alternative functionalization pathways.4-9 Herein, we quantify the effects of hybridization and substitution on the kinetic basicities of a series of organogold compounds, selected to represent a range of the organogold intermediates recently characterized or proposed in the literature.1-3 In addition, we identify a dramatic acid counterion effect in a tandem gold-mediated rearrangement/protodeauration reaction.
In order to probe the relative kinetic basicities of the selected organogold compounds, a series of competitive protodemetalation experiments were performed (Table 1). The strong acid HCl initially was chosen as the proton source to ensure full protodemetalation of the examined compounds. Indeed, addition of 1 equiv of HCl 3 Et2O to phenylgold 1h in CD2Cl2 produced 1 equiv of benzene in less than 10 min, as observed by 1H NMR spectroscopy. Addition of HCl 3 Et2O (1.0 equiv) to a rapidly stirring mixture of two organogold complexes (3.0 equiv each) yielded a mixture of protodemetalation products (benzene and 2) (eq 1). The relative rates of protodemetalation were determined on the basis of the ratios of protodemetalated products 2 and benzene, as observed by 1H NMR spectroscopy. Relative basicities are listed in Table 1 and are normalized to the kinetic basicity of PPh3AuPh, which was defined as 1.0. These studies revealed a striking correlation between hybridization and kinetic basicity. The basicities of organogold compounds conformed to the following trend: sp3 < sp < sp2, as observed with compounds 1c < 1d < 1h < 1m. This trend and the similarity in basicity between sp3- (0.35) and sp- (0.46) hybridized complexes was particularly of note, because the pKa values of the conjugate acids follow a different trend: sp , sp2 95% 1H NMR yield of 6). In contrast to the facile protodemetalation of neutral compounds 1a- n by HCl 3 Et2O, treatment of cationic 6 with HCl 3 Et2O did not result in protodemetalation. Instead, allene 5 was rapidly and quantitatively regenerated, with concurrent formation of PPh3AuCl (
