Cationic Gold Catalyzes ω-Bromination of Terminal Alkynes and

RESEARCH ARTICLE pubs.acs.org/acscatalysis

Cationic Gold Catalyzes ω-Bromination of Terminal Alkynes and Subsequent Hydroaddition Reactions Antonio Leyva-P
erez,*,† Paula Rubio-Marqu
es,† Salem S. Al-Deyab,‡ Saud I. Al-Resayes,‡ and Avelino Corma*,†,‡ †

Instituto de Tecnología Química, Universidad Polit
ecnica de Valencia, Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022 Valencia, Spain ‡ Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

bS Supporting Information ABSTRACT: Orthogonal σ,π-bisfunctionalization of terminal alkynes can be achieved with a cationic gold complex in catalytic amounts. First, the terminal C
H bond is transformed to the corresponding bromoalkyne which is then activated toward nucleophilic attack. This reactivity correlates with the structural nature of isolated gold-alkyne complexes. KEYWORDS: gold catalysis, aurophilic interactions, terminal alkynes, σ,π-bisfunctionalization, bromoalkynes

1. INTRODUCTION Homogeneous and heterogeneous catalysis by gold has gained much attention in the past years,1
7 and new reactions are systematically unveiled. 8
11 In particular, activation of alkynes by gold compounds and nanoparticles has opened unexpected reaction pathways.12
14 Mechanistically, it is accepted that, in general, gold acts as a Lewis acid on the triple bond through π-coordination (Scheme 1, A)15 while, for some reactions of terminal alkynes, a gold alkynylide intermediate is postulated (B). Curiously, the structure of isolated gold-alkynylides compounds does not fit only one of these two simple models but a combination of them: the gold atom presents η1 ,η2-hapticity to the alkyne, together with aurophilic interactions (C).16
18 The potential role of gold as bifunctional σ,π-activator of terminal alkynes has been little explored in catalysis. Of course, neat alkyne-gold complexes are not formed under catalytic conditions, and the structure of the gold intermediate in the reaction differs significantly from that of the corresponding isolated complex. However, an orthogonal σ,π-bisfunctionalization might be expected if significant amounts of the Au-alkyne η1,η2-complex are formed under reaction conditions.19 Here, some results in this regard are presented.

Scheme 1. Coordination of Gold to Alkynes

for this transformation,22
27 and Hg and Pt triflimides (entries 22
23) were able to catalyze the reaction in moderate yields. As it can be seen in Table 1, the solvent also influences the product formation. A comparison between AuPtBu3NTf2 and AgNO3 in different solvents (see Supporting Information, Table S1) shows that the former is highly active in DCM, CH3CN, ethers, and water while, in contrast, the catalytic activity of AgNO3 correlates with the solubility of NBS (acetone . DCM > n-hexane). This solvent-dependence suggests a distinct mechanism for gold and silver as catalysts and reflects the insolubility of the silver alkynylide intermediate (see below). In fact, AuPtBu3NTf2 and AgNO3 catalyze independently the formation of 3a when put together in the reaction medium at low catalytic amounts (see footnote in Supporting Information, Table S1). At this point, the question is how a highly cationic gold complex such as AuPtBu3NTf2 activates so efficiently the terminal σ-CH bond of the alkyne.28
30 In principle, a η2-(π)coordination of gold to the alkyne should be expected, but according to the structure of gold coordination polymers16 (structure C in Scheme 1) and recently isolated cationic goldalkyne complexes,31 a possible η1,η2-hapticity under the reaction conditions could occur. To check this hypothesis, the corresponding

2. RESULTS AND DISCUSSION During the metal-catalyzed bromination of phenylacetylene 1a with N-bromosuccinimide (NBS) 2a, we found that the cationic gold(I) complex AuPtBu3NTf2 is able to activate the terminal C
H bond of 1a to give the corresponding 1-bromophenylacetylene 3a in high yield (Table 1, entries 1 and 16).20 Other Au species (entries 3
10 and 24) and Ag (entries 11
15), Cu (entry 19), Hg (entry 20),21 and Pt (entry 21) salts gave no product. Only AgNO3 (entry 18), the catalyst of choice r 2011 American Chemical Society

Received: April 1, 2011 Revised: April 18, 2011 Published: April 20, 2011 601

dx.doi.org/10.1021/cs200168p | ACS Catal. 2011, 1, 601–606

ACS Catalysis

RESEARCH ARTICLE

Table 1. Formation of 1-Bromophenylacetylene 3a in the Presence of Different Catalysts

Table 2. Metal Coordination Polymers as Substrates for the Reaction

conversion (%)a

run

catalyst

solvent

1

AuPtBu3NTf2

1,4-dioxane

2

none

0

3 4

AuCl3 HAuCl4 3 3H2O

0 0

5

AuNTf2