Article pubs.acs.org/Organometallics
Formation and Reactivity of Gold Carbene Complexes in the Gas Phase Christopher A. Swift and Scott Gronert* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23824 United States S Supporting Information *
ABSTRACT: A series of ligated gold(I) carbenes (where the ligand is Ph3P, Me2S, or an N-heterocyclic carbene, NHC) were formed in the gas phase by a variety of methods. Gold(I) benzylidenes could be formed using Chen’s method of dissociating an appropriate phosphorus ylide precursor. The resulting carbene undergoes an addition reaction with olefins to give an adduct. The adduct undergoes a second gas-phase reaction with an olefin, where presumably a cyclopropanation product is displaced by the second olefin molecule. Both steps in the process were analyzed with linear free energy relationships (i.e., Hammett plots). Under collision-induced dissociation conditions, the adduct undergoes competing processes: (1) dissociation of the cyclopropanation product to give ligated gold(I) species and (2) metathesis to give a more stable gold(I) carbene. Attempts to form less stable gold(I) carbenes in the gas phase by Chen’s approach or by reactions of diazo species with the ligated gold(I) cations were not successfulprocesses other than carbene formation are preferred or the desired carbene, after formation, rearranges rapidly to a more stable species. In accord with other recent work, the data suggest that coordination to a ligated gold(I) cation in the gas phase may not offer sufficient stabilization to carbenes to prevent competition from rearrangement processes.
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INTRODUCTION Gas-phase studies have become a useful tool in examining intermediates in condensed-phase reactions catalyzed by organometallic species.1−4 The key advantage of the gas phase is that short-lived intermediates can be isolated in the inert environment of a mass spectrometer and probed in the absence of side reactions with solvent and other components in reaction mixtures.5,6 Transition-metal-stabilized carbenes have been incorporated in many important catalytic cycles, including cyclopropanation and metathesis processes.7−10 Gold catalysis has become of great interest, due to the unique reactivity that gold exhibits.11 Previously, we have reported the gas-phase synthesis of iron and cobalt carbene complexes by the reaction of a ligated metal with diazoacetate esters.12,13 In those cases, the metal carbenes were prone to rearrangement processes, particularly metal−ligand insertions that converted the carbene to an ylide. Here we apply a variety of approaches to prepare and explore the reactivity of gold(I) carbene complexes in the gas phase. Chen and co-workers have presented several papers focused on the gas-phase formation and reactions of gold(I) benzylidene complexes of the general form LAuCHPh+, where L is an N-heterocyclic carbene (NHC).14−17 The carbenes were not generated directly by electrospray ionization but were formed by the collision-induced dissociation of an ylide precursor (Scheme 1). In their studies, they have shown that the gold(I) benzylidene reacts with alkenes to give addition products and that under collision-induced dissociation conditions (CID) the addition product decomposes either by loss of a cyclopropanation product (combined elements of the benzylidene and alkene) or by metathesis to produce a new gold(I) carbene complex. In the current study, we have tested © XXXX American Chemical Society
Scheme 1. Chen’s Method of Gold(I) Carbene Formation from Ylide Fragmentation
the generality of this approach for forming gold(I) carbenes and explored the kinetics of the bimolecular reactions of the gold(I) benzylidenes. The results are supported by density functional theory (DFT) calculations and highlight the important intermediates on the reaction surface.
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RESULTS AND DISCUSSION Gold(I) Benzylidenes. Chen and co-workers have focused considerable attention on gold(I) benzylidenes that bear an NHC on the gold as the second ligand.14−17 As noted above, they can be formed by the fragmentation of a phosphorus ylide (Scheme 1). They have shown that these gold carbenes react with alkenes to give addition products, which under CID yield cyclopropanes as well as metathesis products.15 Given the nature of their apparatus, it was not possible to examine the kinetics of the addition or the subsequent bimolecular reactions of the addition products. In our quadrupole ion trap, we can complete MSn experiments and therefore can probe each step in the complex reaction process. In the current study, we have adopted Chen’s methodology with triphenylphosphine as the second ligand on gold. When it is subjected to CID, the ylide precursor (Scheme 2) readily loses triphenylphosphine and Received: September 8, 2014
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dx.doi.org/10.1021/om500926v | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
kinetic separation to obtain rate constants for each reaction. This outcome is consistent with previous DFT calculations in related systems which show that the rate-limiting step for the cyclopropanation reactions of gold(I) benzylidenes is the dissociation of the cyclopropane from the metal.20 Kinetic data for the substitution reactions are presented in Table 2. The rate constants for the reactions of the adducts
Scheme 2. Gold(I) Benzylidene Precursor
produces a species with characteristic carbene reactivity. The gold(I) benzylidene was isolated in the ion trap and allowed to react with four olefins: cyclohexene, ethyl vinyl ether, 2,3dihydrofuran, and 1,1-dichloroethylene. It gave addition complexes with each of the olefins, except the most electrondeficient one, 1,1-dichloroethene. The bimolecular rate constants for the olefin reactions are presented in Table 1.
Table 2. Rate Constants for the Reaction of the Addition Complexes with Olefins (Scheme 3)a
1,1-dichloroethylene cyclohexene 2,3-dihydrofuran ethyl vinyl ether
efficiencyc
k b
NR 3.34 7.75 10.10
k
efficiencyb
cyclohexene 2,3-dihydrofuran ethyl vinyl ether
1.40 0.70 1.50
1.66 0.56 1.14
a Rate constants in units of 10−11 cm3 molecule−1 s−1 and efficiencies in units of %. bk/kcoll, where the collision rate is calculated by the method of Su and Bowers.18
Table 1. Rate Constants for the Reaction of the Ph3PAu+ Benzylidene Complex with Olefinsa olefin
olefin
NRb 39 62 76
with the olefins are 30−100-fold lower than those that were observed in the reactions of the gold(I) benzylidene. Analyzing the rate data is more complicated in the substitution reactions because features that make the olefin more nucleophilic will also enhance the binding of the cyclopropane to the gold and attenuate the impact of the substituent. In order to achieve a greater understanding of the electronic effects, we used parasubstituted styrenes to establish linear free energy relationships with the kinetics of the olefin reactions. Styrene, 4fluorostyrene, 4-chlorostyrene, 4-methylstyrene, and 4-methoxystyrene were used in the study. The kinetics are presented in Figure 1. As with the other olefins, the styrenes give addition
Rate constants in units of 10−10 cm3 molecule−1 s−1 and efficiencies in units of %. bReaction too slow to characterize:
