Kinetics and Mechanism of Allene Racemization ... - ACS Publications

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Kinetics and Mechanism of Allene Racemization Catalyzed by a Gold N‑Heterocyclic Carbene Complex Hao Li, Robert J. Harris, Kohki Nakafuku, and Ross A. Widenhoefer* Department of Chemistry, French Family Science Center, Duke University, Durham, North Carolina 27708, United States S Supporting Information *

ABSTRACT: The kinetics of the racemization of 1,3-disubstituted allenes catalyzed by (IPr)AuOTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidine) has been investigated. The rate of gold-catalyzed allene racemization obeyed the following second-order rate law: rate = krac[allene][Au]. An analysis of the rate of the gold-catalyzed racemization of 1-aryl-1,2-butadienes as a function of allene electron donor ability established moderate depletion of electron density on the C1 allenyl carbon atom in the rate-limiting transition state for racemization. Analysis of the temperature dependence of the rate of racemization of 1-(p-tolyl)-1,2-butadiene established the activation parameters ΔH⧧ = 8.3 ± 1 kcal mol−1 and ΔS⧧ = −28 ± 4 eu. These observations were in accord with a mechanism for allene racemization involving turnover-limiting, intermolecular allene exchange followed by rapid allene stereomutation.

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INTRODUCTION The proliferation of gold(I)-catalyzed methods for the functionalization of C−C multiple bonds represents one of the most significant developments in homogeneous catalysis over the past decade.1 In addition to alkynes, alkenes, and allylic alcohols, allenes have proven to be particularly active and versatile substrates for gold-catalyzed functionalization.2 Goldcatalyzed transformations of allenes include cycloaddition with alkenes and dienes3 and hydrofunctionalization with carbon and heteroatom nucleophiles.4 A distinguishing feature of allenes is the potential for axial chirality, and as such, there has been considerable interest in the gold-catalyzed stereospecific functionalization of enantiomerically enriched chiral allenes.5 Although high levels of chirality transfer have been realized, efficient chirality transfer is often compromised by concomitant gold-catalyzed allene racemization.6 Alternatively, we7 and others8 have exploited gold(I)-catalyzed allene racemization to realize the stereoconvergent enantioselective hydrofunctionalization of chiral racemic allenes and propargylic acetates. Despite the relevance of allene racemization to the gold(I)catalyzed functionalization of allenes, our understanding of gold(I)-catalyzed allene racemization has been largely restricted to the information gleaned from computational analyses.9 Toward an experimentally grounded understanding of the coordination chemistry of gold allene complexes, we have investigated the dynamic behavior of well-defined cationic gold π-allene complexes.10 In particular, variable-temperature NMR analysis of the gold(I) π-4,5-nonadiene complex {(P)Au[η2-nPr(H)CCC(H)n-Pr]}+SbF6− (P = P(t-Bu)2o-biphenyl) and related compounds established facile π-face exchange (ΔG⧧298 < 10 kcal mol−1) through chiral η1-allene intermediates (Scheme 1). In comparison, allene stereomutation was not © XXXX American Chemical Society

detected prior to the onset of intermolecular allene exchange with an energy barrier of ΔG⧧298 ≥ 17.4 kcal mol−1. Scheme 1

We subsequently investigated the kinetics of the racemization of axially chiral 1-aryl-1,2-butadienes catalyzed by cationic gold phosphine complexes.11 These studies supported a mechanism involving rapid and reversible intermolecular allene exchange from cationic gold π-allene complexes (I) followed by turnoverlimiting formation of an achiral η1-allylic cation intermediate (II) with energy barriers of ΔG⧧298 = 17−20 kcal mol−1 (Scheme 2). The significant transfer of electron density from Received: April 16, 2016

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DOI: 10.1021/acs.organomet.6b00307 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2

Scheme 3

the allene ligand to the gold phosphine moiety in the transition state for racemization (TS1) was established through Hammett analysis of gold-catalyzed allene racemization as a function of allene and phosphine electron-donor ability. Rapid intermolecular allene exchange relative to racemization was independently confirmed through spin saturation transfer analysis of gold π-allene complexes in the presence of free allene. Recently, Ballesteros and co-workers have directly observed and spectroscopically characterized gold η1-allylic cations generated via ring-opening cycloaddition of 6-aryl-6-(arylethynyl)bicyclo[3.1.0]hex-2-enes.12 N-heterocyclic carbenes (NHCs) such as 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidine (IPr) are common supporting ligands for gold catalysis, including gold-catalyzed allene hydrofunctionalization.13 Owing to the relevance of gold NHC complexes in allene functionalization, we sought to gain information regarding the mechanism and energetics of allene racemization catalyzed by gold IPr complexes. To this end, we have investigated the kinetics of the racemization of 1,3disubstituted allenes catalyzed by cationic gold IPr complexes. Herein we report the results of these investigations along with additional experiments involving allene racemization catalyzed by gold phosphine complexes that reveal some notable differences in the behavior of (IPr)Au+ and (P)Au+ allene complexes. These differences provide new insights into the subtle differences in σ-donor/π-acceptor behavior of phosphines and NHC ligands bound to a cationic gold(I) fragment.

Figure 1. Pseudo-first-order plots for the racemization of (S)-1 (100 mM, 97% ee) catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf (2.5 (×), 5.0 (△), 7.5 (□), and 10 mM (○)) in toluene at 25 °C.

Table 1. Pseudo-First-Order Rate Constants for the Racemization of Axially Chiral Allenes Catalyzed by a 1:1 Mixture of (L)AuCl (L = IPr, P) and AgOTf in Toluene entry

allene

L

[cat.] (mM)

temp (°C)

1 2 3 4 5b 6c 7 8 9 10 11 12 13 14 15 16

(S)-1 (S)-1 (S)-1 (S)-1 (S)-1 (S)-1 (R)-3a (R)-3b (R)-3c (R)-3d (R)-3a (R)-3a (R)-3a (S)-1 (S)-1 (S)-1

IPr IPr IPr IPr IPr IPr IPr IPr IPr IPr IPr IPr IPr P1 P1 P1

2.5 5.0 7.5 10.0 10.0 10.0 0.20 0.20 0.20 0.20 0.20 0.20 0.10 10.0 9.6 10.2

25 25 25 25 25 25 25 25 25 25 9 40 51 25 35 45

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RESULTS Allene Racemization Catalyzed by (IPr)AuOTf. When a toluene solution of (S)-2,3-pentadienyl benzoate ((S)-1, 97% ee) (100 mM) and a catalytic amount of (IPr)AuOTf (5.0 mM; generated in situ from (IPr)AuCl and AgOTf14) was stirred at 25 °C and monitored periodically by HPLC equipped with a chiral stationary phase, (S)-1 underwent complete racemization (≤2% ee) within 2 h with concomitant formation of a small amount (∼5%) of penta-1,3-dien-2-yl benzoate (2) via a formal [3,3]-sigmatropic rearrangement (Scheme 3) previously documented by Gagosz.15 A plot of ln [ee((S)-1)] versus time was linear to >3 half-lives with a pseudo-first-order rate constant of kobs = (5.52 ± 0.14) × 10−4 s−1 (Figure 1; Table 1, entry 2), which established the first-order dependence of the rate of racemization on [(S)-1]. Continued stirring at 25 °C for 24 h led to complete consumption of (S)-1 to form 2 in 88% yield as a 1.2:1 mixture of isomers. To determine the dependence of the rate of allene racemization on catalyst concentration, pseudo-first-order rate

104kobs (s−1)a 3.96 5.52 10.6 14.9 13.7 2.46 5.17 16.7 0.580 0.323 3.28 15.9 11.6 0.543 1.49 5.01

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.13 0.14 0.3 0.7 0.4 0.05 0.05 0.4 0.009 0.005 0.06 0.3 0.3 0.006 0.06 0.19

a

Error limits refer to the standard deviation of the corresponding linear regression plot. bAgOTf/(IPr)AuCl = 2. cAgOTf/(IPr)AuCl = 0.5.

constants for the racemization of (S)-1 catalyzed by (IPr)AuOTf were determined as a function of catalyst concentration from 2.5 to 10.0 mM (Figure 1; Table 1, entries 1−4). A plot of kobs versus catalyst concentration was linear (Figure 2), which established the first-order dependence of the rate on catalyst concentration and overall the following second-order rate law: rate = krac[(S)-1][Au], where krac = 0.146 ± 0.013 s−1 M−1 (ΔG⧧298 = 18.6 ± 0.1 kcal mol−1). In comparison, the secondB


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Figure 3. Plot of log krac versus the Hammett σ+ parameter for the racemization of (R)-3a−d (20 mM) catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf (0.20 mM) in toluene at 25 °C (ρ+ = −1.56 ± 0.02).

Figure 2. Plot of kobs versus catalyst concentration for the racemization of (S)-1 (100 mM, 97% ee) catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf in toluene at 25 °C.

order rate constant for the conversion of (S)-1 to 2 was k2 = (2.57 ± 0.07) × 10−3 M−1 s−1 (ΔG⧧298 = 20.97 ± 0.01 kcal mol−1; see the Supporting Information). The rate of gold(I)catalyzed racemization of (S)-1 was independent of [AgOTf], provided that sufficient silver triflate was present to fully convert (IPr)AuCl to (IPr)AuOTf (Table 1, entries 5 and 6),14 and treatment of (S)-1 (100 mM) with AgOTf (10 mM) in the absence of (IPr)AuCl led to no detectable racemization after 2 h at 25 °C. Interestingly, employment of 0.5 equiv of AgOTf relative to (IPr)AuCl led to reduction in the rate of racemization which far exceeded that predicted by the decreased concentration of (IPr)AuOTf, presumably due to the formation of the catalytically inactive chloride-bridged gold species {[(IPr)Au]2(μ-Cl)}+OTf−.16−19 To evaluate the dependence of the rate of allene racemization catalyzed by (IPr)AuOTf on the electron donor ability of the allene, we determined pseudo-first-order rate constants for the racemization of the 1-aryl-1,2-butadienes (R)(4-X-C6H4)C(H)CC(H)Me (X = H (3a), Me (3b), CF3 (3c), NO2 (3d); 20 mM) catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf (0.20 mM) at 25 °C (eq 1; Table 1,

Figure 4. Eyring plot of the second-order rate constants (krac) for the racemization of (R)-3b (20 mM, 99% ee) catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf (0.20 mM) in toluene over the temperature range 9−51 °C.

of 1-aryl-1,2-butadienes catalyzed by gold phosphine complexes,11 kinetic data are absent regarding the racemization of chiral, 1,3-dialkyl allenes that would allow direct comparisons to the energetics of the racemization of (S)-1 catalyzed by (IPr)AuOTf. To this end, a toluene solution of (S)-1 (100 mM) and a catalytic amount of (P)AuOTf (10.0 mM generated in situ from a 1:1 mixture of (P)AuCl and AgOTf20) was stirred at 25 °C and monitored periodically by HPLC. A plot of ln [ee((S)-1)] versus time was linear to >3 half-lives with a pseudo-first-order rate constant of kobs = (5.43 ± 0.06) × 10−5 s−1, which corresponds to a second-order rate constant of krac = kobs/[Au] = 5.43 × 10−3 M−1 s−1 (Table 1, entry 14). Pseudofirst-order rate constants for the racemization of (S)-1 catalyzed by (P)AuCl/AgOTf were likewise determined at 35 and 45 °C (Table 1, entries 15 and 16). An Eyring plot of the corresponding second-order rate constants provided the activation parameters: ΔH⧧ = 20 ± 1 kcal mol−1 and ΔS⧧ = −2 ± 2 eu (Figure 5), which are fully consistent with our previously proposed mechanism for allene racemization catalyzed by gold phosphine complexes involving turnoverlimiting unimolecular stereomutation of a π-allene complex to a η1-allylic cation.11

entries 7−10). The corresponding second-order rate constants krac (krac = kobs/[Au]) increased 50-fold as the electron donor ability of the allene aryl group increased from X = NO2 (3d) to X = Me (3b). A plot of log krac versus the Hammett σ+ parameter provided an excellent fit with a reaction constant of ρ+ = −1.56 ± 0.02 (Figure 3), which established the depletion of electron density at the C1 allenyl carbon atom in the transition state for racemization. To determine the activation parameters for allene racemization catalyzed by (IPr)AuOTf, pseudo-first-order rate constants for the racemization of (R)-3a catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf in toluene were determined as a function of temperature from 9 to 51 °C (Table 1, entries 7 and 11−13). An Eyring plot of the corresponding second-order rate constants krac provided the activation parameters ΔH⧧ = 8.4 ± 1.3 kcal mol−1 and ΔS⧧ = −28 ± 4 eu (Figure 4). Allene Racemization Catalyzed by (P)AuOTf. Although we have previously investigated the kinetics of the racemization

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DISCUSSION Comparison of Allene Racemization Catalyzed by (IPr)AuOTf and (P)AuOTf. Three points regarding the C


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Organometallics Scheme 4

Figure 5. Eyring plot of the second-order rate constants (krac) for the racemization of (S)-1 (100 mM, 97% ee) catalyzed by a 1:1 mixture of (P)AuCl and AgOTf (∼10 mM) in toluene over the temperature range 25−45 °C.

cationic gold complex (IPr)Au(η2-H2CCMe2) (ΔH⧧ = 8 ± 1 kcal mol−1 and ΔS⧧ = −27 ± 4 eu).21 Similarly, the reaction constant determined for the racemization of 1-aryl-1,2butadienes catalyzed by (IPr)AuOTf (ρ+ = −1.56) is similar to that determined for the equilibrium displacement of trifluoromethanesulfonate from (P)AuOTf with 1-aryl-1,2butadienes (ρ+ = −1.4).11 It should be noted, however, that the former value presumably reflects the depletion of electron density from the attacking allene in transition state for allene exchange (TS2) relative to free allene, whereas the latter value reflects the depletion of electron density of the allene ligand in the ground-state, two-coordinate gold π-allene complex I relative to free allene. Furthermore, our analyses of the intermolecular π-ligand exchange of (P)Au+ π complexes established the presence of competing ligand-dependent and ligand-independent pathways,10 the latter of which may involve transient coordination of the proximal o-biphenyl aryl group that likewise maintains weak coordination to gold in the solid state.10,22,23 However, the observed negative reaction constant for the racemization of 1-aryl-1,2-butadiene catalyzed by (IPr)AuOTf requires that allene exchange occur through a ligand-dependent, associative pathway.24 Our kinetic data for the racemization of 1,3-disubstituted allenes catalyzed by gold phosphine and NHC complexes highlight some notable differences regarding the behavior of (IPr)Au(π-allene)+ and (P)Au(π-allene)+ complexes. For example, the energy barriers for associative allene exchange for (IPr)Au(π-allene)+ complexes must be significantly larger than for (P)Au(π-allene)+ complexes, which presumably reflects the greater steric shielding of the gold center by the aryl isopropyl groups of the IPr ligand relative to the P ligand.25 We have previously documented the strong dependence of the rate of intermolecular π-ligand exchange of two-coordinate gold π complexes on the steric bulk of the supporting ligand, in particular the low energy barriers for π-ligand exchange observed for sterically unencumbered Ph3PAu π-complexes (ΔG⧧ ≤ 10 kcal mol−1).26 As noted above, allene racemization catalyzed by (IPr)AuOTf is faster than is racemization catalyzed by (P)AuOTf. Furthermore, because ligand exchange is turnover-limiting in the former cases, the observed energy barriers for racemization represent the upper limit of the energy barriers for allene stereomutation, which suggests that the energy barriers for unimolecular stereomutation of (IPr)Au(π-allene)+ complexes are much lower than for (P)Au(π-allene)+ complexes. In light of the ligand effects identified in our investigation of allene racemization catalyzed by gold phosphine complexes,11 this is

comparative kinetic data for the racemization of axially chiral, 1,3-disubstituted allenes catalyzed by (IPr)AuOTf and (P)AuOTf are noteworthy. First, the activation parameters determined for the racemization of (R)-3a catalyzed by (IPr)AuOTf (ΔH⧧ = 8.4 ± 1.3 kcal mol−1; ΔS⧧ = −28 ± 4 eu) differ significantly from those determined for racemization of (S)-1 catalyzed by (P)AuOTf (ΔH⧧ = 20 ± 1 kcal mol−1; ΔS⧧ = −2 ± 2 eu). Second, the racemization of 1-aryl-1,2butadienes catalyzed by (IPr)AuOTf is much less sensitive to the electron donor ability of the 2-aryl group (ρ+ = −1.56 ± 0.02) than is racemization catalyzed by (P)AuOTf (ρ+ = −2.8 ± 0.4). Third, (IPr)AuOTf is a more reactive catalyst for allene racemization than is (P)AuOTf, especially for less electron-rich allenes. For example, racemization of 1-(4-nitrophenyl)-1,2butadiene (3d) catalyzed by (IPr)AuOTf at 25 °C is ∼190 times faster than is racemization of 3d catalyzed by (P)AuOTf,11 while racemization of (S)-1 catalyzed by (IPr)AuOTf is ∼28 times faster (ΔΔG⧧ = 1.9 kcal mol−1) than is racemization of (S)-1 catalyzed by (P)AuOTf. Mechanism of Allene Racemization Catalyzed by (IPr)AuOTf. Our observations regarding the racemization of 1,3-disubstituted allenes catalyzed by (IPr)AuOTf, in particular the modest reaction constant for the racemization of 1-aryl-1,2butadienes (ρ+ = −1.56 ± 0.02) and modest enthalpy of activation (ΔH⧧ = 8.3 ± 1 kcal mol−1) and large negative entropy of activation (ΔS⧧ = −28 ± 4 eu) for the racemization of (R)-3b, argue against a mechanism for racemization involving turnover-limiting, unimolecular stereomutation of a gold π-allene complex via an η1-allylic cation intermediate, as was invoked for the racemization of 1-aryl-1,2-butadienes catalyzed by cationic gold phosphine complexes (Scheme 2).11 Rather, these data are consistent with a mechanism for allene racemization involving turnover-limiting, associative allene exchange of π-allene complexes I via the cationic threecoordinate bis(allene) intermediate (S,R)-III followed by rapid allene stereomutation, presumably via an η1-allylic cation intermediate II (Scheme 4). Superimposed on the productive pathway for racemization are degenerate, nonproductive pathways involving intermolecular exchange of coordinated (S)-allene with free (S)-allene and exchange of coordinated (R)-allene with free (R)-allene. Supporting the proposed mechanism, the activation parameters for the racemization of (R)-3b catalyzed by (IPr)AuOTf are not significantly different from those determined for the intermolecular exchange of bound and free isobutylene with the D


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respect to alkene binding.21,22 Most interesting is their computational analysis of cationic metal carbonyl complexes,32 which indicated that, in contrast to nickel tricarbonyl complexes of the form (L)Ni(CO)3 that serve as the basis for the Tolman electronic parameter,34 the C−O stretching frequency of (L)Au(CO)+ complexes is affected only by Au→L π backdonation owing to inefficient transfer of the σ-donor electron density from L to the C−O π* orbital. The calculated C−O stretching frequencies for complexes (P)Au(CO)+ and (IPr)Au(CO)+ are similar, implying comparable Au→L π backdonation for both ligands. However, the calculated total L→Au electron donation is significantly greater for P (0.40 e) than for IPr (0.27 e) owing to greater L→Au σ-donation in the case of P.32 On the basis of these results, it appears plausible that, for the stereomutation of (L)Au(π-allene)+ complexes, electron depletion from the allene ligand in the ground state π-allene complex increases with the increasing Au→L π back-donation to the supporting ligand, which is greater for P than for IPr. Conversely, electron depletion from the allene fragment in the transition state for allene stereomutation (TS1) increases with the decreasing net L→Au electron donation (L→Au σ donation + Au→L π back-donation) from the supporting ligand, which is greater for P than for IPr. Therefore, despite the greater electrophilicity of the (P)Au+ fragment relative to (IPr)Au+ toward alkenes, the latter more effectively accepts electron density from the allene fragment in the transition state for stereomutation owing to lower net L→Au electron donation, thereby decreasing the energy of the transition state for allene stereomutation. As a final comment, it was noted in the Introduction that in our investigation of the fluxional behavior of gold π-allene complexes we were able only to assign a lower limit for the energy barrier for allene stereomutation of ΔG⧧ ≥ 17.4 kcal mol−1 owing to the onset of intermolecular allene exchange.10 The experimentally determined energy barrier for the racemization of (S)-1 catalyzed by (P)AuOTf (ΔG⧧ = 20.5 kcal mol−1) confirms our previous analysis and provides the final value required for the complete energetic mapping of the dynamic behavior of 1,3-dialkylallenes bound to the 12-electron (P)Au+ fragment, albeit with slightly different 1,3-dialkylallenes.

most surprising conclusion drawn from these kinetic investigations. In particular, we previously showed that the rate of racemization of 1-(4-biphenyl)-1,2-butadiene catalyzed by gold (triaryl)phosphine complexes of the form [(4-C6H4X)3P]AuOTf increased significantly with the decreasing electron donor ability of the phosphine ligand (ρ = 2.4),11 consistent with the transfer of electron density from the allene ligand to the (PR3)Au fragment in the transition state for allene stereomutation (TS1). The observations outlined in the preceding paragraph therefore imply that IPr is less electron donating than is P, which appears to contradict both the general perception of IPr as a strong donor ligand to gold13 and our experimental evaluation of the electrophilicity of the 12-electron fragments (P)Au+ and (IPr)Au+ with respect to alkenes.21,22 Specifically, Hammett analyses of the equilibrium binding affinities of parasubstituted vinyl arenes to the 12-electron cationic gold fragments (P)Au and (IPr)Au relative to 3,5-bis(trifluoromethyl)benzonitrile served as a measure of d→π* back-bonding to the alkene ligand and, hence, the electrophilicity of the respective (L)Au+ fragments. Significantly, the reaction constant for vinyl arene complexation to (P)Au+ (ρ = −3.4) was significantly larger than was that for (IPr)Au+ (ρ = −2.4),21,22 which established the greater depletion of electron density from the vinyl arene upon complexation to (P)Au+ in comparison to that to (IPr)Au+ and the greater electrophilicity of the (P)Au+ fragment relative to the (IPr)Au+ fragment with respect to alkene binding. However, it should be noted that both of these values are significantly more negative than are the reaction constants determined for the binding of vinyl arenes to cationic Ag(I) (ρ = −0.77), Pt(II) (ρ = −1.32), and Pd(II) complexes (ρ = −1.44).27 The most obvious rationale to this apparent contradiction is that the relative electrophilicities of the (P)Au+ and (IPr)Au+ fragments do not accurately reflect their ability to accept electron density from the allene ligand in the transition state for allene stereomutation (TS1). Whereas the π-allene ligand binds to gold through a combination of allene→gold σ-donation and gold→allene π-back bonding according to the Dewar−Chatt− Duncanson model,28,29 there is a node on the C2 carbon atom in the HOMO of the η1-allylic cation π system and the η1-allylic cation should therefore behave as pure σ-donor ligand. Therefore, Au→C π* back-bonding in the transition state for allene stereomutation will be diminished relative to the ground state with a concomitant increase in C→Au σ donation. Because the σ-bonding framework of two-coordinate gold complexes involves a three-center, four-electron L−Au−C σ hyperbond through which both ligands compete for σ donation into a partially occupied sd(σ) gold(I) orbital,25 greater L→Au σ donation from P relative to IPr would therefore destabilize the transition state for allene stereomutation (TS1) for (P)Au(π-allene)+ relative to (IPr)Au(π-allene)+. Recent computational work by Belpassi and co-workers directed toward disentangling the σ-donor and π-acceptor behavior of supporting ligands in gold π complexes supports this hypothesis.30−33 First, their computational analysis of (NHC)AuL′ complexes indicates that the extent of Au→NHC π back-donation is dependent on the nature of L′.30 In the case of (L)Au(π-ethylene)+ complexes, Au→π* back-bonding to the ethylene ligand is greater for IPr than for PCy3, implying diminished Au→L back-donation for IPr relative to PCy3,31 which is consistent with our experimental observations regarding the electrophilicity of (L)Au+ gold fragments with

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CONCLUSIONS

We have studied the kinetics of the racemization of axially chiral 1,3-disubstituted allenes catalyzed by (IPr)AuOTf, and we have compared these kinetic data to relevant kinetic data for the racemization of 1,3-disubstituted allenes catalyzed by gold phosphine complexes such as (P)AuOTf. These results support a mechanism for allene racemization catalyzed by (IPr)AuOTf involving turnover-limiting intermolecular π-allene exchange coupled with rapid allene stereomutation, presumably via η1allyic cation intermediates. Although the (P)Au+ fragment possesses greater electrophilicity than does the (IPr)Au+ fragment with respect to alkene binding, allene racemization catalyzed by (IPr)AuOTf was faster than was allene racemization catalyzed by (P)AuOTf. This observation implies that the (IPr)Au+ fragment is better able to accept electron density from the allene ligand in the transition state for allene stereomutation than is the (P)Au+ fragment, which was attributed to the greater σ-donor character of P relative to IPr. E


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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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Kinetic Analysis of Racemization. A solution of (IPr)AuOTf (0.010 mmol) in toluene was prepared by stirring a mixture of (IPr)AuCl (6.2 mg, 1.0 × 10−2 mmol) and AgOTf (2.6 mg, 1.0 × 10−2 mmol) in toluene (1.0 mL) for 5 min at 25 °C.14 To this was added a solution of (S)-1 (37.6 mg, 0.20 mmol) in toluene (1.0 mL) (total volume 2.0 mL, [(S)-1] = 100 mM, [(IPr)AuOTf] = 5.0 mM), and the resulting solution was stirred in a thermostated oil bath at 25 ± 0.5 °C. To monitor reaction progress, aliquots were removed periodically via syringe and analyzed by HPLC equipped with a chiral stationary phase (tr (major) = 21.0 min, tr (minor) = 18.3 min; hexanes:isopropyl alcohol 99.8:0.2 @ 0.5 mL/min). The pseudo-first-order rate constant for racemization (kobs = (5.52 ± 0.14) × 10−4 s−1) was determined from a plot of ln [ee((S)-1))] versus time (Figure 1; Table 1, entry 1).35,36 Pseudo-first-order rate constants for the racemization of (S)-1 catalyzed by a 1:1 mixture of (IPr)AuCl and AgOTf were determined as a function of [catalyst] and [AgOTf] (Table 1, Figure 1, and Figure S1 in the Supporting Information) employing analogous procedures. The second-order rate constant (krac = 0.146 ± 0.013 M−1 s−1) for the racemization of (S)-1 catalyzed by (IPr)AuOTf was determined from a plot of kobs versus [catalyst] (Figure 2). Error limits for kinetic data refer to the standard deviation of the corresponding linear regression plot. Additional experimental details and plots for the kinetic analysis of the racemization of 1-aryl-1,2-butadienes catalyzed by (IPr)AuOTf, the racemization of (S)-1 catalyzed by (P)AuOTf and for the conversion of (S)-1 to 2 are included in the Supporting Information.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00307. General methods, experimental details regarding the synthesis of enantiomerically enriched allenes and gold complexes, kinetic data, and in situ spectroscopic analysis of catalytic reactions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.A.W.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the NSF (CHE-1465209) for support of this research.

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REFERENCES

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Article

Organometallics

kobs[catalyst]) is equal to 2 times the rate constant for enantiomerization (krac = 2kenant).36 (36) (a) Wolf, C. Dynamic Stereochemistry of Chiral Compounds: Principles and Applications; Royal Society of Chemistry: Cambridge, U.K., 2008. (b) Cope, A. C.; Banholzer, K.; Keller, H.; Pawson, B. A.; Whang, J. J.; Winkler, H. J. S. J. Am. Chem. Soc. 1965, 87, 3644.

constant for displacement of (IPr)AuCl from the 12-electron fragment (IPr)Au+ in the chloride-bridged dimer {[(IPr)Au]2(μ-Cl)}+OTf− by allene17 or to dimerization18 and/or precipitation of {[(IPr)Au]2(μCl)}+OTf− in the nonpolar reaction medium. (20) Preisenberger, M.; Schier, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans. 1999, 1645. (21) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009, 131, 6350. (22) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. Chem. Commun. 2009, 6451. (23) (a) Herrero-Gomez, E.; Nieto-Oberhuber, C.; Lopez, S.; BenetBuchholz, J.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5455. (b) Pérez-Galán, P.; Delpont, N.; Herrero-Gomez, E.; Maseras, F.; Echavarren, A. M. Chem. - Eur. J. 2010, 16, 5324. (c) Partyka, D. V.; Robilotto, T. J.; Zeller, M.; Hunter, A. D.; Gray, T. G. Organometallics 2008, 27, 28. (d) Partyka, D. V.; Updegraff, J. B.; Zeller, M.; Hunter, A. D.; Gray, T. G. Organometallics 2009, 28, 1666. (e) Xu, F.-B.; Li, Q.S.; Wu, L.-Z.; Leng, X.-B.; Li, Z.-C.; Zeng, X.-S.; Chow, Y. L.; Zhang, Z.-Z. Organometallics 2003, 22, 633. (f) Li, Q.- S.; Wan, C.-Q.; Zou, R.Y.; Xu, F.-B.; Song, H.-B.; Wan, X.-J.; Zhang, Z.-Z. Inorg. Chem. 2006, 45, 1888. (g) Zhu, Y.; Day, C. S.; Jones, A. C. Organometallics 2012, 31, 7332. (24) Alternatively, we cannot rule out a mechanism for allene racemization catalyzed by (IPr)AuOTf involving rate-limiting, associative displacement of OTf− from (IPr)AuOTf with allene, if (IPr)AuOTf were the predominant species present under catalytic conditions. However, this latter condition was firmly discounted in the case of allene racemization catalyzed by cationic gold phosphine complexes,11 and the close similarity between the activation parameters for allene racemization catalyzed by (IPr)AuOTf and those determined for the associative exchange of isobutylene with (IPr)Au(η2-H2CCMe2)21 supports a mechanism for catalytic allene racemization involving turnover-limiting associative allene exchange from π-allene complex I. It is important to note that whether allene racemization catalyzed by (IPr)AuOTf occurs via turnover-limiting associative displacement of OTf− from (IPr)AuOTf or displacement of allene from I, allene stereomutation is fast relative to ligand exchange and the energy barriers determined for allene racemization catalyzed by (IPr)AuOTf represent the upper limit of the energy barrier for allene stereomutation. (25) (a) Díez-González, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874. (b) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem. 2005, 690, 5407. (c) Strassner, T. Top. Organomet. Chem. 2004, 13, 1. (26) Brooner, R. E. M.; Brown, T. J.; Widenhoefer, R. A. Chem. - Eur. J. 2013, 19, 8276. (27) (a) Fueno, T.; Okuyama, T.; Deguchi, T.; Furukawa, J. J. Am. Chem. Soc. 1965, 87, 170. (b) Kurosawa, H.; Asada, N. J. Organomet. Chem. 1981, 217, 259. (c) Kurosawa, H.; Majima, T.; Asada, N. J. Am. Chem. Soc. 1980, 102, 6996. (28) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79. (b) Chatt, J.; Duncanson, L. J. Chem. Soc. 1953, 2939. (29) (a) Landis, C. R.; Weinhold, F. J. Comput. Chem. 2007, 28, 198. (b) Benitez, D.; Shapiro, N. D.; Tkatchouk, E.; Wang, Y. M.; Goddard, W. A.; Toste, F. D. Nat. Chem. 2009, 1, 482. (c) Ibrahim, N.; Vilhelmsen, M. H.; Pernpointner, M.; Rominger, F.; Hashmi, A. S. K. Organometallics 2013, 32, 2576. (30) Marchione, D.; Belpassi, L.; Bistoni, G.; Macchioni, A.; Tarantelli, F.; Zuccaccia, D. Organometallics 2014, 33, 4200. (31) Ciancaleoni, G.; Biasiolo, L.; Bistoni, G.; Macchioni, A.; Tarantelli, F.; Zuccaccia, D.; Belpassi, L. Chem. - Eur. J. 2015, 21, 2467. (32) Ciancaleoni, G.; Scafuri, N.; Bistoni, G.; Macchioni, A.; Tarantelli, F.; Zuccaccia, D.; Belpassi, L. Inorg. Chem. 2014, 53, 9907. (33) Bistoni, G.; Belpassi, L.; Tarantelli, F. Angew. Chem., Int. Ed. 2013, 52, 11599. (34) Tolman, C. A. Chem. Rev. 1977, 77, 313. (35) The function ln [ee((S)-1)] is equivalent to ln {[(S)-1]t − [(S)1]∞}. The second-order rate constant for racemization (krac = G


Kinetics and Mechanism of Allene Racemization ... - ACS Publications

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