Structure and Dynamic Behavior of Phosphine Gold(I)-Coordinated

Article pubs.acs.org/Organometallics

Structure and Dynamic Behavior of Phosphine Gold(I)-Coordinated Enamines: Characterization of α‑Metalated Iminium Ions Madhavi Sriram,† Yuyang Zhu,† Andrew M. Camp, Cynthia S. Day, and Amanda C. Jones* Department of Chemistry, Wake Forest University, Salem Hall, Box 7486, Winston-Salem, North Carolina 27109, United States S Supporting Information *

ABSTRACT: Cationic gold(I) enamine complexes with the (t-Bu)2(o-biphenyl)phosphine ligand were isolated and characterized by NMR spectroscopy and X-ray crystallography. The complexes display highly distorted coordination modes that are consistent with characterization as α-metalated iminium ions. The barrier to rotation around the formal enamine C−C double bond has been measured in a geminally disubstituted enamine complex, and it is comparable to the barrier to C−C single-bond rotation in electronically restricted alkanes. With additional substitution on the enamine double bond, the complexes remain highly distorted, and the reaction of a mixture of E and Z enamines results in formation of a stereochemically pure gold complex. A survey of binding constants reveals enamines to be significantly stronger donors than any alkenes examined to date, and in the case of a geminally disubstituted enamine, the coordination is stronger even than that of triethylamine. The high stability drives the isomerization of an internal enamine complex generated from an intramolecular hydroamination reaction, to the exocyclic double-bond isomer.

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INTRODUCTION

A number of gold(I) catalyst systems have been developed for the direct hydroamination of alkynes.1,2 Notably, Bertrand2 and Stradiotto1a have independently identified active gold catalysts capable of mediating the addition of strongly metalcoordinating dialkylamines to form imines and enamines and Bertrand’s catalyst could also mediate the addition of ammonia.2d These represent great examples of the potential for using gold in atom-economical processes, but the methods still rely on harsh conditions. Typical alkyne hydroaminations (gold or other metal catalyzed) utilize electron-deficient amines (e.g., aromatic amines, carboxylic amides) for their reduced basicity, and gold-coordinated amines have been identified in the corresponding mechanistic studies.3 In contrast, the structure and coordination strength of enamines have received less attention. An atypical triphenylphosphine gold-bound diaminoalkene has been prepared,4 and two gold-coordinated enamines have been observed in situ from alkyne amination reactions.5a,1a During final preparations of this paper, Zhdanko and Maier characterized a gold enamine complex in the solid state during their mechanistic study of the intramolecular hydroamination reaction.6 We have synthesized a series of gold(I)-coordinated enamines (Figure 1) and elaborate on structural details identified therein, thus furthering our understanding of the gold π-complex structure.7 We have obtained extensive solid-state and solution data on these complexes and confirmed them all to display highly distorted or “slipped” coordination modes; in the cases of coordinated geminally disubstituted enamines (1−3), facile isomerization of the formal CC double bond is observed. © 2014 American Chemical Society

Figure 1. Enamine complexes synthesized from the corresponding enamine, gold phosphine chloride, and sodium salt (all contain SbF6− counterion). Legend: (a) synthesized by intramolecular hydroamination.

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RESULTS AND DISCUSSION Complex Syntheses. With the exception of 3, cationic complexes were prepared in good yields by treating the corresponding free enamine8,9 with (t-Bu)2(o-biphenyl)PAuCl

Received: June 24, 2014 Published: August 11, 2014 4157

dx.doi.org/10.1021/om500670z | Organometallics 2014, 33, 4157−4164

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and NaSbF6 in THF. Complex 3 was isolated under identical conditions from the substoichiometric cyclization of homopropargyl amine 3a, following isomerization of the initially formed internal enamine (3e → 3, Scheme 1). As noted by Scheme 1. Preparation of 3 by Intramolecular Hydroaminationa

a

Figure 2. 1H NMR spectra (a) of 2 (L = (t-Bu)2(o-biphenyl)P in CD2Cl2, 300 MHz) and (b) with 31P decoupling and (c) at −60 °C (500 MHz) and a comparison to select 1H and 13C shifts in 2 and 2a,b.13

L = (t-Bu)2(o-biphenyl)P.

the upfield shift is significantly greater than accounted for by magnetic anisotropy. This was determined by comparison to crude spectra of analogous (t-Bu)3P and Ph3P complexes, which show ca. 1 ppm upfield shifts for the terminal protons.12 In phosphine gold(I) enol ether complexes we found that upfield proton shifts were not a good indicator of the nature of coordination, since upfield shifts were not observed in structurally similar (t-Bu)3P complexes.7b In contrast, the upfield shifts in 1−3 appear to also be indicative of diminished double-bond character in the former enamine C−C double bond. These observations led us to the same conclusion as Zhdanko and Maier, that these complexes may be more appropriately considered α-aurated iminium ions.6 The 13C NMR shifts upon coordination to gold also support the iminium ion character in the complex. For example, the C2−N carbon in 2 appears at δ 187.9 ppm, which is identical with the shift of the tetrafluoroborate salt of the corresponding organic iminium ion.13 In such iminium ion structures, significant rehybridization at C1 is anticipated (generic atom numbering shown in Figure 2). A preliminary survey of a series of gold alkene π complexes suggested to us previously that along the continuum (see Table 1) from symmetric η2-alkene π complex (0% slipped) to η1-alkene complex (100% slipped, gold shifted to one end of the double bond) to sp3-hybridized alkyl gold complex (>100% slipped, gold shifted past the end of the double bond), there would be a steady increase in the twobond coupling constant between C1 and the ligand phosphorus (2JCP).7b In a moderately distorted isobutylene complex (4% slipped), 2JCP = 11.5 Hz.7c In enol ether gold complexes, where slippage approaches 100%, the values are in the range 2JCP = 22−29 Hz.7b In Fürstner’s diaminoalkene complex, 2JCP = 80.2 Hz; in 6 that value is 2JCP = 72.4 Hz.4 In complexes 1−5 the values are in the range 2JCP = 45−49 Hz. On the basis of coupling constants, distortion in 1−5 is attenuated in comparison to complex 6 but enhanced in comparison to hydrocarbon alkenes and enol ethers. The crystal data support this (further discussion below) and are fully consistent with the stronger donor ability of nitrogen. X-ray Crystal Structure of 1 and 3. Crystals of 1 and 3 were obtained by slow diffusion of diethyl ether or hexanes, respectively, into a CH2Cl2 solution at −20 °C, and the solidstate structures reveal additional details about their conformation, including confirming the diastereotopicity of the geminal “vinylic” hydrogens revealed at low temperature (Figures 3 and 4). At room temperature in solution this feature is not detectable, as the protons are identical on the NMR time scale.

Fürstner et al. in their synthesis of the Ph3P analogue of 6, we also found that use of AgSbF6 was detrimental in these reactions, yielding only black material.4 Although crude samples of Ph3P and t-Bu3P gold-bound enamines could be prepared, we had greater success purifying the corresponding (t-Bu)2(obiphenyl)P complexes. Complexes 1 and 2 were the most stable of the series; they could be stored in a −20 °C freezer for up to 5 months. In contrast, hydrolysis of complex 4 to ketone and gold amine was observed after only a few weeks of storage under identical conditions. The most challenging aspect of these studies was the instability of the enamines themselves, which could not be stored for longer than a few days before use; once coordinated to gold, they had a significantly extended shelf life in the solid form. However, hydrolysis was also often observed during NMR experiments, indicating lower stability in solution. Homopropargyl amine 3a was prepared for a preliminary examination of the cyclization kinetics in the presence of gold. When 3a was treated with NaSbF6/(t-Bu)2(o-biphenyl)PAuCl, it was transformed cleanly into enamine 3, without significant formation of the expected enamine 3e. Zhdanko and Maier have explored this alkynamine isomerization in more detail and confirmed the intermediacy of structures analogous to 3e.6 Related double-bond isomerizations have been implicated by product mixtures and H/D exchange in synthetic reactions,10 and the process was also observed in the stoichiometric reaction of 3-pentyn-1-ol.11 Solution-State Structures and NMR Spectroscopy. The most notable feature of the room-temperature 1H NMR spectra of terminal enamine complexes 1−3 is that the formerly distinct vinyl protons are a single peak (equivalent on the NMR time scale), a doublet from coupling to phosphorus (see Figure 2 for insets from the spectra of 2). This two-proton doublet (Figure 2a) simplifies to a singlet in the 31P-decoupled spectrum (Figure 2b). Variable-temperature NMR data on 1 and 2 indicate a dynamic process in solution; at temperatures below −30 °C the two-hydrogen doublet resolves to two doublets of doublets (2JHH‑gem = 4.1 Hz, 3JHP = 8.8 and 9.0 Hz, Figure 2c) indicative of a set of diastereotopic protons. Thus, bond rotation on the NMR time scale, around the former enamine double bond, causes the apparent equivalence at room temperature. The significant upfield shift (from δ 4.17, 4.01 in the free enamine to δ 1.91 in 2, Δav = −2.2 ppm) is in part due to the magnetic anisotropy of the π system of the proximal ligand ophenyl. For example, in the case of 1 (where Δav = −1.9 ppm), 4158


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Table 1. Select Structure Parameters and Generic Atom-Numbering System: Extent of Distortion or “Slippage”

Au−C1 (Å)

Au−C2 (Å)

C1−C2 (Å)

1 3 4 5 6

2.133(3) 2.126(4) 2.136(5) 2.151(4) 2.106(5)

2.769 2.817 2.789 2.656(4) 3.078

1.429(5) 1.410(5) 1.438(8) 1.424(6) 1.440(8)

6d4

2.087(3)

2.977

1.462(5)

C2−N (Å) 1.321(4) 1.305(4) 1.329(7) 1.322(6) 1.345(8) 1.328(8) 1.347(4) 1.352(4)

Figure 3. ORTEP drawing of 1. The counterion (SbF6−) is omitted for clarity. Ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): P1−Au1 = 2.2893(8), Au1−C21 = 2.133(3), C21−C22 = 1.429(5), N1−C22 = 1.321(4); C21−Au1− P1 = 175.15(10), C22−C21−Au1 = 100.2(2).

(N1) (N2) (N3) (N6)

Au−C1−C2 (deg) 100.2(2) 103.8(2) 100.8(3) 93.7(3) 119.3(4) 112.8(2)

Δ (Å) 0.047 0.034 0.056 0.086 0.015 0.007 0.032 0.024

(N1) (N2) (N6) (N3)

Figure 4. ORTEP drawing of one twin of 3. The counterion (SbF6−) is omitted for clarity. Ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): Au1−P1 = 2.2901(9), Au1−C21 = 2.126(4), C21−C22 = 1.410(5), N1−C22 = 1.305(4); C21−Au1−P1 = 178.16(10), C22−C21−Au1 = 103.8(2).

The main features to note are the long distance between the gold center and the C−N carbon (C22−Au1 = 2.769 Å in 1, 2.817 Å in 3), the wide Au−C1−C2 bond angle (C22−C21− Au = 100.2(2)° in 1, 103.8 (2)° in 3), and the nearly full planarity at the N center (Table 1).14,15 There is considerable variation in structural parameters for crystalline enamines; therefore, it is challenging to determine the perturbation induced specifically by the coordination of the gold cation.16 However, these data all support the iminium ion characterization. The dihedral angle between the C1−Au and enamine C2−N bond is 89.8(3)° in 1 and 85.1(4)° in 3, indicating a hyperconjugative interaction between the C−Au bond and the π system of the iminium ion. This presumably contributes to the favored conformation observed in the solid state (see the Newman projection in Figure 5). Dynamic Behavior of Complex 2. Due to the simplicity of its NMR spectra, complex 2 was chosen for a detailed variable-temperature dynamic NMR study. A series of 1H NMR spectra of a CD2Cl2 solution of 2 were collected within the range of −60 to +40 °C (Figure 5). At −60 °C, an ABX pattern is observed with 2J(HH)AB = 4.1 Hz, 3J(HP)AX = 8.8 Hz, and 3 J(HP)BX = 9.0 Hz. Coalescence occurs at around +8.6 °C, and

the subsequent A2X doublet is visible at ≥+34 °C, although the low boiling point of CD2Cl2 prohibits observation of a fully sharpened doublet. Simulation of the data17 provided rate constants, and an Eyring plot using kAB and T provided activation parameters for the process: ΔH⧧ = 13.1 kcal/mol and ΔS⧧ = −2.5 eu. This places the process in a energetic range comparable to the hindered C−C bond rotation of 2,2,3,3tetrabromobutane (ΔH⧧ = 13.5 kcal/mol and ΔS⧧ = −8.4 eu).18 It is important to note that the ability to observe this phenomenon by NMR spectroscopy depends on the presence of two degenerate, enantiomeric conformations and also that the barrier to rotation, while relatively facile, is restricted in comparison to freely rotating C−C single bonds. This restriction could be steric in nature; it could also be a result of the aforementioned C−Au hyperconjugation. Solution and X-ray Crystal Structure of 4, An Internal Enamine. As recently articulated by Zhdanko and Maier,6 the above dynamic feature is crucially important to mechanistic discussions of hydroamination, and enamine isomerization must be considered to play a role in defining the stereochemistry of products. To begin to determine the effect of 4159


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Crystals of complex 4 were obtained from CH2Cl2/Et2O at −20 °C, and the solid-state structure (Figure 6) reveals a level

Figure 6. ORTEP drawing of 4. The counterion (SbF6−) and ligand hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): Au1−P1 = 2.2878(14), Au1−C22 = 2.136(5), C22−C23 = 1.438(8), N1−C23 = 1.329(7), C23−C22−Au1 = 100.8(3), C22−Au1−P1 = 178.09(16).

of distortion comparable to that in 1 and 2, including a Au− C1−C2 angle (C23−C22−Au1 = 100.8(3)° in 4) that is nearly identical with that in 1 and a 13C NMR chemical shift of the C−N carbon that is nearly identical with that in 2 (δ 187.1 ppm in 4). These data are indicative of iminium ion character similar to that in 2. Notably, the torsion angle between the methyl and phenyl substituents (C21−C22−C23−C24 = 21.5(7)°) is intermediate between gauche (30°) and syn (0°); this indicates an approximate cis relationship that corresponds more closely to the E enamine. Facile bond rotation would be demonstrated either by showing that the minor Z isomer can be transformed to complex 4 or by showing that it can be regenerated by displacement from 4 (the latter requires thermodynamic control).20 When [(t-Bu)2(o-biphenyl)PAu(acetonitrile)]+SbF6− was treated with a slight excess of a 1/9 mixture of Z/E enamine 4a at −72 °C, complex 4 was generated cleanly (Scheme 2). The unreacted enamine consistently showed an enhancement of the Z/E ratio, indicating a slight preference for reaction with the E enamine. The difference was, however, small enough that it indicated that the Z enamine was also

Figure 5. Select solid-state data for 1 and variable-temperature NMR (1H, 500 MHz) spectra of 2 in CD2Cl2 and Newman projection depicting the bond rotation. Water appears at δ 1.63 ppm in the first spectrum and then disappears due to artificial referencing to midway between the two signals at the lowest temperature. Red lines are simulated data.

additional substitution on enamine complexation, the coordination of the diethylamine enamine of propiophenone (4a) was examined. The synthetic material routinely contained approximately 10% of the less stable Z isomer; however, only a single isomeric organometallic complex was observed upon reaction with gold. Because the E isomer already predominates after the enamine synthesis, it was not initially clear whether it preferentially coordinated to gold or whether the Z enamine coordinated and then isomerized. In solution, however, no evidence was found of a minor isomer complex. Although facile double-bond isomerization is certainly expected on the basis of the dynamic behavior in 1 and 2, the solution characterization of only one isomer is not sufficient to conclude identical behavior in 4. No dynamic behavior is detectable by NMR spectroscopy from the bond rotation process when only one isomer is observed, since there are no degenerate conformations or exchangeable signals (as there are in 1−3). This is confirmed by 1H NMR spectra in the temperature range from −40 to +60 °C in DCE-d4 that show minimal signal broadening.19

Scheme 2. Reaction of Both the E and Z Isomers to Form a Single Complex

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reacting. On treatment with 1.1 equiv of Me2S, the enamine was only partially and slowly displaced, and in doing so the E enamine was re-enriched, however, not at the full expense of the Z isomer, indicating its concurrent regeneration (i.e., displacement approaches the initial equilibrium ratio). The 1:9.5 ratio was not re-established even after warming to room temperature for 50 min. When DMAP was used to quantitatively and quickly displace the enamine, the Z/E ratio was enriched in the E isomer, suggesting there may have been some kinetic control to this process. These observations support the conclusion that Z to E isomerization is feasible through coordination of the minor enamine and fast rotation to the more stable complex; however, the E to Z isomerization is dependent on both the activation barrier to generate the less stable complex (which is likely quite high) and the rates of displacement from gold (which in some cases is quite slow, see below). Further examination is required to quantify the energetics of these processes. X-ray Crystal Structure of 5. To further probe variations in enamine structure, the commercially available pyrrolidine enamine of cyclohexanone was examined (Figure 7). The

Table 2. Equilibrium Constants for Enamine Displacement of Acetonitrile at −65 °C (0.02 M Gold) or Triethylamine at 25 °C (0.01 M Gold) from [(t-Bu)2(o-biphenyl)PAu]+ in CD2Cl2a

a

L = P(t-Bu)2(o-biphenyl).

strength of Krel = 102, as reported by Widenhoefer et al.21 We did not succeed in isolating a coordination complex of 7a. In order to measure the coordination strength of 1a, 2a, and 4a, analogous experiments were performed with triethylamine as the competitive binder. To our surprise, enamine 1a displaced triethylamine nearly quantitatively; only a negligible amount of coordinated triethylamine remained (Keq = 600). In contrast, equilibrium was attained very slowly in experiments with 4a (performed by adding triethylamine to solutions of 4) and the reaction ratio dropped gradually from K = 30 (within the first 10 min) to K = 3.2 at 30 min to K = 1.5 after 1.5 h. After 20 h the enamine complex had hydrolyzed to propiophenone and diethylamine; thus, we could not conclude when exactly equilibrium was fully established.22 In combination with Keq data obtained by Maier and coworkers, we can put these values on the known scale of binding affinities.23,6 When 4,6-lutidine was compared to triethylamine, Keq = 20 was found in the direction of lutidine. Chart 1 shows a Chart 1. Relative Equilibrium Binding Constants

Figure 7. ORTEP drawing of 5. The counterion (SbF6−) and ligand hydrogen atoms are omitted for clarity. Ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): Au1−P1 = 2.2850(10), Au1−C22 = 2.151(4), Au−C21 = 2.656(4), N1−C21 = 1.322(6), C21−C22 = 1.424(6); C21−C22−Au1 = 93.7(3), C22−Au1−P1 = 173.89(12).

iminium ion character appears to be slightly attenuated, as reflected by both a shorter Au−C2 distance (Au1−C21 = 2.656(4) Å vs 2.769 Å in 1) and narrower Au−C1−C2 angle (Au1−C22−C21 = 93.7(3)° vs 100.2(2)° in 1). Relative Binding Affinity of Enamines. Using 1H NMR spectroscopy, equilibrium constants were obtained for displacement of acetonitrile by enamines. Except for the tetrasubstituted butyrophenone enamine (7a), all precursor enamines examined exhibited significantly stronger coordination in comparison to acetonitrile (data shown for 4a and 7a; Table 2). In fact, meaningful values could not be obtained for 4a, 2a, and 1a, since the amount of intact acetonitrile complex was too small to measure. Lower limits were estimated, however, which reveal both the significant coordination strength of enamines and the weakening of that coordination when β-disubstitution is introduced (Krel ≥ 103 for 4a vs 7a). In contrast, isobutylene and 2,3-dimethyl-2-butene displayed a relative coordination

relative scale using the aforementioned literature data. The high sensitivity of enamines to hydrolysis prevents a more quantitative analysis at this time; however, these results suggest that, in addition to deactivation by the amine,3,6 enamine coordination itself may play a role in inhibiting the catalytic activity of gold in hydroamination reactions. Comparison to Fü rstner’s Enediamine. The higher stability of π complexes with strong donor ligands such as alkylphosphines and N-heterocyclic carbenes is well-known and has precluded a broad analysis of ligand effects on π-complex structure.7a For example, Widenhoefer recently showed that a series of triphenylphosphine gold(I) π complexes decomposed at temperatures above −20 °C to (Ph3P)2Au+.24 Thus, Fürstner’s diaminoalkene complex (see Figure 8, structure 6d) represents one of only two such complexes characterized in 4161


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in 6 likely result from the adventitious combination of the pendent biphenyl and the aromatic imidazolium ring, rather than an intrinsic ligand effect.

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CONCLUSION In summary, we have synthesized a series of cationic gold phosphine enamine complexes incorporating the (t-Bu)2(obiphenyl)P ligand. They are highly distorted and show features consistent with α-metalated iminium ions. Significant rehybridization at the gold-coordinated carbon is suggested by long distances from the C−N carbon to gold, large 13C−31P coupling constants, and wide Au−C−C angles approaching those of tetrahedral centers. Presumably due to the highly distorted coordination observed in terminal enamines, there is a much higher sensitivity to substitution that destabilizes the internal enamines, and this is reflected in the observed wider range of equilibrium binding constants in comparison to previously reported alkenes and enol ethers. The strength of coordination to gold is significantly greater than that of any alkene or enol ether examined to date, and this appears also to provide the driving force for isomerization of a more substituted enamine to a less substituted one. The high distortion leads to a diminishing of double-bond character in the enamine, and the barrier to rotation has been measured for a geminally disubstituted enamine. This feature suggests that enamine stereochemistry in gold-catalyzed hydroamination reactions is thermodynamically driven. However, the higher steric demands of more substituted enamines likely raises this barrier, and the energetics of the process were not determined, nor was it determined whether the weakly coordinating tetrasubstituted enamines can undergo isomerization.26 Finally, enamine coordination is competitive with amine coordination, suggesting that it too may play a role in deactivating gold catalysts and inhibiting hydroamination reactions.

Figure 8. Comparison to Fürstner’s enediamine complex.4

the solid state.25 We saw this as an opportunity to prepare the parallel (t-Bu)2(o-biphenyl)P complex and expand our data trove for analyzing the impact of the ligand on π-complex structure (and ultimately, reactivity). Complex 6 was prepared in good yield by the general procedure, and crystals were obtained by slow diffusion of diethyl ether into a CH2Cl2 solution at −20 °C. In the solid state, 6 shows similarities to 6d (as well as 1 and 3−5), including the elongated ene C1−C2 bond and wide Au− C1−C2 angle. In fact, the angle is even wider in 6, and in contrast to many π complexes with the (t-Bu)(o-biphenyl)P ligand, the biphenyl ring orients itself neither toward the terminal end of the enamine (as in 1) nor underneath the ene double bond (Figure 9). In 1 and 3−5 the plane of the biphenyl

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EXPERIMENTAL SECTION

General Methods. All enamines were prepared under an inert atmosphere using argon and were purified by vacuum distillation. Gold precursors were weighed in an oxygen- and moisture-free glovebox, and the complexes were prepared in a fume hood under argon. Chemical reagents used in the syntheses were purchased from various chemical suppliers and used as received without further purification. Solvents were either purchased anhydrous or purified by distillation or by use of a solvent purification system. Spectra were recorded with NMR spectrometers operating at either 300 or 500 MHz. NMR spectra using CD2Cl2 were referenced to the solvent signal at 5.32 ppm for 1H spectra and at 53.84 ppm for 13C spectra. All of the spectra were recorded at 25 °C unless otherwise specified. NMR data processing was carried out using MestreNova. Elemental analyses were performed by Complete Analysis Laboratories (Parsippany, NJ). XRD data were collected using the APEX CCD diffractometer system at Wake Forest University. Synthesis of Gold Complexes. [(t-Bu)2(o-biphenyl)PAu][N,Ndiethyl-4-methylpenta-1,3-dien-2-amine]+SbF6− (1). The synthesis of 1 is a representative procedure. In a 10 mL flame-dried round bottomed flask in a glovebox were placed (t-Bu2)(o-biphenyl)PAuCl (53.08 mg, 0.10 mmol), NaSbF6 (25.9 mg, 0.10 mmol), a stir bar, and anhydrous THF (2 mL). The reaction flask was capped with a septum and transferred to a fume hood. Enamine (0.055 mL, 0.3 mmol) was added, and the reaction mixture was stirred for 1 h in the absence of light under argon. THF was removed under vacuum. Anhydrous CH2Cl2 (2 mL) was added to the resulting slurry, and the solution was filtered through Celite (2×). The combined organic solvents were

Figure 9. ORTEP drawing of 6. The counterion (SbF6−) is omitted for clarity. Ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): for 6, Au1−P1 = 2.2972(13), Au1−C21 = 2.106(5), N1−C22 = 1.345(8), N2−C22 = 1.328(8), C21−C22 = 1.440(8); C22−C21−Au1 = 119.3(4), C21−Au1−P1 = 166.11(15).

ring is nearly perpendicular (angles 81−89°) to the plane of atoms in the formal π system (plane defined by C1−C2−N) and oriented toward the least hindered side. In 6, the biphenyl is oriented toward the imidazolium ring instead. This also appears to have the effect of widening the Au−C1−C2 angle even further (from ∼113° in 6d to ∼119° in 6) and bending the P−Au−C1 angle (from nearly linear, ∼176°, in 6d, to slightly bent, ∼166°, in 6). There is a close contact between the para H of the biphenyl ring and the backside double bond of the imidazolium; we propose this observed conformation to be a result of electrostatic attraction between the phosphine ligand and the aromatic cation.12 In fact, the unique structural features 4162


Organometallics

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(d, J = 6.1 Hz), 135.0 (d, J = 1 Hz), 133.8 (d, J = 7.3 Hz), 133.7, 131.7 (d, J = 1.9 Hz), 130.33, 130.3, 130.2, 129.7, 129.4, 129.0, 128.6, 128.1, 127.9 (d, J = 6.5 Hz), 127.6, 126.5 (d, J = 41.6 Hz), 126.1, 55.9 (d, J = 46.2 Hz), 48.9, 44.8, 38.33 (d, J = 22.2 Hz), 38.2 (d, J = 21.7 Hz), 31.3 (d, J = 6.6 Hz), 31.0 (d, J = 6.6 Hz), 16.0 (d, J = 4.3 Hz), 13.9, 12.7. 31 P NMR (120 MHz, CD2Cl2): δ 64.6. Anal. Calcd (found) for C33H46AuF6NPSb: C, 43.06 (43.20); H, 5.04 (5.08); N, 1.52 (1.50). [(t-Bu)2(o-biphenyl)PAu][1-(cyclohex-1-en-1-yl)pyrrolidine]+SbF6− (5). Complex 5 was prepared according to the procedure to prepare 1. The product was obtained as an off-white solid (70% yield). The product was recrystallized with CH2Cl2/Et2O at −20 °C. 1 H NMR (500 MHz, CD2Cl2): δ 7.89−7.86 (m, 1H), 7.61−7.48 (m, 5H), 7.29−7.14 (m, 3H), 3.37−3.70 (m, 1H), 3.44−3.36 (m, 2H), 3.17−3.09 (m, 1H), 2.80−2.27 (m, 2H), 2.06−1.60 (m, 11H), 1.42 (d, 9H, JHP = 15.5 Hz), 1.25 (d, 9H, JHP = 15.3 Hz). 13C NMR (125 MHz, CD2Cl2): δ 179.4 (d, JCP = 3.6 Hz), 149.5 (d, JCP = 14.8 Hz), 144.0 (d, JCP = 6.2 Hz), 134.7, 133.5 (d, JCP = 7.6 Hz), 131.5 (d, JCP = 1.9 Hz), 130.2, 130.1, 129.0, 128.9, 127.9 (d, JCP = 6.5 Hz), 127.8, 126.4 (d, JCP = 40.1 Hz), 59.6 (d, JCP = 45.5 Hz), 50.8, 50.1, 38.2 (d, JCP = 21.3 Hz), 37.9 (d, JCP = 21.6 Hz), 31.1 (d, JCP = 6.4 Hz), 30.8 (d, JCP = 6.5 Hz), 27.6, 27.5, 25.4, 24.9, 23.6, 23.5. 31P NMR (120 MHz, CD2Cl2): δ 66.1. Anal. Calcd (found) for C30H44AuF6NPSb: C, 40.84 (40.93); H, 5.03 (4.99); N, 1.59 (1.54). [(t-Bu)2(o-biphenyl)PAu][1,3-dimethyl-2-methylene-2,3-dihydro1H-imidazole]+SbF6− (6). Complex 6 was prepared according to the procedure to prepare 1. The product was obtained as an off-white solid (65 mg, 74% yield). The complex was recrystallized from CH2Cl2/ Et2O at −20 °C. 1 H NMR (300 MHz, CD2Cl2): δ 7.87 (m, 1H), 7.58−7.36 (m, 5H), 7.29−7.13 (m, 3H), 6.76 (s, 2H), 3.45 (s, 6H), 1.42 (d, JHP = 9 Hz, 2H), 1.31 (d, JHP = 15 Hz, 18H). 13C NMR (125 MHz, CD2Cl2): δ 157.7 (d, J = 4.7 Hz), 149.8 (d, J = 15.3 Hz), 144.2 (d, J = 5.9 Hz), 135.1 (s), 133.4 (d, J = 7.7 Hz), 131.0 (d, J = 2.2 Hz), 130.3 (s), 128.7 (s), 127.6 (d, J = 36.7 Hz), 127.6 (d, J = 6 Hz), 127.4 (s), 119.0 (s), 38.1 (d, J = 21.3 Hz), 34.4 (s), 31.0 (d, J = 6.6 Hz), 21.9 (d, J = 74 Hz). Anal. Calcd (found) for C26H37AuF6N2PSb: C, 37.12 (36.99); H, 4.43 (4.66); N, 3.33 (3.32). 31P{1H} NMR (202.5 MHz, CD2Cl2): δ 65.7.

removed under vacuum, and the resulting crude product was triturated with anhydrous Et2O (3×) to give the desired product as an off-white solid (84 mg, 95% yield). The product was recrystallized using CH2Cl2/Et2O at −20 °C. 1 H NMR (300 MHz, CD2Cl2): δ 7.90−7.79 (m, 1H), 7.58−7.18 (m, 8H), 5.82 (br s, 1H), 3.46−3.41 (m, 4H), 1.89 (d, J = 1 Hz, 3H), 1.83 (d, JHP = 9 Hz, 2H), 1.71 (d, J = 0.5 Hz, 3H), 1.37 (d, JHP = 15.4 Hz, 18H), 1.30 (t, 3H, J = 7.2 Hz), 1.11 (t, J = 7.2 Hz, 3H). 13C NMR (125 MHz, CD2Cl2): δ 183.5 (d, J = 3.7 Hz), 149.7 (d, JCP = 14.8 Hz), 144.3 (d, JCP = 6.4 Hz), 143.9 (CCMe2), 134.9 (d, JCP = 1.8 Hz), 133.7 (d, JCP = 7.6 Hz), 131.5 (d, JCP = 2.3 Hz), 130.4, 129.0, 128.0 (d, JCP = 6.2 Hz), 127.5, 126.8 (d, JCP = 40.8 Hz), 119.7 (CCMe2), 47.6, 45.6, 45.2 (d, JCP = 48.5 Hz), 38.0 (d, JCP = 22.1 Hz), 31.2 (d, JCP = 6.6 Hz), 25.8, 20.7, 13.3 (d, J = 1.25 Hz), 12.5. 31P NMR (120 MHz, CD2Cl2): δ 65.5 ppm. Anal. Calcd (found) for C30H46AuF6NPSb: C, 40.74 (40.86); H, 5.24 (5.18); N, 1.58 (1.59). [(t-Bu) 2 (o-biphenyl)PAu][N,N-diethyl-1-phenylethene-1-amine]+SbF6−(2). Complex 2 was prepared according to the procedure to prepare 1. The product was obtained as an off-white solid (≥99%). The product was recrystallized with CH2Cl2/Et2O at −20 °C. 1 H NMR (500 MHz, CD2Cl2): δ 7.94−7.88 (m, 1H), 7.57−7.46 (m, 5H), 7.36−7.09 (m, 8H), 3.60 (q, J = 6.9 Hz, 2H), 3.33 (q, 2H, J = 7.3 Hz), 1.90 (d, JHP = 8.8 Hz, 2H), 1.42 (t, J = 7.3 Hz, 3H), 1.41 (d, JHP = 15.4 Hz, 18H), 1.08 (t, J = 7.1 Hz, 3H). 13C NMR (125 MHz, CD2Cl2): δ 187.7 (d, J = 3.3 Hz), 149.5 (d, JCP = 14.7 Hz), 144.1 (d, JCP = 6.2 Hz), 136.1, 134.7, 133.5 (d, JCP = 7.7 Hz), 131.3 (d, JCP = 2.3 Hz), 130.6, 130.1, 129.1 (br), 128.7 (br), 127.8 (d, JCP = 6.2 Hz), 127.4, 127.1, 126.7 (d, JCP = 41.5 Hz), 48.2, 47.4 (d, JCP = 47.7 Hz), 45.5, 38.1 (d, JCP = 21.9 Hz), 31.0 (d, JCP = 6.5 Hz), 14.1, 12.0. 31P NMR (120 MHz, CD2Cl2): δ 65.7 ppm. Anal. Calcd (found) for C32H44AuF6NPSb: C, 42.40 (42.42); H, 4.89 (4.85); N, 1.55 (1.47). [(t-Bu)2(o-biphenyl)PAu][2-methylene-1-phenylpyrrolidine]+SbF6− (3). (t-Bu)2(o-biphenyl)phosphine chloride (65.80 mg, 0.124 mmol) and NaSbF6 (32.01 mg, 0.124 mmol) were placed in a flask and dissolved in THF (4 mL). The solution was stirred briefly, and N(pent-3-yn-1-yl)aniline (39.40 mg, 0.247 mmol) was added to the solution. The solution was stirred for 1 h under argon and then filtered through Celite and triturated in ethyl ether to yield a white powder. The product was recrystallized at −20 °C by dissolution in CH2Cl2/ hexanes to give colorless crystals (44.8 mg, 41% yield). 1 H NMR (300 MHz, CD2Cl2): δ 7.91−7.86 (m, 1H), 7.63−7.45 (m, 5H), 7.33−7.10 (m, 6H), 7.06 (d, J = 3.6 Hz, 2H), 4.12 (t, J = 6.6 Hz, 2H), 3.15 (t, J = 7.8 Hz, 2H), 2.22 (pentet, J = 7.5, 2H), 1.85 (d, JHP =8.5 Hz, 2H), 1.39 (d, JHP = 15.3 Hz, 18H). 13C NMR (125 MHz, CD2Cl2): δ 189.9 (d, J = 3.5 Hz), 149.7 (d, J = 15.3 Hz), 144.0 (d, J = 6.4 Hz), 138.3(s), 134.8 (d, J = 1.4 Hz), 133.6 (d, J = 7.2), 131.5 (d, J = 1.9 Hz), 130.8 (s), 130.2 (s), 130.1 (s), 129.0 (s), 128.0 (d, J = 6.2 Hz), 127.6 (s), 126.7 (d, J=41.1 Hz), 125.4 (s), 60.1 (s), 39.5 (d, J = 49.6 Hz), 38.6 (s), 38.2 (d, J = 21.4), 31.2 (d, J = 6.6 Hz), 21.21 (s). 31 1 P{ H} NMR (202.5 MHz, CD2Cl2): δ 65.9. Anal. Calcd (found) for C31H40AuF6NPSb: C, 41.82 (41.76); H, 4.53 (4.59); N, 1.57 (1.55). [(t-Bu)2(o-biphenyl)PAu][N,N-diethyl-1-phenylprop-1-en-1-amine] + SbF 6 − (4). In a glovebox, [(t-Bu) 2 (o-biphenyl)PAu][acetonitrile]+SbF6− (36.7 mg, 0.47 mmol) was weighed into a flame-dried 10 mL round-bottomed flask equipped with a stir bar. Anhydrous CH2Cl2 (2 mL) was added, and the flask was capped with a septum. The reaction flask was then transferred into a fume hood and cooled to −78 °C using an acetone/dry ice bath. N,N-Diethyl-1phenylprop-1-en-1-amine (10.4 μL, 0.5 mmol) was added, and the reaction mixture was stirred at −60 °C for 10 min under argon. The cooling bath was removed, and the solvents were removed in vacuo. The crude product was triturated with hexanes (2×) and dried under vacuum. The product was obtained as an off-white solid (45 mg, 98% yield). The product was recrystallized with CH2Cl2/Et2O at −20 °C. 1 H NMR (300 MHz, CD2Cl2): δ 7.96−7.90 (m, 1H), 7.60−7.40 (m, 9H), 7.3−7.2 (m, 2H), 7.04−6.98 (m, 2H), 3.47−3.11 (m, 4H), 1.95 (dq, JHP = 9.5, JHH = 5.7 Hz, 1H), 1.46 (d, JHP = 15.5 Hz, 9H), 1.39 (d, JHP = 15.5 Hz, 9H), 1.38 (hidden t, 3H), 0.99 (t, J = 7.1 Hz, 3H), 0.96 (dd, JHH = 5.7, JHP = 2.8 Hz, 3H). 13C NMR (125 MHz, CD2Cl2): δ 187.1 ppm (d, J = 3.3 Hz), 149.5 (d, J = 15.2 Hz), 144.9

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files giving characterization data and isolation details for free enamines, gold enamines with the t-Bu3P and Ph3P ligands, and gold amines, experimental details regarding equilibrium binding studies and DNMR simulation of 2, full details of analysis and crystallographic data for all crystal structures (1, 3−6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.C.J.: [email protected]. Author Contributions †

Madhavi Sriram and Yuyang Zhu share equal authorship.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgement is made to Wake Forest University for startup funds. We thank Dr. Marcus Wright for assistance in the NMR facility. We wish to acknowledge the National Science Foundation for the funding of the purchase of the X-ray equipment (Award No. 0234489). We also thank Ellen M. Petryna for preparation and characterization of {[(t-Bu)2(obiphenyl)P]Au(NEt3)}+SbF6− and Justin K. Piedad for assistance with enamine syntheses. 4163


Organometallics

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Article

(18) Roberts, J. D.; Hawkins, B. L.; Bremser, W.; Borcic, S. J. Am. Chem. Soc. 1971, 93, 4472−4479. (19) Thanks to a reviewer comment, we also note the possibility that the two conformations have identical spectra or that the rotation is fast even at low temperatures. We agree that these scenarios are unlikely. (20) In the recent work by Zhdanko and Maier,6 the authors prepared a single gold complex from (E)-4-(1,2-diphenylvinyl) morpholine. Clear 1H NMR shift assignments were not made due to impurities and signal broadening. The signal broadening was used as evidence of a bond rotation analogous to that observed in 1 and 2. They then wrote that “therefore, the same compound would be formed from the corresponding Z isomer.” Without the Z isomer in hand, this cannot be definitely shown. (21) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. Chem. Commun. 2009, 6451−6453. (22) For the characterization of amine complexes, see the Supporting Information. (23) Zhdanko, A.; Ströbele, M.; Maier, M. E. Chem. Eur. J. 2012, 18, 14732−14744. (24) Brooner, R. E. M.; Brown, T. J.; Widenhoefer, R. A. Chem. Eur. J. 2013, 19, 8276−8284. (25) Shapiro, N. D.; Toste, F. D. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2779−2782. (26) Although such structures do not represent alkyne amination products, they are key intermediates in gold-catalyzed processes, and gold-mediated isomerization has been proposed: Montserrat, S.; Faustino, H.; Lledós, A.; Mascareñas, J. L.; López, F.; Ujaque, G. Chem. Eur. J. 2013, 19, 15248−15260.

REFERENCES

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(12) See the Supporting Information for more details. (13) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am. Chem. Soc. 2005, 127, 7805−7814. (14) These values are very close to values predicted by recent computational studies: Ciancaleoni, G.; Rampino, S.; Zuccaccia, D.; Tarantelli, F.; Belanzoni, P.; Belpassi, L. J. Chem. Theory Comput. 2014, 10, 1021−1034. (15) In a Pt(IV) enamine, the Pt−C−C angle is 96.4 °C; the authors assign this to be intermediate between π-complex characterization and iminium alkyl: Béthegnies, A.; Daran, J.-C.; Poli, R. Organometallics 2013, 32, 673−681. (16) Brown, K. L.; Damm, L.; Dunitz, J. D.; Eschenmoser, A.; Hobi, R.; Kratky, C. Helv. Chim. Acta 1978, 61, 3108−3135. (17) Reich, H. J. J. Chem. Educ. 1995, 72, 1086. 4164


Structure and Dynamic Behavior of Phosphine Gold(I)-Coordinated

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