Synthesis, Structure, and Reactivity of Palladium Proazaphosphatrane

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Synthesis, Structure, and Reactivity of Palladium Proazaphosphatrane Complexes Invoked in C−N Cross-Coupling Adrian D. Matthews,† Gregory M. Gravalis,† Nathan D. Schley,‡ and Miles W. Johnson*,† †

Department of Chemistry, University of Richmond, Richmond, Virginia 23173, United States Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States

‡

Organometallics Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/08/18. For personal use only.

S Supporting Information *

ABSTRACT: The preparation and reactivity of elusive palladium proazaphosphatrane complexes that represent putative intermediates in C−N cross-coupling reactions are described. Variable transannulation in these compounds, as determined by X-ray crystallography, validates the previously untested hypothesis that proazaphosphatranes undergo conformational changes to stabilize catalytic intermediates. The competence of these complexes as catalytic intermediates is supported through stoichiometric and catalytic coupling reactions, providing the first examples of discrete proazaphosphatrane complexes employed in cross-coupling.

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INTRODUCTION The use of conformationally flexible ligands has emerged as a highly effective strategy for the cross-coupling of sterically encumbered electrophile−nucleophile pairs.1 These supporting ligands undergo structural changes in response to steric crowding from reactive ligands, promoting cross-coupling between sterically bulky coupling partners, including aryl halides with two ortho substituents. Additionally, the dynamic steric profiles of flexible ligands may promote both oxidative addition and reductive elimination.1e Proazaphosphatranes, commonly known as Verkade’s superbases,2 are perhaps the most ubiquitous of these ligands. These caged, bicyclic molecules undergo dramatic structural deformation due to transannulation, the intramolecular interaction between the axial nitrogen (Nax) and phosphorus in these molecules, upon binding to an electrophile to form an azaphosphatrane (eq 1).3

between the initial examination of proazaphosphatranes as ligands for transition metals7 and a recent re-examination of this area. Yang and co-workers have determined the Tolman electronic and steric parameters8,9 of nickel tricarbonyl proazaphosphatrane complexes, demonstrating that proazaphosphatranes exhibit highly variable cone angles (152−207°) depending on substitution at the equatorial nitrogen (Neq) and are among the most electron-donating phosphine ligands known.9 The Martinez group has provided evidence that these ligands exhibit electronic characteristics akin to both traditional phosphines and N-heterocyclic carbenes, as demonstrated in the synthesis of silver and gold complexes.10 Despite these advances in understanding the steric and electronic properties of proazaphosphatranes as ligands, the reactivity of their complexes has only been examined with regard to transmetalation.10 Surprisingly, palladium complexes of these ligands remain unreported despite their invocation in catalysis. Insight into the coordination chemistry of such complexes may assist in harnessing proazaphosphatranes for new reactions and in improving established ones. The effectiveness of proazaphosphatranes as ligands in catalysis is largely predicated on three hypotheses: (1) oxidative addition is promoted due to electron donation to the metal center from transannulation;5d (2) steric bulk at Neq facilitates reductive elimination;5d and (3) transannulation increases with electron deficiency at the metal center.11 To evaluate these hypotheses, we have synthesized and examined catalytic intermediates that may be encountered in Buchwald− Hartwig amination (BHA) using the most prolifically employed proazaphosphatrane, triisobutylphosphatrane (L). This model system was chosen due to the thoroughly studied

The combination of a palladium source and one of these strong bases4 generates a complex that can form C−N5 and C−C1c,6 bonds from challenging substrates. Furthermore, the facile and modular synthesis of proazaphosphatranes permits systematic tuning of reactivity.5c Although the application of proazaphosphatranes in catalysis is well-studied, the coordination chemistry of these ligands remains relatively unexplored. Over two decades elapsed © XXXX American Chemical Society

Received: July 2, 2018

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DOI: 10.1021/acs.organomet.8b00453 Organometallics XXXX, XXX, XXX−XXX

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Organometallics mechanism of BHA (Scheme 1)12 and the multiple reports of amination using L as a supporting ligand.5 Scheme 1. General Mechanism for C−N Cross-Coupling

dissociation of L from complex 1 may explain why oxidative addition complexes are not directly accessible from complex 1. Determination of kinetic or thermodynamic parameters for the interconversion between complex 1 and oxidative addition complexes 2 is precluded by the lack of formation of arylpalladium(II) halide in the absence or presence of exogenous L as well as the low solubility of 1.17 Electron-deficient complexes 2a and 2b were characterized by X-ray crystallography as dimers (Figure 2). Though L has a

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RESULTS AND DISCUSSION A palladium bis(proazaphosphatrane) complex (1) is readily prepared in high yield by treating Pd2dba3 with excess L (eq 2).13 Notably, the same palladium source and solvent are used

Figure 2. ORTEP diagrams of complex 2a (left) and 2b (right). Ellipsoids are shown at 50% probability. Hydrogens and solvent molecules are excluded for clarity.

in many catalytic reactions.15 Previous attempts in the literature to access this complex were unsuccessful,5b potentially due to the exceedingly low solubility of the complex in common organic solvents. The solid-state structure of complex 1 (Figure 1) shows a linear complex with a transannular Nax−P distance of 3.482(2) Å, similar to the distance observed in nickel(0) complexes.9

cone angle greater than that of P(t-Bu)3 (200° vs 182°), the latter permits isolation of monomeric palladium(II) aryl halide complexes.18 The favorability of a dimer in the solid state of the proazaphosphatrane complexes may be attributed to the flexibility of the isobutyl substituents, as has been observed by the Shaughnessy group with neopentyl phosphines,1c,19 and a lack of a stabilizing agostic interaction trans to the aryl ligand.18 Complexes similar to compounds 2a−d are excellent synthons for amido complexes that are implicated in BHA. Palladium amides are synthetically challenging but have provided a wealth of information concerning C−N bond formation.20 Consistent with observations that C−N crosscoupling is most facile with an electron-rich amido ligand and electron-deficient aryl ligand,20a treatment of trifluoromethylsubstituted complex 2b with KNPh2 at low temperatures21 results in a red solution that subsequently darkens to form palladium black and the corresponding triarylamine (eq 4). In

Figure 1. ORTEP diagram of complex 1. Ellipsoids are shown at 50% probability. Hydrogens are excluded.

Oxidative addition by palladium bis(phosphine) complexes is well-documented;14 however, treatment of 1 with a variety of aryl halides to form an oxidative addition complex was unsuccessful. An orthogonal, low-temperature approach using Pd(COD)(CH 2 TMS) 2 provides access to a series of palladium(II) proazaphosphatrane complexes with varied aryl ligands (eq 3). These complexes decompose in solution at room temperature to form mixtures that include 1, biaryls,15 and palladium black.16 Their solution-state stability tracks with the electron-withdrawing ability of the aryl substituent, with half-lives varying from minutes in the case of electron-rich complex 2d to days for electron-poor complexes 2a and 2b. This lack of thermal stability and a potentially high barrier to B

DOI: 10.1021/acs.organomet.8b00453 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Key Distances in Proazaphosphatrane La

contrast, reaction of methoxy-substituted complex 2d with the electron-poor amide KN[3,5-(CF3)2C6H3]2 results in a similar red solution, but the color persists even at room temperature (eq 5). Compound 3 can be isolated from this solution and characterized crystallographically as a three-coordinate amido complex (Figure 3). Unlike other palladium phosphine complexes featuring the same amido and aryl ligands, 3 does not undergo reductive elimination to form the corresponding triarylamine.20b

1 2ab 2b 3

A (Å)

B (Å)

C (Å)

D (Å)

3.482(2) 3.260(6) 3.265(7) 3.206(3)

0.714(1) 0.644(4) 0.628(4) 0.628(2)

2.563(2) 2.515(5) 2.522(5) 2.513(2)

0.205(2) 0.107(7) 0.122(8) 0.067(3)

a One N(i-Bu)CH2CH2 group is excluded for clarity. bAverage of four distances in the unit cell.

neighboring methylene groups). This is most dramatic when comparing 1 and 3. The combined distances B and D are approximately 0.276 Å shorter in 3 than 1. Whereas 1 clearly possesses a proazaphosphatrane ligand with Nax protruding away from phosphorus in an exo conformation,23 L on complex 3 is nearly planar at Nax, demonstrating quasi-atrane character.2a Taken together, these comparisons offer the first support for the hypothesis that proazaphosphatranes undergo varying degrees of transannulation in catalytic intermediates as a function of the electron density of the metal to which the ligand is bound. Such interactions may prevent ligand dissociation during catalysis and make palladium sufficiently electron-rich to oxidatively add C−Cl bonds under mild conditions.1c,5d,6a

Figure 3. ORTEP diagram of 3. Ellipsoids are shown at 50% probability. Hydrogens are excluded for clarity.

The catalytic viability of proposed BHA intermediates 1, 2b, and 3 was tested. Both 1 and 2b promote the coupling of 4bromobenzotrifluoride and diphenylamine in high yield under literature conditions (eq 5).5b To our knowledge, these reactions represent the only examples of well-defined proazaphosphatrane complexes being used as precatalysts. Furthermore, this reactivity shows their potential relevance in the industrially essential BHA reaction. Complex 3 shows no reactivity, consistent with the inability of this compound to undergo C−N bond formation stoichiometrically; however, this molecule is a stable model system for a key intermediate.

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CONCLUSIONS In summary, previously putative catalytic intermediates in the palladium proazaphosphatrane-catalyzed BHA have now been prepared for the first time. These complexes exhibit catalytic and stoichiometric reactivity that is consistent with their presence on a catalytic cycle. Importantly, the structure of the proazaphosphatrane changes in response to the oxidation state of palladium and the coordination sphere of the metal. These results substantiate the long-held hypothesis that the extent of transannulation in a proazaphosphatrane ligand changes throughout a catalytic process. Synthetic and computational studies are underway to further interrogate the role of transannulation on catalysis and to harness this phenomenon in developing new reactivity.

The palladium complexes that we have synthesized provide new insight into the transannulation of proazaphosphatranes and permit the first examination of transannular distance (A in Table 1) as a function of the ligand set on a single metal.22 The greatest transannular distance is observed in 1 (3.482(2) Å). This length is likely due to the electron-rich palladium center and not the mutually trans disposition of the two proazaphosphatranes as a trans-platinum(II) dichloride bis(proazaphosphatrane) complex has a relatively short transannular distance of 3.250 Å.7a Moreover, electron-poor complexes 2a and 2b exhibit transannular distances shorter than those of bis(proazaphosphatrane) complex 1. Amido complex 3, which has a single L-type donor ligand and a highly electron-deficient amido group trans to L, has the shortest transannular distance of the series (3.206(3) Å). Notably, in all four cases, the distance between the plane formed by the equatorial nitrogens and the methylene groups neighboring Nax (C) is nearly invariable. The change in transannular distance is due to the shortening of distances B (the Neq plane to phosphorus) and D (Nax to the plane formed by its

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out in a nitrogen-filled MBraun LABstar Pro glovebox unless otherwise noted. All glassware was oven-dried overnight at greater than 110 °C prior to use. All liquid aryl halides were passed through activated alumina and stored over 3 Å molecular sieves prior to use. 4-Bromobenzotrifluoride (Aldrich), bromobenzene (Aldrich), 4-bromobenzonitrile (Oakwood), and 4-bromoanisole (Aldrich) were purchased from commercial sources. Anhydrous pentane was purchased from Aldrich and stored over 3 Å molecular sieves prior to use. All other solvents were collected from a Glass Contour solvent purification system, degassed, and stored over 3 Å molecular sieves. Anhydrous deuterobenzene (C6D6) was purchased from Aldrich and stored over 3 Å molecular sieves. Pd2(dba)3 (Aldrich), (COD)Pd(CH2TMS)2 (Strem), and 2,8,9-triisopropyl-2,5,8,9-tetraaza-1phosphabicyclo[3,3,3]undecane (L, Aldrich) were used as received from their respective vendors. KNPh224 and KN[3,5-CF3(C6H3)]220b were prepared by literature procedures. 1,3,5-Trimethoxybenzene was purified by sublimation. Authentic samples of biaryl compounds were C

DOI: 10.1021/acs.organomet.8b00453 Organometallics XXXX, XXX, XXX−XXX

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Organometallics prepared by literature methods.25 Chromatography was performed using SiliaFlash F60 silica. Elemental analyses were performed by Midwest Microlab, LLC. Spectroscopy. 1H, 13C{1H}, 19F{1H}, and 31P{1H} spectra were collected on Bruker AV-300 and AV-500 NMR spectrometers at ambient temperature unless otherwise noted. 1H NMR chemical shifts (δ) are reported in parts per million (ppm) relative to the solvent (7.16 ppm for C6D5H). 13C NMR spectra were referenced relative to the solvent signal (128.06 ppm for C6D6). 31P{H} and 19F{H} were referenced using the absolute reference function of the MNova 9.0.1 NMR software package. Infrared spectra were recorded on a Nicolet iS10 FT-IR spectrometer. In all dilute 1H NMR spectra, a small, broad artifact is observed at 1.37 ppm; a spectrum of C6D6 is provided for reference in the Supporting Information. When possible, 1H and 13C resonances were assigned using COSY, HSQC, and NOESY experiments. Due to the low solubility and/or thermal stability of some complexes, the signal-to-noise is low in many 13C NMR spectra. An inseparable impurity observed at ca. 0.6 ppm by 1H NMR in some spectra may be the product of cyclometalation, a decomposition product suggested in the literature.5b Pd(P(i-BuNCH2CH2)3N)2 (1). To a 20 mL scintillation vial in the glovebox were added Pd2(dba)3 (27.4 mg, 0.030 mmol, 1.0 equiv) and toluene (2 mL). P(i-BuNCH2CH2)3N (46.2 mg, 0.135 mmol, 4.5 equiv) in toluene (2 mL) was added to the stirred palladium solution. Within 20 min, the solution changed from a dark wine color to a deep burgundy. The reaction mixture was stirred for 16 h, during which time the reaction mixture became green and heterogeneous. The reaction mixture was diluted with toluene until homogeneous, approximately 40 mL, and passed through a pad of Celite to remove solid palladium. The resulting yellow solution was concentrated to a brown solid. The crude solid was suspended in Et2O and cooled to −35 °C, and the supernatant was then decanted. This suspension− decantation process was repeated until the supernatant was colorless, indicating the absence of dibenzylideneacetone. Residual solvent was evaporated under vacuum to yield the desired complex as an analytically pure, colorless solid (39.6 mg, 0.050 mmol, 83% yield). Xray quality crystals were grown from a 5:1 Et2O/toluene mixture at −35 °C. 1H NMR (500 MHz, C6D6): δ 3.33 (ap q, J = 6.1 Hz, 5.64 Hz, 12H, i-PrCH2), 2.87 (br s, 12H, CH2Nax), 2.75 (t, J = 5.0 Hz, 12H, CH2Neq), 2.27 (sept, J = 6.8 Hz, 6H, (CH3)2CH)), 1.14 (d, J = 6.6 Hz, 36H, CH3). 13C NMR (126 MHz, C6D6): δ 60.0 (t, JC−P = 16.5 Hz, i-PrCH2), 52.0 (CH2Nax), 48.0 (CH2Neq), 29.5 (t, JC−P = 3.00 Hz, Me2CH), 21.6 (CH3). 31P NMR (202 MHz, C6D6): δ 134.5. IR (ATR, cm−1): 2949, 2926, 2823, 1117, 1014, 836, 701. Anal. Calcd for C36H78N8P2Pd: C, 54.63; H, 9.93; N, 14.16. Found: C, 54.30; H, 9.71; N, 14.01. General Procedure for the Synthesis of Oxidative Addition Complexes (Complexes 2a−2d). A 20 mL scintillation vial was charged with (COD)Pd(CH2TMS)2 (38.9 mg, 0.10 mmol, 1.0 equiv) and pentane chilled to −35 °C (4 mL). To this solution was added a chilled solution of pentane (8 mL) containing proazaphosphatrane (34.2 mg, 0.10 mmol, 1.0 equiv) and aryl halide (3.0 equiv). The reaction mixture was warmed to room temperature and stirred for 16−18 h, at which point the precipitate was permitted to settle and the supernatant was decanted. The resulting solid was again stirred in pentane and the mother liquor decanted. Residual solvent was removed in vacuo to yield the desired product. Reactions conducted on a 0.05 mmol scale in (COD)Pd(CH2TMS)2 were conducted identically but on half the scale. For all complexes, small quantities of pentane remain despite prolonged exposure to vacuum. [(N(CH2CH2Ni-Bu)3PPd(C6H4-p-CN)Br]2 (2a). This synthesis was performed on a 0.05 mmol scale using the general procedure. The product was isolated as a pale yellow solid (25.2 mg, 0.0399 mmol, 80% yield). X-ray quality crystals were grown from a saturated toluene solution at −35 °C. 1H NMR (500 MHz, C6D6): δ 7.61 (dd, JC−P = 8.0 Hz, 5.3 Hz, 2H, H meta to Pd), 6.96 (d, J = 7.8 Hz, 2H, H para to Pd), 2.98 (br s, 6H, i-PrCH2), 2.60 (br s, 6H, ethylene CH2), 2.37 (t, J = 4.8 Hz, 6H, ethylene CH2), 1.98 (br s, 3H, (CH3)2CH), 0.93 (d, J = 6.6 Hz, 18H, CH3). 13C NMR (126 Hz, C6D6): δ 161.2 (d, JC−P = 16.1 Hz, CN), 138.4 (d, J = 5.0 Hz, Csp2−H), 129.5 (Csp2−H) 119.9

(ipso), 107.4 (ipso), 57.7 (br, i-PrCH2), 51.0 (ethylene CH2), 49.6 (ethylene CH2), 28.8 (d, J = 3.8 Hz, (CH3)2CH), 21.9 (CH3); note: one aromatic carbon was not observed. 31P NMR (202 MHz, C6D6): δ 94.8 ppm. IR (ATR, cm−1): 2954, 2220 (nitrile), 1570, 1470, 1390, 1174, 1031, 811. Anal. Calcd for C50H86Br2N10P2Pd2: C, 47.59; H, 6.87; N, 11.10 Found: C, 47.43; H, 6.73; N, 11.22. [(N(CH2CH2Ni-Bu)3PPd(C6H4-p-CF3)Br]2 (2b). This synthesis was performed on a 0.10 mmol scale using the general procedure. The product was isolated as a pale yellow solid (43.6 mg, 0.065 mmol, 65% yield). X-ray quality crystals were grown from a 2:1 pentane/ toluene mixture at −20 °C. 1H NMR (500 MHz, C6D6): δ 7.74 (dd, J = 7.9 Hz, 5.4 Hz, 2H, H meta to Pd), 7.24 (d, J = 7.9 Hz, 2H, H ortho to Pd), 3.04 (br s, 6H, i-PrCH2), 2.62 (br s, 6H, ethylene CH2), 2.39 (t, J = 5.0 Hz, 6H, ethylene CH2), 2.00 (br s, 3H, (CH3)2CH)), 0.98 (d, J = 6.6 Hz, 18 H, CH3). 13C NMR (126 MHz, C6D6): δ 137.9 (C meta to Pd), 123.4 (C ortho to Pd), 57.8 (i-PrCH2), 51.1 (ethylene CH2), 49.7 (ethylene CH2), 28.8 ((CH3)2CH), 22.0 (CH3); note: ipso carbons were not observed. 31P NMR (202 MHz, C6D6): δ 95.8 ppm. 19 F (282 MHz, C6D6): δ −61.6 ppm. IR (ATR, cm−1): 2955, 2927, 2872, 1585, 1390, 1318, 1160, 1037, 1009, 850. Combustion analysis was not pursued because an impurity at 0.58 ppm (1H NMR) was observed in varying quantities despite recrystallization. [(N(CH2CH2Ni-Bu)3PPd(C6H5)Br]2 (2c). This synthesis was performed on a 0.05 mmol scale using the general procedure. The product was isolated as an off-white solid (21.7 mg, 0.0359 mmol, 72% yield). 1H NMR (500 MHz, C6D6): δ 7.71 (t, J = 6.5 Hz, 2H, Ar), 7.03 (t, J = 7.4 Hz, 2H, Ar), 6.87 (t, J = 7.2 Hz, 1H, p-H), 3.17 (br s, 6H), 2.70 (br s, 6H), 2.46 (t, J = 5.1 Hz, 6H), 2.13 (s, 3H, (CH3)2CH)), 1.04 (d, J = 6.6 Hz, 18H, CH3). 13C NMR (126 Hz, C6D6): δ 137.8 (d, J = 5.0 Hz), 123.1, 58.1, 51.3, 49.6, 28.9, 22.0; note: two signals were not observed. 31P NMR (202 MHz, C6D6): δ 97.6. IR (ATR, cm−1): 2955, 2860, 1561, 1396 1103, 1009, 805. Combustion analysis was not pursued due to the thermal instability of the complex. [(N(CH2CH2Ni-Bu)3PPd(C6H4-p-OMe)Br]2 (2d). This synthesis was performed on a 0.10 mmol scale using the general procedure. The product was isolated as a pale yellow solid (35.6 mg, 0.0560 mmol, 56% yield). 1H NMR (500 MHz, C6D6): δ 7.56 (dd, J = 8.4, 5.5 Hz, 2H, Ar), 6.77 (d, J = 8.2 Hz, 2H, Ar), 3.38 (s, 3H, OMe), 3.20 (br s, 6H), 2.73 (br s, 6H), 2.47 (ap t, J = 5.0 Hz, 6H), 2.15 (br s, 3H, (CH3)2CH)), 1.05 (d, J = 6.6 Hz, 18H, CH3). 31P NMR (202 MHz, C6D6): δ 97.5. IR (ATR, cm−1): 2951, 2864, 1480, 1464, 1388, 1183, 1117, 1014, 836. Combustion analysis and 13C NMR data were not collected due to the low thermal stability of the complex. (N(CH2CH2Ni-Bu)3PPd(C6H4-p-OMe)(N[3,5-CF3(C6H3)]2) (3). Thawing THF (2 mL) was added to a vial containing complex 2d (19.8 mg, 0.031 mmol, 1.0 equiv), and the suspension was frozen in the glovebox cold well. KN[3,5-CF3(C6H3)]2 (15.5 mg, 0.031 mmol, 1.0 equiv) was dissolved in THF (4 mL) and layered onto the frozen suspension of 2d. The suspension was permitted to thaw and mix with the amido salt solution, changing within 30 min from a yellow suspension to a homogeneous red solution. The reaction mixture was concentrated, and the residue was dissolved in pentane. The pentane solution was passed through a syringe filter, and the resulting solution was concentrated to saturation. The pentane solution was stored at −35 °C until crystals formed. The supernatant was decanted, and residual solvent was removed in vacuo, yielding the desired compound as orange crystals (15.5 mg, 0.016 mmol, 50% yield). Xray quality crystals were grown from a saturated pentane solution at −35 °C. 1H NMR (500 MHz, C6D6): δ 7.91 (s, 4H, o-H 3,5(CF3)2C6H3), 7.31 (s, 2H, p-H 3,5-(CF3)2C6H3), 7.02 (dd, J = 8.7, 4.4 Hz, 2H, anisyl H ortho to Pd), 6.45 (d, J = 8.6 Hz, 2H, anisyl H meta to Pd), 3.21 (s, 3H, OCH3), 2.66 (dd, J = 11.4, 7.2 Hz, 6 H, iPrCH2), 2.43−2.3 (m, 6H, ethlyene), 2.32−2.24 (m, 6H, ethylene), 1.42 (sept, J = 6.5 Hz, 3H, (CH3)2CH)), 0.80 (d, J = 6.5 Hz, 18H, CH3). 13C NMR (126 Hz, C6D6): δ 158.6 (ipso), 156.1 (ipso) 134.6 (anisyl C ortho to Pd), 133.0 (ipso), 132.8 (ipso), 120.7 (o-C 3,5(CF3)2C6H3), 114.3 (d, J = 4.0 Hz, anisyl C meta to Pd), 110.9 (p-C 3,5-(CF3)2C6H3) 55.4 (d, J = 15.4 Hz, i-PrCH2) 54.9 (OCH3) 50.4 (ethylene CH2), 47.8 (ethylene CH2), 30.2 (i-PrCH2), 20.3 (CH3); D

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Organometallics note: CF3 not observed. 31P NMR (202 MHz, C6D6): δ 101.8. 19F (282 MHz, C6D6): δ −62.8. IR (ATR, cm−1): 2951, 1386, 1271, 1161, 1117. Anal. Calcd for C41H52F12N5OPPd: C, 49.93; H, 5.26; N, 7.03. Found: C, 49.09; H, 6.01; N, 6.82. Analytically pure material was not obtained despite recrystallization; however, the structure can be proposed with confidence based on NMR and crystallographic data. Use of 2b as a Precatalyst. In a glovebox, 2b (0.0040 mmol, 2.7 mg, 2 mol %) was dissolved in toluene (1 mL) and added to a vial containing NaOt-Bu (0.300 mmol, 28.8 mg). Additional toluene (1.5 mL) was used to transfer residual 2b. The reaction mixture immediately became dark pink. To the mixture was added Ph2NH in toluene (1.5 mL). To the vial was added a 0.015 M stock solution of L (0.015 M in toluene, 0.003 mmol, 200 μL, 2 mol %) followed by 4-bromobenzotrifluoride (0.20 mmol, 28 μL). The vial was removed from the glovebox and stirred at 80 °C for 16 h. The reaction mixture became a dark green-black. The mixture was filtered through a medium porosity frit with additional toluene and concentrated on a rotary evaporator to yield a green-black solid. The crude material was purified by column chromatography (2.5% ethyl acetate/97.5% hexane) to yield a white solid (59.4 mg, 0.190 mmol, 95% yield). 1 H NMR data for the product match that in the literature.26 Use of 1 as a Precatalyst. The above reaction was performed identically but with 1 (3.2 mg, 0.0040 mmol, 2 mol %) in place of 2b, and without the addition of L. The desired product was isolated as a white solid (61.1 mg, 0.195 mmol, 98% yield). Stoichiometric Coupling of 2b with 4-Bromobenzotrifluoride. In a glovebox, 2b (0.010 mmol, 6.7 mg) was dissolved in THF (1 mL) and frozen in a cold well. The mixture was removed from the cold well once frozen, and to the vial Ph2NK (0.013 mmol, 2.7 mg) was transferred using THF (2 mL). The reaction mixture was stirred for 15 min at ambient temperature, during which time it changed from yellow to red to black. The solution was filtered through a syringe filter; the filter was washed with DCM, and the resulting orange solution was concentrated. To the residue was added a stock solution of 1,3,5-trimethoxybenzene in CDCl3 (200 μL, 0.05 M) and additional CDCl3. The yield of Ph2N(p-C6H4CF3) was determined by 1 H NMR relative to the internal standard. First run: 29% yield. Second run: 35% yield. X-ray Crystallography. A suitable crystal of each sample was selected for analysis and mounted in a polyimide loop. All measurements were made on a Rigaku Oxford Diffraction Supernova Eos CCD with filtered Cu Kα or Mo Kα radiation at a temperature of 100 K. Using Olex2,27 the structure was solved with the ShelXT structure solution program using direct methods and refined with the ShelXL refinement package28 using least squares minimization. Complex 1. The structure was refined without restraint. Complex 2a. Three disordered toluene molecules were modeled as rigid bodies using the method of Guzei.29 Their thermal parameters were refined with similarity restraints. A disordered isopropyl group was modeled as a 2-position disorder using similarity restraints placed on the thermal parameters. Complex 2b. The structure was refined without restraint. Complex 3. The structure was refined without restraint.

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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nathan D. Schley: 0000-0002-1539-6031 Miles W. Johnson: 0000-0002-7009-6138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The University of Richmond is acknowledged for start-up funds (M.W.J.) and for undergraduate summer fellowships (A.D.M. and G.M.G.). Funding from Vanderbilt University is gratefully acknowledged (N.D.S.). Miriam A. Bowring, C. Wade Downey, Allegra L. Liberman-Martin, and Kristine A. Nolin are thanked for helpful discussions.

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REFERENCES

(1) (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. Sterically Demanding, Bioxazoline-Derived N-Heterocyclic Carbene Ligands with Restricted Flexibility for Catalysis. J. Am. Chem. Soc. 2004, 126, 15195−15201. (b) Raders, S. M.; Moore, J. N.; Parks, J. K.; Miller, A. D.; Leißing, T. M.; Kelley, S. P.; Rogers, R. D.; Shaughnessy, K. H. Trineopentylphosphine: A Conformationally Flexible Ligand for the Coupling of Sterically Demanding Substrates in the Buchwald−Hartwig Amination and Suzuki−Miyaura Reaction. J. Org. Chem. 2013, 78, 4649−4664. (c) Su, W.; Urgaonkar, S.; McLaughlin, P. A.; Verkade, J. G. Highly Active Palladium Catalysts Supported by Bulky Proazaphosphatrane Ligands for Stille CrossCoupling: Coupling of Aryl and Vinyl Chlorides, Room Temperature Coupling of Aryl Bromides, Coupling of Aryl Triflates, and Synthesis of Sterically Hindered Biaryls. J. Am. Chem. Soc. 2004, 126, 16433− 16439. (d) Hu, H.; Qu, F.; Gerlach, D. L.; Shaughnessy, K. H. Mechanistic Study of the Role of Substrate Steric Effects and Aniline Inhibition on the Bis(trineopentylphosphine)palladium(0)-Catalyzed Arylation of Aniline Derivatives. ACS Catal. 2017, 7, 2516−2527. (e) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. An NHeterocyclic Carbene Ligand with Flexible Steric Bulk Allows Suzuki Cross-Coupling of Sterically Hindered Aryl Chlorides at Room Temperature. Angew. Chem., Int. Ed. 2003, 42, 3690−3693. (f) Lavallo, V.; Canac, Y.; Präsang, C.; Donnadieu, B.; Bertrand, G. Stable Cyclic (Alkyl)(Amino)Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the Difference. Angew. Chem., Int. Ed. 2005, 44, 5705−5709. (2) (a) Verkade, J. G. Atranes: New Examples with Unexpected Properties. Acc. Chem. Res. 1993, 26, 483−489. (b) Verkade, J. G.; Kisanga, P. B. Proazaphosphatranes: A Synthesis Methodology Trip from Their Discovery to Vitamin A. Tetrahedron 2003, 59, 7819− 7858. (3) For theoretical studies, see: (a) Windus, T. L.; Schmidt, M. W.; Gordon, M. S. Theoretical Investigation of Azaphosphatrane Bases. J. Am. Chem. Soc. 1994, 116, 11449−11455. (b) Galasso, V. Theoretical Study of the Structure and Bonding in Phosphatrane Molecules. J. Phys. Chem. A 2004, 108, 4497−4504. (4) Laramay, M. A. H.; Verkade, J. G. The “Anomalous” Basicity of P(NHCH 2 CH 2 ) 3 N Relative to P(NMeCH 2 CH 2 ) 3 N and P(NBzCH2CH2)3N: A Chemical Consequence of Orbital Charge Balance? J. Am. Chem. Soc. 1990, 112, 9421−9422. (5) (a) Venkat Reddy, C. R.; Urgaonkar, S.; Verkade, J. G. A Highly Effective Catalyst System for the Pd-Catalyzed Amination of Vinyl Bromides and Chlorides. Org. Lett. 2005, 7, 4427−4430. (b) Urgaonkar, S.; Verkade, J. G. Scope and Limitations of Pd2(dba)3/P(iBuNCH2CH2)3N-Catalyzed Buchwald−Hartwig Amination Reac-

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00453. Tabulated X-ray crystallographic data and NMR spectra (PDF) Accession Codes

CCDC 1850176−1850179 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The E

DOI: 10.1021/acs.organomet.8b00453 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics tions of Aryl Chlorides. J. Org. Chem. 2004, 69, 9135−9142. (c) Kim, S. H.; Kim, M.; Verkade, J. G.; Kim, Y. A Tuned Bicyclic Proazaphosphatrane for Catalytically Enhanced N-Arylation Reactions with Aryl Chlorides. Eur. J. Org. Chem. 2015, 2015, 1954−1960. (d) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. P[N(i-Bu)CH2CH2]3N: A Versatile Ligand for the Pd-Catalyzed Amination of Aryl Chlorides. Org. Lett. 2003, 5, 815−818. (e) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. P(i-BuNCH2CH2)3N: An Effective Ligand in the Palladium-Catalyzed Amination of Aryl Bromides and Iodides. J. Org. Chem. 2003, 68, 452−459. (6) (a) You, J.; Verkade, J. G. A General Method for the Direct αArylation of Nitriles with Aryl Chlorides. Angew. Chem., Int. Ed. 2003, 42, 5051−5053. (b) Nandakumar, M. V.; Verkade, J. G. Pd2dba3/P(iBuNCH2CH2)3N: A Highly Efficient Catalyst for the One-Pot Synthesis of Trans-4-N,N-Diarylaminostilbenes and N,N-Diarylaminostyrenes. Tetrahedron 2005, 61, 9775−9782. (c) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. Pd/P(i-BuNCH2CH2)3N: An Efficient Catalyst for Suzuki Cross-Coupling of Aryl Bromides and Chlorides with Arylboronic Acids. Tetrahedron Lett. 2002, 43, 8921−8924. (7) (a) Xi, S. K.; Schmidt, H.; Lensink, C.; Kim, S.; Wintergrass, D.; Daniels, L. M.; Jacobson, R. A.; Verkade, J. G. Bridgehead-Bridgehead Communication in Untransannulated Phosphatrane ZP(ECH2CH2)3N Systems. Inorg. Chem. 1990, 29, 2214−2220. (b) Tang, J. S.; Laramay, M. A. H.; Young, V.; Ringrose, S.; Jacobson, R. A.; Verkade, J. G. Stepwise Transannular Bond Formation between the Bridgehead Atoms in Z-P(MeNCH2CH2)3N Systems. J. Am. Chem. Soc. 1992, 114, 3129−3131. (8) Tolman, C. A. Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev. 1977, 77, 313−348. (9) Thammavongsy, Z.; Kha, I. M.; Ziller, J. W.; Yang, J. Y. Electronic and Steric Tolman Parameters for Proazaphosphatranes, the Superbase Core of the Tri(pyridylmethyl)azaphosphatrane (TPAP) Ligand. Dalton Trans 2016, 45, 9853−9859. (10) Chatelet, B.; Nava, P.; Clavier, H.; Martinez, A. Synthesis of Gold(I) Complexes Bearing Verkade’s Superbases. Eur. J. Inorg. Chem. 2017, 2017, 4311−4316. (11) Urgaonkar, S.; Xu, J.-H.; Verkade, J. G. Application of a New Bicyclic Triaminophosphine Ligand in Pd-Catalyzed Buchwald− Hartwig Amination Reactions of Aryl Chlorides, Bromides, and Iodides. J. Org. Chem. 2003, 68, 8416−8423. (12) Shekhar, S.; Ryberg, P.; Hartwig, J. F.; Mathew, J. S.; Blackmond, D. G.; Strieter, E. R.; Buchwald, S. L. Reevaluation of the Mechanism of the Amination of Aryl Halides Catalyzed by BINAP-Ligated Palladium Complexes. J. Am. Chem. Soc. 2006, 128, 3584−3591. (13) For a related synthesis, see: Paul, F.; Patt, J.; Hartwig, J. F. Structural Characterization and Simple Synthesis of {Pd[P(oTol)3]2}. Spectroscopic Study and Structural Characterization of the Dimeric Palladium(II) Complexes Obtained by Oxidative Addition of Aryl Bromides and Their Reactivity with Amines. Organometallics 1995, 14, 3030−3039. (14) Barrios-Landeros, F.; Carrow, B. P.; Hartwig, J. F. Effect of Ligand Steric Properties and Halide Identity on the Mechanism for Oxidative Addition of Haloarenes to Trialkylphosphine Pd(0) Complexes. J. Am. Chem. Soc. 2009, 131, 8141−8154. (15) For a similar observation, see: Hooper, M. W.; Utsunomiya, M.; Hartwig, J. F. Scope and Mechanism of Palladium-Catalyzed Amination of Five-Membered Heterocyclic Halides. J. Org. Chem. 2003, 68, 2861−2873. (16) See Supporting Information for a full byproduct profile. (17) For parameters for a related system, see: Roy, A. H.; Hartwig, J. F. Directly Observed Reductive Elimination of Aryl Halides from Monomeric Arylpalladium(II) Halide Complexes. J. Am. Chem. Soc. 2003, 125, 13944−13945. (18) Stambuli, J. P.; Bühl, M.; Hartwig, J. F. Synthesis, Characterization, and Reactivity of Monomeric, Arylpalladium Halide Complexes with a Hindered Phosphine as the Only Dative Ligand. J. Am. Chem. Soc. 2002, 124, 9346−9347.

(19) Hill, L. L.; Moore, L. R.; Huang, R.; Craciun, R.; Vincent, A. J.; Dixon, D. A.; Chou, J.; Woltermann, C. J.; Shaughnessy, K. H. Bulky Alkylphosphines with Neopentyl Substituents as Ligands in the Amination of Aryl Bromides and Chlorides. J. Org. Chem. 2006, 71, 5117−5125. (20) (a) Arrechea, P. L.; Buchwald, S. L. Biaryl Phosphine Based Pd(II) Amido Complexes: The Effect of Ligand Structure on Reductive Elimination. J. Am. Chem. Soc. 2016, 138, 12486−12493. (b) Yamashita, M.; Hartwig, J. F. Synthesis, Structure, and Reductive Elimination Chemistry of Three-Coordinate Arylpalladium Amido Complexes. J. Am. Chem. Soc. 2004, 126, 5344−5345. (21) Low temperatures mitigate decomposition of 2d. (22) For a list of transannular distances in proazaphosphatrane and azaphosphatrane, see the supporting information of: Thammavongsy, Z.; Khosrowabadi Kotyk, J. F.; Tsay, C.; Yang, J. Y. Flexibility Is Key: Synthesis of a Tripyridylamine (TPA) Congener with a Phosphorus Apical Donor and Coordination to Cobalt(II). Inorg. Chem. 2015, 54, 11505−11510. (23) Marín-Luna, M.; Alkorta, I.; Elguero, J. Theoretical Study of the Geometrical, Energetic and NMR Properties of Atranes. J. Organomet. Chem. 2015, 794, 206−215. (24) Glock, C.; Görls, H.; Westerhausen, M. Electronic, Steric, and Ligand Influence on the Solid-State Structures of Substituted Sodium and Potassium Anilides. Eur. J. Inorg. Chem. 2011, 2011, 5288−5298. (25) Cheng, G.; Luo, M. Homocoupling of Arylboronic Acids Catalyzed by CuCl in Air at Room Temperature. Eur. J. Org. Chem. 2011, 2011, 2519−2523. (26) Hirai, Y.; Uozumi, Y. Clean Synthesis of Triarylamines: Buchwald-Hartwig Reaction in Water with Ambiphilic ResinSupported Palladium Complexes. Chem. Commun. 2010, 46, 1103− 1105. (27) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341. (28) Sheldrick, G. A. Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (29) Guzei, I. A. An Idealized Molecular Geometry Library for Refinement of Poorly Behaved Molecular Fragments with Constraints. J. Appl. Crystallogr. 2014, 47, 806−809.

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DOI: 10.1021/acs.organomet.8b00453 Organometallics XXXX, XXX, XXX−XXX