Communication pubs.acs.org/Organometallics
Synthesis and Structure of Cationic Phosphine Gold(I) Enol Ether Complexes Yuyang Zhu, Cynthia S. Day, and Amanda C. Jones* Department of Chemistry, Salem Hall, Wake Forest University, Winston-Salem, North Carolina 27109, United States S Supporting Information *
ABSTRACT: Cationic {[(t-Bu) 2 (o-biphenyl)P]Au(enol ether)}+SbF6− and {[(t-Bu)3P]Au(enol ether)}+SbF6− complexes were isolated and characterized by NMR spectroscopy and X-ray crystallography. For three of the four complexes characterized in the solid state, gold coordinates to the electronrich bond in a η1 fashion, while a 2,3-dihydrofuran complex displays η2 coordination.
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ationic gold catalysts have demonstrated utility in a broad array of reactions involving activation of multiple bonds.1 The pioneering work by Teles et al. demonstrated the efficiency with which gold catalyzes the hydration and methoxylation of alkynes.2 To date, the highest efficiency, reactivity, and reliability of gold-catalyzed processes is in the arena of nucleophilic addition to activated π systems such as alkynes and allenes. In gold-catalyzed additions of oxygen nucleophiles to alkynes, enol ethers can be observed, which are themselves activated alkenes and easily react further.3,4 Such processes to form carbonyl and ketal products are, however, usually facilitated by acid. Although enol ether complexes are key intermediates in these processes and the subject of theoretical consideration,5 few experimental studies have been focused on understanding their reactivity and structural features. Encouraged by others’ successes in the arena of gold π-complex synthesis,6,7 we thus sought to examine a series of complexed enol ethers to further understand these structures, as only a single example, a complexed enol ether diene, has previously been prepared.6b,8,9 Access to these complexes is an important step toward exploring their reactivity and involvement in catalytic cycles. Initial attempts to generate enol ether complexes with the relatively electron deficient triphenylphosphine ligand led to rapid formation of material resembling polymeric ether compounds by 1H NMR spectroscopy, an initial reminder of the high activity of gold as a π Lewis acid.6a Heeding others’ observations6,7,15 of the higher stability of π complexes with strong ligand donors (including N-heterocyclic carbenes), we switched to (t-Bu)3PAuCl or (t-Bu)2(o-biphenyl)PAuCl in order to take advantage of the 31P NMR handle. Addition of the enol ethers to Celite-prefiltered mixtures of AgSbF6 and (tBu)2(o-biphenyl)PAuCl followed ( 2σ(I)) = 0.0377, wR2(F2) = 0.0960, GOF = 1.028; for 1b, C16H35AuF6OPSb, M = 707.13, monoclinic, P21 (racemic twin), T = 193(2) K, a = 8.2207(12) Å, b = 15.516(2) Å, c = 9.7572(14) Å, β = 111.4860(10)°, V = 1158.1(3) Å3, Z = 2, R1(I > 2σ(I)) = 0.0299, wR2(F2) = 0.0777, GOF = 1.051; for 3a, C24H33AuF6OPSb, M = 801.19, monoclinic, P21/c, T = 193(2) K, a = 9.0067(9) Å, b = 25.080(2) Å, c = 11.9594(11) Å, β = 92.5960(10)°, V = 2698.7(4) Å3, Z = 4, R1(I > 2σ(I)) = 0.0251, wR2(F2) = 0.0646, GOF = 1.028; for 4a, C25H35AuF6OPSb, M = 815.22, monoclinic, Cc, T = 193(2) K, a = 11.7396(15) Å, b = 14.8313(19) Å, c = 16.241(2) Å, β = 92.3860(10)°, V = 2825.3(6) Å3, Z = 4, R1(I > 2σ(I)) = 0.0359, wR2(F2) = 0.0812, GOF = 0.873. (12) Jiang, H.; Li, H.; Wu, T.; Han, S. Chem. Phys. Lett. 2005, 406, 489−494. (13) Although structurally very similar, our values are in contrast with those calculated by Sanguramath et al.,6b who report a value of 45% slippage for [(t-Bu)2(o-biphenyl)PAu][2,3-dimethoxybutadiene] [SbF6], in comparison to the 91% calculated for their structure by our method. (14) Soriano, E.; Marco-Contelles, J. Top. Curr. Chem. 2011, 302, 1− 29. (15) The parent catalysts display a weak η1-arene interaction: (a) Herrero-Gómez, E.; Nieto-Oberhuber, C.; López, S.; BenetBuchholz, J.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5455− 5459. (b) Pérez-Galán, P.; Delpont, N.; Herrero-Gómez, E.; Maseras, F.; Echavarren, A. M. Chem. Eur. J. 2010, 16, 5324−5332. (16) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308−4320.
2.98 (dd, JHP = 6.4 Hz, JHH = 1.9 Hz, 1H), 2.68 (ddq, JHP = 6.1 Hz, JHH = 1.9, 0.7 Hz, 1H), 2.31 (s, 3H), 1.39 (d, JHP = 16.2 Hz, 9H), 1.37 (d, JHP = 16.2 Hz, 9H). 13C NMR (125 MHz, CD2Cl2): δ 193.8, 149.0 (d, J = 13.8 Hz), 144.0 (d, J = 6.9 Hz), 134.3 (d, J = 2.8 Hz), 133.8 (d, J = 7.6 Hz), 132.0 (d, J = 2.3 Hz), 130.3 (d, J = 11.8 Hz), 129.6 (d, J = 4.5 Hz), 128.3 (d, J = 6.9 Hz), 128.2 (s), 125.22 (d, 1JCP = 45.8 Hz), 59.1 (s), 58.1 (d, J = 26.8 Hz), 38.2 (d, 1JCP = 23.5 Hz), 38.0 (d, 1JCP = 23.3 Hz), 31.0 (d, 2JCP = 6.4 Hz), 30.9 (d, 2JCP = 6.3 Hz), 25.6 (s). 31 1 P{ H} NMR (202.5 MHz, CD2Cl2): δ 66.4. Anal. Calcd (found) for C24H35AuF6OPSb: H, 4.39 (4.29); C, 35.89 (35.86).
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, figures, and CIF files giving general methods and characterization and isolation details for compounds 2a−4a and 1b−4b, experimental details regarding exchange kinetics and equilibrium binding studies, X-ray crystallographic data, slippage calculation instructions, and preliminary computational data. 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:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Acknowledgment is made to Wake Forest University for startup funds in addition to support from the Science Research Fund. We thank Dr. Marcus W. 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. Acknowledgement is made to Prof. Akbar Salam and Dr. David Chin for technical support in the use of Gaussian 03. Computations were performed on the Wake Forest University DEAC Cluster, a centrally managed resource with support provided in part by the University.
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REFERENCES
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dx.doi.org/10.1021/om300893q | Organometallics 2012, 31, 7332−7335
