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Synthesis of an Azaphosphatriptycene and Its Rhodium Carbonyl Complex Yu Cao,† Jonathan W. Napoline,†,§ John Bacsa,‡ Pamela Pollet,† Jake D. Soper,† and Joseph P. Sadighi*,† †

School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta Georgia 30322, United States

‡

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ABSTRACT: A 10-aza-9-phosphatriptycene is accessible on a gram scale, in three laboratory steps from commercially available precursors. Infrared spectroscopy of a rhodium(I) carbonyl complex bearing this ligand reflects the weak σ-donor/strong π-acceptor character of the phosphine; the solid-state structure reveals moderate steric demand. This ligand supports highly active catalysts for the hydroformylation of cyclic alkenes.

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Herein we report the preparation of an azaphosphatriptycene ligand via a straightforward sequence. This sequence affords the ligand reproducibly, on a gram scale. We have examined this phosphine as a supporting ligand for rhodiumcatalyzed hydroformylation, in which weak σ-donor and strong π-acceptor ligands have enabled remarkable advances in activity.9−21 Although the rigid, polycyclic phosphabarrelene framework has been found to be highly effective,22,23 phosphatriptycenes had not been explored in this context. A combination of this ligand with a suitable rhodium precatalyst gives rise to an isolable complex for spectroscopic and structural study or to a highly active catalyst system for hydroformylation of terminal and cyclic olefins. Scheme 1a shows the synthesis of 2,7,14-tri-tert-butyl-10aza-9-phosphatriptycene (1) from the corresponding tris(2bromoaryl)amine, prepared in two facile and high-yield steps24,25 from commercial precursors. Triple lithium− bromine exchange, as used to prepare tris(2-lithiophenyl)phosphine,4,5 proved effective in generating the tris(2lithiophenyl)amine intermediate. Crystalline, air-stable tris(2,4-di-tert-butylphenyl) phosphite proved more convenient than triphenyl phosphite as the electrophile and gave higher yields; we believe that increased steric demand favors closing of the bicyclic framework over intermolecular side reactions. Crystallization of 1 from methanol effectively removes the byproduct 2,4-di-tert-butylphenol and allows isolation of the air-stable product in 71% yield based on tris(2-bromoaryl)amine. The 31P NMR resonance for 1 appears at very high field, δ −77.0 ppm (CDCl3), similar to that of 10-aza-9phosphatriptycene.1 This value falls well upfield from those of

olycyclic phosphines have been the subject of considerable interest for their combination of structural rigidity and unusual electronic properties. Phosphatriptycenes, for example, possess a high fraction of s orbital character in the phosphorus lone pair as a result of their constrained C−P−C angles.1,2 The initial report on the first ligand in this class, 10-aza-9phosphatriptycene, noted its extremely high field 31P chemical shift of −80 ppm.1 A 9-phosphatriptycene variant displayed electronic character more similar to that of triphenylstibine than that of triphenylphosphine.3 Tsuji, Tamao, et al. synthesized a series of 9-phospha-10-silatriptycenes, studying a phosphine selenide and a platinum(II) complex to gain insight into their electronic properties.4 The difficulty of early phosphatriptycene syntheses may have limited their exploration in catalysis, but more recent approaches afford easier access. Efficient generation of tris(2lithiophenyl)phosphine enabled the synthesis of 10-sila-9phosphatriptycenes4,5 and, more recently, salts of the anionic 9-phosphatriptycene-10-phenylborate.6,7 Phosphatriptycenes have been examined as ligands in catalytic reactions such as the Stille coupling.3 More recently, a silica-tethered 10-sila-9phosphatriptycene supported the effective Suzuki−Miyaura coupling of chloroarenes.5 The parent 10-aza-9-phosphatriptycene was synthesized from tris(2-bromophenyl)amine,1 but more substituted tris(2-bromoaryl)amines are easier to prepare. Hellwinkel et al. synthesized the trimethyl-substituted 10-aza-9-phosphatriptycene from tri-p-tolylamine by a sequence of bromination, halogen−lithium exchange, and quenching with triphenyl phosphite.8 They reported difficulties in reproducing this route, however, and subsequently adapted Bickelhaupt’s elegant but synthetically demanding route for 9-phosphatriptycene. © XXXX American Chemical Society

Received: February 7, 2019

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DOI: 10.1021/acs.organomet.9b00081 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

95.00(13) to 95.14(13)°; those in [(Ph3P)Rh(acac)(CO)] range from 103.37(9) to 104.24(9)°. The effective cone angle for 1 in complex 2 is about 151°, somewhat larger than that of 145° for triphenylphosphine.29 The C−N−C angles range from 107.5(2) to 108.9(2)°. Combinations of 1 with the precatalyst [Rh(acac)(CO)2] were examined in the hydroformylation of 1-octene, for comparison with the well-studied triphenylphosphine.30 The results are summarized in Table S1. Ligand 1 proved comparable to triphenylphosphine in several respects. The key advantage of 1 was the high activity of the catalyst system formed in situ, at ligand:metal ratios lower than those preferred for triphenylphosphine.31 In light of the modest selectivity but high activity of the 1: [Rh] system, we examined the hydroformylation of less reactive substrates. The Ph3P/[Rh] catalyst system exhibits low activity in the hydroformylation of cyclic alkenes under mild conditions.31 In contrast, bulky π-acidic phosphite ligands enable high catalyst activity toward these substrates.12 Cyclohexene was examined first (Table 1). Because

Scheme 1. Synthesis of a 10-Aza-9-phosphatriptycene and Its Rh(I) Carbonyl Complex

a Conditions: THF/n-C5H12, −78 °C, 2 h. bConditions: Ar = 2,4-ditert-butylphenyl; THF/n-C5H12, −78 °C to room temperature, then reflux, 96 h. Yield for reactions 1 and 2: 71%. cConditions: PhCH3, room temperature, 20 min. Yield: 58%.

9-phosphatriptycene (δ −64.8 ppm),2 9-phospha-10-silatriptycenes (δ ca. −44 ppm),4,5 and the 9-phosphatriptycene-10phenylborate anion (also δ ca. −44 ppm).6,7 The rthodium(I) complex [(1)Rh(acac)(CO)] (2) was prepared by reaction of 1 with [Rh(acac)(CO)2] (Scheme 1b). The 31P NMR spectrum of 2 displays a doublet resonance at δ −1.3 ppm, with 1JP−Rh = 189 Hz. For comparison, the corresponding triphenylphosphine complex gives δ +48.6 ppm and 1JP−Rh = 177 Hz.26 The infrared stretching frequency νCO, reflecting the σ-donor and π-acceptor character of supporting ligands,27 is 1985 cm−1 for complex 2. This value falls between those of 1975 cm−1 for [(Ph3P)Rh(acac)(CO)] and 2006 cm−1 for {[(PhO)3P]Rh(acac)(CO)}.28 The solid-state structure of 2 was determined by singlecrystal X-ray diffraction (Figure 1). Complex 2 is planar with a Rh−P bond distance of 2.2080(8) Å, slightly shorter than that of 2.2418(9) Å in [(Ph3P)Rh(acac)(CO)].26 Within the bound phosphatriptycene the C−P−C angles range from

Table 1. Hydroformylation of Cyclohexenea

entry

reaction time (h)

reaction temp (°C)

pressure (bar)

aldehyde yield (%)

aldehyde TOF (h−1)

1 2b 3 4 5

2 2 2 2 4

100 100 80 100 100

20 20 20 15 20

85 73 49 64 100

2130 1830 1230 1610 1250

a

Conditions unless specified otherwise: [Rh(acac)(CO)2] 0.020 mol %; ligand 1 0.040 mol %; [cyclohexene]initial = 0.86 M. bConditions: [cyclohexene]initial = 0.56 M.

addition/elimination gives no change, and no alcohol or alkane byproducts were detected using this system, alkene conversion and the yield of aldehyde are identical. Quantitative conversion was observed using 0.020 mol % [Rh] precatalyst and 0.040 mol % 1 (entry 5). Next we examined the reactivity of this catalyst system toward electron-rich heteroatom-substituted alkenes. Formyl furan and formyl pyran derivatives, for example, are relevant to natural product synthesis.13 Results for the hydroformylation of 2,3-dihydrofuran and 3,4-dihydro-2H-pyran are shown in Scheme 2. The conversion of 2,3-dihydrofuran to a mixture of the 2and 3-formyl derivatives proceeded under mild conditions. The selectivity for 2-formyltetrahydrofuran, while moderate, was higher than that of the Ph3P/[Rh] system under comparable conditions.13 The hydroformylation of 3,4-dihydro-2H-pyran proceeded to lower conversion under similar conditions, perhaps due to greater hindrance about the CC bond and the absence of ring strain,33 and displayed no significant preference for functionalization at the 2- or 3-positions. In conclusion, a functionalized azaphosphatriptycene has been synthesized by a concise route from inexpensive precursors. The broad synthetic accessibility of triarylamines34−36 should permit considerable variation on this framework. Consistent with its expected weak σ basicity and strong π acidity, this ligand combines with rhodium to give

Figure 1. Solid-state structure of [(1)Rh(acac)(CO)], with ellipsoids at 50% probability. Calculated hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh1−P1 2.2080(8), Rh1−C1 1.821(3), Rh1−O2 2.067(2), Rh1−O3 2.044(2), C1−O1 1.142(4); P1−Rh1−CO 86.43(10), C2−P1−C13 95.00(13), C2−P1−C22 95.14(13), C13−P1−C22 95.13(13).32 B

DOI: 10.1021/acs.organomet.9b00081 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics Scheme 2. Hydroformylation of Cyclic Enol Ethersa

(3) Agou, T.; Kobayashi, J.; Kawashima, T. Evaluation of sigmadonating ability of a 9-phosphatriptycene and its application to catalytic reactions. Chem. Lett. 2004, 33, 1028−1029. (4) Tsuji, H.; Inoue, T.; Kaneta, Y.; Sase, S.; Kawachi, A.; Tamao, K. Synthesis, Structure, and Properties of 9-Phospha-10-silatriptycenes and their Derivatives. Organometallics 2006, 25, 6142−6148. (5) Iwai, T.; Konishi, S.; Miyazaki, T.; Kawamorita, S.; Yokokawa, N.; Ohmiya, H.; Sawamura, M. Silica-Supported Triptycene-Type Phosphine. Synthesis, Characterization, and Application to PdCatalyzed Suzuki−Miyaura Cross-Coupling of Chloroarenes. ACS Catal. 2015, 5, 7254−7264. (6) Drover, M. W.; Nagata, K.; Peters, J. C. Fusing triphenylphosphine with tetraphenylborate: introducing the 9-phosphatriptycene10-phenylborate (PTB) anion. Chem. Commun. 2018, 54, 7916− 7919. (7) Konishi, S.; Iwai, T.; Sawamura, M. Synthesis, Properties, and Catalytic Application of a Triptycene-Type Borate-Phosphine Ligand. Organometallics 2018, 37, 1876−1883. (8) Hellwinkel, D.; Schenk, W.; Blaicher, W. Heterotriptycenes; structural calculations and NMR relations. Chem. Ber. 1978, 111, 1798−1814. (9) Trzeciak, A. M.; Ziółkowski, J. J. Perspectives of rhodium organometallic catalysis. Fundamental and applied aspects of hydroformylation. Coord. Chem. Rev. 1999, 190−192, 883−900. (10) Franke, R.; Selent, D.; Börner, A. Applied Hydroformylation. Chem. Rev. 2012, 112, 5675−5732. (11) Pruett, R. L.; Smith, J. A. Low-pressure system for producing normal aldehydes by hydroformylation of.alpha.-olefins. J. Org. Chem. 1969, 34, 327−330. (12) van Leeuwen, P. W. N. M.; Roobeek, C. F. Hydroformylation of less reactive olefins with modified rhodium catalysts. J. Organomet. Chem. 1983, 258, 343−350. (13) Polo, A.; Real, J.; Claver, C.; Castillón, S.; Bayón, J. C. Lowpressure selective hydroformylation of 2,3- and 2,5-dihydrofuran with a rhodium catalyst. Unexpected influence of the auxiliary ligand tris(ot-butylphenyl) phosphite. J. Chem. Soc., Chem. Commun. 1990, 600− 601. (14) van Rooy, A.; Orij, E. N.; Kamer, P. C. J.; van den Aardweg, F.; van Leeuwen, P. W. N. M. Hydroformylation of oct-1-ene with extremely high rates using rhodium catalysts containing bulky phosphites. J. Chem. Soc., Chem. Commun. 1991, 1096−1097. (15) Jongsma, T.; Challa, G.; van Leeuwen, P. W. N. M. A mechanistic study of rhodium tri(o-t-butylphenyl)phosphite complexes as hydroformylation catalysts. J. Organomet. Chem. 1991, 421, 121−128. (16) van Rooy, A.; Orij, E. N.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Hydroformylation with a Rhodium/Bulky Phosphite Modified Catalyst. Comparison of the Catalyst Behavior for Oct-1-ene, Cyclohexene, and Styrene. Organometallics 1995, 14, 34−43. (17) Fernández, E.; Ruiz, A.; Claver, C.; Castillón, S.; Polo, A.; Piniella, J. F.; Alvarez-Larena, A. Regio- and Stereoselective Hydroformylation of Glucal Derivatives with Rhodium Catalysts. Organometallics 1998, 17, 2857−2864. (18) Selent, D.; Wiese, K.-D.; Röttger, D.; Börner, A. Novel Oxyfunctionalized Phosphonite Ligands for the Hydroformylation of Isomeric n-Olefins. Angew. Chem., Int. Ed. 2000, 39, 1639−1641. (19) Breit, B. Highly regioselective hydroformylation under mild conditions with new classes of π-acceptor ligands. Chem. Commun. 1996, 2071−2072. (20) Breit, B.; Winde, R.; Harms, K. Phosphabenzene−rhodium catalysts for the efficient hydroformylation of terminal and internal olefins. J. Chem. Soc., Perkin Trans. 1 1997, 1, 2681−2682. (21) Breit, B.; Winde, R.; Mackewitz, T.; Paciello, R.; Harms, K. Phosphabenzenes as Monodentate π-Acceptor Ligands for RhodiumCatalyzed Hydroformylation. Chem. - Eur. J. 2001, 7, 3106−3121. (22) Breit, B.; Fuchs, E. Phosphabarrelene-rhodium complexes as highly active catalysts for isomerization free hydroformylation of internal alkenes. Chem. Commun. 2004, 694−695.

a

Conditions: [Rh(acac)(CO)2] 0.050 mol %, ligand 1 0.10 mol %; [enol ether]initial = 1.2 M. bReaction proceeded to 70% completion. Yields were determined by GC with respect to internal standard.

active catalyst systems for the hydroformylation of internal cyclic alkenes, including cyclic enol ethers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00081. Experimental procedures and spectral and crystallographic data (PDF) Accession Codes

CCDC 1857239 contains 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 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 for J.P.S.: [email protected]. ORCID

Jake D. Soper: 0000-0002-1961-8076 Joseph P. Sadighi: 0000-0003-1304-1170 Present Address §

Department of Chemistry, Saint Anselm College, Manchester, New Hampshire 03102, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the American Chemical Society Petroleum Research Fund (Award Number 54739-ND3) for generous support of this project and Peach State Laboratories, Inc. for support to J.W.N. Profs. Charles L. Liotta and Christopher W. Jones kindly allowed us the use of their groups’ Parr reactors and gas chromatographs, and Dr. Robert A. Braga allowed the use of an FT-IR spectrometer. We thank them and Prof. E. Kent Barefield for helpful discussions.

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REFERENCES

(1) Hellwinkel, D.; Schenk, W. Azaphosphatriptycene. Angew. Chem., Int. Ed. Engl. 1969, 8, 987−988. (2) Jongsma, C.; de Kleijn, J. P.; Bickelhaupt, F. Phosphatriptycene. Tetrahedron 1974, 30, 3465−3469. C

DOI: 10.1021/acs.organomet.9b00081 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (23) Fuchs, E.; Keller, M.; Breit, B. Phosphabarrelenes as Ligands in Rhodium-Catalyzed Hydroformylation of Internal Alkenes Essentially Free of Alkene Isomerization. Chem. - Eur. J. 2006, 12, 6930−6939. (24) Michinobu, T.; Tsuchida, E.; Nishide, H. 3,4′-Bis[bis(t-butyland methoxy-phenyl)amino]stilbene Bis(cation radical)s and Their Electrochemical and Magnetic Properties. Bull. Chem. Soc. Jpn. 2000, 73, 1021−1027. (25) Talipov, M. R.; Hossain, M. M.; Boddeda, A.; Thakur, K.; Rathore, R. A search for blues brothers: X-ray crystallographic/ spectroscopic characterization of the tetraarylbenzidine cation radical as a product of aging of solid magic blue. Org. Biomol. Chem. 2016, 14, 2961−2968. (26) Brink, A.; Roodt, A.; Steyl, G.; Visser, H. G. Steric vs. electronic anomaly observed from iodomethane oxidative addition to tertiary phosphine modified rhodium(I) acetylacetonato complexes following progressive phenyl replacement by cyclohexyl [PR3 = PPh3, PPh2Cy, PPhCy2 and PCy3]. Dalton Trans 2010, 39, 5572−5578. (27) de Montauzon, D.; Poilblanc, R. Electrochemical investigation of coordination compounds: I. The oxidationreduction mechanism of CoI, RhI and IrI systems containing trivalent phosphorus and carbonyl ligands. J. Organomet. Chem. 1975, 93, 397−404. (28) Trzeciak, A. M.; Ziółkowski, J. J. Infrared and NMR, 1H, 19F, 31 P studies of Rh(I) complexes of the formula:[Rh(β-diketone)(CO)X(P)Y](x= 0, 1, 2; y= 0, 1, 2; x+ y = 2; P= PPh3 or P(OP) 3). Inorg. Chim. Acta 1985, 96, 15−20. (29) Tolman, C. A. Phosphorus ligand exchange equilibriums on zerovalent nickel. A Dominant role for steric effects. J. Am. Chem. Soc. 1970, 92, 2956−2965. (30) Evans, D.; Osborn, J. A.; Wilkinson, G. Hydroformylation of alkenes by use of rhodium complex catalysts. J. Chem. Soc. A 1968, 3133−3142. (31) Brown, C. K.; Wilkinson, G. Homogeneous hydroformylation of alkenes with hydridocarbonyltris-(triphenylphosphine)rhodium(I) as catalyst. J. Chem. Soc. A 1970, 2753−2764. (32) The numbering scheme is different from the organic numbering system for triptycenes. These numbers are used for consistency with the CIF. (33) Polo, A.; Claver, C.; Castillón, S.; Ruiz, A.; Bayón, J. C.; Real, J.; Mealli, C.; Masi, D. Regioselective hydroformylation of cyclic vinyl and allyl ethers with rhodium catalysts. Crucial influence of the size of the phosphorus cocatalyst. Organometallics 1992, 11, 3525−3533. (34) Yamamoto, T.; Nishiyama, M.; Koie, Y. Palladium-catalyzed synthesis of triarylamines from aryl halides and diarylamines. Tetrahedron Lett. 1998, 39, 2367−2370. (35) Surry, D. S.; Buchwald, S. L. Selective Palladium-Catalyzed Arylation of Ammonia: Synthesis of Anilines as Well as Symmetrical and Unsymmetrical Di- and Triarylamines. J. Am. Chem. Soc. 2007, 129, 10354−10355. (36) Hoi, K. H. C.; Coggan, J. A.; Organ, M. G. Pd-PEPPSI-IPentCl: An Effective Catalyst for the Preparation of Triarylamines. Chem. Eur. J. 2013, 19, 843−845.

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DOI: 10.1021/acs.organomet.9b00081 Organometallics XXXX, XXX, XXX−XXX