Letter Cite This: Org. Lett. 2018, 20, 7965−7968
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Diphosphination of 1,3-Dienes with Diphosphines under VisibleLight-Promoted Photoredox Catalysis Nobutaka Otomura, Koji Hirano,* and Masahiro Miura* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
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S Supporting Information *
ABSTRACT: A diphosphination of 1,3-dienes with tetraaryldiphosphines proceeds under Ir(ppy)3-promoted photoredox catalysis to form the corresponding 1,4-diphosphino-2-butenes in good yields with good regioselectivity. The key to success is the addition of a Br+ additive. Subsequent double bond hydrogenation successfully delivers the 1,4-bis(diphenylphosphino)butane (DPPB) derivatives with uniquely large bite angles. Thus, the present method can provide a facile access to DPPB-type ligands, which are of great importance in transition metal catalysis, from readily available diene substrates.
B
straightforward manner, but there is only one successful example in the literature, where the parent 1,3-butadiene reacted with especially reactive tetramethyldiphosphine under harsh conditions (neat, >100 °C).11 Thus, the development of new protocols to accommodate more versatile diene and phosphine substrates is greatly appealing. Herein, we report a bromine cation initiated diphosphination of 1,3-dienes with tetraaryldiphosphines (Ar2P−PAr2) under Ir(ppy)3-promoted photoredox catalysis. The reaction proceeds smoothly under mild conditions (2.4 W blue LED irradiation and ambient temperature) with good regioselectivity. The successive hydrogenation of obtained diphosphinated compounds readily affords the corresponding DPPB-type bidentate ligands. Our optimization studies commenced with isoprene (1a) and tetraphenyldiphosphine (Ph2P−PPh2; 2a) as model substrates (Table 1). On the basis of our previous work,9c treatment of 1a with 2a in the presence of Ir(ppy) 3 photocatalyst (PC) and Br+ additive, NBS, under blue LED irradiation (2.4 W) was followed by S8 quenching to furnish the desired 1,4-diphosphinated product 3aa-S in 84% 1H NMR yield (65% isolated yield) with a 94:6 E/Z ratio (entry 1). The corresponding 1,2-diphosphinated product was not detected at all. Notably, the reaction was rapid and completed within just 10 min. Given that the role of NBS generates the truly active Br−PPh2 species in situ,9c we then tested the direct addition of Br−PPh2 instead of NBS. Pleasingly, the yield of 3aa-S increased to 96% 1H NMR yield (87% isolated yield; entry 2). Although other halogen cation sources were also investigated (entries 3−9), NBS proved to be optimal from the viewpoint
isphosphines are indispensable ancillary ligands in transition metal catalysis because they can greatly affect the activity and selectivity in the catalytic reactions.1 Particularly, the bite angle of chelating bisphosphine ligands is a pivotal factor to determine their electronic as well as steric nature.2 Thus, the rapid and concise preparation of bisphosphine ligands with both small and large bite angles is an important research subject in synthetic community. In this context, the diphosphination across C−C unsaturated bonds has recently received significant attention as the straightforward access to the above bisphosphines from relatively simple starting substrates.3 The research groups of Oshima/ Yorimitsu4 and Ogawa,5 independently, developed the radical diphosphination of alkynes with diphosphines (R2P−PR2) under typical radical conditions. Subsequent double bond reduction can provide the corresponding 1,2-bis(diphenylphosphino)ethanes (DPPEs) efficiently. More recently, Ogawa reported the radical phosphinylphosphination of more robust and simple alkenes with unsymmetrical diphosphines (R2(O)P−PR2).6 The catalytic double hydrophosphination of alkynes is also a good alternative to prepare the DPPE structure.7 Our group also recently focused on the organophosphorus chemistry and succeeded in the diphosphination reactions of some alkynes8 and alkenes9 with diphosphines or silylphosphines under transition-metal-catalyzed or visiblelight-promoted photoredox conditions. The aforementioned advances recently appeared in the diphosphination reaction leading to DPPEs with relatively small bite angles;10 however, the related synthesis of 1,4-bis(diphenylphosphino)butanes (DPPBs) with larger bite angles still remains largely elusive. Theoretically, the regioselective diphosphination of 1,3-dienes with diphosphines can form the DPPB-type skeletons in a © 2018 American Chemical Society
Received: November 5, 2018 Published: December 6, 2018 7965
DOI: 10.1021/acs.orglett.8b03534 Org. Lett. 2018, 20, 7965−7968
Letter
Organic Letters Table 1. Optimization for Diphosphination of Isoprene (1a) with Tetraphenyldiphosphine (2a) under Visible-LightPromoted Photoredox Catalysisa
entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15g 16 17h 18i
PC
additive
Ir(ppy)3
NBS
Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)3 Ir(ppy)2(dtbpy)PF6 [Ir{dFCF3ppy}2(bpy)]PF6 Ru(bpy)3(PF6)2 Eosin Yf Ir(ppy)3 Ir(ppy)3 none none Ir(ppy)3
BrPPh2 Br1 Br2 Br3e Br4 NIS NCS NFSI NBS NBS NBS NBS none NBS NBS NBS BrPPh2
Scheme 1. Diphosphination of Various 1,3-Dienes 1 with Tetraphenyldiphosphine (2a) under Visible-LightPromoted Photoredox Catalysisa
yield (%),b E/Zc 84 (65), 94:6 91 (79), 89:11d 96 (87), 95:5 59, 87:13 85, 95:5 81, 94:6 61, 93:7 4, n.d. 0, n.d. 20, 54:46 44, 95:5 75, 95:5 14, 99:1 24, 99:1 0, n.d. 12, 99:1 15, 99:1 12, 99:1 0, n.d.
a
Reaction conditions: 1 (0.50 mmol), 2a (0.25 mmol), Ir(ppy)3 (0.0013 mmol), NBS or BrPPh2 (0.050 mmol), DCE (1.5 mL), 2−4 h, N2, blue LED irradiation (2.4 W), ambient temp then S8 (0.75 mmol of S atom), 30 min, N2. Isolated yields are shown. The Br+ source used (NBS or BrPPh2) is indicated in parentheses. b31P NMR yield. cWith 1f (0.75 mmol) and BrPPh2 (0.10 mmol). dWith 1j (0.75 mmol, (E,E)/(E,Z)/(Z,Z) = 71:23:6) and BrPPh2 (0.10 mmol).
a
Reaction conditions: 1a (0.50 mmol), 2a (0.25 mmol), PC (0.0013 mmol), additive (0.050 mmol), DCE (1.5 mL), 10 min, N2, blue LED irradiation (2.4 W), ambient temp then S8 (0.75 mmol of S atom), 30 min, N2. b1H NMR yield. Isolated yield in parentheses. cDetermined by 31P NMR of the crude mixture. d1.0 mmol scale, 2 h. eWith 0.025 mmol of Br3. fGreen LED irradiation. gIn dark. hUV irradiation (254 nm). iWith 0.50 mmol of BrPPh2 instead of 2a.
(1c, e−g), and 2,3-disubstituted (1d, h, and i) terminal dienes underwent the regioselective 1,4-diphosphination efficiently, and the corresponding DPPB-type derivatives 3ba-S−3ia-S resulted in good yields. The best additive (NBS or Br−PPh2) was dependent on the diene substrate, but Br−PPh2 generally showed better reactivity. Notably, 2-aryl-1,3-butadienes afforded the corresponding Z-isomers preferably, which is more attractive for the ligand application (3ea-S−3ga-S). Gratifyingly, 1,2-dimethylenecycloheptane (1h) and 2,3diphenyl-1,3-butadiene (1i) also gave the Z-1,4-diphosphinated products 3ha-S and 3ia-S selectively. The high 1,4regioselectivity was observed also in the reaction of internal 2,4-hexadiene (1j): the syn/anti ratio was poor, but the corresponding 1,4-adduct 3ja-S was successfully formed. On the other hand, other less substituted internal dienes and cyclic diene gave a mixture of 1,2- and 1,4-diphosphinated products (Scheme 2): 1,3-pentadiene (1k) produced separable 1,2- and 1,4-adducts (3ka-S and 3ka′-S) in a 2:1 ratio. Similar results were obtained in the reactions of 3-methyl-1,3-pentadiene (1l) and 1,3-cyclohexadiene (1m).12 The salient feature of photoredox catalysis is its accommodation of functionalized diphosphines 2, which can be easily prepared from the corresponding hydrophosphines and chlorophosphines. Thus, the electron-donating and electronwithdrawing DPPB-type ligands 3hb-S and 3hc-S were readily available from the simple diene (Figure 1).13 The obtained bisphosphine sulfide 3aa-S was readily reduced to the corresponding bisphosphine 3aa by using
of reaction efficiency, availability, and cost. As the photosensitizer, Ir(ppy)3 showed the highest performance of those tested (entries 10−13). We also confirmed that NBS, blue LED irradiation, and Ir(ppy)3 all were essential for a satisfactory yield (entries 14−16). The simple UV irradiation also resulted in the poor reaction efficiency (entry 17). The replacement of Ph2P−PPh2 (2a) with a stoichiometric amount of Br−PPh2 gave no detectable amount of diphosphinated product (entry 18). Additionally, the reaction could be easily conducted on a 1.0 mmol scale, thus indicating the good reliability and practicality of the present protocol (entry 1). With the conditions in entries 1 and 2 in Table 1, we examined the scope and limitation of 1,3-dienes with tetraphenyldiphosphine (2a) (Scheme 1). In addition to isoprene (1a), the parent butadiene (1b) and 2-substituted 7966
DOI: 10.1021/acs.orglett.8b03534 Org. Lett. 2018, 20, 7965−7968
Letter
Organic Letters Scheme 2. Diphosphination of Some Internal 1,3-Dienes 1 with Tetraphenyldiphosphine (2a) under Visible-LightPromoted Photoredox Catalysis
Scheme 4. Plausible Mechanism
initially generate Br−PPh2, which was confirmed by 31P NMR analysis.9c A single-electron transfer from the excited Ir(III)* to Br−PPh2 is followed by fragmentation to form the phosphinyl radical 5. Actually, Br−PPh2 efficiently quenched the luminescence of Ir(ppy)3.9c The regioselective radical addition to 1a at the terminal position (5 to 6) and subsequent back electron transfer to Ir(IV) species completes the Ir catalytic cycle (6 to 7). A concurrently formed allylic cation 7 is trapped with the diphosphine 2a preferably at the more sterically accessible terminal position, thus leading to the observed high 1,4-regioselectivity (7 to 8).18 The sterically less favored 1,2-regioisomer 8′ is not formed, but in cases of internal dienes 1k−1m, the 1,4- and 1,2-additions are sterically competitive to give a mixture of regioisomers (Scheme 2); the construction of an adjacent phosphinated C−C bond (1,2regioisomer; 3ka′-S−3ma′-S) is generally sterically unfavored, but the formation of P−C bond at the sterically hindered internal carbon (1,4-regioisomer; 3ka-S−3ma-S) is also not so preferable. The resulting phosphonium cation 8 directly enters the second catalytic cycle through the single-electron transfer, giving the phosphinyl radical 5 along with the release of desired product 3aa.19 At present, we cannot completely exclude the possibility of a radical chain mechanism, but a light ON/OFF experiment20 and unsuccessful result with the simple UV irradiation (Table 1, entry 17) can support the aforementioned Ir(III)-promoted photoredox cycle.21 In conclusion, we have developed a regioselective diphosphination of 1,3-dienes with diphosphines under visible-light-promoted photoredox catalysis. The present strategy accommodates versatile diene substrates to form the corresponding 1,4-diphosphinated products, namely, DPPBtype ligands in good yields under mild conditions. Thus, the relatively simple hydrocarbons can be readily transformed into the DPPB-type bidentate ligands with large bite angles, which are of potent interest in the development of new transition metal catalysts. Further elucidation of the reaction mechanism and application to phosphination of additional simple hydrocarbon materials are currently under investigation in our laboratory.
Figure 1. Structure and yields of 1,4-diphosphinated products 3-S from 1h and substituted tetraaryldiphosphines 2. The reaction conditions were slightly modified by using 40 mol % of NBS.
Schwartz reagent, Cp2Zr(H)Cl (Scheme 3a).14 Moreover, 3aa could also be directly isolated without S8 quenching under Scheme 3
otherwise identical conditions. Additionally notable is the successful hydrogenation of 3aa-S and 3aa-O15 in the presence of Crabtree’s catalyst,16 [Ir(cod)(PCy3)(Py)]PF6 (Scheme 3b): the saturated DPPB-type backbone 4 can be readily constructed in good overall yields from the simple isoprene. On the basis of our findings and literature information,17 we are tempted to propose that the mechanism of reaction of isoprene (1a) with tetraphenyldiphosphine (2a) is as follows (Scheme 4). The diphosphine 2a partially reacts with NBS to 7967
DOI: 10.1021/acs.orglett.8b03534 Org. Lett. 2018, 20, 7965−7968
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Organic Letters
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(8) (a) Okugawa, Y.; Hirano, K.; Miura, M. Org. Lett. 2017, 19, 2973. (b) Okugawa, Y.; Hayashi, Y.; Kawauchi, K.; Hirano, K.; Miura, M. Org. Lett. 2018, 20, 3670. (9) (a) Okugawa, Y.; Hirano, K.; Miura, M. Angew. Chem., Int. Ed. 2016, 55, 13558. (b) Otomura, N.; Okugawa, Y.; Hirano, K.; Miura, M. Org. Lett. 2017, 19, 4802. (c) Otomura, N.; Okugawa, Y.; Hirano, K.; Miura, M. Synthesis 2018, 50, 3402. (10) For additional examples using highly reactive PCl3, Cl2P−PCl2, F2P−PF2, or Me2P−PMe2 under harsh conditions, see: (a) Chatt, J.; Hussain, W.; Leigh, G. J.; Ali, H. M.; Picket, C. J.; Rankin, D. A. J. Chem. Soc., Dalton Trans. 1985, 1131. (b) Burg, A. B. J. Am. Chem. Soc. 1961, 83, 2226. (c) Drieß, M.; Haiber, G. Z. Anorg. Allg. Chem. 1993, 619, 215. (d) Morse, K. W.; Morse, J. G. J. Am. Chem. Soc. 1973, 95, 8469. While somewhat unique, some highly activated substrates such as acrylate and propiolate also undergo the bisphosphination: (e) Dodds, D. L.; Haddow, M. F.; Orpen, A. G.; Pringle, P. G.; Woodward, G. Organometallics 2006, 25, 5937. (f) Burck, S.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2007, 46, 2919. (g) Hajdók, I.; Lissner, F.; Nieger, M.; Strobel, S.; Gudat, D. Organometallics 2009, 28, 1644. (11) Hewertson, W.; Taylor, I. C. J. Chem. Soc. C 1970, 1990. (12) The E/Z stereochemistry of 3aa-S and 3ea-S were assigned by NOESY analysis while the Z-stereochemistry of 3ia-S was determined after the derivatization. The stereochemistry of other products are assigned by analogy (see the Supporting Information for details). On the other hand, the relative stereochemistry (syn/anti) of 3ma′-S was confirmed by X-ray analysis (CCDC 1881728), and those of 3ja-S and 3ma-S are thus assigned by analogy. Attempts to apply 1-phenyl1,3-butadiene and ethyl sorbate remained unsuccessful under the present conditions. (13) Unfortunately, tetracyclohexyldiphosphine (Cy2P−PCy2) did not give any detectable amount of phosphinated products. (14) (a) Zablocka, M.; Delest, B.; Igau, A.; Skowronska, A.; Majoral, J.-P. Tetrahedron Lett. 1997, 38, 5997. (b) Saito, M.; Nishibayashi, Y.; Uemura, S. Organometallics 2004, 23, 4012. Also, see ref 9a. (15) With H2O2 aq. instead of S8 at the quenching step, the bisphosphine oxide 3aa-O was readily prepared in 84% yield under otherwise identical conditions. (16) Crabtree, R. Acc. Chem. Res. 1979, 12, 331. We also note that neither 3aa-S nor 3aa-O underwent the hydrogenation by Pd/C or Pd(OH)2/C catalyst even under forcing conditions (refluxing MeOH). (17) Selected reviews on visible-light-promoted photoredox catalysis: (a) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527. (b) Narayanam, J. M. R.; Stephenson, C. R. Chem. Soc. Rev. 2011, 40, 102. (c) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (d) Ravelli, D.; Protti, S.; Fagnoni, M. Chem. Rev. 2016, 116, 9850. (e) Koike, T.; Akita, M. Acc. Chem. Res. 2016, 49, 1937. Recent advances in organophosphorus chemistry under photoredox catalysis: (f) Luo, K.; Yang, W.-C.; Wu, L. Asian J. Org. Chem. 2017, 6, 350. (18) The product E/Z ratio may also be kinetically determined in this step due to the steric factors. We confirmed that the E/Z ratio did not change during the course of the reaction with 1a and 1f. (19) For recent examples of radical generation from phosphoniums under visible-light-promoted photoredox catalysis, see: (a) Lin, Q.-Y.; Xu, X.-H.; Zhang, K.; Qing, F.-L. Angew. Chem., Int. Ed. 2016, 55, 1479. (b) Miura, M.; Funakoshi, Y.; Nakahashi, J.; Moriyama, D.; Murakami, M. Angew. Chem., Int. Ed. 2018, 57, 15455. (20) See the Supporting Information for details. (21) A energy transfer mechanism is also plausible, but the performance of photocatalysts observed in Table 1 seems to be less dependent on the triplet energy. (a) Arias-Rotondo, D. M.; McCusker, J. K. Chem. Soc. Rev. 2016, 45, 5803. (b) Teegardin, K.; Day, J. I.; Chan, J.; Weaver, J. Org. Process Res. Dev. 2016, 20, 1156.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03534. 1
H, 13C{1H}, (PDF)
19
F{1H}, and
31
P{1H} NMR spectra
Accession Codes
CCDC 1881728 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 Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Koji Hirano: 0000-0001-9752-1985 Masahiro Miura: 0000-0001-8288-6439 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP 15H05485 (Grant-in-Aid for Young Scientists (A)) and 18K19078 (Grant-in-Aid for Challenging Research (Exploratory)) to K.H. and JP 17H06092 (Grant-in-Aid for Specially Promoted Research) to M.M. We thank Dr. Yuji Nishii (Osaka University) for his assistance in obtaining X-ray diffraction data.
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REFERENCES
(1) (a) Metal-Catalyzed Cross-Coupling Reactions; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004. (b) Knowles, W. S. Angew. Chem., Int. Ed. 2002, 41, 1998. (c) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008. (2) (a) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299. (b) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100, 2741. (c) Freixa, Z.; van Leeuwen, P. W. N. M. Dalton Trans. 2003, 1890. (3) For a recent review, see: Hirano, K.; Miura, M. Tetrahedron Lett. 2017, 58, 4317. (4) Sato, A.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2005, 44, 1694. (5) Kawaguchi, S.-i.; Nagata, S.; Shirai, T.; Tsuchii, K.; Nomoto, A.; Ogawa, A. Tetrahedron Lett. 2006, 47, 3919. (6) Sato, Y.; Kawaguchi, S.-i.; Nomoto, A.; Ogawa, A. Angew. Chem., Int. Ed. 2016, 55, 9700. (7) (a) Kamitani, M.; Itazaki, M.; Tamiya, C.; Nakazawa, H. J. Am. Chem. Soc. 2012, 134, 11932. (b) Di Giuseppe, A.; De Luca, R.; Castarlenas, R.; Pérez-Torrente, J. J.; Crucianelli, M.; Oro, L. A. Chem. Commun. 2016, 52, 5554. (c) Yuan, J.; Zhu, L.; Zhang, J.; Li, J.; Cui, C. Organometallics 2017, 36, 455. (d) Bookham, J. L.; McFarlane, W.; Thornton-Pett, M.; Jones, S. J. Chem. Soc., Dalton Trans. 1990, 3621. While somewhat limited in scope, KO-t-Bu is also known to promote the double hydrophosphination; see: (e) Bookham, J. L.; Smithies, D. M.; Wright, A.; Thornton-Pett, M.; McFarlane, M. J. Chem. Soc., Dalton Trans. 1998, 811.
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NOTE ADDED AFTER ASAP PUBLICATION Table 1 entry 2 was re-inserted on December 7, 2018. 7968
DOI: 10.1021/acs.orglett.8b03534 Org. Lett. 2018, 20, 7965−7968
