Article pubs.acs.org/Organometallics
C‑Trifluoromethyl-Substituted 1,2-Oxaphosphetane Complexes: Synthetic and Structural Study Andreas W. Kyri, Gregor Schnakenburg, and Rainer Streubel* Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Straße 1, 53121 Bonn, Germany S Supporting Information *
ABSTRACT: Recently, the first synthetic route to 1,2-oxaphosphetane complexes was described, but the formation of too many isomers was a clear drawback and hampered further studies. Herein, we present significant advances with this problem using trifluoromethyl epoxide (4), 1,1′-bis(trifluoromethyl) epoxide (5), and differently substituted Li/Cl phosphinidenoid complexes 1−3 (R = CH(SiMe3)2, CPh3, C5Me5), thus giving 1,2-oxaphosphetane complexes 6−10 with high selectivity.
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INTRODUCTION While 1,2σ5λ5-oxaphosphetanes I (Scheme 1) are well-known intermediates in the Wittig reaction,1 their P(III) analogues II
Scheme 2. Synthetic Protocol for 1,2-Oxaphosphetane Complexes using Li/Cl Phosphinidenoid Complexes
Scheme 1. 1,2σ5λ5-Oxaphosphetanes I, 1,2σ3λ3Oxaphosphetanes II, and 1,2σ3λ3-Oxaphosphetane Complexes III
remained unknown for a long time. Similarly, 1,2-oxaphosphetane complexes III were unknown, and only one attempt was described, proposing a 1,2-oxaphosphetane complex as a reactive intermediate, but leading finally to a mixture of a phosphirane and a dioxaphospholane complex.2 In recent years, we demonstrated that Li/Cl phosphinidenoid complexes are versatile building blocks leading to various new ring systems with aldehydes,3 ketones,4 imines,5 or alkynes.3 However, also 1,1′-bifunctional acyclic complexes were obtained in good yields and with high selectivity by formal insertion into the O−H bond of alcohols.6,7 This versatile nucleophilic building block also enabled access to 1,2oxaphosphetane complexes VIa−c via formal insertion into a C−O bond of the epoxides, displaying additionally high group tolerance (Scheme 2).8 It is known that epoxides react preferentially at the least hindered ring carbon atom,9 leading most often to high regioselectivities. Somehow similarly, 1,2-oxaphosphetane complexes VIa,b were formed regioselectively, but instead of two four different isomers were observed in the case of the PCH(SiMe3)2 derivative due to atropisomerism at the exocyclic P−C bond.10 In the case of styrene oxide a different regioisomer was obtained, thus disfavoring the least hindered © XXXX American Chemical Society
ring carbon atom and favoring a reaction at the benzylic position; the regiochemistry of VIc was again unequivocally established by single-crystal X-ray diffraction studies.8 As questions concerning regio- and stereoselectivity remained, we decided to examine this aspect. Furthermore, we describe a stepwise approach to obtain isomerically pure 1,2-oxaphosphetane complexes. In addition, the quest of steric tolerance of this epoxide ring expansion method is tackled for the first time.
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RESULTS AND DISCUSSION In order to achieve enhanced selectivity, we decided to examine the effect of sterically demanding substituents at the epoxide. Therefore, tert-butyl epoxide was employed, but this reaction utterly failed and only E and Z diphosphene complexes, known as self-condensation products of the Li/Cl phosphinidenoid complex 1, were observed.3,11 To overcome this decreased reactivity of the epoxide, while preserving a steric demand, we used the trifluoromethyl epoxide 7 and 1, under otherwise identical conditions (Scheme 3). Received: November 30, 2015
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DOI: 10.1021/acs.organomet.5b00974 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 3. Synthesis of 1,2-Oxaphosphetane Complexes using Li/Cl Phosphinidenoid Complexes (solv. = Et2O, THF)
Figure 1. Molecular structure of complex 9 in the crystal. Ellipsoids are set at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected bond lengths in Å and angles in deg: P−W 2.4672(11), P−O1 1.687(3), P−C2 1.858(4), P−C4 1.802(4), C2− P−O1 78.83(17), folding angle 1.9.
The increased steric hindrance of 7 (in comparison to propylene oxide) led only to a small change of the isomeric ratio and four isomers of 9 were still obtained (ratio 49:14:31:6; Table 1). Nevertheless, a significant change in solubility properties was observed, so that the workup led to a mixture of an isomer-enriched final product (ratio 85:12:3: 2σ(I)) = 0.0280, wR2 (for all data) = 0.0500, final R = 0.0340, goodness of fit 1.017, ΔF(max/min) = 1.396/−1.803 e Å−3. Crystal Data for 10. Suitable single crystals of 10 were obtained from a concentrated diethyl ether solution at 4 °C. Data were collected with a Bruker X8-KappaApexII diffractometer equipped with a lowtemperature device at 100 K by using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by Patterson methods (SHELXS-97)14 and refined by full-matrix least squares on F2 (SHELXL-97):14 C27H18F3O6PW, Mr = 710.23, crystal dimensions 0.11 × 0.09 × 0.04 mm3, triclinic, space group P1,̅ Z = 2, a = 9.6002(10) Å, b = 10.5444(11) Å, c = 13.8429(14) Å, α = 71.032(3)°, β = 76.087(3)°, γ = 75.469(4)°, V = 1263.3(2) Å3, dcalcd = 1.867 g cm−3, μ = 4.699 mm−1, T = 100 K, transmission factors (min/max) 0.5035/0.7460, empirical absorption correction, 2θmax = 51.996°, no. of unique data 4756, Rint = 0.0286, R1 (for I > 2σ(I)) = 0.0237, wR2 (for all data) = 0.0511, final R = 0.0298, goodness of fit 1.056, ΔF(max/min) = 1.17/−0.83 e Å−3. Crystal Data for 11. Suitable single crystals of 11 were obtained from a concentrated n-pentane solution at 4 °C. Data were collected with a Bruker X8-KappaApexII diffractometer equipped with a lowtemperature device at 100 K by using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by Patterson methods (SHELXS-97)14 and refined by full-matrix least squares on F2 (SHELXL-97):14 C18H18F3O6PW, Mr = 602.14, crystal dimensions 0.1 × 0.1 × 0.06 mm3, monoclinic, space group P21/c, Z = 4, a = 9.86(2) Å, b = 14.30(3) Å, c = 16.92(3) Å, α = 90°, β = 94.00(11)°, γ = 90°, V = 2381(8) Å3, dc = 1.680 g cm−3, μ = 4.969 mm−1, T = 100 K, transmission factors (min./max.) 0.3735/0.7459, empirical absorption correction, 2θmax = 55.986°, no. of unique data 5670, Rint = 0.3262, R1 (for I > 2σ(I)) = 0.1136, wR2 (for all data) = 0.3744, final R = 0.2554, goodness of fit 0.852, ΔF(max/min) = 2.79/−3.9 e Å−3. Crystal Data for 12a. Suitable single crystals of 12a were obtained from a concentrated n-pentane solution at 4 °C. Data were collected with a Bruker X8-KappaApexII diffractometer equipped with a lowtemperature device at 100 K by using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by Patterson methods (SHELXS-97)14 and refined by full-matrix least squares on F2 (SHELXL-97):14 C16H21F6O6PSi2W, Mr = 694.33, crystal dimensions 0.05 × 0.04 × 0.01 mm3, triclinic, space group P1,̅ Z = 2, a = 8.9748(9)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00974. Crystallographic data for 9−11, 12a, and 13 (CIF)
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AUTHOR INFORMATION
Corresponding Author
* R.S.: tel, (+49) 0228 73-5345; e-mail, r.streubel@uni-bonn. de. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the DFG foundation (STR 411/29-1 and SFB 813 “Chemistry at Spin Centers”) and cost action CM1302 “Smart Inorganic Polymers” (SIPs) for financial support. The authors thank Prof. Dr. A. C. Filippou and Prof. Dr. D. Menche for the use of the X-ray facilities and C. Rödde for the X-ray diffraction data collections.
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DOI: 10.1021/acs.organomet.5b00974 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.5b00974 Organometallics XXXX, XXX, XXX−XXX
