Organometallics 2007, 26, 831-837
831
New Enantiomerically Pure Alkylimido Molybdenum-Based Alkylidene Complexes. Synthesis, Characterization, and Activity as Chiral Olefin Metathesis Catalysts Tatiana S. Pilyugina,† Richard R. Schrock,*,† Peter Mu¨ller,† and Amir H. Hoveyda‡ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467 ReceiVed October 30, 2006
Molybdenum olefin metathesis catalysts that contain aliphatic 1-phenylcyclohexylimido (NPhCy) and 2-phenyl-2-adamantylimido (NPhAd) groups and (S)-Biphen or (R)-Trip ligands (Biphen ) 3,3′-di-tertbutyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate; Trip ) 3,3′-bis(2,4,6-triisopropylphenyl)-2,2′-binaphtholate) have been prepared. Their catalytic activity and enantioselectivity in desymmetrization reactions such as ring-closing metathesis of amines and lactams and ring-opening/cross-metathesis of substituted norborneols with styrene were compared to the results obtained with the only known alkylimido catalyst, Mo(NAd)(CHCMe2Ph)[(S)-Biphen]. The activities and enantioselectivities provided by these new chiral complexes vary significantly, but in virtually all instances explored were not superior to the adamantylimido analogues. Introduction
Scheme 1. Chiral Mo Complexes Bearing Aryl- and Adamantylimido Ligands
The first isolable (symmetric) olefin metathesis catalysts of the type Mo(NR)(CHR′)(OR′′)21 that contain an alkylimido (adamantylimido) ligand, as opposed to an arylimido ligand, were synthesized in 1993.2 Adamantylimido complexes have been used for olefin cross-metathesis,3 ring-opening polymerization (ROMP),4 and living polymerization of substituted acetylenes.5 In the last several years, we have designed and synthesized chiral, enantiomerically pure Mo-based imido alkylidene complexes that promote a variety of olefin metathesis reactions such as asymmetric ring-closing metathesis (ARCM) and asymmetric ring-opening/cross-metathesis (AROM/CM) with a high enantioselectivity (>90% ee).6 The majority of these catalysts are four- or five-coordinate species that bear arylimido functionalities and a chiral diolate (Scheme 1). In 2003, we disclosed the synthesis and selected enantioselective processes catalyzed by the first enantiomerically pure alkylimido catalyst, Mo(NAd)(CHCMe2Ph)[(S)-Biphen] (Ad ) 1-adamantyl; Biphen2) 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-di* Corresponding author. E-mail:
[email protected]. † Massachusetts Institute of Technology. ‡ Boston College. (1) Schrock, R. R. Chem. ReV. 2002, 102, 145-180. (2) Oskam, J. H.; Fox, H. H.; Yap, K. B.; McConville, D. H.; O’Dell, R.; Lichtenstein, B. J.; Schrock, R. R. J. Organomet. Chem. 1993, 459, 18-198. (3) Fox, H. H.; Schrock, R. R.; O’Dell, R. Organometallics 1994, 13, 635-639. (4) Oskam, J. H.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 1183111845. (5) Schrock, R. R.; Luo, S.; Zanetti, N. C.; Fox, H. H. Organometallics 1994, 13, 3396-3398. (6) (a) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592-4633. (b) Schrock, R. R. J. Mol. Cat. A: Chem. 2004, 213, 2130. (c) Schrock, R. R.; Jamieson, J. Y.; Dolman, S. J.; Miller, S. A.; Bonitatebus, P. J. Jr.; Hoveyda, A. H. H. Organometallics 2002, 21, 409417. (d) Zhu, S. S.; Cefalo, D. R.; La, D. S.; Jamieson, J. Y.; Davis, W. M.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 82518259. (e) Dolman, S. J.; Hultzsch, K. C.; Pezet, F.; Teng, X.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 2004, 126, 10945-10953. (f) Alexander, J. B.; Schrock, R. R.; Davis, W. M.; Hultzsch, K. C.; Hoveyda, A. H.; Houser, J. H. Organometallics 2000, 19, 3700-3715.
olate; 2a, Scheme 1).7 We were surprised and pleased to find that this new chiral complex is often superior to the analogous arylimido alkylidenes, sometimes dramatically so, in terms of catalytic activity and selectivity. Therefore, we became interested in preparing other alkylimido catalysts and attempted to determine what property, if any, of alkylimido species is responsible for their effective promotion of asymmetric metathesis reactions. Herein, we report several new alkylimido species and evaluate their efficacy in a variety of catalytic ARCM and AROM/CM reactions that involve substrates that have proven challenging for arylimido-containing substrates. (7) Tsang, W. C. P.; Jernelius, J. A.; Cortez, G. A.; Weatherhead, G. S.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 2591-2596.
10.1021/om061001o CCC: $37.00 © 2007 American Chemical Society Publication on Web 01/20/2007
832 Organometallics, Vol. 26, No. 4, 2007 Scheme 2. Synthesis of Chiral Mo-Based Complexes 7a and 7b
Pilyugina et al. Table 1. Crystal Data and Structure Refinement for Complex 7a empirical formula fw temperature wavelength cryst syst space group unit cell dimens
Results and Discussion Synthesis of Alkylimido Mo-Based Olefin Metathesis Catalysts. New imido groups were chosen on the basis of the requirement that an imido group be sufficiently bulky to prevent bimolecular decomposition of at least the initial neopentylidene or neophylidene. Therefore, we judged that the imido group should contain a quaternary carbon bound to the imido nitrogen. Two imido groups that fulfill these requirements, and can be readily prepared, are 1-phenylcyclohexylimido (NPhCy) and 2-phenyl-2-adamantylimido (NPhAd). We employed phenyl groups in the hope that catalysts that contain these imido groups would be more crystalline and therefore could be isolated more readily. We selected (S)-Biphen and (R)-Trip ligands (Biphen2) 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate; Trip2- ) 3,3′-bis(2,4,6-triisopropylphenyl)-2,2′-binaphtholate) from the pool of available chiral chelating diolates since these ligands have been utilized successfully in previous asymmetric metathesis studies.6a The sequence leading to the Mo(NR)(CHCMe2Ph)(diolate*) catalysts (R ) NPhCy, NPhAd; diolate* ) (S)-Biphen, (R)Trip) is the one that has been employed for all imido alkylidene compounds.8 As illustrated in Scheme 2, 1-phenylcyclohexyl amine 3, easily prepared in two steps from 1-phenylcyclohexanol,9 and sodium molybdate react to produce the bis(imido)dichloride complex 4 (>98% conv, 90 °C, 24 h); the product is isolated as a 1,2-dimethoxyethane adduct, which is typical of bis(imido) dichloride complexes of this type. Alkylation yields the dineophyl bisimido complex 5, which upon treatment with 3 equiv of triflic acid in cold diethyl ether in the presence of DME is converted into 6, which is then converted into Mo(NR)(CHCMe2Ph)(diolate*) complexes 7a and 7b. Contrary to our expectations, 7a and 7b are highly soluble in all common solvents, but can be isolated as crystalline solids from concentrated pentane solutions at -20 to -30 °C. In the 500 MHz 1H NMR spectrum of compound 7a in C6D6 the alkylidene resonance at 10.7 ppm can be attributed to a syn alkylidene on the basis of the value for the CH coupling constant (8) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875-3886. (9) Jirgensos, A.; Kauss, V.; Kalvinsh, I.; Gold, M. R.; Danysz, W.; Parsons, C. G.; Quack, G. Eur. J. Med. Chem. 2000, 35, 555-565.
volume Z density (calcd) absorp coeff F(000) cryst size θ range for data collection index ranges no. of reflns collected no. of indep reflns completeness to θ ) 29.57° absorp corr max. and min. transmn refinement method no. of data/restraints/ params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute struct param largest diff peak and hole
C46H59MoNO2 753.88 100(2) K 0.71073 Å orthorhombic P2(1)2(1)2(1) a ) 12.9341(10) Å, R ) 90° b ) 16.5919(16) Å, β ) 90° c ) 19.2463(18) Å, γ ) 90° 4130.3(6) Å3 4 1.212 Mg/m3 0.354 mm-1 1600 0.15 × 0.05 × 0.04 mm3 1.90 to 29.57° -17 e h e 17, 0 e k e 23, 0 e l e 26 93 372 11 592 [R(int) ) 0.0647] 100.0% semiempirical from equivalents 0.9860 and 0.9489 full-matrix least-squares on F2 11 592/0/463 1.095 R1 ) 0.0365, wR2 ) 0.0828 R1 ) 0.0416, wR2 ) 0.0856 -0.01(2) 1.443 and -0.315 e Å-3
(121 Hz). 1H NMR spectra of a sample of 7a in toluene-d8 remain unchanged in the range -80 to 100 °C; there is no evidence for any significant amount of the anti isomer in this temperature interval. An X-ray crystallographic analysis of 7a (Table 1) shows it to have the expected structure (Figure 2). Structural parameters for 7a lie within the range of values observed for similar molybdenum complexes. The Mo-C(1) bond distance (1.889(2) Å) and Mo-C(1)-C(2) bond angle (144.51(16)°) are comparable to the analogous distance and angle in 2a (1.867(4) Å and 149.3(3)°). The Mo(1)-N(1) distance (1.7185(18) Å) is also approximately the same as in 2a (1.709(3) Å). Chiral complex 7b is obtained as a THF adduct, which is typical of complexes that contain binaphtholate ligands.6a Only one alkylidene resonance is observed in the 1H NMR spectrum of 7b (at 14.0 ppm, 22 °C), while the alkylidene carbon resonance in the 13C NMR spectrum is found at 308 ppm (JCH ) 145 Hz); the large value for JCH suggests that the alkylidene ligand is in the anti configuration with the alkyl substituent pointing away from the imido group. The spectrum does not change upon heating a toluene-d8 sample of 7b to 100 °C. However, when the solution is cooled to -70 °C, two alkylidene resonances are observed (Figure 2). We propose that these two resonances correspond to the diastereomeric THF adducts. It is not clear which resonance corresponds to which Mo complex diastereomer and what the precise identity of each diastereomer (i.e., the stereochemistry of THF binding) is. The intensities of the alkylidene resonances suggest that the two species are approximately equal energetically at -70 °C. In Figure 2 we show THF binding to the two CNO faces trans to one of the binaphtholate oxygens. Addition of 10 equiv of THF yields the same variable-temperature 1H NMR spectra between 10 and -70 °C. This finding suggests that the two diastereomers interconvert without dissociation of THF by a mechanism that is unclear at the present time. A good possibility is successive “turnstile” operations in which three ligands rotate with respect to the other two. Mo complex 7b is analogous to 2b (Scheme
Alkylimido Mo-Based Complexes
Organometallics, Vol. 26, No. 4, 2007 833 Scheme 3. Synthesis of Mo Bisimido Complexes 9 and 10 and Chiral Mo-Based Complexes 11a and 11b
Figure 1. Thermal ellipsoid drawing of the structure of 7a. Selected distances (Å) and angles (deg): Mo-N(1) ) 1.7185(18), MoC(1) ) 1.889(2), Mo-O(1) ) 2.0070(15), Mo-O(2) ) 2.0140(16), O(1)-Mo-O(2) ) 129.45(6), N(1)-Mo-C(1) ) 105.97(9), Mo-C(1)-C(2) ) 144.51(16), Mo-N(1)-C(50) ) 167.18(15).
Figure 2. Variable-temperature 1H NMR spectrum of a sample of 7b in toluene-d8. Two alkylidene resonances at -70 °C correspond to the two diastereomers of the anti THF adduct.
1);10 2b also is exclusively an anti alkylidene THF adduct (δHR ) 13.8 ppm in toluene-d8, JCH ) 145 Hz). In stark contrast to the new alkylidene 7b, however, cooling the sample of 2b to -70 °C does not lead to splitting of the alkylidene resonance.10 Therefore, either only one diastereomer is present under these conditions or two are present whose interconversion is too facile for their alkylidene resonances to be resolved. Chiral Mo complexes 11a and 11b (Scheme 3) were also synthesized. The requisite 2-phenyl-2-adamantylamine (8) is prepared in a three-step sequence from 2-adamantanone.11 Bis(10) Tsang, W. C. P. Ph.D. Thesis, Massachusetts Institute of Technology, 2003. (11) Kalir, A.; Balderman, D. Org. Synth. 1981, 60, 104-108.
(imido) dichloride complexes (9 and 10, Scheme 3) are synthesized as described previously. However, 9 does not contain coordinated dimethoxyethane. The yields of Mo(NPhAd)2Cl2 are atypically low (20-40%), and the reaction is not easily reproduced. We ascribe the absence of DME to the significant steric bulk of the imido ligand. If the same reaction is performed in THF, a five-coordinate complex is obtained in which one molecule of THF is bound in the coordination sphere (10 in Scheme 3). The NMR characteristics of 11a and 11b are analogous to those of 7a and 7b, respectively; that is, 11a exists predominantly as a syn alkylidene, while the alkylidene in 11b is anti. However, the 1H NMR spectrum of 11b (δHR ) 14.06 ppm), like that of 2b (Scheme 1), does not change upon cooling to -80 °C (toluene-d8). As a result, it is not known with certainty whether compound 11b exists as a mixture of diastereomeric THF adducts, as with 7b, and interconversion is too facile for the alkylidene resonances to be resolved at -80 °C, or whether only one diastereomeric adduct is present. Asymmetric Olefin Metathesis Reactions. Initial studies, involving representative substrates, indicate that the new alkylimido complexes can effectively promote enantioselective olefin metathesis reactions. The results of these are summarized in Tables 2-6. We have previously demonstrated that complex 2a exhibits a different profile of reactivity and selectivity than analogous arylimido complexes when applied to certain classes of metathesis substrates. One of the reactions in which 2a is a significantly better catalyst (versus arylimido complexes) is tandem asymmetric ring-opening metathesis/cross-metathesis (AROM/CM) of norbornene derivatives with monosubstituted olefins. The results for the AROM/CM reaction between syn7-(benzyloxy)bicyclo[2.2.1]hept-2-ene (12) and styrene12 are (12) La, D. S.; Sattely, E. S.; Ford, J. G.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 7767-7778.
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Table 2. Mo-Catalyzed AROM/CM Reactions
Table 5. Mo-Catalyzed ARCM Reactions of Amide 19
entry
chiral complex
temp (°C)
yield (%) 13b
yield (%) 14b
ee (%) 13c
1 2 3 4 5 6
2a 7a 11a 2b 7b 11b
25 25 25 25 75 75
56 36
