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Organometallics 2006, 25, 1412-1423
Reactions of M(N-2,6-i-Pr2C6H3)(CHR)(CH2R′)2 (M ) Mo, W) Complexes with Alcohols To Give Olefin Metathesis Catalysts of the Type M(N-2,6-i-Pr2C6H3)(CHR)(CH2R′)(OR′′) Amritanshu Sinha, Lourdes Pia H. Lopez, Richard R. Schrock,* Adam S. Hock, and Peter Mu¨ller Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed NoVember 14, 2005
The reaction between M(NAr)(CH-t-Bu)(CH2-t-Bu)2 (M ) Mo, W, Ar ) 2,6-i-Pr2C6H3) and various alcohols (1-adamantyl-OH, t-BuOH, ArOH, (CF3)2CHOH, (CF3)2MeCOH, CF3Me2COH, (CF3)3COH, C6F5OH) in pentane or toluene yields either complexes of the type M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR), through direct addition of ROH across a Mo-C bond, or complexes of the type M(NAr)(CH2-t-Bu)3(OR), through direct addition of ROH across a ModC bond. The trineopentyl species appear to be formed when the alcohol has a relatively low pKa value. The outcome also can depend on whether the alcohol is employed neat or in benzene, and mixtures are observed in some circumstances. The conversion of M(NAr)(CH2-t-Bu)3(OR) into M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) was shown to be unimolecular in several examples. Preliminary experiments have shown that M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) complexes are surprisingly active catalysts for various metathesis reactions, although conversion is sometimes limited by decomposition of intermediate alkylidenes to yield dimeric species that contain MdM bonds. In contrast, M(NAr)(CH-t-Bu)(CH2-t-Bu)2 species are virtually inactive for metathesis. X-ray structures are reported for Mo(NAr)(CH2-t-Bu)3(OC6F5), Mo(NAr)(CH-t-Bu)(CH2-t-Bu)[OSi(O-t-Bu)3], [Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5)]2, and Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5)(PMe3). Introduction We have been interested in olefin metathesis by imido alkylidene complexes of Mo and W,1 most recently asymmetric metathesis reactions catalyzed by complexes of the type Mo(NR)(CHR′)(diolate) (R′ ) CMe3, CMe2Ph) that contain an enantiomerically pure biphenolate or binaphtholate ligand.2 As the number of catalyst variations increases, it would be highly desirable to prepare catalysts through protonation of two monoanionic ligands in an imido alkylidene precursor with an alcohol. Catalysts then could be prepared and evaluated in situ, a possibility that would create opportunities for rapid throughput or combinatorial screening of catalysts for activity, selectivity, enantioselectivity, etc., if other products that might be formed in such reactions are catalytically inactive. For this reason we began to explore reactions between alcohols and dineopentyl imido alkylidene species of the type M(NAr)(CH-t-Bu)(CH2t-Bu)2 (M ) Mo, W, Ar ) 2,6-i-Pr2C6H3), in the hope that 2 equiv of neopentane would be lost and M(NAr)(CH-t-Bu)(OR)2 species generated. This particular precursor also could be added to a silica surface (SisurfOH) in order to create a relatively welldefined surface-supported catalyst.3 Related reactions involving Ta(CH-t-Bu)(CH2-t-Bu)3,4 W(C-t-Bu)(CH2-t-Bu)3,5 and Re(Ct-Bu)(CH-t-Bu)(CH2-t-Bu)26 have been employed for this purpose so far.3 It has been believed that the lifetime of surfacesupported species would be greater than the lifetime of analogous species in solution as a consequence of slow (1) Schrock, R. R. Chem. ReV. 2002, 102, 145. (2) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (3) Cope´ret, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J.-M. Angew. Chem., Int. Ed. 2003, 42, 156. (4) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359. (5) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.; Rocklage, S. M.; Pedersen, S. F. Organometallics 1982, 1, 1645.
bimolecular (in metal) decomposition at relatively low temperatures.7,8 In two preliminary communications we showed that addition of alcohols to M(NAr)(CH-t-Bu)(CH2-t-Bu)2 did not lead to bis(alkoxide) species but to M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) species.9,10 Only one example of a complex of this general type had been reported11 (prepared through addition of Ph3SiOH to Mo(N-t-Bu)(CH-t-Bu)(CH2-t-Bu)2), although it was not characterized and its olefin metathesis activity was not explored. On the basis of what we know about 1-adamantylimido bis(alkoxide) complexes,12,13 and the fact that Mo(NAd)(CH-t-Bu)(CH2-t-Bu)2 is unstable with respect to bimolecular decomposition to yield di-tert-butylethylene,14 we suspect that the tertbutylimido ligand is “too small” to prevent bimolecular decomposition of Mo(N-t-Bu)(CH-t-Bu)(CH2-t-Bu)(OSiPh3) and formation of products that prevent crystallization of these already highly soluble compounds. In this paper we present the full details of the reactions between M(NAr)(CHR)(CH2R′)2 (M ) Mo, W; R, R′ ) CMe3, CMe2Ph) and selected alcohols. Some M(NAr)(CHR′)(CH2-t-Bu)(OR) complexes decompose in a (6) Edwards, D. S.; Biondi, L. V.; Ziller, J. W.; Churchill, M. R.; Schrock, R. R. Organometallics 1983, 2, 1505. (7) Chabanas, M.; Cope´ret, C.; Basset, J.-M. Chem. Eur. J. 2003, 9, 971. (8) LeRoux, E.; Taoufik, M.; Chabanas, M.; Alcor, D.; Baudouin, A.; Cope´ret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Lesage, A.; Hediger, S.; Emsley, L. Organometallics 2005, 24, 4274. (9) Sinha, A.; Schrock, R. R. Organometallics 2004, 23, 1643. (10) Lopez, L. P. H.; Schrock, R. R. J. Am. Chem. Soc. 2004, 126, 9526. (11) Ehrenfeld, D.; Kress, J.; Moore, B. D.; Osborn, J. A.; Schoettel, G. J. Chem. Soc., Chem. Commun. 1987, 129. (12) Alexander, J. B.; Schrock, R. R.; Davis, W. M.; Hultzsch, K. C.; Hoveyda, A. H.; Houser, J. H. Organometallics 2000, 19, 3700. (13) Tsang, W. C. P.; Jernelius, J. A.; Cortez, A. G.; Weatherhead, G. S.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125, 2591. (14) Pilyugina, T. S.; Schrock, R. R.; Hock, A. S.; Mu¨ller, P. Organometallics 2005, 24, 1929.
10.1021/om050978a CCC: $33.50 © 2006 American Chemical Society Publication on Web 02/16/2006
Olefin Metathesis Catalysts
Organometallics, Vol. 25, No. 6, 2006 1413
Table 1. Relevant Proton and Carbon NMR Data for Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(X) Compounds Reported in This Paper X CH2-t-Bu Cl OAd OCMe3 ORF3 ORF6 OCH(CF3)2 ORF9 OAr OC6F5 (OC6F5)(PMe3) a
Mo δ(HR)
Mo δ(CR)
9.50 11.71a 11.71 11.63
255.0
11.90 11.80 12.21 11.99 12.07 13.3 (anti) 10.8 (syn)
283.6 284.5
275.7 275.9
277.7 286.5 309.7 (anti) 278.7 (syn)
W δ(HR) (JHW, Hz)
W δ(CR) (JCH, Hz)
6.74 (15) 8.83 (16) 8.86 (15)c 8.80 (14) 8.88 (16) 9.00 (15) 8.96 (15)b 9.39 (15)c 9.37 (16) 9.29 (15)
247.2 (102) 261.0 (114) 253.11 (110) 253.6 (106) 256.0 (109) 259.2 (115) 263.3 (107)c 255.1 (116) 260.5 (105)
See ref 14. b Prepared in situ. c See ref 10.
bimolecular fashion to yield species that contain a MdM bond that is not bridged by imido or alkoxide groups that are present.10 These and other types of MdM species will be reported elsewhere. It should be noted that N-2,6-diisopropylphenyl (NAr) ligands are the most sterically demanding of the imido ligands available in compounds of this type today; three other imido ligands that have been reported in Mo(NR)(CHR′)(OR′′)2 compounds (inter alia) are N-2,6-dimethylphenyl, N-2,6-dichlorophenyl, and N-1-adamantyl.2,15 Reactions analogous to those discussed here have not yet been carried out with complexes that contain these “smaller” imido ligands. Steric properties are so important that similar results could not be guaranteed, nor even expected.
of the decomposition product or products is (are) not known. Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 also has been prepared in 90% yield through addition of neopentyllithium to Mo(NAr)Np3Cl at -30 °C.14 It is straightforward to prepare “mixed” alkyl/alkylidene compounds such as Mo(NAr)(CHCMe2Ph)(CH2-t-Bu)2 (1b) through alkylation of Mo(NAr)(CHCMe2Ph)(triflate)2(dme) with neopentyl Grignard reagent and Mo(NAr)(CH-t-Bu)(CH2CMe2Ph)2 (1c) through alkylation of Mo(NAr)(CH-t-Bu)(triflate)2(dme) with neophyl Grignard reagent. There is no evidence for intramolecular R-proton migration that would lead to alkyl/ alkylidene scrambling in any of these species in solution at room temperature. This type of intramolecular transfer or migration of an R-hydrogen from an alkyl R-carbon to an alkylidene R-carbon is known to be slow in all high-oxidation-state systems so far where it has been tested.1,17,18 There is also no evidence that alkyl or alkylidene ligands transfer readily between metals. Reactions between 1a and the alcohols shown in eq 1 (Ad ) 1-adamantyl) in benzene or toluene (22 °C, 0.1-0.2 M, up to 24 h reaction time) give isolable complexes of the type Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) (2a-d), which we will call the type B product. The product of a reaction between 1a and (t-
Results Syntheses of Molybdenum Complexes. Addition of 2 equiv of neopentylmagnesium chloride in ether to Mo(NAr)(CH-tBu)(triflate)2(dme)16 (Ar ) 2,6-i-Pr2C6H3, dme ) 1,2-dimethoxyethane) in ether gives red-orange Mo(NAr)(CH-t-Bu)(CH2-tBu)2 (1a) in 93% isolated yield. Highly soluble 1a can be prepared on a large scale and used in “crude” form for further reactions if pure Mo(NAr)(CH-t-Bu)(triflate)2(dme) and accurately titrated Me3CCH2MgCl are employed. NMR spectra of 1a in C6D6 show resonances for HR at 9.50 ppm and for CR at 255.0 ppm (JCH ) 108 Hz) for the alkylidene ligand and at 2.76 and 0.62 ppm (δ(CH2); JHH ) 12 Hz) for the diastereotopic methylene protons in the neopentyl ligands. The neopentylidene chemical shifts are what one might expect for a high-oxidationstate syn alkylidene complex1,17,18 and should be compared to δ(HR) 9.22 ppm and δ(CR) 249.3 ppm (JCH ) 106 Hz) in Mo(N-t-Bu)(CH-t-Bu)(CH2-t-Bu)211 and δ(HR) 6.74 ppm, δ(CR) 247.2 ppm (JCH ) 102 Hz), and δ(CH2) 2.72 and 0.41 ppm in W(NAr)(CH-t-Bu)(CH2-t-Bu)2.19 No alkylidene resonance for the anti isomer of Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 could be found. Relevant proton and carbon NMR data for all compounds reported here are given in Table 1. Mo(NAr)(CH-t-Bu)(CH2t-Bu)2 is relatively stable thermally. When a toluene solution is heated to 60 °C, 24% decomposition is observed after 1 day, 46% after 3 days, and 100% decomposition in 6 days. The nature (15) Schrock, R. R. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 1, p 8. (16) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875. (17) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1. (18) Schrock, R. R. In Reactions of Coordinated Ligands; Braterman, P. R., Ed.; Plenum: New York, 1986. (19) Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park, L. Y.; DiMare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.; Kru¨ger, C.; Betz, P. Organometallics 1990, 9, 2262.
BuO)3SiOH is also an isolable, crystalline species that has been reported elsewhere.20 We can confirm that AdOH or t-BuOH adds across a Mo-C bond directly, since addition of ROH to 1b gives Mo(NAr)(CHCMe2Ph)(CH2-t-Bu)(OR) exclusively. In contrast, the reaction between 1a and C6F5OH in benzened6 or neat (CF3)2CHOH at 22 °C gives the trineopentyl species 3e or 3a shown in eq 2 rapidly (minutes) and exclusively. We
will call the trineopentyl species the type A product. (The results (20) Blanc, F.; Cope´ret, C.; Thivolle-Cazat, J.; Basset, J.-M.; Sinha, A.; Schrock, R. R. Angew. Chem., Int. Ed., in press.
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of alcohol addition are summarized in Table 2.) When solutions of 3e or 3a are heated to 60 °C over a period of hours, neopentane evolves smoothly to yield Mo(NAr)(CH-t-Bu)(CH2t-Bu)(OR) species (OR ) OC6F5 (2e), OCH(CF3)2 (2a)) quantitatively. The conversion of 3e to 2e was shown to be unimolecular with k ) 1.0 × 10-4 s-1 in benzene-d6 at 60 °C. (All rate constants for reactions of this type are gathered in Table 3.) Addition of C6F5OH to Mo(NAr)(CHCMe2Ph)(CH2-t-Bu)2 yields Mo(NAr)(CH2CMe2Ph)(CH2-t-Bu)2(OC6F5), which upon heating is transformed into a mixture of the expected three compounds (approximately 1:1:1) shown in eq 3, along with neopentane and tert-butylbenzene.
Sinha et al. Table 2. Formation of Complexes of the Types M(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) (B) and M(NAr)(CH2-t-Bu)3(OR) (A) ROH
pKa in H2O21-23
Me3COH 1-adamantanol Me2(CF3)COH 2,6-i-Pr2C6H3OH Me(CF3)2COH (CF3)2CHOH (CF3)3COH C6F5OH
19.2 18 12.6 11 9.8 9.3 5.4 5.5
a
Mo
W
B B A or Ba A+B A
B B B B B A+B A A
B B
Neat only.
Table 3. Rate Constants (10-5 s-1) for Conversion of M(NAr)(CH2-t-Bu)3X into M(NAr)(CH2-t-Bu)(CH-t-Bu)X in C6D6 at 60 °C (Except Where Noted)
When (CF3)3COH is added to 1a in benzene-d6, both Mo(NAr)(CH-t-Bu)(CH2-t-Bu)[OC(CF3)3] (2f) and Mo(NAr)(CH2t-Bu)3[OC(CF3)3] (3f) are formed in a ratio of 1:1.2, while the reaction between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 and neat (CF3)3COH affords a mixture which contains >95% 3f and
