Organometallics 1995, 14, 5615-5621
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Physical Organic Chemistry of Transition Metal Carbene Complexes. 6. Kinetics and Mechanism of the Thermal and Photochemical Hydrolysis of (C0)5M=C(OMe)CH2Ph (M = Cr, W)in Aqueous Acetonitrile1 Claude F. Bernasconi” and Weitao Sun Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064 Received July 14, 1995@
An extensive kinetic study of the hydrolysis of (benzy1methoxycarbene)pentacarbonylchromium(O), 7 4 r , and a more limited study of the hydrolysis of its tungsten analog, 7-W, in 50%acetonitrile-50% water is reported. Both lead to P-methoxystyrene as the main organic product; with 7-W, phenylacetaldehyde is a significant second product, while with 7-Cr only traces of phenylacetaldehyde are found. The kinetic results which include the demonstration of general base catalysis and of a kinetic solvent isotope effect are consistent with a mechanism in which 7-M (M = Cr, W) undergoes a rapid, reversible deprotonation of the a-carbon to form 7-M-, followed by rate-limiting reaction of 7-M- with water, H30+, or buffer acids. This latter step most likely involves protonation of the carbene carbon concerted with cleavage of the C-Cr bond. The hydrolysis of both 7-Cr and 7-M is strongly accelerated by light; a limited study of this photochemical pathway at high pH, which also leads to P-methoxystyrene as the principal organic product, suggests a mechanism which most likely involves a ligand exchange between CO and acetonitrile prior to the waterinduced rate-limiting product-forming step. Despite the explosive development of the chemistry of Fischer type transition metal carbene complexes (‘‘Fischercarbenes”) over the last 20 years,2the hydrolysis of these compounds has, thus far, only received scant attention. In 1993 Aumann et al.3 reported a product study of the hydrolysis of several Fischer carbenes of the type 1 (R = Ph, CH=CHPh, C4H3S, CH=CHC4H3S, OD
(CO)5Cr=C\ 1
R
+ H20 + C6HIZN4+
EtOH + RCH=O+ (CO)SC~C~H,~N, (1)
EtO-, yields 3, a type of carbene complex known4 to break down to the corresponding aldehyde 2.
The hydrolysis of (methoxymethy1carbene)pentacarbonylchromium(O),4a, and (ethoxymethylcarbenelpentacarbonylchromium(O), 4b, in 50% acetonitrile-50% water yields analogous products as eq 1, Le., acetaldehyde and methanol in the case of 4a and acetaldehyde
2
(CO),Cr =C\
and CGCPh) in THF containing small quantities of water. In the presence of the tricyclic amine urotropine ( C ~ H I ~ Naldehydes ~), such as 2 are formed in 290% yield with all R groups except when R is CHECPh; in this latter case the triple bond undergoes nucleophilic attack by the amine. The reaction was assumed to occur via a tetrahedral intermediate, T-, which, after loss of Abstract published in Advance ACS Abstracts, November 1,1995. (1)Part 5: Bernasconi, C. F.; Flores, F. X.; Sun, W. J. Am. Chem. SOC.1995,117, 4875. (2) For recent reviews see: (a) Dotz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Deerfield Beach, FL, 1983.(b) Dotz, K. H. Angew. Chem., Int. Ed. Engl. 1984,23,587.(c) Gallop, M. A.; Roper, W. R. Adv. Organomet. Chem. 1986,25, 121.(d) Wulff, W. D. Adu. Met.-Org. Chem. 1989,I , 209.(e) Schubert, U.,Ed. Advances in Metal Carbene Chemistry; Kluwer: Dordrecht, Holland, 1989.(f) Dotz, K. H. New J. Chem. 1990,14,433. (g) Wulff, W. D. In Comprehensive Organic Synthesis; Trost, B. M . , Fleming, I., Eds.; Pergamon Press: New York, 1990;Vol. 5. (h) Schmidt, M. A.; Miller, J. R.; Hegedus, L. S. J. Organomet. Chem. 1991,413, 143.(i) Veillard, A. Chem. Rev. 1991,91,743. (3) Aumann, R.; Hinterding, P.; Kruger, C.; Goddard, R. J . Organomet. Chem. 1993,459,145. @
/OR CH3
4a (R = CH3) 4b (R = CH3CH2)
and ethanol in the case of 4b.l However, on the basis of a kinetic analysis of these reactions, a mechanism was proposed that is quite different from that suggested by Aumann et al. for eq 1. This mechanism, whose key feature is that it involves the conjugate base of the carbene complex, 4-, rather than a T--like adduct as the intermediate, is shown in Scheme l a 5This mechanism is apparently more efficient for carbene complexes with acidic a-hydrogen than the alternative pathway via nucleophilic attack by OH- on the carbene carbon. The evidence for the mechanism of Scheme 1 can be summarized as fo1lows.l (4) Fischer, E. 0.;Massbol, A. Chem. Ber. 1967,100,2445. ( 5 ) In the presence of buffers, conversion of 4- to 5 involves the
buffer acid instead of water as the proton donor, while in acidic solution H30+ takes on the role of proton donor.
0276-733319512314-5615$09.00/0 0 1995 American Chemical Society
Bernasconi and S u n
5616 Organometallics, Vol. 14, No. 12, 1995
organic product is not the corresponding aldehyde, PhCH&H=O, but B-methoxystyrene. It will be shown that this result does not necessitate the postulation of a different mechanism for the hydrolysis of 7-Cr and 7-W but actually adds further support to the mechanism of hydrolysis of 4a,b shown in Scheme 1. We also report that the hydrolysis of 7-Cr and 7-W is strongly accelerated by W light.
Scheme 1
4
H
OR
'H
H' 5
CH,CH=O + (CO)&kOH-
+ ROH
Scheme 2
4-
6
Results General Features. All experiments were conducted in 50% acetonitrile-50% water (vlv). Rates were measured in HC1 and KOH solutions as well as in acetate, N-morpholine,and triethylamine buffers, spanning a pH range from 2.0 t o 14.2. The main hydrolysis products of 7-Cr are /3-methoxystyrene, 8, and (CO)&rX this PhCH= CHOCH3 8
(1) The reversible deprotonation of 4, whose rate had been measured previously,6 is much faster than the hydrolysis reaction and hence acts as a rapid preequilibrium in front of the rate-limiting k H z o step. (2) The break in the pH-rate profiles (log kobsd vs pH, with kobsd being the observed pseudo-first-order rate constant) occurs at pH = 12.35 f 0.15 for 4a and pH = 12.80 & 0.15 for 4b, consistent with p g H = 12.50 for 4a and 12.97 for 4b.6 (3) The rates are substantially reduced when the reactions are conducted in 50% acetonitrile-50% D20. At pH >> where kobsd = k H 2 0 , k H z o / k D z o ratios in the order of 4 and 7 were determined for 4a,b, respectively. This constitutes a substantial primary kinetic isotope effect and indicates a rate-limiting step that involves a proton transfer. (4) Even though vinyl ethers are stable in basic solution, it is plausible that complexation of ROCH=CH2 with Cr(CO)j can activate them toward OH--promoted hydr~lysis.~ Support for this contention comes from control experiments in which acetaldehyde was produced when EtOCH=CH2 was added t o a basic solution of (CO)&r(CH3CN)generated by W irradiation of Cr(CO)6 in acetonitrile. In Scheme 1 the k H 2 0 step implies that protonation of the carbene carbon of 4- is concerted with cleavage of the Cr-C bond. The kinetic data are, however, also consistent with a two-step process in which rate-limiting protonation of 4- on the metal is followed by a rapid reductive elimination (Scheme 2). This alternative could not be rigorously excluded, but the concerted mechanism was preferred on the grounds that Scheme 2 requires the pKa of 6 to be unrealistically high. In this paper we report results of a detailed product and kinetic study of the hydrolysis of (benzylmethoxycarbene)pentacarbonylchromium(O), 7 4 3 , and a more
gH
limited study of the hydrolysis of the tungsten analogue, 7-W. Even though the reactions of 7-Cr and 7-W show similar kinetics as the hydrolysis of 4a,b, the major
latter is probably present as a mixture where X may be OH-, the buffer base, CH3CN and perhaps H20, as shown previous1y.l The identity of 8 was confirmed by HPLC and lH NMR analysis; both methods showed a 290% yield of 8 which is formed in an EIZ ratio of approximately 6. Furthermore, trace amounts of PhCH2CH=O were detected by lH NMR which, however, were too small to quantify. On the other hand, the hydrolysis of 7-W yielded about 25% PhCHzCH=O and 270% of 8 , the latter in an EIZ ratio of approximately 5.9. Kinetic Measurements. 7-Cr. At pH 11.5 were also performed in an Applied Photophysics DX. 17MV stopped-flow apparatus at 320 and 360 nm and monochromator entrance and exit slit widths of 2 mm. The data relating to the influence of the light intensity on the rate of conversion of 7-M to products were obtained in a 0.1 M KOH solution by varying the monochromator slit width from 0.1 to 8 mm; again, both the entrance and exit slit had the same setting. Time resolved spectra (Figure 1)were taken at 25 "C in a Hewlett Packard HP8452A diode array spectrophotometer equipped with a deuterium and a tungsten lamp; [7-Crlo = 0.18 mM, [KOH] = 0.0008 M, and p = 0.10 M (KC1). Under these conditions the observed process represents mainly the thermal reaction. pH and pKa Measurements. The pH in 50%acetonitrile50% water was determined according t o eq 734with pH,,,,
pH = pH,,,,
+ 0.18
(7)
referring to the reading of a pH meter calibrated with standard aqueous buffers. p e H values of acetic acid, N-methylmorpholine, and triethylamine were measured as the pH of buffer solutions prepared in a 1:l ratio of B:BH. Prior to a kinetic run,the buffer solutions of a given B to BH ratio were adjusted for constant pH by adding microliter amounts of HC1 or KOH, as needed.
Acknowledgment. This research was supported by Grant CHE-9307659 from the National Science Foundation. OM950544N (33) Strohmeier, W. Z.; Gerlach, K. 2.Nuturforsch. 1961,15B, 622. (34)Allen, A. D.; Tidwell, T. T. J . Am. Chem. SOC.1987,109, 2774.
