Organometallics 1983, 2,462-463
462 I 0.10
t l Figure 2. Dependence of the observed first-order rate constant on concentrationof the initial oxidant (CH2C12,2 “C, initially 1 atm of CO,and Cp(PPh3)COFeCH3= 0.10-0.15 M): circles, ferricinium ion as oxidant; triangles, acetylferricinium ion as oxidant.
showed only the acyl couple (4 + e- + 2). This behavior is consistent with partial electrogeneration of the alkyl cation radical 3 that then rapidly catalyzes the carbonylation of l to 2 in the region near the electrode. The mechanism is Scheme I is strongly supported by kinetic studies in CH2C1, at 2 “C. Runs were initiated by injection of a small volume of a solution of ferricinium or acetylferricinium tetrafluoroborate into solutions of 1 (10-15 mM), saturated with CO at 1 atm (ca. 6 mM), and the progress of the reaction was monitored by following the decrease in absorbance at 500 nm, associated with the conversion of 1 to 2. Under these conditions of [l]> [CO], first-order kinetics were observed, for which the rate constants were independent of the concentration of 1, but linearly dependent on the initial concentration of the oxidizing agent, [0Xli. Thus the experimental rate law is given by rate = k[CO][OXIi
k = 500 f 100 M-’s-’
(2)
in which there is zero-order dependence on [ 11, and k has been evaluated from the slope of the plot in Figure 2. Under conditions where [CO] >> [l], a striking linear decrease in absorbance with time was observed. It continued until the absorbance corresponding to complete conversion to 2 was reached, when a sharp break occurred and the absorbance did not change further. This zero-order behavior is fully consistent with the rate law in eq 2. With assumption of steady-state concentrations of 3 and 4, the reaction sequence in Scheme I leads to the rate expression (3)
Provided he,[ I] >> k,[CO] (a reasonable condition, since the electron self-exchange reaction for related ferrocene derivatives is very r a ~ i d )the , ~ above expression simplifies to eq 2 in which the experimentally determined constant (7) The observed first-order dependence on the concentration of carbon monoxide is consistent with either an associative process or with a two-step mechanism involving a 15-electron intermediate for which the concentration of CO is insufficient to detect rate saturation.
k is to be equated with the Fe(II1) carbonylation rate constant kc. Thus,the rate of carbonylation of the formally iron(II1) methyl complex 3 is conservatively estimated to be 107-108 faster than the carbonylation rate of the formally Fe(I1) species. This difference between the rates of carbonylation in the Fe(I1) and Fe(II1) states is most likely attributable to a substantial reduction in the d a donor ability of the metal center on oxidation, which allows a bent (or perhaps side-on) CO configuration in the activated complex to be energetically much more a c c e s ~ i b l e . The ~ ~ ~crucial role of a back-bonding in the migratory insertion is also attested to by the ability of an external entering group, which is a strong a acid such as CO, to drive the reaction on Fe(I1) thermodynamically much farther than a weaker a acid such as NCCH, .(Kco/KNcc~s > 1015).’0 It must be pointed out that parts per million concentration of an oxidant is, in principle, sufficient for the redox-catalyzed pathway to carry the entire reaction, so that interpretation of kinetic and stereochemical results must be done with caution. As this type of redox catalysis is most likely not restricted to iron, nor even to carbonylation reactions, the implications for organometallic reactivity may well be far-reachingal’ Acknowledgment. We wish to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Graduate School of Boston University for support of this research. Registry No. 1, 12100-51-5;2, 12101-02-9. (8) Pladziewicz, J. R.; Espenson, J . H. J. Am. Chem. SOC.1973, 95, 56-63. (9) Hoffman, R.; Berke, H. J. Am. Chem. SOC.1978,100,7224-7236. (10) KAN= W0M-I3 and Kco must be >lo5 since no 1 is detectable after the catalyzed carbonylation of 1. (11) Enhanced reactivity toward ligand substitution has been reported in other 17-valence-electron organometallic species. (a) Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J.Am. Chem. SOC.1982,104,3034-3043. (b) McCullen, S. B.; Walker, H. W.; Brown, T. C. Zbid. 1982, 104, 4008%4010. (c) Hepp, A.’F.;Wrighton, M. S. Ibid. 1981,103,1258-1261. (d) Absi-Halabi, M.; Brown, T. L. Zbid. 1977, 99, 2982-2988.
Cp2M(s-trans q4-butadiene) Complexes from Zlrconocene and Hafnocene Dlhalldes and “Magnesium Butadiene” Ulrlch Dorf, Klaus Engel, and Gerhard Erker” Abteilung fur Chemie der Ruhr-Universitat 0-4630 Bochum 1, Germany Received September 28, 1982
Under the kinetic control reaction of “magnesium butadiene” (2) with Cp,MCI, complexes la-c (Cp = q-C5H5, q-C,Me,; M = Zr, Hi) gives (strans -q4-butadiene)metallocenecomplexes 4a-c, the products expected from a stepwise reaction through Cp,MCI-substituted crotylmagnesium halide intermediates 3. Summary:
(2-Butene-1,4-diyl)magnesium(2; magnesium butadiene), conveniently obtained as an oligomer upon treatment of butadiene with magnesium metal in THF,l has been shown to be a versatile reagent for the preparation of (1) Pujita, K.; Ohnuma, Y.; Yasuda, H.; Tani, H. J . Organomet. Chem. 1976, 113, 201.
0276-7333/S3/2302-0462$01.50/00 1983 American Chemical Society
Organometallics 1983, 2, 463-465 Scheme I
'c I 2 -
1
463
6 98.6 (Cp), 92.8 (d, J = 147 Hz, CH), 57.8 (dd, J = 142, 148 Hz, CH,)) is formed. Clearly 4b is the thermodynamically less favored (butadiene)hafnocene isomer, since at 60 "C in benzene solution it completely rearranges to (s-cis-butadiene)hafnocene(6b: lH NMR (CHFCl,, -120 "C) 6 5.17, 4.66 (s, 5 H each, Cp) 5.0 (m, 2 H, CH), 2.74, -0.73 (m, 2 H each, CH,); 13CNMR (benzene-d6,ambient temperature) 6 102.2 (Cp), 114.5 (d, J = 156 Hz, CH), 45 (t, J = 140 Hz, CH,)). The thermally induced rearrangement 4b 6b can be described by a first-order rate law. A Gibbs activation energy of AGtGO0C= 24.7 f 0.3 kcal mol-' has been obtained for the isomerization process. (s-tram-Butadiene)permethylzirconocene (4c: 13CNMR (benzene-de)6 11.7 (CH,), 66.2 (dd, J = 153, 142 Hz, CHJ, 102.0 (d, J = 147 Hz, CH), 110.0 (Cp-C)) is formed exclusively upon treatment of IC with 2 at 25 "C. Isomerization to the thermodynamically favored (s-cis-butadienelisomer (6c: 13CNMR (benzene-& ambient temperature) 6 11.8, 12.0 (CH,), 56.5 (dd, J = 150, 145 Hz,CH,), (CH-hidden under solvent), 119.5, 119.4 (Cp-C)) has an even larger activation barrier: equilibration 4c e 6c slowly goes to completion (40:60) upon prolonged thermolysis at 140 "C ( 7 1 / 2 i= 10 h). From the observed formation of 4 under kinetic control in cases where (s-trans-butadiene)metallocenesare of about equal (4a,c) or even substantially lower thermodynamic stability (4b) than their s-cis-diene isomers 6a-c the reaction path followed can be deduced. Initial transition metal to carbon bond formation between 1 and the magnesium reagent 2 yields 3. This bimetallic intermediate shows the expected reactivity of a CpzMC1-substituted crotyl Grignard reagenta4 Preferred intramolecular nucleophilic attack by the internal allylic carbon center (a) leads to a formal analogue of a three-membered metallacyclic reaction product: the (7,-butadiene)metallocene intermediate 5, from independent trapping experiments6s8 known to equilibrate much more rapidly with 4 than with 6.
-
r
i 1
(q4-diene)transition-metal complexes from metal halides.2 Interestingly, the seemingly obvious reaction mechanism to account for this observation-formation of a metallacyclopentene intermediate via bond formation between the metal center and the most negatively charged terminal carbon atoms of the C4chain-clearly is incompatible with the reaction course established for electrophilic attack on "butadiene dianion" eq~ivalents.~ The outcome of such stepwise reactions appears to be determined by the reactivity of the substituted allyl anion intermediate4formed first. Consequently, under kinetic control the formation of three-membered not five-membered ring systems in the product-determining step is favored starting from "[C4H62-]"and gem dihalides.',, Using suitable reaction conditions, we now have obtained experimental evidence that this reaction pattern is also retained in the reaction of transition-metal dihalides la-c with "magnesium butadiene" At ambient temperature the mutual rearrangement of the (q4-butadiene)zirconoceneisomers is known to be fast (4a 6a: AG*10.50C= 22.7 f 0.3 kcal mol-l)! As expected, the well-known, independently accessible equilibrium mixture of (s-cis- and s-trans-butadiene)zirconocenecomplexes 6a and 4a (4555) is obtained by treating an ethereal suspension of 2 with zirconocene dichloride at 25 "C. In contrast, performing this reaction as well as the subsequent workup below -10°C yields only (s-trans-butadienezirconocene (4a: 13CNMR (toluene-d,, -10 "C) 6 99 (Cp), 96 (d, J = 152 Hz, CH), 59 (dd, J = 149, 159 Hz, CH,); 75% isolated yield). Within the accuracy of the low-temperature NMR (lH, 13C) analysis, under kinetic control exclusively isomer 4a is formed in the reaction of la with "magnesium butadiene" 2. Analogous results have been obtained on reacting 2 with hafnocene dichloride (lb) and bi~(~~-pentamethylcyclopentadieny1)zirconocene dichloride (IC). At 0 "C (strans-butadiene)hafnocene (4b:' 13C NMR (benzene-d6)
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(2)Wreford, S. S.;Whitney, J. F. Inorg. Chem. 1981,20, 3918,and references therein. (3)Bahl, J. H.; Bates, R. B.; Beavers, W. A,; Mills, N. S. J. Org. Chem. 1976,41,1620. Richter, W. J. Angew. Chem. 1982,94,298. (4)Benkeser, R. A. Synthesis 1971,347. (5)The reaction of 2 with la,bunder thermodynamic control has been reportad recently: Yasuda, H.; Kajihara, Y.;Mashima, K.; Nagasuna, K.; Lee, K.; Nakamura, A. Organometallics 1982,1, 388. (6)Erker, G.;Wicher, J.; Engel, K.; Rosenfeldt, F.; Dietrich, W.; Krtiger, C. J. Am. Chem. SOC.1980,102, 6344. Erker, G.;Engel, K.; Krtiger, C. 9th International Conference on Organometallic Chemistry, Toronto 1981,Abstr. 2 E 74.
0276-733318312302-0463$01.50/0
Acknowledgment. Financial aid from the Minister fiir Wissenschaft und Forschung des Landes NordrheinWestfalen is gratefully acknowledged. Registry No. la, 1291-32-3; lb, 12116-66-4; IC, 54039-38-2; 2, 70809-00-6; 4a/6a, 75374-50-4; 4b/6b, 80185-89-3; 4c/6c, 84498-96-4. (7)For 'H NMR data see: Benn, R.; Schroth, G. J. Organomet. Chem. 1982,228,71. (8)Erker, G.;Dorf, U.;Engel, K.; Atwood, J. L.; Hunter, W. E. Angew. Chem. 1982,94,915,916.
Possible Formation of Ferrabenzene and Its Novel Conversion to 1,3-Diphenyl-2-methoxyferrocene R. Ferede and Neli 1.Allison" Department of Chemistry, University of Arkansas Fayetteville, Arkansas 7270 1 Received August 23, 1982
Summary: Introduction of ( E , € ) -1,4-dilithio-l,4-diphenylbutadiene (2) to q5-C5H,Fe(C0)J followed by alkylation with (CH,),OBF, gives l,3-diphenyl-2-methoxyferrocene (4). Plausible intermediates in the formation of 4 include a ferrabenzene complex 3. After reductive elimination, loss of a carbonyl, and alkylation, 3 gives 4. 0 1983 American Chemical Society
