1501
J . Am. Chem. SOC.1992, 114, 1501-1502
On the Allylic Rearrangements in Metal Oxo Complexes: Mechanistic and Catalytic Studies on M ~ O ~ ( a l l y l o x o ) ~ ( C H ~and C N Analogous )~ Complexes
0
L\
J. Belgacem, J. Kress, and J. A. Osborn*
RO'
~
II IN
.., /OR Mo.
o
L
Laboratoire de Chimie des MPtaux de Transition et de Catalyse, URA au CNRS 424 UniversitP Louis Pasteur Institut Le Bel, 4, rue Blaise Pascal 67000 Strasbourg, France Received September 9, 1991
OR = allyloxo L = CH,CN, pyridine, 1/2 bipyridyl
Figure 1.
The 1,3 transposition of a hydroxy group on an allyl moiety using metal oxo catalysts is of both fundamental and practical importance. The isomerization of allyl alcohols can be catalyzed by complexes such as VO(OR),I or WO(OR)42(R = alkoxy or siloxy) at 130-200 OC, or by V O ( a ~ a c )or~ M 0 0 ~ ( a c a c at ) ~ 25 OC when activated by Me3SiOOSiMe3.3 Similar rearrangements have also been invoked in the mechanism of acrolein (and acrylonitrile) formation through the oxidation of propene on heterogeneous bismuth molybdate catalysts at 350-450 OC (Sohio p r ~ s s ) .The ~ determination of the detailed mechanism of these processes has been hampered by the absence of well-defined molecular complexes in which such a rearrangement takes place under mild conditions. We report herein studies on the complexes Mo02(allyloxo)zL2(allyl = CMe2CH=CH2or CH2CH=CMe2) and their imido analogues for which such allyl arrangements can be readily observed. Further, these complexes also effect the catalytic isomerization of allyl alcohols at 25 "C. By treatment of MoO2ClZwith lithium allyloxide (2 equiv) in CH,CN at -10 "C, followed by filtration to remove the LiCl formed, the complexes Mo02(allyloxo)2Lz(L = CH3CN, a; pyridine; or 'I2bipyridyl, b) can generally be isolated as white crystals after the addition of L. Their molecular structure (Figure 1) is analogous to that found for the corresponding alkoxo comp l e x e ~ according ,~ to their 'H NMR and IR spectra. When, ( la)6 is synhowever, MoO,(OCM~~CH=CH~)~(CD~CN), thesized in this fashion in CD3CN and then studied by 'H NMR at 25 "C, the growth of new resonances indicates the progressive conversion of l a into complexes 2a and 3a6 containing the rearranged OCHzCH=CMe, ligands (Scheme I).
Y
L
i
i
Y =O,NR L = CD,CN
Figure 2.
and 3a cannot be determined. On addition of bipyridyl to this solution, 3b6 can be isolated quantitatively. The same equilibration takes place (albeit more slowly) when starting from 3a. By measurement of the initial rate of conversion of l a to 2a for varying M) we find that -d[la]/dt concentrations of l a (3.6-10 X = k,[la] kz[laI2,with kl = 3 X s-' and kz = M-'
+
S-1.
The analogous imido complexes MoO(NR)(OCMezCH= CH2)2Lzcan also be synthesized and shown to isomerize similarly, although significantly more slowly. The following relative rates could be established qualitatively: 0 > NC6H4-4-NO2> NC,H, > NBu'. When R = But (L = CD,CN, 4a6), the rearrangement Scheme I is too slow to be observed before the oxidative dehydrogenation MOO~(OCM~,CH=CH~)~(CD~CN), + of the allyloxo ligands,' which occurs a t ca. 80 "C in this case. la The first-order term in the kinetic expression indicates that the Moo2(OCMe2CH=CH2) (OCHzCH=CMez) (CD3CN)2 allyl rearrangement takes place intramolecularly in a mononuclear 2a complex. Transfer of the allyl groups would occur to a vicinal MoO,(OCH~CH=CM~~)~(CD~CN), oxo ligand,* probably via a [3,3] sigmatropic shift (Figure 2) 3a similar to a Claisen rearrangement.' The greatly reduced rate At equilibrium (after ca. 5 h), the ratio of the IH resonances of this process when one oxo ligand is replaced by a more electron of the ligands OCMe2CH=CH2 to those of OCHzCH=CMe2 donating imido ligand would indicate that in the transition state is ca. 1:3, but the relative concentrations of the complexes la, 2a, some accumulation of negative charge takes place on Mo, with the migrating allyl group possessing some cationic character? The second-order kinetic term can be interpreted as arising from the ( I ) Chabardes, P.; Kuntz, E.; Varagnat, J. Tetrahedron 1977,33, 1775. presence of small quantities of a more rapidly rearranging dimeric (2) Hosogai, T.; Fujita, Y . ;Ninagawa, Y . ;Nishida, T. Chem. Lett. 1982, species which is in equilibrium with monomer in solution.1° 357. (3) Matsubara, S . ; Okazoe, T.; Oshima, K.; Takai, K.; Nazaki, H. Bull. Chem. SOC.Jpn. 1985,58, 844. (4) (a) Grasselli, R. K.; Burrington, J. D. Adu. Catal. 1981,30, 133. (b) Grasselli, R. K. J . Chem. Educ. 1986,63, 216. (5) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg. Chem. 1984,23, 1021. (6) Selected 'H NMR data (CD3CN, d ppmj are as follows. la: 6.18 (dd, 1 H, 3 J ~b - 17.3 HZ, 3 J ~ a=~10.6 c HZ, =CH,), 5.24 (dd, 1 H, 3 J q b ~ ,= 17.3 Hz,qH,, = 1.7 Hz,=CHbH,), 5.06 (dd, 1 H, 3JHcHa = 10.6 Hz, JHcHb = 1.7 Hz, = C H & ) , 1.48 (s, 6 H, CMe2). 39: 5.66 (t, 1 H, 3JHH = 9.5 Hz, = C H ) , 4.78 (d, 2 H, 3JHH = 9.5 Hz, CH2), 1.75 and 1.68 (2 s, 6 H, ==€Me2). 3b: 9.37 (d, 2 H, H6,6,),8.35 (d, 2 H, H3,3r),8.18 (m, 2 H, H5,5,),7.68 (m, 2 H, Hq4,),4.81 (t, 2 H, )JHH = 8.9 Hz, =CH), 4.27 (d, 4 H, 3JHH = 8.9 Hz, CH;), 1.46 and 1.28 (2 s, 12 H, =CMe2). 4s: 6.02 (dd, 2 H, 3JyHb = 17.4 HZ, 3 J ~ a=~10.7 c HZ,
