Our findings also support olefin co-ordination prior to oxida-
tive addition of dihydrogen. Sinou and co-workers 4a have
shown that the addition of α-acetamidocinnamic acid (ACA)
to [Rh(L–L)(H2O)2]ϩ (L–L = tetra sulfonated BDPP) gives an
olefin chelate complex in water. The 31P NMR spectrum of this
complex exhibits a doublet of doublets at δ 56.2 (JRh–P = 169,
JP–P = 43 Hz) and a doublet of doublets at δ 43.6 (JRh–P = 157,
JP–P = 43 Hz) and we observe an approximate weighted average
of these features in the spectrum of the AAA complex. There is
no indication that the bis(aqua) complex undergoes oxidative
addition of H2 and the postulated catalytic cycle is thus the
same as that found for the bis(methanol) complex (Scheme 1).
the corresponding bis(methanol) system. However, in water
there is also a dramatic effect of pH on the rate of hydrogen-
ation. First it should be noted that the very high rates at low pH
made a detailed kinetic investigation impossible, but yet it
seems reasonable to assume that the basic mechanistic pattern,
rapid olefin co-ordination followed by rate determining oxid-
ative addition of hydrogen, is the same at low pH. This implies
that oxidative addition of H2 is faster at low pH and has also
been found for [Ir(CO)Cl(TPPMS)2].6 However, the rate
enhancements brought about by pH changes for this latter
complex are relatively small. In our system we see large effects
and a leap in reactivity between pH 3.2 and 4.5, indicative of
the involvement of a protolysis step. It can probably not be
ascribed to protolysis at the metal centre, since the 31P NMR
experiments point to a pKa of the water ligands higher than 4.2,
and furthermore the leaving ligand (whether water or hydrox-
ide) is not present in the transition state. The key feature instead
seems to be the protolysis of the olefin and the implications this
will have on its co-ordination. Between pH 3.2 and 4.5 the AAA
goes from being almost completely protonated to being com-
pletely deprotonated. As shown by 31P NMR this changes the
mode of co-ordination and the carbonyl complex obviously
undergoes oxidative addition much faster than the carboxylate
complex. Thus, both the earlier found pH effect on oxidative
addition and the fact that the reactant in reaction 1 changes
(Scheme 1) account for the observed pH dependence. The com-
plex [Rh(DPPB)Cl(solv)] has previously been used as a catalyst
for the hydrogenation of ACA in organic solvents.17 The hydro-
genation rate is comparable to those rates observed for the
[Rh(DPPBTS)(NBD)]ϩ catalysed hydrogenations of AAA at a
pH higher than the pKa for the substrate. The reason for the
extremely high rates observed at low pH is, however, hard to
rationalise and must thus be further investigated.
+
COOH
P
P
S
S
NHC(O)CH3
Rh
1
4
K1
+
C
+
HOOC
P
NH
O
P
NH
Rh
Rh
P
COOH
CH3
O=C
H
P
+H2, k
2
+
HOOC
3
NH
H
P
Rh
O=C
P
H
Scheme 1 Proposed catalytic cycle for the hydrogenation of AAA with
complex 4.
Acknowledgements
We gratefully acknowledge TFR (Swedish Research Council for
Engineering Sciences) for financial support.
Assuming reaction (2) (Scheme 1) to be rate determining gives
a rate law of the form (2) which if K1[olefin] ӷ 1 reduces to
v = kK1[olefin][H2][Rh](1 ϩ K1[olefin])
eqn. (3) which if kobs = k[H2] is consistent with the experimental
v = k[H2][Rh] (3)
(2)
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So far everything in the present bis(aqua) system agrees with
2874
J. Chem. Soc., Dalton Trans., 1999, 2871–2875