Regioselective Hydrometalation of Alkenes Reveals the Amphipolar Nature of the Pd-H Bond in Heterogeneous Hydrogenation

Full text of DOI:10.1021/jo971602c

8618
J . Org. Chem. 1997, 62, 8618-8619
Ta ble 1
Regioselective Hyd r om eta la tion of Alk en es
Revea ls th e Am p h ip ola r Na tu r e of th e
P d -H Bon d in Heter ogen eou s
Hyd r ogen a tion
J inquan Yu and J onathan B. Spencer*
University Chemical Laboratory, Lensfield Road, Cambridge
CB2 1EW, U.K.
Received August 28, 1997
Catalytic hydrogenation using heterogeneous transi-
tion-metal catalysts, such as palladium on carbon, is an
important synthetic method widely used for the last 50
years, for example in the selective reduction of alkynes
to cis-alkenes.1 Recently, there has been much interest
in developing heterogeneous catalysts modified with
chiral auxiliaries that can be used for asymmetric reduc-
tion of prochiral alkenes and ketones.2 To understand
the fundamental mechanism at a molecular level, a huge
research effort has been concentrated on finding a link
between hydrocarbon reactivity on the metal surface and
organometallic chemistry in solution.3-8 Even though
evidence has been provided that â-elimination occurs on
the metal surface9 in a manner analogous to that that
has been rigorously proved in organometallic chemistry,10
the key step of hydrometalation, which occurs prior to
â-elimination, has eluded detailed investigation.
Recently, we reported a new method for probing the
hydrometalation step in homogeneous hydrogenation
that has established how the metal hydrogen addition
takes place in organometallic chemistry.11 This new
finding allows for the first time a direct comparison to
be carried out between hydrometalation on the metal
surface and that in organometallic chemistry under
normal catalytic conditions. By studying the location of
the deuterium in electronically polarized trans-alkenes,
formed by the isomerization of the cis-alkenes in the
presence of palladium catalyst and deuterium, we have
been able to determine the regioselectivity of hydrometa-
lation on the metal surface. The results reveal for the
first time that the hydrometalation on the metal surface
bears great similarity to that established in organome-
tallic chemistry that occurs by two possible electronic
modes (a Mδ+ - Hδ- or b Mδ- - Hδ+, Scheme 1).
Intriguingly, modification of palladium with lead acetate
(Lindlar catalyst)12 is shown to dramatically enhance
mode a , thus affording the first clue to the origin of
superior selectivity of this catalyst.
a
If CH3CH2OH is used as the solvent instead of CH3CH2OD
the same distribution of deuterium occurs in the trans-alkenes
1-9, however, the incorporation is slightly lower because dilution
with hydrogen takes place by the exchange of the protons in the
solvent with the deuterium on the palladium.15 Cis/trans refers
b
to the deuterium being cis/trans to the aromatic group. Some
compound was identified with two deuteriums in the terminal
position of the double bond. The proportion of this compound
increased with a longer reaction time as did that of the monodeu-
terated species. c The same result was obtained in benzene.
Following the hydrometalation of the double bond of
cis-alkene, either reductive elimination can occur to give
the alkane or â-elimination can take place to afford the
trans-alkene (Scheme 1). The location of the deuterium
in the trans-alkene formed by the latter pathway acts as
a reporter of the regioselectivity of hydrometalation
(Table 1). The results show that alkenes containing
solely electron-withdrawing groups conjugated to the
double bond (1 and 2) have deuterium located only
remote to these functional groups. These alkenes are
polarized so that the carbon remote to the carbonyl group
is electron deficient and would be consistent with hydro-
metalation occurring by mode a (Mδ+ - Hδ-). Those
alkenes with electron-donating groups conjugated to the
double bond (3 and 4) are also found to contain deuterium
only remote to the functional group. The electronic
polarity in these alkenes is the reverse of that in
compounds 1 and 2 with the carbon remote to the
aromatic groups being electron rich and would facilitate
addition by mode b (Mδ- - Hδ+). The level of deuterium
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Communications
J . Org. Chem., Vol. 62, No. 25, 1997 8619
Sch em e 1
incorporation into the trans-alkenes (Table 1) is very close
to that found with homogeneous palladium catalyst and
infers that â-elimination (which should result in 100%
deuterium incorporation) is not the only mechanism that
may be operating.11 These results demonstrate that
hydrometalation is operating on the surface of metal
catalysts with great similarities to that found in orga-
nometallic chemistry.
Ta ble 2
Alkenes containing difunctional groups conjugated to
the double bond (5-9) can potentially promote both
modes of addition. Comparison of compound 6 with 7 is
particularly revealing as the structural difference be-
tween these two compounds is minimal so that the
electronic effect of the methoxy group can be clearly
identified (Table 1). The increase in the amount of
deuterium located remote to the aromatic ring of 7
suggests that the strong electron-donating ability of the
p-methoxy-substituted ring promotes addition by mode
b. This illustrates that the regioselectivity of hydro-
metalation of difunctional alkenes is very sensitive to
electronic effects.
Using the same substrate with different catalysts, the
regioselectivity of deuteration should reveal the relative
intrinsic electronic properties of the transition metal to
promote mode a or b. This will allow a correlation to be
made between reactivity and the basic electronic property
of the catalyst. The Lindlar catalyst (palladium treated
with lead acetate) has much better selectivity than
unmodified palladium for the reduction of alkynes to cis-
alkenes.12 Although this catalyst has been used to great
effect for over 40 years, the origin of its improved
properties has remained obscure. When the cis isomers
of compounds 6, 7, and 9 (Table 2) were isomerized with
the Lindlar catalyst and deuterium, the results clearly
show that the modification of palladium in the Lindlar
catalyst promotes mode a (Pdδ+ - Hδ-). Since alkynes
are more readily attacked by nucleophiles than alkenes,13
the improved selectivity of the Lindlar catalyst could be
due to the modification of the palladium enhancing mode
a , and hence, less over-reduction of the cis-alkene occurs.
When the results for heterogeneous palladium (Table
1) are compared to those obtained for the homogeneous
palladium catalyst,11 it is observed that mode a (Mδ+
δ-) is also enhanced in the homogeneous system,
-
H
probably owing to the phosphine ligands donating elec-
trons into the palladium. The homogeneous palladium
catalyst also has higher selectivity than that of palladium
on carbon,14 thus being consistent with the rationale put
forward for the Lindlar catalyst where an increase in
mode a correlates with the preferential reduction of
alkynes.
This study identifies the amphipolar nature of the
palladium hydrogen bond in heterogeneous hydrogena-
tion and reveals the detailed similarities between surface
and organometallic chemistry. The discovery that pal-
ladium is balanced in its ability to react either by mode
a or mode b opens the way for tuning the electronic
properties of the metal to favor one of these processes.
This should facilitate the logical design of new catalysts
for many key reactions used in the laboratory and
industry.
Ack n ow led gm en t. We thank the Royal Society for
the award of a Royal Society University Research
Fellowship (J .B.S.) and the British Council and the
Chinese Goverment for the award of a Sino-British
Friendship Scholarship (J .Y.).
(13) Strozier, R. W.; Caramella, P.; Houk, K. N. J . Am. Chem. Soc.
1979, 101, 1340.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and spectroscopic and analytical data for compounds 1-9
(6 pages).
(14) Takaya, H.; Noyori, R. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol.
8, pp 443-463.
(15) Horiuti, J .; Polanyi, M. Nature 1933, 132, 818.
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