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Catalytic Dehydrogenation of Cyclohexene on Silica Overlayer Films.

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[I] G. Witt~g,M. Rieber, Justus Liebigs Ann. Chem. 562 (1949) 187: G. Wittig, K. Claul3, h i d . 577(1952) 26, 578 (1952) 136.
[2] P. J . Wheatley, C. Wittig, J . Chem. Soc. 1962, 251: P. J . Wheatley, ibrd.
1964. 2206.
131 P. J. Wheatley, J . Cliem. Soc. 1964, 3718: A. L. Beauchamp, M. J. Bennett, F. A. Cotton, J . Am. Chem. Sac. YO (1968) 6675.
141 C. Brabant, 3. Hubert, A. L. Beauchamp, Can. J. Chem. 51 (1979)
2952.
[5] D. H. R. Barton, J. P. Finet, Pure Appl. Chem. 59 (1987) 937.
161 D. H. R. Barton. N. Y. Bhatnagar, J:C. Blazejewski, B. Charpiot, J:P.
Finet, D. J. Lester, W. H. Motherwell. M. T. B. Papoula, S. P. Stanforth,
J . Chem. Soc. Perkin Trans. I 1985. 2657, 2667.
171 D. Hellwinkel, M. Bach, Justus Liebigs Ann. Chem. 720 (1969) 198.
[8] Bi(p-CH,C,H<), is apparently formed a s a finely crystalline, violet
powder by the reaction of Bi@-CH,C,H,),CI, with p-CH,C,H,Li in the
molar ratio I :2 in ether below - 10°C. I t decomposes with decoloration
already at - 10 “C.
19) Bi(C,3H5)-(Cc,F5)2:
A solution of CoHsLi (freshly prepared from C,FsBr
and n-C,H,Li) in ether was added dropwise at -78°C 10 a suspension
of Bi(C,H,),CIZ in anhydrous ether. The mixture was slowly warmed t o
room temperature, filtered, and the recovered solid recrystallized from
ether. Pure (CbH5)1(C6F5)2was obtained in the form of orange, watersensitive crystals, m.p. I04 “C “F-NMR (ether, CHCll ext.):
6(0)= - 118.6,6(m)= - 160.2,6@)= - 155.5. Correct elemental analysis.
The crystals are triclinic a n d (systematically) twinned.
[lo] Siemens diffrdctometer with low-temperature facility, after H. Dietrich,
H. Dierks, Messtechnik 78 (1970) 184.
Catalytic Dehydrogenation o Cyclohexene
on Silica Overlayer Films**
By Jeffrey M . Cogen, Khosro Ezaz-Nikpay.
Ronald H . Fleming, Scott M . Baumann, and
Wilhelm F. Maier*
Heterogeneous dehydrogenation of cyclohexene 1 to
benzene 3 has received attention as a model reaction for
dehydrogenation processes.[’] This reaction is not stoichiometric (see temperature dependence of the product
distribution in Fig. 3) and requires the presence of dihydrogen.
2
1
3
The reaction is not only catalyzed by noble metals, but
also by spillover hydrogenl2I as well as on carbonaceous
overlayers on Pt.L3,41
Chemically bound hydrogen atoms on
Te were proposed as active sites for cyclohexene dehydrogenation in Te-NaX zeolites.151The reaction also occurs on
gold covered Ptl6l and may be due to active spillover hydrogen on the Au surface.[’] These studies indicate that an
exposed transition metal surface may not be necessary to
catalyze this reaction.
To test the importance of an exposed transition metal
surface the catalytic properties of Pt films supported on Si
wafers with and without silica overlayers were compared
(see Fig. 1). It was hoped that dihydrogen would diffuse
readily through the overlayer to become activated (dissociated) at the underlayer. This activated hydrogen could
then migrate back to the oxide surface and was expected to
~~
[*I
[**I
Prof. Dr. W. F. Maier, J. M. Cogen, K. Ezaz-Nikpay
Department of Chemistry
University of California
Berkeley, C A 94720 (USA)
Vr. K. H. Fleming, Dr. S. M . Baumann
Charles Evans & Associates
Redwood City, C A 94063 (USA)
This study was supported by the National Institutes of Health (Grant
GM-33386-01). We thank Dr. R. S . Rai for the SEM analyses.
1182
0 V C H Verlagsgesellschaft mbH. 0-6940 Weinheim. I987
exhibit catalytic reactivity different from that of the exposed transition metal surface.
\
I H H
SiOz
Pt
Si
Fig. I . Schematic representation of the overlayer catalysts.
The film catalysts were prepared by deposition of
l o + 1.5 nm of Pt by electron beam evaporation at lo-’
torr onto polished silicon single-crystal wafers. Overlayers
were deposited at a rate of 30-40 nm/min by electron
beam evaporation of S O z (from quartz) and the thickness
was followed by a quartz crystal thickness monitor. The
films were then transferred to a gas-phase flow reactor to
test the catalytic activity at atmospheric pressure with dihydrogen or dideuterium as carrier gas. These SiOJPt/Si
films catalyze the hydrogenation and dehydrogenation of
cyclohexene, while in control experiments Si02/Si proved
to be catalytically inert.
The effect of the silica overlayer thickness on the rate
of conversion of 1 to 2 and 3 at 250°C was studied for
Si02/Pt/Si.[81 The reaction rate per unit of geometric surface area drops exponentially with increasing overlayer
thickness up to about 30 nm after which the rate stabilizes
at about 1/100 (3 orders of magnitude above the background) of the activity of exposed metal and becomes
nearly independent of thickness (overlayer thicknesses u p
to 2200 nm were investigated). During this study we noted
that the activity decreases with decreasing deposition rate
of the S i 0 2 overlayer. Annealing of the overlayer catalyst
prior to the reaction also results in a reduction of activity.
These data suggest that catalytic activity requires defects in
the overlayer film which can serve as carriers of the dissociated hydrogen and/or dihydrogen.
To exclude edge effects the F’t was deposited through a
window leaving an outer frame (2 mm) of uncovered silicon so that the subsequent Si02 deposition completely
covered the edges of the Pt film. On such films the masked
(Pt-free) area showed no catalytic activity providing evidence against lateral Pt diffusion. As expected from the
low concentration (based on simple geometrical estimates)
of exposed edge platinum on unmasked films there was no
detectable difference in the activity of masked and unmasked overlayer films. Scanning electron microscopy
(SEM) of new and used overlayer catalysts gave no evidence for cracks or pores in the overlayer. Subsequent
deposition of silica or gold onto used overlayer films (to
fill any cracks or pores) did not decrease catalytic activity.
Relative rates of deuteriation of n-hexane and n-octane
(first order in hydrocarbon) on overlayer catalysts correlate with boiling point (as with F’tr91)and not with hydrocarbon diffusion rates. These experiments are evidence
against cracks or pores as carriers of catalytic activity.
The dependence of catalytic activity on S O z overlayer
thickness is most likely due to hydrogen recombination
processes. However, due to analytical limitations we can
not rule out partial accessibility o f the Pt films with thin
silica overlayers. For this reason subsequent experiments
were conducted on thicker ( > 60 nm) S O z overlayers for
which possible exposure of Pt could be addressed.
Auger electron spectra (AES) and thickness profiles of
such films are very clean and no transition metal diffusion
into the overlayer or to the surface has been detected.[“”
0570-0833/87/1111-1182 $ 02.50/0
Angew. Chem. Inr. Ed. Engl. 26 (1987) No. I 1
However, the rather low sensitivity of AES (about 0.5% for
Pt) is not suitable for trace analysis. The most sensitive
method for trace analysis of such layered thin films is secondary ion mass spectrometry (SIMS, sensitivity limit
< 0.0005 atom-% for Pt):' 'I which was used to analyze representative thin film catalysts before and after use. Rutheford back scattering spectrometry (RBS, sensitivity limit = 0.005 atom-% for Pt)[''l was also used for chemical analysis and thickness measurements of the overlayer films.
SIMS depth-profiling is based on sputtering which leads to
unavoidable tailing of one layer into the next, while RBS is
nondestructive and provides a much better depth resolution
and accuracy. The RBS data confirmed film thicknesses in
all cases. RBS and SIMS (see Figure 2) gave no evidence for
Pt at or near the oxide surface. The RBS profiles show that
the thickness of the films is homogeneous and that the surface of the SiOz is continuous and not rough. Although very
close to exact stoichiometry, a slight oxygen deficiency in
the silica overlayer was sometimes noted by RBS (33-36'10
Si :67-64% 0). Catalytic activity due to platinum on the
silica surface at concentrations below the detection limit of
our analytical techniques (= lo8 Pt atoms/cm2) was excluded by the lack of activity after impregnation and activation of SiO,/Si films with readily detectable Pt concentrations of 108-10i4P t / ~ m ~ . ~ ' ~ l
due to cracks or pores or diffusion of hydrocarbons
through the silica layer.
Upon prolonged use of these films RBS and SIMS indicate sometimes that there is a slow diffusion of Pt into the
silica overlayer (concentration 30 ppm Pt in S O z ) which is
currently under investigation. I n this study each experiment was run on an unused film for time periods too short
for significant Pt diffusion (confirmed by SIMS and RBS).
The effect of temperature on the rate (turnover frequency, N I , molecules cm-' sec-') and selectivity of a 72 nm
SiO,/Pt/Si was remarkably similar to that on a Pt/Si catalyst (Fig. 3). Over the temperature range studied the 3 / 2
ratio was slightly lower on the SiO,/Pt/Si. The activation
energies for the overall reaction of cyclohexene are:
Pt/Si = 2.09 _t 5 kJ/mol, SiOz/Pt/Si = 16.3 f 5 kJ/mol ;
for formation of benzene: Pt/Si = 82.8 k 14 kJ/mol,
SiO,/Pt/Si = 92.5 _t 6 kJ/mol. The differences between the
exposed Pt and the overlayer films are not believed to
be significant.
100
1061
40
C6H, ( P t l
I
A
/
-
1810,l
#H6
A.
21
1000/ T [K-']
d [nrnl
-
83 nm Si 36 0 6 4
0 9 - 9 nrn P t 83 SI 17
08-
0
3 nm P t 50 SI 50
0
20
40
60
d [nrn]
-
80
170
190
,
210
,
,
,
230
,
250
T ior1-
Fig. 3. Comparison o f t h r Arrhenius plots (left) and the temperature dependence of the product composition (right) of cyclohexene conversion (low conversion conditions) on a 7 2 nm SiOJPt/Si film with that on an exposed Pt/
Si film.
1
0 7-
-
22
. .
100
Fig. 2. Representative SIMS (left) and KBS (right) depth profiles of a used,
active Si02/Pt/Si catalyst. Chemical composition of overlayers is given in
atom-% in the upper left corner of the RBS profile. IC = ion counts, d=layer
thickness, A F = atomic fraction.
Single crystal experiments have shown that prompt and
extensive carbonation occurs on Pt during dehydrogenation of 1 while the reaction continues.131To test for sensitivity of SIMS for carbon at the Si02-Ptinterface, we used
a Pt/Si film for the catalytic reaction and then deposited
the silica overlayer. The SIMS profile of this film indicated
extensive carbonation at the Si0,-Pt interface showing that
SIMS can detect low carbon concentrations in these overlayers. Other overlayer films show no detectable increase
of the carbonation below the S i 0 2 surface upon use. This
also serves as evidence against catalysis at the interface
Angen,. Chem Inr. Ed. Enyl. 26 (1987) No. I I
Since isotopic product distributions have been useful as
characteristic fingerprints for substrate catalyst interactions it was hoped to observe differences between the deuteriation patterns obtained on the oxide and on the Pt surface upon substitution of dideuterium for dihydrogen as
carrier gas. However, the isotopic distributions of the products benzene and cyclohexane are very similar and show
only a slightly higher deuterium content o n Pt/Si relative
to the products obtained on SiOz/Pt/Si.
The similarity of the catalysis on the oxide overlayer to
that on the Pt film suggests that this dissociated hydrogen"41 (whatever its nature on the surface) and not the exposed metal represents the active site for this catalytic
reaction. The presence of active hydrogen on a surface
may then be more important than the chemical nature of
the surface. This conclusion is consistent with the wide variety of surfaces known to catalyze this reaction."-61
In summary, inert silicon dioxide thin films become catalytically active in the presence of a platinum underlayer.
Additional studies in our laboratory have shown that this
phenomenon is not limited to SiOz on Pt, but has been
observed with many other over-underlayer combinations.
The most promising potential features of such catalystsinduction of catalytic activity on new surfaces and the resistance to poisoning due to protection of the transition
metal by the overlayer-have now to be investigated.
0 VCH Verlag.~ge,sellschajimbH. 0.6940 Weinhetm. 1987
Received: June 16, 1987;
revised: September 3, 1987 [Z 22YX 1E)
German version: Anyew. Cliem. 99 (1987) 1222
0570-0833/87/1111-118~ $ 02.50/0
1183
[I] P. Tetenyi, K. Schachter, Acta Chim. Acad. Sci. Hung. 50 (1966) 129: J.
E. Germain, M. Cathala, Chem. Ind. (London) 58 (1971) 1389; V. V. Lunin, A. E. Agronomov, L. K. Denisov, Vestn. Mosk. Univ. Ser. 2 Khim.
17 (1976) 103; J. K. S. Wang, K. Wolf, R. D. Heydine, Stud. Surf. Sci.
Catal. 19 (1984) 561: A. Clearfield, J . Mol. Catal. 27 (1984) 251; S. A.
Nyarady, R. E. Severs, J . Am. Chem. Sac. 107 (1985) 3726; R. H. Crabtree, C. P. Parnell, OrganometaNics 4(1985) 519.
[2] W. C. Conner, Jr., G. M. Pajonk, S. J. Teichner, Adu. Catal. 34 (1986) I ;
S. Khoobiar, J . Phys. Chem. 68 (1964) 41 1 ; S. Khoobiar, R. E. Peck, €3. J.
Reitzer, Proc. Inr. Congr. Catal. 3rd. 1965. 338; P. A. Sermon, G. C.
Bond, J . Chem. SOC.Faraday Trans. I 76 (1980) 889.
[3] S. M. Davis, G. A. Somorjai, J . Catal. 65 (1980) 78; S . M. Davis, F.
Zaera, G. A. Somorjai, ibid. 77 (1982) 439.
[4] E. Segal, R. J. Madon, M. Boudart, J. Catal. 52 (1978) 45.
[51 G. L. Price, Z. R. Ismagilov, J. W. Hightower, J. Cutal. 73 (1982) 361.
[6] J. W. A. Sachtler, M. A. Van Hove, J. P. Biberan, G. A. Somorjai, Phys.
Reu. Lett. 45 (1 980) 160I .
[7] B. J. Wood, H. Wise, J . Catal. 5 (1966) 135.
[S] Unless otherwise mentioned, conditions used in all experiments were:
mol/min; cyatmospheric pressure; dihydrogen flow rate of 4.5 x
mol/min (syringe pump); catalyst
clohexene addition rate of 2.6 x
(previously unused) prereduction for 15 min at 250 "C. Capillary G C
was used for product analysis. Dideuterium was substituted for hydrogen in deuteriation experiments (analyzed by GC-MS).
191 C. B. Lebrilla, W. F. Maier, J . Am. Chem. SOC.I08 (1986) 1606.
[lo] A. B. McEwen, G. P. Meeker, W. F. Maier, unpublished results.
[ I l l W. Heiland, E. Taglauer, Methods Exp. Phys. 22 (1985) 299.
[I21 C. A. Evans, Jr., R. J. Blattner, Annu. Reu. Muter. Sci. 8 (1978) 181; W.
K. Chu, J. W. Mayer, M. A. Nicolet: Backscattering Spectromerry, Academic Press, New York 1978.
(131 A. B. McEwen, W. F. Maier, R. H. Fleming, S. M. Baumann, Nature
(London) 329 ( 1987) 53 I .
[I41 Artifacts d u e to Brmsted sites can safely be excluded due to the lack of
activity of the oxide overlayer in the absence of transition metal underlayer and the lack of detectable rearrangement to methylcyclopentane o r
methylcyclopentene.
Transition State n-Solvation by Aromatic Rings:
An Electronic Contribution to Diels-Alder Reaction
Diastereoselectivity**
study we have focused on the Diels-Alder reactions recently reported from this laboratory.
.Me
..'
R'
1 a-j
During the course of our investigations of the DielsAlder cyclpaddition reactions of a,@&aturated acyloxazolidinones 1 [Eq. (a)] we observed that oxazolidinone dienophile lg (R = benzyl) consistently provided diastereofacia1 discrimination superior to that of the closely related
congener Id (R = isopropyl ; cf. Table l).12m.n1
Since we expected the steric requirements of an appended isopropyl
Table I. Variation of diastereoface shielding in oxazolidinone crotonate/isoprene Diels-Alder cycloadditions and oxazolidinone butyrate/methyl iodide
alkylations (Eq. (a) and (b)].
1/2
R
Ratio D J D ? [a]
Ratio A I / A z [a]
a
Ph
Me
Et
i-C,H,
CH~(C-C,H,,)
I-C,H,
CH2Ph
CH2@-MeOC6H4)
CH2@-CIC6H4)
CH2@-CF,C,H,)
2.06
3.83
5.50
5.34
9.68
> 100 [b]
20.7
23.3
22.8
21.0
4.24
6.95
9.21
9.85
17.4
67.1
16.7
16.9
15.8
16.8
b
C
d
By David A . Evans,* Kevin T. Chapman,
Deborah Tan Hung,and Alan T. Kawaguchi
e
"n-Stacking" interactions between aromatic rings and
prochiral carbon atoms have been implicated as stereochemical control elements in a broad range of stereodifferIn each of the cited cases,
entiating organic
enhanced reaction diastereoselection could be attributed to
the imposition of a transition state n-interaction between a
resident aromatic ring and prochiral carbon(s). It has
proven difficult, however, to unequivocally dissect such
noncovalent interactions from the accompanying steric effects which together define reaction stereo'selectivity. In
the most comprehensive study reported to date for a given
system, WhiteseN et al. have established that the presence
of a proximal phenyl group is essential for high diastereoselectivity in certain glyoxylate ene reaction^.^^^-^] As was
concluded in this study, however, these results do not demand the presence of an eiectronic effect, but impiy that
some property of the phenyl ring, be it size, shape, or electronic character, is essential for high stereoselectivity. In
view of the tremendous potential utility of such n-stacking
interactions in asymmetric synthesis design, we have attempted to define a protocol which might unequivocally
demonstrate the presence of such a transition state electronic effect for a given reaction of interest. In the present
h
f
g
[*] Prof. D. A. Evans, Dr. K. T. Chapman, D. T. Hung, A. T. Kawaguchi
Department of Chemistry, Harvard University
Cambridge, MA 02138 (USA)
[**I This work was supported by the National Science Foundation, Washington DC (USA).
1184
0 VCH Verlagsgesellschajt mbH, 0-6940 Weinheim, 1987
Et,AICI
-30 f 0.5 "C
i
i
[a] Ratios determined by capillary vapor phase chromatography. [b] Diastereoselectivity beyond the limits of detection.
group to exceed those of a benzyl group, we attributed the
enhanced facial bias provided by lg to '%tacking". In an
effort to verify that we were observing an electronic interaction which was associated with the aromatic n-system,
the reaction diastereoselectivity of the fully reduced cyclohexylmethyl derivative l e was also evaluated. We noted a
significant decrease in the diastereoselectivity of this cycloaddition relative to that of lg despite the increased
steric bulk of the appended side-chain (Table 1). Thus,
some property of the phenyl ring of l g is important to the
observed diastereofacial bias. It seemed reasonable to expect that a charge-transfer interaction between the aromatic ring and the electron deficient dienophilic moiety in
l g should manifest itself not only in increased diastereoselectivity, but also in decreased reactivity. Accordingly, a
relative rate comparison between l e and l g in the cycloaddition with isoprene was carried out. Contrary to expectation, it was found that the phenyl-substituted dienophile Ig actually reacts slightly faster (rate ratio: 1.3) than
does its fully saturated counterpart l e . This result suggests
that little, if any, electronic reorganization accompanies
the alleged n-stacking in the cycloaddition of l g . Addi-
0570-0833/87/1111-l184 $ 02.50/0
Angew. Chem. Int. Ed Engl. 26 (1987) No. l l
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