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mer-[Cr(CO)3(4-norbornadiene)(2-ethene)] A Key Compound in Understanding the Mechanism of the Photocatalytic Diene Hydrogenation with Chromium Carbonyl Complexes.

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6-H).2.31 (ddq, lH;12-H),2.51 (dd, lH;4-H),2.54(dd,lH;e-H),3.63
(d. 1 H; 13-H), 3.74 (s, 3H; OMe), 3.80 (ddd, 1 H; 7-H). 3.98 (dd, 1 H;
9-H), 4.70 (dddd, 1 H; 5-H) 5.13 (s, 1 H ; 2-H), 5.44 (4.1 H; 15-H), 5.47(dd,
1H: 10-H), 5.55 (dd, 1 H ; 11-H); J4,,.=16.8, J4,,=4.6, Jq,,=10.3,
J5,,=6.2, J,,,.=2.7, J6,,.=13.8, J 6, , =8. 4, J,,,=7.0. J,,,=7.3.
6.9. J,1,,3
= 9.1, J1s,16
= 6.7 Hz.
[8] M. Fetizon, M. Jurion, J. Chem. Soc., Chem. Commun. 1972, 382; H.
Redlich. B. Schneider, Liebigs Ann. Chem. 1983, 412.
(91 M. Kinoshita. S. Mariyama, Bull. Chem. SOC.Jpn. 48 (1975) 2081.
[lo] Y.S. Cheng, W. L. Liu, S. Chen, Synthesis 1980. 223.
[ll] The isomeric ratio was determined by 'H NMR spectroscopy by integra= 4.70, dA.",&= 4.04) and their assignment
tion of the 4-H signals
was confirmed by NOE experiments.
[12] a ) An approach to explaining these difficulties is provided by the finding
that. on treatment with butyllithium in hexamethylphosphoramide (HMPA) or THFiHMPA at -78°C in the absence of a carbonyl component,
20 affords the diphenylphosphane oxide (i) (m.p. = 9 2 ° C [%]iO- 5
(CHCI,); 53%). This abnormal course of a phosphonium salt deprotona-
mev-[Cr( CO),(q4-norbornadiene)(q2-ethene)]:
A Key Compound in Understanding the
Mechanism of the Photocatalytic Diene
Hydrogenation with Chromium Carbonyl
Complexes **
By Dietmar Chmielewski, Friedrich- Wilhelm Grevels,*
Jurgen Jacke, and Kurt Schaffner
Dedicated to Professor Horst Prinzbach on the occasion
of his 60th birthday
The photocatalytic hydrogenation of norbornadiene
(NBD) with [Cr(CO),] or [Cr(CO), (q4-nbd)] as precursor of
the catalytically active species" -41 affords norbornene
(NBN) and nortricyclene (NTC) in a ratio of about 1 : 3
(Scheme 1). Conjugated dienes undergo selective cis 1,4-hy-
1. n - B u L i
Me &:Ph2
2. HZO
+ HZ
[Cr(CO)d or
Scheme 1. Norbornene (NBN) and nortricyclene (NTC) are formed in a ratio
of approximately 1 : 3. Norbornadiene = NBD.
tion. which has not been previously documented, is possibly due to the fact
that deprotonation does not occur in a position u to phosphorus, but
rather at the allylic C atom bearing the alkoxy group. This, together with
participation of the 4,5 double bond, seemingly results in formation of the
cyclic intermediate (ii); the subsequent elimination of benzene, without
precedent at pentavalent phosphorus [12 b], then leads, after aqueous
workup, to the hydroxyphosphorane (iii), which undergoes cleavage of the
P-C bond to give (i), thereby forming the best resonance-stabilized moiety
(12~1.The abnormal course of the deprotonation seems to be limited to
alkenyl(tripheny1)phosphonium salts with a 3-alkoxy-4-ene group in the
aliphatic moiety. Compounds without 3-alkoxy groups undergo normal
deprotonation and smoothly afford the desired olefination products 112d];
b) D. Seyferth, J. K. Heeren, W. B. Hughes Jr., J. Am. Chem. SOC.84(1962)
1764; c) H. J. Bestmann, Angew. Chem. 77 (1965) 609; Angew. Chem. Int.
Ed. 6 i g l . 4 (1965) 583; d) see, e.g., H. J. Bestmann, K. H. Koschatzky, 0.
Vostrowsky, Liebigs Ann. Chem. 1982,1478; J. Balsevich, Can.J. Chem. 61
(1983) 1053; K. Sato, 0. Miyamoto, S. Inoue, E Furusawa, Y Matsuhashi. Chem. Lett. 1983, 725.
(131 M. Julia, J. M. Paris, Tetrahedron Lett. 1973, 4833; P. J. Kocienski, B.
Lythgoe, S. Ruston, J. Chem. SOC.Perkin Trans 1,1978,829; P. J. Kocienski, B. Lythgoe, I. Waterhouse, ibid. 1980, 1045.
[14] D. A. Evans, R. L. Dow, T. L. Shih, J. M. Takacs, R. Zahler, J. Am. Chem.
Sor. I12 (1990) 5290.
[15] J. F. Wolfe, T. M. Harris, C. R. Hauser,J. Urg. Chem. 29(1964) 3249; S. N.
Huckin. L. Weiler, Tetrahedron Lett. 1971,4835; Can. J. Chem. 52 (1974)
[16] H. Meyer. D. Seebach, Justus Liebigs Ann. Chem. 1975,2261 ;T. Reffstrup,
P. M. Boll, Acta Chem. Scand. 8 3 0 (1976) 613; H. Achenbach, J. Witzke,
Z . Narurforschg. B35 (1980) 1459; Y.Tanabe, M. Miyakado, N. Ohno, H.
Yoshioka, Chem. Lett. 1982, 1543; W. C. Groutas, T. L. Huang, M. A.
Stanga, M. J. Brubaker, M . K. Moi, J. Heterocycl. Chem. 22 (1985) 433.
[17] P. Brownbridge, T. H. Chan, M. A. Brook, G. J. Kang, Can. J. Chem. 61
(1983) 688.
1181 H. Hagiwara, K. Kimura, H. Uda, J. Chem. SOC.Chem. Commun. 1986,
860; W. S . Johnson, A. B. Kelson, J. D. Elliot, Tetrahedron Lett. 29 (1988)
[19] Fluka Chemie AG, Buchs, Schweiz.
[20] a ) A. B. Smith. P. A. Levenberg, P. J. Jerris, R. M. Scarborough, Jr., P. M.
Wovkulich, J. Am. Chem. Soc. 103 (1981) 1501 ;b) S . Sakaki, Y.Sugita, M.
Sato. C. Kaneko, J. Chem. SOC.Chem. Commun. 1991,434.
[21] M. Demuth, A. Palomer, H.-D. Dhuna, A. K. Dey, C. Kruger, Y-H. Tsay,
Angew. Chem.98(1986) 1093; Angew. Chem. Int. Ed. Engl.25(1986) 1117.
[22] The X-ray structure analysis was performed by Prof. H . 1 Lindner, Technische Hochschule Darmstadt.
Angew. Chem. h i . Ed. Engl. 30 (1991) No. 10
drogenation.['-'] Much work has addressed questions
about the photochemical activation as well as the subsequent
course of the reaction. The most recent IR spectroscopic
results,[9,'1 obtained both by conventional spectroscopy at
low temperature in noble-gas matrices and in liquid noble
gases and by time-resolved spectroscopy at room temperature, indicate that [Cr(C0),(q4-nbd)] is transformed intofacand rner-[Cr(CO),(y4-nbd)] by photolytic detachment of
CO. Both complex fragments" '1 add hydrogen,["] which is
subsequently transferred to the norbornadiene ligand. This
provides a plausible explanation for the formation of the
two hydrogenation products (mer isomer, 3,2-hydrogenation -+ NBN; fac isomer, homo 1,Chydrogenation -+ NTC;
Scheme 2).
Scheme 2.
We describe here the photochemical synthesis and the use
of the thermally labile complex mer-[Cr(CO),(y4-nbd) ($ethene)] (l),whose ethene ligand is readily exchanged. Thus,
1 offers a potential source of the reactive [Cr(CO), (q4-nbd)]
fragment, which, in turn, obviates the photochemical induc[*] Prof. Dr. F.-W. Grevels, Dipl.-Chem. D. Chmielewski, Dr. J. Jacke,
Prof. Dr. K . Schaffner
Max-Planck-Institut fur Strahlenchemie
Stiftstrasse 34-36, W-4330 Mulheim an der Ruhr (FRG)
[**I This work was supported by the Commission of the European Community
(contract SCI-0007-C, EDB).
Verlagsgesellschajt mbH, W-6940 Weinheim, 1991
tion step. The catalytic hydrogenation of norbornadiene can
then be carried out in the dark, thus affording deeper insight
into the reaction mechanism.
Complex 1 is obtained in crystalline, analytically pure
form by irradiation of [Cr(CO), (q4-nbd)] in ethene-saturated solution at - 50°C, followed by workup below - 30°C
(Scheme 3). The structural assignment is supported by a
comparison of the spectroscopic data with those of the
analogous tungsten complex,['2] which was investigated by
X-ray analysis. Solid 1 is stable for a limited period of time
even at room temperature. Its lability is revealed in solution,
where it decomposes above - 10 "C to give insoluble products and [Cr(CO),(q4-nbd)]. If other ligands are present
(L = trimethylphosphite, E-cyclooctene, CO, 3CO), they
smoothly displace the coordinated ethene to form [Cr(CO),h4-nbd)
Scheme 3. Product distribution after catalysis: NBN : NTC
1 : 3.2.
In H,-saturated solution, 1 catalyzes the hydrogenation of
norbornadiene present in excess. Without irradiation, roughly
100 turnovers take place for a substrate-to-catalyst ratio of
200 : 1, until catalysis ceases after about 2 h. The product
distribution (NBN :NTC = 1 :3.2) is nearly identical to that
obtained in the photoinduced catalysis with [Cr(CO),(q4nbd)]. Detachment of ethene from 1 seems to be the ratedetermining step, since the gradual loss of catalytic activity
parallels the decrease in concentration of 1 observable by JR
spectroscopy. Complex 1 decomposes with formation of
[Cr(CO),(q4-nbd)] together with traces of [Cr(CO),], both of
which are catalytically inactive in the dark. When the reaction is performed with D, instead of H,, endo-[D,]NBN and
[D,]NTC are formed in a ratio of 1 :2.2. Both this H/D
isotope effect and the selective endo deuteration are consistent with the results of the photoinduced catalysis with
[Cr(C0),(q4-nbd)], which was carried out earlier by others"]
and was repeated by us.
Our results indicate that photolytic CO detachment from
[Cr(C0),(q4-nbd)] and loss of ethene from 1 generate the
same catalyst system. Earlier suggestions that cis-vacant
[Cr(CO), (endo-q2-nbd)]is the active species[2-41in the formation of norbornene (NBN) are not supported by our results. Moreover, we have obtained no evidence that the mer[Cr(CO),J structure of 1 is retained in the active catalyst and
thus could favor even transiently the 1,2-hydrogenation
(+. NBN). Instead, a rapid merlfuc rearrangement of the
[Cr(C0),(q4-nbd)] fragment presumably occurs, so that the
catalysis affords the two products, NBN and NTC, in a
constant ratio from the very beginning. It is unlikely, therefore, that the product ratio will be controllable by selective
photochemical generation of mer- orfuc-[Cr(CO),(q4-nbd)].
Q VCH Vedagsgesellschafl mbH. W-6940 Weinheim.1991
Analogous conclusions apply to the hydrogenation of
conjugated dienes,['
as shown by catalytic experiments
with 1 in which, instead of norbornadiene, isoprene was used
as the substrate (Scheme 4). After displacement or hydro-
+ H2 ca. 15 turnoversC
Scheme4. The NBD contained in 1 is partly hydrogenated (NBD:
NTC r2: 1: 3), partly displaced without hydrogenation.
genative release of the NBD ligand ( -+ NBN :NTC = 3 :3
to 1 :3 3 , the [Cr(CO),] unit is available for hydrogenation
of isopropene. Without irradiation, up to 15 turnovers took
place. As observed in the photoinduced catalysis,[61isoprene
almost selectively undergoes 1,4-hydrogenation (+. 2methyl-2-butene), despite the mer structure of the [Cr(CO),]
skeleton in complex 1. From this we conclude that the
stereoselective photolytic formation of the fuc-[Cr(CO),(q41,3-diene)] fragment['31 is not a necessary condition for the
photocatalytic selective 1,4-hydrogenation of conjugated
With respect to the pressure dependence of the NBD hydrogenation,I4' the use of 1 also provides new insight. First
of all, it should be noted that, by analogy with the photoinduced catalysis,[41a high pressure of H, also favors the
formation of norbornene when the reaction is carried out in
the dark with 1 as catalyst (NBN : NTC = 1 : 1.4 under
50 bar H,; a small amount of norbornane is also formed).
When D, is used, the ratio of products formed under high
pressure is likewise shifted in favor of 1,2-addition
([D,]NBN : [D,]NTC = 1 :1.1 under 60 bar D,; Scheme 5).
Scheme 5.
endo-(D2]N BN em-[D2] N BN
Surprisingly, however, the 'H NMR spectrum of [D,]NBN,
isolated by preparative gas chromatography, reveals that
exclusive endo 1,2-deuteration has not taken place here. Instead, a mixture of 52 % endo and 48 % ex0 1,2-deuterated
norbornene is present. This finding clearly shows that the
increased formation of NBN under high pressure is not
due to a favoring of the mer-[Cr(CO), (q4-nbd)] species over
the fuc species, but rather to an additional reaction path
involving exo q2-coordinated norbornadiene. Owing to the
absence of the stabilizing chelate effect, an intermediate of
this kind should have an considerably shorter lifetime compared with the above-mentioned species with q4-NBD, in
consequence of which this hydrogenation path becomes
kinetically important only at higher H, or D, pressures.
The mer-[M(CO),(q4-nbd) (q2-olefin)]complexes are generally more stable than the corresponding fuc isomers because of the trans-orthogonal orientation of the olefin with
respect to one of the C=C bonds of norbornadiene.["]
Identification of the fuc isomer in the case of chromium
complexes was achieved only with E-cyclooctene (ECO),
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 10
which has elsewhere['41 manifested a special coordination
ability. Irradiation (d = 289 nm) of [Cr(C0),(q4-nbd)] and
ECO in 2-methylpentane at - 40°C affords mer- and juc[Cr(C0),(q4-nbd) (q2-eco)], which are easily distinguished
on the basis of their CO stretching bands (rner complex,
f[cm-'] = 1997(m), 1925 (sst); fac complex <[cm-'] =
1968, 1904, 1876). The mer complex, which is more stable, is
the only observed product at room temperature. In order to
determine the quantum yield (@ = 0.18 at 300 nm, 0.11 at
365 nm), its formation was followed quantitatively by IR
spectroscopy. If this reaction is assumed to serve as a model
for the generation of the catalytically active [Cr(CO),] complex (Qind), then the relation eta, = Gindx (turnovers) gives a
rough estimate of 10 to 20 for the quantum yield of the
photocatalytic hydrogenation of norbornadiene (Qca,). The
experimentally determined values (1.9,['] ca. 0.3[,') are considerably lower, probably because the active [Cr(CO),] complex generated in situ is rapidly transformed into inactive
[Cr(CO),(q4-nbd)] by the CO still present in the system.
The stability of [M(C0),(q4-nbd)(q2-olefin)] complexes
increases markedly in the series Cr < Mo < W. As a result,
the catalytically active species is trapped by excess substrate
(NBD) as [M(CO), (q4-nbd)(q2-nbd)] in the cases of tungsten and molybdenum.['0] The analogous chromium complex, by contrast, has not yet been detected. Thus, the drastic
decrease in the catalytic activity in the series Cr $
Mo > WI3] is readily explained.
In analogy to the hydrogenation, norbornadiene can also
be hydrosilylated with silanes (R,SiH), both photocatalytically with [Cr(CO), (q4-nbd)] and in the dark with 1. As we
will report in detail elsewhere, both variants afford endo 1,2-,
exo 1,2-, and homo 1,4-adducts with comparable product
Experimental Procedure
Preparation of 1: [Cr(C0),(q4-nbd)] (1.00 g, 3.9 mmol) was irradiated in
200 mL of ethene-saturated pentane at - 50°C in an immersion lamp apparatus (Solidex glass, i.
2 280 nm; Philips HPK 125-W Hg lamp) for about 3 h,
until the starting material (Cco [cm-'1 = 2043, 1961, 1946, 1917) had largely
reacted. The temperature was maintained at I - 30°C during workup. After
addition of about 2 g of silica gel (to absorb readily decomposable by-products), the solution was stirred for several minutes and then evaporated to dryness under vacuum. The residue was extracted into 3 x 10 mL of ethene-saturated pentane, transferred to a column (silica gel 60, Merck; 2 x 15 cm), and eluted
with additional solvent. The first of the two yellow fractions was concentrated
under vacuum to about 50mL and cooled to -78° C whereupon pure mer[Cr(C0),(q4-nbd)(q2-ethene)] (0.46 g, 46 %) crystallized out. M.p. = 46°C;
correct elemental analysis (C, H, Cr); IR (n-hexane): F[cm-'] = 2005(m), 1933
(sst) (CO). 'H NMR (400 MHz, C,D,, 243 K): 6 = 0.85 (m, 2 H), 1.85 (s, br,
4 H, ethene), 3.00 (s, br, 2 H), 3.16 (s, br, 2 H), 3.89 (s, br, 2 H). "C NMR
(100.6 MHz, C,D,, 243 K): 6 = 47.41 (CH), 52.78 (ethene), 55.66 (CH), 59.95
(CH,). 79.47 (CH). 229.45 (CO). 232.62 (2 CO). The second yellow fraction of
the chromatographic run contained residual, unreacted starting material.
Catalytic hydrogenations with 1: A solution of 1 (4-6mM) and NBN (0.20.8 M)or isoprene in n-hexane (with addition of n-octane as internal standard)
was prepared at - 35 "C and hydrogen was admitted to the solution, which was
warmed to room temperature and then stirred in the dark. Samples were treated
with aqueous cerium(1v) ammonium nitrate solution to destroy the complex
and then analyzed by gas chromatography. In pressure experiments, the precooled solution was sucked rapidly into a previously evacuated autoclave and
H, or D2 was admitted.
Received: May 6, 1991 [Z 4611 IE]
German version: Angew. Chem. 103 (1991) 1361
CAS Registry numbers:
1, 124815-42-5; NBN, 498-66-8; NBD, 121-46-0; NTC, 279-19-6;
[Cr(C0),(qSi4-nbd)], 12146-36-0; isoprene, 78-79-5; 2-methyl-2-butene, 51335-9.
[I] G. Platbrood, L. Wilputte-Steinert, Bull. Soc. Chim. Belg. 82 (1973) 733735.
[2] G . Platbrood, L. Wilputte-Steinert, J. Orgunomet. Chem. 70 (1974) 393405.
Angert.. Chem. lnt. Ed. Engl. 30 (1991) No. 10
0 VCH VerlugsgesellschuflmbH,
[3] D. J. Darensbourg, H. H. Nelson, 111, M. A. Murphy. 1 Am. Chem. SOC.
99 (1977) 896-903.
[4] a) M. J. Mirbach, D. Steinmetz, A. Saus, J. Orgunomel. Chem. 168 (1979)
C13-Cl5; b) M. J. Mirbach, T. N. Phu, A. Saus, ibid. 236 (1982) 309320.
[5] a) J. Nasielski, P. Kirsch, L. Wilputte-Steinert, J. Orgunomel. Chem. 27
(1971) C 13-C 14; b) G. Platbrood, L. Wilputte-Steinert. Terruhedron
Lett. 1974, 2507-2508.
[6] M. Wrighton, M. A. Schroeder, J. Am. Chem. Soc. 95 (1973) 5764-5765.
(71 a) G. Platbrood, L. Wilputte-Steinert, J. Orgunomer. Chem. 70 (1974) 407412; b) G. Platbrood, L. Wilputte-Steinert, J. Mol. Curul. 1 (1975/76)265273.
[8] I. Fischler, M. Budzwait, E. A. Koerner von Gustorf, J. Orgunomer.
Chem. 105 (1976) 325-330.
[9] E-W. Grevels, J. Jacke, W. E. Klotzbiicher, K . Schaffner, R. H. Hooker,
A. J. Rest, J. Orgunomel. Chem. 382 (1990) 201-224.
[lo] a) S. A. Jackson, P. M. Hodges, M. Poliakoff. J. J. Turner, E-W. Grevels.
J. Am. Chem. SOC.112(1990) 1221-1233; b) P. M. Hodges, S . A. Jackson,
J. Jacke, M. Poliakoff, J. J. Turner, E-W Grevels, ibid. 112 (1990) 12341244.
[ l l ] The occupation of the free coordination site by a solvent molecule is
omitted for clarity.
1121 F.-W Grevels, J. Jacke, P. Betz, C. Kriiger, Y-H. Tsay, 0rgunomerullic.s 8
(1989) 293-298.
[I31 W. Gerhartz, E-W. Grevels, W. E. Klotzbiicher, E. A. Koerner von Gustorf, R. N. Perutz, Z . Nururjorsch. B40 (1985) 518-523.
[14] a) F.-W. Grevels, V. Skibbe, J. Chem. Soc. Chem. Commun. 1984,681 -683;
b) H. Angermund, E-W. Grevels, R. Moser, R. Benn, C. Kriiger,
M. J. RomZo, Orgunomefallics 7 (1988) 1994-2004; c) F:W. Grevels,
J. Jacke, W. E. Klotzbiicher, S. Ozkar, V. Skibbe, Pure Appl. Chem. 60
(1988) 1017- 1024.
The Mechanism of Interaction of Triplet
3-Methylcyclohex-2-en-1-one with Maleo- and
fumarodinitrile: Evidence for Direct Formation of
Triplet 1,4-Biradicalsin (2 + 21 Photocycloadditions without the Intermediacy of Exciplexes **
By David I. Schuster,* George E. Heibel, and Jan Woning
Dedicated to Professor Kurt Schaffner on the occasion of his
60th birthday
The mechanism of [2 21 photocycloaddition reactions
between cyclic enones and alkenes continues to be of interest.['] There seems to be general agreement that triplet 1,4-biradicals are intermediates in these reactions."
the question of whether exciplexes of enone triplet excited
states (TI) and alkenes are precursors to the 1,4-biradicals
remains u n r e ~ o l v e d . ~ ~ ~ ' ]
In acetonitrile, the high-energy triplets of acetophenone
(E, = 73.8 kcalmol-'),'81 benzophenone (E, = 68.6 kcal
mol - '),[*I
and bicyclo[4.3.0]non-l(6)-en-2-one (BNEN,
ET = 74 kcalmol- I)['] interact with maleo- (MN) or fumarodinitrile (FN, E, = 48 kcalmol- ' ) [ l o ] exclusively by energy transfer. Thus, in acetonitrile the T , state of BNEN is
quenched by both F N and M N at the diffusion-controlled
rate (k, = (4.5 & 0.7) x lo9 M-'s-' ).[ I 1 ] Irradiation of these
ketones in the presence of F N leads exclusively to cis-trans
isomerization of the alkene. The MN :F N photostationary
state ratios are 49 :51, 48 : 52, and 44 : 56, respectively. No
[2 + 21 cycloadducts could be detected in these systems. On
Prof. D. I. Schuster, Dr. G. E. Heibel, Dr. J. Woning
New York University, Faculty of Arts and Science
Department of Chemistry
New York, NY 10003 (USA)
[**I This work was supported by the National Science Foundation (Grant
CHE-890099). We thank Prof. D . Wink and Dr. JDewan for X-ray
studies and Dr. M . Burru and Dr. J. C . Scaiuno (National Research Council, Canada) for their cooperation in obtaining flash photolysis data.
W-6940 Weinheim. 1991
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carbonyl, understanding, ethene, compounds, mechanism, hydrogenation, complexes, chromium, mer, diener, norbornadiene, photocatalytic, key
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