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Homogeneously Catalyzed Dimerization of Isoprene to 2 7-Dimethyl-2 trans-4 6-octatriene at Chromium.

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(3)
0 ' c1
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4
1
Products
Products
if the dye laser is triggered 4.5ps after the N2 photolysis
laser; it contains contributions from the photoproduct, from
unchanged starting material, and from the solvent. Figure
1b shows the spectrum recorded after spectrum 1a without
photolysis laser; it corresponds with that of ( I ) , except for
the broad CCI4 solvent band at 1539cm-'. Finally, Figure
1c shows the difference spectrum, i. e. the Raman spectrum
of the photochemically generated transient in the form in
which it exists 4.5 ps after the photoflash.
In the spectrum of ( I ) in C C 4 , we assign the band at
1680cm-' to the C=O stretching vibration, and that at 1570
cm
to the non-aromatic C==C stretching vibration; the
bands at 1600, 1296, 1260, and 2170cm-' are characteristic
for the benzoyl moiety. The difference IR spectrum of ( 1 )
in an irradiated and an unirradiated KBr disct4]reveals new
bands at 1638cm-' Cv(C=O)] and at 1610,1520, 1490, 1380,
and 1284 cm (decay time z 2 h at 300 K). We assign these
bands to the radical (2), since according to model calculations
its bond orders are similar to those of an unsubstituted naphthalene. In particular, the decrease in the frequency of the
(C=O) stretching vibration is consistent with the smaller
n-bond order of this bond in the radical. In contrast, the
Raman spectrum of the photoproduct in CC14 (Fig. 1 c) shows
no substantial bands at frequencies appearing in KBr and assigned to the radical. The strongest line appears at 1495cm-',
others at 1190, 1320, 1400, 1575, and 1700 cm-I. We thus
conclude that two different photoproducts predominate in
KBr and CC14, respectively. An increased C=O frequency
(1700cm-') compared to that observed for ( 1 ) would be
in agreement with Zweegers"'] assignment as cation ( 3 ) , since
the model calculation yields a higher x-bond order. As yet,
we cannot explain the high intensity of the line at 1495cm-';
whether a resonance effect is responsible is presently under
investigation.
-'
means that the transient electronic spectrum obtained in a
flash-photolysis experiment provides no decisive information.
More characteristic differences are to be expected in the vibrational spectra, even in the case of similar compounds.
The aim of our present studies was to record the first
highly time-resolved Raman spectra of transient photoproducts
in order to determine their structure. For this purpose the
light pulse of a NJaser (Lambda Physik M2000) is focused
into the reaction cell. A dye laser (Zeiss 807103, 1 9 W 21 000cm-') is triggered after a delay time of At (>0.2ps);
its beam follows the trace of the NZ photolysis laser, The
Raman light generated by the photoproduct is spectrally analyzed in a double grating spectrograph (Coderg U F 76). After
amplification by an image converter (EM1 9912), which is
externally gated during the "photofla~h"[~],
the spectrum is
recorded with an optical multichannel analyzer (PAR 1205).
The spectra can be immediately displayed or fed to a computer
for further processing (e.g. for difference spectra).
The Raman spectrum of the transient from (I) in CCI4
(300 K) was detected in the time interval 0.2 ps (apparatus
limit) to 100ms. Figure l a shows the spectrum one obtains
Received: February 9, 1979 [Z 187 IE]
German version: Angew. Chem. 91, 348 (1979)
a)
CAS Registry numbers:
( l ) , 17735-13-6; (2), 23390-09-2; ( 3 ) , 69707-05-7
[l] F. Feichrmayr, G. Scheibe, 2. Naturforsch. 8 1 3 , 51 (1958); G. Scheibe,
Chem. Ing. Tech. 31, 321 (1959); G. Scheibe, F. Feichtmclyr, J. Phys.
Chem. 66, 2449 (1962).
[2] F. P. A. Zweegers, Dissertation, Universitat Leiden 1978.
[3] F . Ddrr, u! Hub, S . Schneider, J. R. Taylor, EM1 Appl. Note: Application
of a negative H. V.-square wave generator for gating image intensifier
tubes.
[4] 19: Hub, Dissertation, Technische Universitat Miinchen 1979.
A
K
A
I
1
1200
1300
I
1400
I
I
1500
1600
-
1700
vfcm-'
Fig. 1 . Raman spectra of a 10-'M solution of ( I ) in CCL: a) probe laser
delay 4.5 ps (FL=20576cm-'); h) without photolysis laser; c) difference spectrum.
324
Homogeneously Catalyzed Dimerization of Isoprene
to 2,7-Dimethy l-2, tvans-4,6-octatriene at C h r o m i u m [ * * ]
By Heindirk tom Dieck and Alois Kinzel"]
Dedicated to Professor Horst Pommer on the occasion of his
60th birthday
In our studies on homogeneously catalyzed oligomerizations
of dienes on dad-modified metal catalysts (dad = 2,4-diaza-1,3diene) we found that a chromium-dad system is able to transform isoprene in a highly specific manner into 2,7-dimethyl2,trans-4,6-octatriene ( I ). The reaction involves tail-to-tail
linkage with a double hydrogen shift.
['I
Prof. Dr. H. tom Dieck, Dip].-Chem. A. Kinrel
Institut riir Anorganische und Angewandte Chemie der Universitat
Martin-Luther-King-Platz 6, D-2000 Hamburg 13 (Germany)
[**I This work was supported by the Deutsche Forschungsgemeinschaft
and BASF AG.
Angen?. Chem. Int. E d . Engl. I 8 ( 1 9 7 9 ) No. 4
0 Verlag Chemie, GmbH, 6940 Weinhelm, 1979
0570-0833/79,'0404-0324 $ 02.50/0
Open-chain dimers of isoprene are formed by numerous
catalyst systems. The head-to-tail dimer 2,6-dimethyl-1,3,6octatriene ( 5 ) is then formed-often only in poor yield. Only
palladium['] is found to give not only polymers but also
2,7-dimethyl-I ,3,7-octatriene, which undergoes base-catalyzed
rearrangement to ( 1 ) in modest yield.
The catalytic direct synthesis of (1) preferably starts with
generation of the chromium(0) complex (3 ) from chromiurn(II1)
compounds and the dad-ligand N,N'-ethanediylidenebis(2,4dimethyl-3-pentanamine) (2).
R
R
R = ( C H3)2CH-CH-C H( C H S ) ~
I
Activation of (3) with Et'AlOEt in the presence of isoprene
(molar ratio ca. 1 :2 : 500) leads to the best result obtained
so far: a small amount of trimeric (10 %) and higher products
is accompanied by 80 % of (1 ).
In the simplified version of the reaction, compound (3)
is not isolated. Reduction in THF is followed by evaporation
to dryness, (3) is extracted from the residue with isoprene
and then activated with Et'AlOEt. A chromium(in) compound, the ligand (2), and AIR3 as reducing agent also gives
(1) as the sole dimer on reaction in a steel autoclave, admittedly together with much polymeric material; the yield of
( 1 ) is improved if ligand (2) is added in excess.
Chromium complexes of type (3) bearing other substituents
R give less specific reactions; the products contain no ( I ) .
When R = cyclohexyl reaction gives inter alia 2,7-dimethyl1,3,6-octatriene ( 4 ) , the intermediate probably present before
the final hydrogen shift, alongside the head-to-tail dimer 2,6dimethyl-1,3,6-octatriene ( 5 ).
(41
(51
The exclusive formation of the tail-to-tail dimer ( 1 ) on
a catalyst system (3) suggests a strong steric interaction during
the primary C-C linkage.
Similar steric control leading to tail-to-tail linkage in metaldiazadiene catalysis has also been observed in the V(dad)3/
AIR3 system['].
Compound ( I ). m.p. 35-37"C. b.p. 110--115"C~55 torr,
forms the expected Diels-Alder adduct 3,3-dimethyl-6(2-methyl-I -propenyl)-4-cyclohexene-1,1,2,2-tetracarbonitrile
with tetracyanoethylene, while only a yellow-orange chargetransfer complex is formed with maleic anhydride. The nonuniform bromination gives inter a h 2,3,6,7-tetrabromo-2,7dimethyl-trans-4-octene in ca. 10 % yield['].
tOmmol), (2)k3I (5.04g, 20mmol), and sodium (0.69g,
30mmol) are stirred together in T H F (150ml) at room temperature for 20 h. The brown solution is evaporated to dryness,
the residue taken up in n-hexane (150ml), the solution filtered
through a fine frit and evaporated down. Compound (3)
precipitates in a freezer as a dark-brown solid which is washed
with acetonitrile. It deliquesces, turning reddish brown, on
exposure to air: MS: m/e=556 (M'), 304 (dadCr'), 278
[(dad),Cr'+], etc.; VIS spectrum (n-hexane): 13250, 15750,
17290 (sh), 21 100 (sh), 24690, 28170 cm-'.
Dimerization of isoprene to give (1 ): Dimerization was
performed in a flame-treated glass ampoule in a steel autoclave
under n-pentane back-pressure at 120°C. Compound (3)
(265.1 mg) is transferred under argon into an ampoule. After
freshly distilled isoprene (25 ml) and Et'AlOEt have been
pipetted into the ampoule, it is frozen, evacuated at oil-pump
vacuum, and sealed. After 20h at 120°C (volume contraction
ca. 10 %) the reaction solution no longer contains isoprene.
Distillation affords 13g (80%) of ( 1 ) and 1.8g (11 %) of
trimer; the rest is undistillable. UV (cyclohexane): 34 250,
35710, 37 170cm-'; 270-MHz 'H-NMR spectrum (CDC13,
int. TMS): Ci=1.76 (s, 6H), 1.78 (s, 6H), 5.78-6.35 (m, 4H,
AA'BB' system); "C-NMR (CDC13, int. TMS): 6 = 18.3, 26.1
(CHJ), 125.7, 126.7 (=CH-),
134.3 (=C<).
Received: December 27, 1978 [Z I90 IE]
German version: Angew. Chem. 91. 344 (1979)
CAS Registry numbers:
( I ) , 42588-68-1; (2), 59725-31-4; ( 3 ) , 69795-69-3; Cr(acac),, 21679-31-2
[ I ] A . D . Jose!, J. Org. Chem. 39, 139 (1974).
[2] A. Kinrrl, Projected Dissertation, Universitat Hamburg, 1979.
[3] H. tom Dieck, M . Scohoda, Chem. Ber. 109. 1657 (1976).
Novel Titanium Halides Containing Metal-Metal
Bonds: Ti7ClI6and Ti7BrI6
By Harald Schafer, Reinhard Laumanns, Bernt Krebs, and
Gerald Henkel"]
Dedicated to Professor Horst Pommer on the occasion of' his
60th birthduy
Among the halides of titanium, no stoichiometric compound
having a composition between TiX2 and TiX3 has so far
been reported. The thermal disproportionation of TiC13 is
usually formulated as involving formation of TiC14,g and
TiC12,$.However, it initially leads to low-order products of
varying composition['] before ending up as TiCI'. The latter
compound is also of low order and can spontaneously ignite
in moist air. To explain this behavior it was assumed-as
in the case of the TiBr3 analogue[']-that
there is a large
range of homogeneous solid solutions.
The observation that the trihalides already disproportionate
at 400-50O0C, i.e. at temperatures at which atoms could
not be very mobile in the solid state, posed the question
whether the equilibration can be improved with the aid of
transport reagents such that well crystallized solid phases
are formed. Aluminum halides are suitable as transporting
agents for lower titanium halides; the gaseous complexes
TiA1C16 and TiCl2.nA1Cl3 are already well
Our
experiments led to the discovery of the crystalline compounds
Ti7ClI6and Ti7Br16.
Experimental
Synthesis of (3): Synthesis was successful only in flametreated Schlenk tubes under re-purified argon in anhydrous
Tris(2,4-pentanedionato)chromium(r11) (3.49g,
solvents.
Angew. Chrm. Inr. Ed Engl. 18 ( 1 9 7 9 ) No. 4
[*I
Prof. Dr. H. Schafer, Dipl.-Chem. R. Laumanns, Prof. Dr. B. Krebs,
Dr. G. Henkel
Anorganisch-chemisches Institut der Universitat
Gievenbecker Weg 9, D-4400 Miinster (Germany)
325
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dimerization, homogeneous, dimethyl, octatriene, transp, chromium, catalyzed, isoprene
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