Aldol-type condensation reactions of cyclic ketones by the W(CO)6/CCl4/UV system

Article abstract of DOI:10.1016/S0022-328X(00)00180-7

The cyclic ketones can be converted to their aldol-type condensation products by the W(CO)₆/CCl₄/UV system. The reaction was monitored by recording the IR and GC-MS spectra of the reaction mixture. The results support a mechanism that includes an intermediate carbene complex of tungsten.

Full text of DOI:10.1016/S0022-328X(00)00180-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2000) 252–255
Note
Aldol-type condensation reactions of cyclic ketones by the
W(CO)6/CCl4/UV system
C¸ . Bozkurt
:
Abant Izzet Baysal Uni6ersity, Department of Chemistry, Faculty of Arts and Science, Bolu 14280, Turkey
Received 2 August 1999; received in revised form 27 January 2000
Abstract
The cyclic ketones can be converted to their aldol-type condensation products by the W(CO)6/CCl4/UV system. The reaction
was monitored by recording the IR and GC-MS spectra of the reaction mixture. The results support a mechanism that includes
an intermediate carbene complex of tungsten. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Metal carbonyls; Photochemistry; Tungsten hexacarbonyl; Cyclic ketones; Aldol condensation; Metal carbenes
1. Introduction
It has been known that olefin metathesis reactions
can be catalyzed by the species that are generated
photochemically from hexacarbonyl tungsten and car-
bon tetrachloride [1,2]:
W(CO) /CCl /UV
This paper describes our results with several cyclic
ketones with n=3–6.
6
4
2R1–CHꢀCH–R2 ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢂ R1–CHꢀCH–R1+
R2–CHꢀCH–R2
The nature of the active catalyst has not been fully
understood in photochemical metathesis reactions
though there has been substantial research on this
subject [3,4]. The carbene mechanism [5] which is com-
monly accepted for olefin metathesis reactions, has also
been considered for the related reactions catalyzed by
the W(CO)6/CCl4/UV system. We obtained some evi-
dence for the existence of carbene intermediates in a
previous study using benzaldehyde as a carbene-trap
for the metathesis of linear olefins [6]. Surprisingly
when cyclohexanone was applied as the carbene-trap,
the only reaction product observed was the partially
deoxygenated dimer of the ketone.
2. Experimental
2.1. Reagents
W(CO)6 (Merck) was used without further purifica-
tion. Cyclic ketones (cyclopentanone, cyclohexanone,
cycloheptanone, and cyclooctanone, all from Merck);
CCl4 and n-hexane were distilled under argon
atmosphere.
2.2. Typical reaction procedure
It is therefore of interest to examine the Wittig-type
reactions of cyclic ketones in this photochemical
system.
The reactions were carried out in a quartz photo-
chemical reactor (Hereaus Laboratory-UV-Reactor
System/2) with a mercury lamp (TQ 150) as the light
source. All manipulations were performed under argon
using standart Schlenk techniques at room temperature.
E-mail address: cetin@ibu.edu.tr (C¸ . Bozkurt)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00180-7

C¸ . Bozkurt / Journal of Organometallic Chemistry 603 (2000) 252–255
253
A 200 ml solution of W(CO)6 (5.0×10−3 mol l−1
)
min−1, carrier gas He, 10 ml min−1) and HP 5971
mass spectrometer connected to a HP 486/33 N com-
puter preloaded with HPG 1034 MS Chem. Station
Software.
in n-hexane was irradiated for 10 min, then the cyclic
ketone (5.0×10−2 mol l−1) was added under continu-
ous irradiation. CCl4 (0.150 mol l−1) was then intro-
duced after 30 min of the addition of the ketone. IR
and GC-MS data of the mixtures were periodically
recorded by withdrawing samples for the analyses.
Irradiation was stopped at 30 min after the addition
of CCl4. The reaction mixture obtained was concen-
trated by vacuum to ca. 2 ml and separated through a
silicagel column with n-hexane–diethlyether (1:1) elu-
ent. The condensation product thus isolated was further
analyzed.
3. Results and discussion
GC-MS studies revealed that the expected condensa-
tion reactions with the cyclic ketones used have oc-
curred. The condensation products of cyclopentanone
and cyclohexanone (2-cyclopentylidencyclopentanone
(I) and 2-cyclohexylidencyclohexanone (II)) have been
isolated while those of the high boiling cycloheptanone
and cyclooctanone (2-cycloheptylidencycloheptanone
(III) and 2-cyclooctylidencyclooctanone (IV)) could not
have been separated through the column.
Additional experiments were performed to determine
the effect of the W/ketone ratio on the reaction. Sam-
ples withdrawn were analysed by GC using the external
standard method.
The characteristic properties of I and II were eventu-
ally investigated (Table 1) and found to be identical
with authentic samples [7].
2.3. Physical measurements
We have focused our studies on cyclohexanone con-
densation to obtain some information about the mecha-
nistic pathways of this photochemical process. In order
to determine the role of the amount of W(CO)6 on the
conversion of cyclohexanone to II, the concentration of
W(CO)6 was kept constant and that of cyclohexanone
was varied. Formation of II has been found to increase
with the W(CO)6/cyclohexanone ratio from 1/1 to 1/10
(Fig. 1) but never exceeding the initial concentration of
W(CO)6. This trend clearly indicates that the reaction
does not follow a catalytic pathway.
When CCl4 was replaced by CHCl3 or CH2Cl2 no
condensation product was observed which emphasizes
the role of CCl4 as a reagent rather than a solvent.
Fig. 2 shows the changes in the IR spectra of the
W(CO)6 solution in n-hexane upon irradiation and the
addition of the cyclohexanone. The peak at ca. 1981
cm−1 in curve (a) represents the characteristic carbonyl
stretching in W(CO)6 (Oh). Irradiation leads to the
photodissociation of W(CO)6 and the peaks appearing
at 2078, 1961, and 1946 cm−1 suggest that the forma-
tion of the coordinatively unsaturated W(CO)5 (C4v)
intermediate (curve (b)). The light yellow color ob-
served at this stage supports the existence of W(CO)5
[8]. Introduction of cyclohexanone to this solution
upon continuous irradiation resulted in the appearance
of three new bands at 2075 (w), 1933 (s), and 1909 (m)
cm−1 and in the disappearance of the former bands
(curve (c)). The presence of new peaks is consistent with
the M(CO)5L structure [9] which can be assigned as the
W(CO)5–cyclohexanone adduct characterized by 2075
(A11), 1933 (E), and 1909 (A21) frequencies. The intensi-
ties and positions of these bands are parallel to those
reported for the W(CO)5–acetone adduct [10]. Upon
photolysis for 30 min, the peaks related to the adduct
intensified while the peak at 1981 cm−1 corresponding
FTIR spectra were recorded on a JASCO 430 spec-
trometer. GC-MS analyses were performed using com-
bined system HP 5890 GC (30 m, ¥=0.2 mm,
capillary column coated with Hp-1% dimethyl-
polysiloxan gum, heating from 50 to 250°C with 10°C
Table 1
Physical data for I and II
I
II
IR frequencies
(cm−1) (KBr
pellet)
Maxima of mass
signals (m/z)
Elemental analysis
3430–3300wb;
2932s; 2858s;
1728vs; 1600w
178, 149, 135, 121,
107, 79, 67
3410–3300wb; 2936s;
2854s; 1729vs; 1600w
150, 149, 122, 107,
79, 67
Found: C, 78.0; H, Found: C, 77.9; H,
9.67%. Calc.: C,
80.9; H, 10.1%
54–55
9.41%. Calc.: C,
80.0; H, 9.33%
49–51
M.p. (°C)
Fig. 1. Variation of the yield of II with the W(CO)6/cyclohexanone
ratio:
No. of moles of II
Yield=
×100.
No. of initial moles of W(CO)6

254
C¸ . Bozkurt / Journal of Organometallic Chemistry 603 (2000) 252–255
Fig. 2. The changes in the IR spectra of the W(CO)6 solution in n-hexane upon irradiation. (a) W(CO)6 solution, (b) after photolysis of the
solution, (c) after the addition of the cyclohexanone.
to unreacted W(CO)6 weakened. GC-MS analysis of the
reaction mixture at this step showed that no condensa-
tion reaction takes place. The IR data of the other
cyclic ketones studied are more or less the same as
recorded for the W(CO)5–cyclohexanone adduct [11].
No change was observed in the peak at 1951 cm−1
through photolysis and also upon addition of different
cyclic ketones which led us to consider it related to an
unidentified impurity.
(2)
Addition of CCl4 to the system yields CHCl3 and
C2Cl6 by radicalic reactions (4, 5) and II (5).
UV
W(CO)6+CCl4“W(CO)5Cl+CO+CCl3
UV
(3)
(4)
2CCl3“C2Cl6
Addition of CCl4 caused a reduction in the intensities
and all three bands disappeared after 30 min of irradia-
tion without the appearance of any extra band due to
the formation of a new intermediate. Meanwhile, a blue
precipitate was observed and the IR spectrum of this
precipitate showed no absorption related to a carbonyl
complex, as reported previously for photocatalytic
metathesis reactions [4].
(5)
Reaction 6, representing the formation of II, is quite
similar to the well-known Wittig-type reactions of or-
ganic carbonyl compounds with Schrock-type alkyli-
dene complexes [13].
GC-MS analysis of the solution containing CCl4
indicated the formation of II, CHCl3, and
C2Cl6.Chloroform and hexachloroethane are known to
be derived from CCl4 in olefin metathesis reactions
catalyzed by the W(CO)6/CCl4/UV system [6,12]; how-
ever, the observation of II from refers to a novel field of
chemistry for the application of metal carbonyls.
(6)
Though we have not detected carbenic complexes of
tungsten pentacarbonyl spectroscopically, it is plausible
to propose that the active species that yield II are in
carbene nature. The W-alkyl complex described in reac-
tion 5, may give the proposed carbene by a-H abstrac-
tion in a way similar to the formation of the initiative
carbenes in olefin metathesis reactions [14].
4. Conclusion
Irradiation of W(CO)6 produces initially the coordi-
natively unsaturated complex W(CO)5 (1) which coordi-
nates the ketone in the second step (2).
UV
References
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437.
W(CO)6“W(CO)5+CO
(1)

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