Formation of carbenoid intermediates in the reaction of ditertiobutyl ketone with low-valent titanium reagents

Article abstract of DOI:10.1016/S0022-328X(00)00651-3

Treatment of ^tBu₂CO with TiCl₄ and Li(Hg) in THF gave the hydrocarbon ^tBu₂CH₂ as the major product (40% yield); 60% of the total quantity of ^tBu₂CH₂ was lib

Full text of DOI:10.1016/S0022-328X(00)00651-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 617–618 (2001) 744–747
Communication
Formation of carbenoid intermediates in the reaction of
ditertiobutyl ketone with low-valent titanium reagents
Claude Villiers a, Alain Vandais b, Michel Ephritikhine a,*
a Ser6ice de Chimie Mole´culaire, DSM, DRECAM, CNRS URA 331, CEA Saclay, 91191 Gif sur Y6ette, France
b Ser6ice des Mole´cules Marque´es, DSV, DBCM, CEA Saclay, 91191 Gif sur Y6ette, France
Received 2 August 2000; accepted 1 September 2000
Abstract
t
t
Treatment of Bu2CO with TiCl4 and Li(Hg) in THF gave the hydrocarbon Bu2CH2 as the major product (40% yield); 60% of
t
the total quantity of Bu2CH2 was liberated after hydrolysis of the reaction mixture. Similar experiments with THF-d8 and D2O
afforded Bu2CHD and Bu2CD2, indicating that carbenoid species Bu2Cꢀ[Ti] are likely intermediates in the reaction of Bu2CO
with low-valent titanium reagents. © 2001 Elsevier Science B.V. All rights reserved.
t
t
t
t
Keywords: Carbenoid species; McMurry reaction; Ditertiobutyl ketone; Low-valent titanium reagents
Although the McMurry reaction proved to be very
efficient for the synthesis of sterically hindered and
1. Introduction
strained olefins, it failed in the reductive coupling of
Recently we reported that metallopinacols are not,
ditertiobutyl ketone [3,4], and the long awaited tetrater-
contrary to generally accepted ideas, the only interme-
tiobutylethylene could never be prepared, despite many
diates in the so-called McMurry reaction, i.e. the reduc-
synthetic approaches [5,6]. We decided however to ex-
tive coupling of carbonyl compounds into alkenes by
t
amine in detail the reactivity of Bu2CO towards low-
means of low-valent titanium reagents [1,2]. Careful
valent titanium reagents, with the hope of gaining
analysis of the products obtained by treatment of
further insight into the species which are actually
i
R2CO (R=Me, Et, Pr) with the MCl4–Li(Hg) system
formed under the conditions of the McMurry reaction;
(M=Ti, U) revealed that the coupling process could
the results presented here confirm the formation of
also proceed via a carbenoid species, a route which is
carbenoid species in the reaction of sterically encum-
followed preferentially by the most sterically encum-
bered ketones.
bered ketones (Scheme 1). Thus, acetone was quantita-
tively transformed into a uranium pinacolate which was
isolated and further deoxygenated into tetramethyl-
2. Results and discussion
ethylene; in contrast, tetraisopropylethylene resulting
i
from coupling of Pr2CO was not produced from a
Ditertiobutylketone (1) reacted for 24 h at 20°C in
tetrahydrofuran (THF) with TiCl4 and Li(Hg) in the
pinacolate species but rather from a carbenoid inter-
mediate which also explains the formation of 2,4-di-
molar ratio of 1:1:4 to give, after hydrolysis, a mixture
methyl-2-pentene by H migration.
of the corresponding alcohol 2 (25%) and alkane 3
(40%) as the major products (Eq. (1)), smaller amounts
of the alkenes 4 (20%), 5 (5%) and 6 (8%), and a trace
* Corresponding author. Tel.: +33-1-69086436; fax: +33-1-
69086640.
E-mail address: ephri@nanga.saclay.cea.fr (M. Ephritikhine).
of the cyclopropyl derivative 7 (ca. 1%). These com-
pounds were characterized by their 1H-NMR and
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00651-3

C. Villiers et al. / Journal of Organometallic Chemistry 617–618 (2001) 744–747
745
(1)
Table 1
about 60% of the total quantity of 3 was liberated after
Percentages of 3-d0, 3-d1 and 3-d2 produced in the reaction of tBu2CO
with the TiCl4–Li(Hg)system in THF and THF-d8
hydrolysis. Deuterolysis of the reaction mixture led to
t
t
the formation of Bu2CHD 3-d1 and Bu2CD2 3-d2 in
the ratio 75:25; this ratio was equal to 60:40 when the
reaction was performed in THF-d8 and in that case the
alkane 3 which was first formed before deuterolysis was
partially deuteriated, with [3-d0]:[3-d1]=73:27. The al-
cohol 2 produced in each of these experiments was not
deuteriated at the 1 position.
Before deuterolysis
After deuterolysis
THF
THF-d8
THF
THF-d8
3-d0
3-d1
3-d2
100
0
0
73
27
0
0
75
25
0
60
40
Incorporation of deuterium into the fraction of 3
which is liberated upon deuterolysis clearly supports the
presence of carbenoid species [Ti]ꢀCtBu2 in reaction (1);
these would be transformed into the alkane, via an
alkyl intermediate [Ti]CHtBu2, by successive addition of
H atoms (Scheme 2). The actual carbenoid and alkyl
species could not be separated from the alkoxide
tBu2CHOM (M=Li or Ti) and their exact composition
was not determined. It is possible that such species
adopt a dinuclear structure with bridged oxo and
alkylidene or alkyl ligands, [Ti]2(m-O)(m-CtBu2) or
[Ti]2(m-O)(m-CHtBu2), similar to those obtained by reac-
tion of ketones with lanthanide metals or electron-rich
metal complexes [9].
GCMS spectra which were identical to those of authen-
tic samples. Other minor products, which represent
about 1% of the total yield, were not identified.
To our knowledge, formation of these hydrocarbons
has never been noticed in such McMurry reactions of 1,
and the alcohol 2 was the only product characterized.
However, reduction of ketones into the corresponding
alkanes during McMurry reactions was previously ob-
served in a few cases [7]; the mechanism of formation of
these alkanes was not elucidated.
The presence of the cyclopropyl compound 7 among
the products provided a strong indication of the in-
volvement of carbenoid species in reaction (1), since 7
was the major hydrocarbon resulting from thermal or
photochemical decomposition of the carbene precursors
Treatment of 1 with the TiCl4–Li(Hg) system in
THF-d8, which afforded non-deuteriated
2 after
deuterolysis and partially deutriated 3 (3-d1:3-d0=
27:73) in the first step of the reaction, revealed that
hydrogen atoms were provided by the ketone. It is quite
plausible that formation of H atoms has to be related
to that of isobutylene 4 which would itself result from
H abstraction from a tertiobutyl radical. The lack of C5
t
tBu2CN2PPh3 [6] or Bu2CN2 [8].
While the hydrocarbons 3–7 were observed before
hydrolysis of the reaction mixture, it was important to
note that the alkane 3 was also formed, together with
the alcohol 2, upon addition of water (Table 1). In fact,
Scheme 1. The dual nature of the mechanism of the McMurry reaction.

746
C. Villiers et al. / Journal of Organometallic Chemistry 617–618 (2001) 744–747
Scheme 2. Formation of the alkane 3.
t
’
hydrocarbons among the products suggests that a Bu
standard Schlenk-vessel and vacuum-line techniques or
in a glove box. Solvents were dried and deoxygenated by
standard methods and distilled before use. Deuterated
radical could not be produced by cleavage of the car-
benoid or alkyl species. On the other hand, dissociation
t
1
’
of Bu from the ketyl radical would give an acyl
solvents were dried over Na–K alloy. The H-NMR
derivative [Ti]COtBu which would be unstable and de-
spectra were recorded on a Bruker DPX-200 instrument
and were referenced internally using the residual protio
solvent resonances relative to TMS (l 0). The GLC
analyses were performed on a Chrompack CP 9002
apparatus equipped with a capillary CP Wax 57 CB
column. The mass spectra were obtained using a
Hewlett–Packard 6890-5973 instrument operating in the
ionization mode and equipped with a HP 23 (60 m)
chromatography column. The commercial compounds
(Aldrich) TiCl4 and 5 were used as received; lithium
amalgam (1.05% Li) was prepared by addition of Li to
Hg in boiling p-cymene [10]. The hydrocarbons 3 [11],
6 [12] and 7 [6] were synthesized by published methods.
compose by decarbonylation into a titanium carbonyl
t
’
[Ti]CO and another Bu radical.
The results presented here confirm that carbenoid
species can be readily formed by reaction of sterically
hindered ketones R2CO with low-valent titanium
reagents. The first step is the electron transfer from the
metal to the carbonyl, giving the ketyl radical [Ti]–O–
’
CR2which is further deoxygenated into [Ti]ꢀCR2. When
R=iPr, this carbenoid species was found to undergo
facile rearrangement by a-H migration into 2,4-dimethyl-
2-pentene, the major product, or react with another
molecule of ketone to give the coupling alkene, tetraiso-
propylethylene; addition of H atoms onto [Ti]ꢀCiPr2 also
i
occurred, affording a small amount of Pr2CH2 [2]. The
t
carbenoid species issued from 1 exhibited a distinct
behaviour since it did not react with the ketone, certainly
because of steric hindrance, and its rearrangement into
7 by migration of a b-hydrogen atom was not favoured;
the alkane 3 was then formed preferentially. In fact, the
species [Ti]ꢀCtBu2, which were present in the reaction
mixture before hydrolysis, appeared to be much more
stable than [Ti]ꢀCiPr2, thus permitting a more direct
characterization by deuterolysis.
Formation of 5 and 6 is difficult to explain, although
it apparently results from CH2 migration between two
ditertiobutylmethylene fragments; we are currently try-
ing to determine the way in which these alkenes are
produced. We are also studying the reactions of the
carbenoid species [Ti]ꢀCtBu2 with various substrates,
especially those with aldehydes and ketones which are
less sterically encumbered than 1, in order to prepare
cross-coupling products and confirm that such carbenoid
species are intermediates in the reductive coupling of
carbonyl compounds. This work will be presented in a
forthcoming paper.
3.2. Reactions of Bu2CO with TiCl4 and Li(Hg)
These reactions were monitored by NMR spec-
troscopy. In a typical experiment, an NMR tube was
charged with TiCl4 (6.3 ml, 0.058 mmol) and 1.05%
Li(Hg) (170 mg, 0.232 mmol Li) in THF or THF-d8 (0.4
ml). The ketone 1 (10 ml, 0.058 mmol) was introduced
into the tube. The mixture was stirred at 20°C for 24 h
by attaching the tube perpendicular to the axis of an
electrical rotor. The solvent and the volatile products of
the reaction were transferred under vacuum into another
NMR tube cooled in liquid nitrogen. The NMR, chro-
matography and mass spectrometry analyses showed the
formation of 3 (15%), 4 (20%), 5 (5%), 6 (8%) and 7 (1%).
When the reaction was performed in THF-d8, the ratio
[3-d0]:[3-d1] was equal to 73:27. The non-volatile products
of the reaction were deuterolyzed (10 ml of D2O) in
THF-d8 (0.4 ml), leading to the formation of the alcohol
2 (25%) and the alkane 3 (25%). The ratio [3-d1]:[3-d2] was
equal to 75:25 and 60:40 when the reaction was per-
formed in THF and THF-d8, respectively.
3. Experimental
References
3.1. General procedures
[1] M. Ephritikhine, Chem. Commmun. (1998) 2549.
[2] C. Villiers, M. Ephritikhine, Angew. Chem. Int. Ed. Engl. 36
(1997) 2380.
All reactions were carried out under argon using

C. Villiers et al. / Journal of Organometallic Chemistry 617–618 (2001) 744–747
747
C1. (c) K.G. Pierce, M.A. Barteau, J. Org. Chem. 60 (1995)
2405.
[3] A. Fu¨rstner, B. Bogdanovic, Angew. Chem. Int. Ed. Engl. 35
(1996) 2442.
[8] J.E. Gano, R.H. Wettach, M.S. Platz, V.P. Senthilnathan, J.
Am. Chem. Soc. 104 (1982) 2326.
[9] (a) Z. Hou, H. Yamazaki, Y. Fujiwara, H. Taniguchi,
Organometallics 11 (1992) 2711. (b) M.H. Chisholm, K. Folting,
[4] T. Letcka, in: A. Fu¨rstner (Ed.), Active Metals, Preparation,
Characterization, Applications, VCH, Weinheim, 1996, p.
85.
[5] (a) A. Krebs, W. Born, B. Kaletta, W.U. Nickel, W. Ru¨ger,
Tetrahedron Lett. 24 (1983) 4821. (b) J. Dannheim, W. Gram,
H. Hopf, C. Parrodi, Chem. Ber. 120 (1987) 871.
[6] T.G. Back, D.H.R. Barton, M.R. Britten-Kelly, F.S. Guziec, J.
Chem. Soc. Perkin Trans. 1 (1976) 2079.
J.A. Klang, Organometallics
9 (1990) 602, 607. (c) M.H.
Chisholm, K. Folting, K.C. Glasgow, E. Lucas, W.E. Streib,
Organometallics 19 (2000) 884.
[10] J. Alexander, G.S.K. Rao, J. Chem. Educ. 4 (1970) 277.
[11] S. Hellmann, H.D. Beckhaus, C. Ru¨chardt, Chem. Ber. 116
(1983) 2219.
[7] (a) D. Lenoir, H. Burghard, J. Chem. Res. (1980) 4715. (b) T.L.
Chen, T.H. Chan, A. Shaver, J. Organomet. Chem. 268 (1984)
[12] L. Fitjer, U. Quabeck, Syn. Commun. 15 (1985) 855.
.