Titanium-catalyzed hydroalumination of conjugated dienes: Access to fulvene-derived allylaluminium reagents and their diastereoselective reactions with carbonyl compounds

Article abstract of DOI:10.1002/chem.201304775

The described titanium-catalyzed hydroalumination of conjugated dienes opens up a new way to allylaluminium reagents. The reaction is carried out by using diisobutylaluminium hydride (DIBAL-H) and a catalytic amount of [Cp ₂TiCl₂] (Cp=cyclopentadienyl). When applied to mono- and disubstitued pentafulvenes, this reaction proceeds in a highly endocyclic manner. The formed allylaluminium compounds react regio- and stereoselectively with both aldehydes and ketones to afford homoallylic alcohols that are suitable synthons for functionalized cyclopentanones. An extension of this methodology to simple dienes was also investigated. In the proposed mechanism, the initially formed bimetallic species (Ti/Al) are involved in the two possible catalytic cycles with a direct hydroalumination or/and a hydrotitanation followed by a titanium to aluminium transmetallation. Aluminate your chemistry! The described titanium-catalyzed hydroalumination of conjugated dienes opens up a new way to allylaluminum reagents. When applied to mono- and disubstitued pentafulvenes, this reaction proceeds in a highly endocyclic manner producing allylaluminium compounds that react with aldehydes and ketones to afford homoallylic alcohols (see scheme; DIBAL-H=diisobutylaluminium hydride, Cp=cyclopentadienyl).

Full text of DOI:10.1002/chem.201304775

DOI: 10.1002/chem.201304775
Full Paper
&
Hydroalumination
Titanium-Catalyzed Hydroalumination of Conjugated Dienes:
Access to Fulvene-Derived Allylaluminium Reagents and Their
Diastereoselective Reactions with Carbonyl Compounds
Jomy Joseph,[a] Florian Jaroschik,[a] Dominique Harakat,[a] K. V. Radhakrishnan,[b] Jean-
Luc Vasse,[a] and Jan Szymoniak*[a]
Abstract: The described titanium-catalyzed hydroalumina-
tion of conjugated dienes opens up a new way to allylalumi-
nium reagents. The reaction is carried out by using diisobu-
tylaluminium hydride (DIBAL-H) and a catalytic amount of
[Cp2TiCl2] (Cp=cyclopentadienyl). When applied to mono-
and disubstitued pentafulvenes, this reaction proceeds in
a highly endocyclic manner. The formed allylaluminium com-
pounds react regio- and stereoselectively with both alde-
hydes and ketones to afford homoallylic alcohols that are
suitable synthons for functionalized cyclopentanones. An ex-
tension of this methodology to simple dienes was also in-
vestigated. In the proposed mechanism, the initially formed
bimetallic species (Ti/Al) are involved in the two possible cat-
alytic cycles with a direct hydroalumination or/and a hydroti-
tanation followed by a titanium to aluminium transmetalla-
tion.
Introduction
Transition-metal-catalyzed hydroalumination of unsaturated
carbon–carbon bonds has gained wide interest for selective
access to reactive aluminium reagents under mild condi-
tions.[1,2] Numerous substrates, including internal and terminal
alkenes and alkynes, as well as nonconjugated dienes have
been investigated. In contrast, examples using conjugated
dienes are scarce and no reactivity studies towards electro-
philes other than protolysis have been reported.[3] The con-
trolled hydroalumination of such substrates would open up
a new pathway to allylicaluminium reagents, which are inter-
esting nucleophiles for a number of stereoselective transforma-
tions.[4]
Scheme 1. Access to allylaluminium reagents.
To date, allylic aluminium compounds have mainly been syn-
thesized by direct metal insertion into allylic halides or by salt
metathesis using Grignard or lithium reagents (Scheme 1a).[4]
However, for certain substrates no allylic halide or metal pre-
cursors can be accessed. Among those, fulvenes are prominent
examples as the corresponding 3-halo-5-methylenecyclopent-
1-enes or their metalated analogues have not been reported
(Scheme 1a). Fulvenes have found widespread use in cycload-
dition reactions in which they can act as 2-, 4- or 6-p-electron
components.[5,6] However, the selective transformation of the
endocyclic diene system by methods other than cycloaddition
reactions is largely unexplored.[7] Such methods would poten-
tially launch fulvene as a suitable synthon for functionalized
cyclopentanones.
[a] Dr. J. Joseph, Dr. F. Jaroschik, Dr. D. Harakat, Dr. J.-L. Vasse,
Prof. J. Szymoniak
Institut de Chimie Molꢀculaire de Reims
CNRS (UMR 7312) and Universitꢀ de Reims
BP 1039, 51687 Reims Cedex 2 (France)
Fax: (+33)3-26-91-32-44
We recently reported a new titanium-mediated activation of
the endocyclic diene moiety in pentafulvenes.[8] The initial ap-
proach involved the well-known formation of h3-allyltitano-
cenes from dienes following the sequence shown in
Scheme 2.[9]
E-mail: jan.szymoniak@univ-reims.fr
[b] Dr. K. V. Radhakrishnan
National Institute for Interdisciplinary Science and Technology
CSIR-NIIST, Industrial Estate P. O.
We noticed, however, that the classical reaction conditions
(2RMgCl+[ Cp2TiCl2]) (Cp=cyclopentadienyl) did not apply to
fulvenes. In this case, no allyltitanocene intermediate was
Trivandrum-19, Kerala (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304775.
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Scheme 2. Formation of h3-allyltitanocenes from pentafulvenes and their ad-
dition to aldehydes.
Figure 1. Plausible pathways for the stoichiometric allylmetalation.
formed as confirmed by quenching the reaction with PhCHO.
We therefore used diisobutylaluminium hydride (DIBAL-H)
(2 equivalents) instead of RMgCl. Firstly, DIBAL-H was already
associated with [Cp2TiCl2] in some cases to give an efficient al-
lylmetalation reaction.[10] Second, we demonstrated that DIBAL-
H alone reacted very slowly towards fulvenes at room temper-
ature. Initial experiments performed under rigorously stoichio-
metric conditions, by using benzaldehyde as the electrophile,
afforded the corresponding homoallylic alcohol 2a in 10 to
30% yields. Performing the reaction at À508C, increased the
yield to 40%. Finally, the use of an excess of DIBAL-H (3 equiv-
alents) provided 2a in 60% isolated yield, without competitive
reduction of the aldehyde. Cycloheptyl- and adamantyl-derived
fulvenes and other aromatic and aliphatic aldehydes were
tested to afford the homoallylic alcohols with total regio- and
stereoselectivity, up to 72% isolated yields (Scheme 3).[8]
regio- and stereoselectivities. We have demonstrated that this
new approach also applies to simple dienes.
Results and Discussion
In the first experiments, DIBAL-H (1.1 mmol) was added to
a THF solution of fulvene (0.5 mmol) and [Cp2TiCl2] (1 mol%) at
room temperature. After stirring for 16 h at the same tempera-
ture, benzaldehyde (0.5 mmol) was added at À608C and the
reaction was continued for 1 h. Basic workup (aqueous
NaHCO3) resulted in the exclusive formation of cyclopentadien-
yl homoallylic alcohol 2’c related to the exocyclic attack of the
hydride (Table 1, entry 1). By increasing the catalyst loading to
Table 1. Optimization studies.
[Cp2TiCl2][a] [mol%]
1c [equiv]
t [h]
T
2c [%][b]
2’c[b]
Scheme 3. Synthesis of homoallylic alcohols from pentafulvenes.
1
2
3
4
5
6
7
1
10
20
20
10
5
1.0
1.0
1.0
1.5
1.5
1.5
1.5
16
16
16
16
4
RT
RT
RT
508C
508C
508C
508C
–
52
23
<5
–
–
5
28
55
99
99
72
–
The significant increase of the yield upon addition of 3
versus 2 equivalents of DIBAL-H, indicates that an excess of
DIBAL-H is crucial for promoting an efficient allylmetallation. At
this stage, one may assume that an allylaluminium intermedi-
ate could be generated under these reaction conditions.
A plausible mechanistic rationale for this reaction involves
the formation of the bimetallic species I,[8]which would act as
the hydrotitanation agent to form an allyltitanium intermediate
(path a). The transmetallation from titanium to aluminium
could afford the allylaluminium species (path b). A third path-
way, which would involve a direct hydroalumination could also
operate (path c) (Figure 1).
4
4
–
91
[a] Based on amount of aldehyde used. [b] NMR spectroscopic yields.
10 mol%, the endo addition product 2c also started to form
and the ratio of exo/endo products turned to be nearly 1:1
with a total yield of 51% (entry 2). With a catalyst loading of
20 mol%, exocyclic attack was almost suppressed but the yield
remained moderate (entry 3). A rise in temperature proved to
be beneficial and at 508C the homoallylic alcohol was obtained
in 99% yield, with total endo selectivity and an exclusive anti
stereoselectivity with respect to the vicinal protons (entry 4).
Further, at this temperature the catalyst loading was reduced
to 10 mol% and reaction time to 4 h without having any detri-
mental effect on the yield and regio- or diastereoselectivities
(entry 5). However, a further decrease of the catalyst loading to
5 mol% led again to the formation of some 2’c, albeit in low
The aforementioned mechanistic scenarios led us to the as-
sumption that a titanium-catalyzed hydroalumination of conju-
gated dienes might be possible.
In this paper, we would like to disclose the development of
a new catalytic system based on [Cp2TiCl2] and DIBAL-H that
allows the highly selective endocyclic hydroalumination of
pentafulvenes (Scheme 1b). The successive allylalumination of
carbonyl compounds is high yielding and displays very good
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quantities (entry 6). In the absence of titanium complex under
the optimised conditions exclusive formation of 2’c was ob-
served in high yield (entry 7). Small amounts of fulvene re-
duced products are also formed in all the cases. It is important
to note that other aluminium hydrides (Red-Al, Lithium tri-tert-
butoxyaluminium hydride), boron hydride (l-selectride) and
Grignard reagents failed to produce this novel titanium-cata-
lysed hydrometallation.
The very good results obtained with the new catalytic
system and disubstituted pentafulvenes, led us next to the in-
vestigation of monosubstituted pentafulvenes. These are more
challenging substrates in terms of achieving endocyclic selec-
tivity because the highly reactive exocyclic C-6 position is read-
ily exposed. Indeed, under stoichiometric hydrotitanation con-
ditions, only reduction products of fulvene were observed.
However, under catalytic conditions, we were pleased to find
that the endocyclic positions were again the preferred site of
reactivity. A variety of monosubstituted fulvenes carrying
4-N,N’-dimethylamino-, 4-bromo-, 4-methyl-, and 4-methoxy-
phenyl substituents 1e–i were studied and the resulting ho-
moallylic alcohols 3e–i were obtained in moderate to good
yields with high regio- and diastereoselectivity (Table 3).
With the optimized conditions in hand, the substrate scope
of this new catalytic reaction was further explored. Aliphatic,
aromatic, a,b-unsaturated and heteroaromatic aldehydes
showed excellent reactivity and afforded the corresponding
homoallylic alcohols in good yields and with total diastereose-
lectivity according to 1H NMR spectroscopy (Table 2). In the
Table 2. Reactions of disubstituted pentafulvenes with aldehydes.[a]
Table 3. Reactions of monosubstituted pentafulvenes with aldehydes.[a]
[a] Reaction conditions: Fulvene (0.75 mmol), [Cp2TiCl2] (0.05 mmol),
DIBAL-H (1.1 mmol), THF 508C, 4 h, then aldehyde (0.5 mmol), À608C,
1 h. [b] Isolated yields. [c] NMR spectroscopic yields calculated by using
1,2-tetrachloroethane as the internal standard.
Through selective endocyclic dihydroxylation followed by oxi-
dative cleavage of the exocyclic double bond, the relative con-
figuration of these new homoallylic alcohols was established
and it was again found to be anti with respect to vicinal pro-
tons.
Allylation of aldehydes is known to proceed in a highly dia-
stereoselective manner with several allylic metal reagents.[11] In
contrast, achieving such a level of selectivity with ketones is
rare.[12] Recently, the highly diastereoselective addition of cyclic
allylalanes onto methylketones was described.[4a] We thus de-
cided to investigate ketones in our catalytic conditions
(Table 4). At first, acetone was employed, affording the corre-
sponding alcohol 4a in 94% yield. A series of unsymmetrical
ketones were used next. Acetophenone derivatives provided
good to excellent diastereoselectivities (4b–d), probably due
to a different steric demand within the ketone. In contrast,
chalcone led to a 1:1 mixture of diastereomers 4e along with
a 1,4-addition product. To maintain a significant size differen-
tiation between the two branches of the electrophile, ketones
containing an alkynyl substituent, which constitutes an alkyl
[a] Reaction conditions: Fulvene (0.75 mmol), [Cp2TiCl2] (0.05 mmol),
DIBAL-H (1.1 mmol), THF, 508C, 4 h, then aldehyde (0.5 mmol), À608C,
1 h. [b] Isolated yields. [c] Reaction time before adding electrophile is
12 h.
case of cinnamaldehyde, the 1,2-addition product 2m was
formed exclusively. It is noteworthy that compounds with reac-
tive functional groups, for example ÀOH, ÀNO2, and even
ÀCOOH, were also tolerated under the reaction conditions
(Table 2, 2 f–h.) When the adamantyl group in fulvene was re-
placed with two phenyl or two ethyl groups (fulvenes 1a, d)
still high selectivity could be achieved (2a, n).
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Table 4. Reactions of adamantyl-derived pentafulvene1c with ketones.[a]
Figure 2. Proposed reaction mechanism.
[a] Reaction conditions: 1a (0.75 mmol), [Cp2TiCl2] (0.05 mmol), DIBAL-H
(1.1 mmol), THF, 508C, 4 h, then ketone (0.5 mmol). [b] Isolated yields.
formation of allylicalumininum species in the case of the p-
N,N’-dimethylaminophenylfulvene (1e) by HRMS (B, Figure 2).
We also noticed that for the hydrotitanation, the classical
[Cp2TiCl2]/iPrMgX system or any other transition-metal/DIBAL-H
combination ([Cp2ZrCl2], NiCl2, CuCl2) cannot be applied to ful-
venes.[8] This may indicate that the catalytically active hydride
involves a cooperative Al/Ti association.
chain precursor, were tested. In these cases, alcohols 4 f–h
were obtained in a diastereoselectively pure form. Interestingly,
functional groups could be incorporated to the alkynyl chain,
for example, a protected alcohol in 4g. The use of a ketone
containing a linear alkyl and an alkynyl chain opens up the
way to diastereoselective access to tertiary alcohol 4h bearing
two comparable alkyl chains.
Based on these results and on our previous mechanistic
studies for the stoichiometric reaction,[8] a plausible mechanism
is shown in Figure 2. Starting from the initially formed trivalent
complex [Cp2Ti(m-Cl)2Al(iBu)2], chloride for hydride exchanges
could generate two bimetallic hydride intermediates I and
II(A).
Two reaction pathways could then operate. In the first, the
bimetallic species A could give direct hydroalumination to
yield the allylaluminium B (path a). Alternatively, the bimetallic
species could act as a fulvene hydrotitanation agent to give
the p-allyltitanocene III, which transmetallates to B and regen-
erates the catalytically active hydride (path b). Path a is possi-
bly favoured under the catalytic conditions, as referred to the
stoichiometric variant, (see Figure 1), however, path b cannot
be ruled out at this stage. To date we cannot exclude that
even more complex species are involved in this transforma-
tion.
This approach, involving fulvene-derived allylaluminium spe-
cies, is of particular interest since it constitutes an alternative
to cyclopentanone–ketone cross-aldol reactions.[13] The corre-
sponding products can be readily obtained in two steps from
the homoallylic alcohol: hydrogenation of the endocyclic
double bond followed by oxidative cleavage of the exocyclic
double bond (Scheme 4). After the oxidation step, adamanta-
Scheme 4. Transformation of homoallylic alcohols to cross-aldol products.
To examine the generality of this new catalytic system, the
optimized reaction conditions were applied to simple dienes,
that is, cyclohexadiene, cyclooctadiene, and isoprene. In these
cases, a longer reaction time (12 h) was necessary to obtain
the corresponding homoallylic alcohols in good yields
(Table 5). Cyclic dienes reacted with higher diastereoselectivity,
whereas in the case of isoprene complete loss of diastereose-
lectivity was observed. With simple dienes, the catalyst loading
can even be reduced to 1 mol%, probably due to the relatively
decreased competition from other reactive sites.
none can be fully recovered and reused in the formation of
starting fulvene.
Mechanistic studies of this new titanium-catalyzed hydroalu-
mination reaction were hampered by the fact, that the initial
reaction between titanocene dichloride and DIBAL-H yielded
paramagnetic TiIII species.[8] Hence, the reaction could not be
followed by NMR spectroscopy.[14] We therefore attempted to
detect intermediate species by ESI-MS studies. Whereas the
catalytic species remained elusive, we were able to confirm the
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H was completed, the reaction vessel was inserted into a preheated
oil bath at 508C. After 4 h at the same temperature, the mixture
was cooled to À508C and aldehyde or ketone (0.5 mmol) was
added. The reaction was continued for 1 h at the same tempera-
ture and then quenched by adding a saturated aqueous solution
of NaHCO3. The mixture was further diluted with ether (25 mL) and
stirred for another 2 h. The organic phase was separated and con-
centrated under reduced pressure. The residue was purified by
column chromatography to give the corresponding homoallylic al-
cohol.
Table 5. Generalisation to simple dienes.[a]
General procedure for homoallylic alcohols derived from
simple dienes and benzaldehyde/ketones
A standard Schlenk tube with magnetic stir bar was charged with
[Cp2TiCl2] (2.5 mg, 0.01 mmol). The system was evacuated and
back-filled with argon (3 times). Dry THF (5 mL) was added fol-
lowed by the diene (1 mmol). DIBAL-H (1m in THF, 1.1 mL) was
added over 2 min at room temperature. When the addition of
DIBAL-H was completed the reaction vessel was inserted into a pre-
heated oil bath at 508C. After 12 h at the same temperature, the
mixture was cooled to À508C. Then, benzaldehyde or ketone
(0.5 mmol) was added. The stirring was continued for 1 h at the
same temperature. The reaction was quenched by adding a saturat-
ed aqueous solution of NaHCO3, and the mixture was further dilut-
ed with Et2O (25 mL) and stirred for another 2 h. The organic
phase was separated and concentrated under reduced pressure.
The residue was purified by column chromatography to give the
corresponding homoallylic alcohol.
[a] Reaction conditions: Diene (2 mmol), [Cp2TiCl2] (0.05 mmol), DIBAL-H
(1.1 mmol), THF, 508C, 12 h, then electrophile (0.5 mmol), À608C, 1 h.
[b] Isolated yields.
Conclusion
In summary, we have developed a new titanium-catalyzed hy-
droalumination that allows the formation of allylaluminium re-
agents from conjugated dienic systems. With mono and disub-
situted pentafulvenes, hydroalumination took place in a highly
endoselective manner. The formed allylaluminium reagents are
excellent nucleophiles for aldehydes and ketones and allowed
the diastereoselective formation of homoallylic alcohols. The
easy accessibility of cross-aldol products from these homoallyl-
ic alcohols emphasizes the potential of the present methodolo-
gy. Simple dienes also form allylaluminium reagents under the
reaction conditions thereby making this novel catalytic system
suitable for broad applicability. Investigations regarding the
basis of site selectivity, exact nature of catalytic species, and
applications to other classes of electrophiles are currently un-
derway.
Synthesis of 2-(1’-hydroxy-1’-methyl)benzylcyclopentanone
(6b)
A mixture of 4b (1 mmol) and Pd/C (10%, 100 mg, 0.1 mmol) in
MeOH (5 mL) was stirred under an atmosphere of hydrogen for
1 h. The reaction mixture was diluted with CH2Cl2 (10 mL) and fil-
tered through a pad of Celite. The filtrate was evaporated to dry-
ness and the crude product of 5b was directly used without purifi-
cation.
The crude mixture 5b was dissolved in a 1:1 mixture of CH3CN:CCl4
(20 mL). NaIO4 (1.278 g, 6 mmol) was added with stirring. Then a so-
lution of RuCl3·H2O (22.5 mg, 0.1 mmol) in water (10 mL) was
added in one portion. The reaction was continued for 5 h. The re-
action mixture was diluted with water (10 mL) and extracted with
CH2Cl2 (3ꢁ15 mL). The organic layer was evaporated to dryness
and the residue was purified by column chromatography to give
the ketone 6b (142 mg, 70%) as a colourless oil.[13] 1H NMR
(250 MHz, CDCl3, 258C, TMS): d=7.46 (m, 2H), 7.34 (m, 2H), 7.25
(m, 1H), 4.58 (brs, 1H),2.44 (m, 2H), 2.16 (m, 1H), 1.82 (m, 1H),
1.67 (m, 2H),1.58 ppm (s, 3H); 13C NMR (62.5 MHz, CDCl3, 258C,
TMS): d=222.41, 145.9, 128.10, 127.00, 125.16, 74.97, 59.90, 39.89,
27.13, 23.54, 20.00 ppm; HRMS-ESI: m/z: elemental analysis calcd
(%) for C13H16NaO2: 227.1048; found: 227.1050 [M+Na]+.
Experimental Section
General remarks
All reactions involving organometallics were conducted under an
atmosphere of argon. Prior to use, THF was distilled over sodium-
benzophenoneketyl. [Cp2TiCl2], DIBAL-H, and aldehydes were pur-
chased from Aldrich. 1H and 13C NMR spectra were recorded in
CDCl3on a Bruker AC-250 spectrometer. Mass spectra were record-
ed on a Micromass Q-TOF micro MS spectrometer. All fulvenes
(1c–j) except diphenylfulvene 1a were prepared by the procedure
reported by Little.[15] Purification of the compounds was done
through flash column chromatography on silica gel.
Acknowledgements
General procedure for homoallylic alcohols derived from ful-
venes and aldehydes/ketones
Financial support from the Indo-French Center for the Promo-
tion of Advanced Research (IFCPAR-CEFIPRA) is greatly ac-
knowledged. Financial support by the CNRS, Conseil Regional
Champagne Ardenne, Ministry of Higher Education and Re-
search (MESR) and EU-programme FEDER to the PlAneT CPER
A standard Schlenk tube with magnetic stir bar was charged with
fulvene (0.75 mmol) and [Cp2TiCl2] (12.5 mg, 0.05 mmol). The
system was evacuated and back filled with argon (three times). Dry
THF was added (5 mL) followed by slow addition of DIBAL-H (1m
in THF, 1.1 mL) at room temperature. When the addition of DIBAL-
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·
dienes
·
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Received: December 5, 2013
Published online on && &&, 0000
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&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Aluminate your chemistry! The de-
scribed titanium-catalyzed hydroalumi-
nation of conjugated dienes opens up
a new way to allylaluminum reagents.
When applied to mono- and disub-
stitued pentafulvenes, this reaction pro-
ceeds in a highly endocyclic manner
producing allylaluminium compounds
that react with aldehydes and ketones
to afford homoallylic alcohols (see
scheme; DIBAL-H=diisobutylaluminium
hydride, Cp=cyclopentadienyl).
&
Hydroalumination
J. Joseph, F. Jaroschik, D. Harakat,
K. V. Radhakrishnan, J.-L. Vasse,
J. Szymoniak*
&& – &&
Titanium-Catalyzed Hydroalumination
of Conjugated Dienes: Access to
Fulvene-Derived Allylaluminium
Reagents and Their Diastereoselective
Reactions with Carbonyl Compounds
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ