Metal carbene dimerization: Versatile approach to polyalkynylethenes

Article abstract of DOI:10.1002/anie.200801584

(Chemical Equation Presented) Heads or tails? Chemo-, regio-, and stereoselective access to cross- and linear conjugated poly-ynene scaffolds from alkynyl-derived group six metal carbenes is reported. The metal carbenes shown (M=Cr, W; TIPS=triisopropylsi

Full text of DOI:10.1002/anie.200801584

Angewandte
Chemie
DOI: 10.1002/anie.200801584
Synthetic Methods
Metal Carbene Dimerization: Versatile Approach to
Polyalkynylethenes**
JosØ Barluenga,* Diana de Sµa, Arµnzazu Gómez, Alfredo Ballesteros, Javier Santamaría,
Ana de Prado, Miguel Tomµs, and Angel L. Suµrez-Sobrino
In memory of Lorenzo Pueyo
The last years have witnessed an increasing interest in
conjugated organic molecules and polymers due primarily
to their potential applications as advanced materials, molec-
ular wires, and switches, chemo- and biosensors, nonlinear
optics (NLO), organic conductors, etc.[1] The main focus has
been on molecules containing an ethene unit, which is
surrounded by alkynyl or mixed alkynyl/alkenyl branches,
because of their planarity, two-dimensional conjugation, and
potential for acetylenic scaffolding.[2] Whereas the area of
polyethynylethenes has been a motif of special emphasis, the
access to hybrid ethynyl/ethenyl ethenes has been explored to
a lesser extent.[3] Despite the fact that some dimerization
reactions involving carbenoid species[4] or palladium cataly-
sis[3] have been disclosed, most work has focused on protocols
carbene complexes 2–5 (the R groups are defined in
Schemes 1–4)[10] in THF by sequential treatment of alkoxy-
carbene complexes 1 with various lithium acetylides and
trimethylsilyl triflate at À808C. The solution was then
warmed to room temperature in the presence of a substoi-
chiometric amount of a nucleophile to provide an arsenal of
different types of dimers (6–12; see below; Scheme 1–6). The
types of metal carbene structures studied were based on the
number of ethyne units in the dimer precursors: 1) alkynyl
carbenes 2, 2) cross-conjugated diynyl carbenes 3, 3) linear-
conjugated diynyl carbenes 4, and 4) cross-conjugated triynyl
carbenes 5. As both chromium and tungsten complexes work
similarly in terms of efficiency, only the organic framework of
reagents 2–5 is outlined throughout the text, schemes, and
tables (for the specific metal used in each case see the
Supplementary Information). All the yields reported for
dimers 6–12 refer to the one-pot process and represent the
overall yield from metal carbenes 1.
À
À
for dimer preparation by multiple C(sp2) C(sp) and C(sp) C(sp)
coupling processes,[5] which generally require protection/
deprotection steps.
Recently, we advanced that in situ generated, non-hetero-
atom stabilized alkynyl carbene complexes of group six
metals dimerize below room temperature.[6] This finding
indicates that transition-metal carbene complexes might
actually be synthetically useful precursors of complex poly-
unsaturated molecules.[7,8] Described herein is a rapid, one-
pot process to access a diverse array of enyne-based frame-
works by a simple nucleophile-induced dimerization reaction
of non-heteroatom–metal carbenes 2–5, which are formed
in situ from readily available Fischer methoxycarbene com-
plexes 1 [Eq. (1)].[9]
The working model is outlined in Equation (1). This single
process involves the low-temperature generation of alkynyl
[*] Prof. Dr. J. Barluenga, D. de Sµa, A. Gómez, Dr. A. Ballesteros,
Dr. J. Santamaría, Dr. A. de Prado, Prof. Dr. M. Tomµs,
Dr. A. L. Suµrez-Sobrino
Instituto Universitario de Química Organometµlica “Enrique
Moles”, Unidad Asociada al C.S.I.C.
Universidad de Oviedo
C/Juliµn Clavería, 8, 33006 Oviedo (Spain)
Fax: (+34)985-103-450
E-mail: barluenga@uniovi.es
Homepage: http://uniovi.es/emoles/barluenga/index.htm
[**] This research was supported by the MEC, Spain (CTQ2004-08077
and CTQ2007-61048; fellowships to D.S. and A.G.) and the
Principado de Asturias (GE-EXP01-11). We are grateful to Dr. A.
Soldevilla (Universidad de La Rioja) for his assistance in the X-ray
crystallographic analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801584.
Scheme 1. Diethynylethene (DEE) scaffolds 6 from tail-to-tail dimeriza-
tion of metal carbene complex 2. PMP=4-MeOC6H4; TMS=Me3Si.
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The access to the diethynylethene scaffold (DEE) was
first accomplished by starting from metal the alkynyl carbene
(2). Thus, addition of KOtBu (0.5 equiv) to a solution of
complex 2 (THF, À808C), generated in situ from 1, with
subsequent warming to room temperature, the removal of
volatiles, and then column chromatography afforded the
dimeric products (6) in overall yields ranging from 68 to 95%
from 1 (Scheme 1).[10] In all cases, the homocoupling takes
place exclusively in a tail-to-tail fashion, with the trans
stereoisomer being solely formed. Other than the participa-
tion of alkyl and aryl groups, valuable TMS (compounds 6c,f,
and i) and alkenyl (compounds 6g–i) functional groups were
efficiently incorporated.
Scheme 3. Tetraethynylethene (TEE) scaffold 8 from tail-to-tail dimeri-
zation of metal carbene complex 4. Tol=4-Me-C6H4.
with complete selectivity wherein the Cb atom, rather than the
Cd atom, is solely involved.
The alkynyl homologated cross-conjugated triynyl car-
bene complexes (5) are very attractive because they may
produce adducts of greater complexity. Moreover, the chemo-
selectivity is another interesting goal to be addressed as
linear- or cross-conjugated structures might result
(Scheme 4). Our first observation made it clear that the
According to the regiochemistry observed, the synthesis
of the more complex dibutadynylethene (DBDE) structure
would be feasible by using cross-conjugated dialkynylcar-
benes 3 (Scheme 2). Thus, symmetrical carbenes 3a and b
Scheme 2. Dibutadynylethene (DBDE) scaffolds 7 from tail-to-tail
dimerization of metal carbene complex 3. TIPS=(Me2CH)3Si.
Scheme 4. Hexaethynylethene (HEE) scaffolds 9–10 from tail-to-tail
dimerization of metal carbene complex 5. TIPS=(Me2CH)3Si.
(R1 = R2 = Ph or PMP) dimerized as expected to afford 7a
and b, respectively (56–88% yield), as the only products. In
the case of unsymmetrical carbenes 3c–h, tail-to-tail dimeri-
zation products 7c–h were obtained in overall yields ranging
from 48 to 87% with complete chemo-, regio-, and stereose-
lectivity under the standard reaction conditions.[10] Signifi-
cantly, these results also make clear that the resulting isomer
can be predicted on the basis of the nature of R1 and R2, and
selectivity, and consequently the structure of the adduct not
only depends on the substituent of the monoalkyne moiety,
but also on the ability to control the reaction at either Cb
center (for comparison see the above discussion on the
dimerization of the structurally analogous diynylcarbene
complexes 3, Scheme 3). Thus, the dimerization of complex
5a (R2 = nPr) led exclusively to dihexatriynylethene (DHTE)
scaffold 9 (49% yield), wherein the dimerization occurred
through the Cb carbon atom appended to the nPr group. On
the contrary, hexatriynylcarbene 5b (R2 = TIPS) cleanly
dimerized by coupling of the Cb center of the diyne branch
À
on the proposed mechanism (initial nucleophilic attack at Cb
1
2
2
À
R versus Cb R ; see Scheme 6). The directing effect of R
over R1 can be explained by invoking either steric effects
(compounds 7c and d: preference of Ph over tBu and TIPS,
respectively; compounds 7e and f: preference of TMS over
tBu and TIPS, respectively) or electron-based effects (com-
pounds 7g–h; preference of nPr over Ph and PMP).
À ꢀ
(Cb C C-Ph), rather than through the Cb carbon atom
appended to the TIPS group, to provide the mixed
1,2-diethynyl-1,2-dibutadiynylethene (DEDBDE) structure
(10; 37% yield).[10]
Moreover, the suitability of butadiynylcarbene metal
complex 4 towards dimerization is exemplified in Scheme 3.
In this case, the reaction allowed the preparation of the
tetraethynylethene (TEE) scaffold (8) in good yield (60%).[10]
The tail-to-tail dimerization reaction was found to take place
Finally, additional efforts were made to expand this
method by undertaking the head-to-tail and head-to-head
dimerization. After screening several protocols (e. g. nature
and amount of nucleophile, dimerization conditions, metal-
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Chemie
exchange/dimerization sequence) the head-to-tail dimeriza-
tion was accomplished with excellent selectivity in the case of
alkenyl-substituted carbene complexes of the types 2 and 4
(Scheme 5). Thus, addition of 3,5-dichloropyridine (3,5-DCP;
would lead to intermediate II, which would afford adducts
6–10 upon the elimination of tert-butoxide and the metal.[11]
On the contrary, zwitterionic species III undergoes conjugate
addition by the allenyl metallate structure to carbenes 2 and 4
to generate intermediate IV, which evolves into dimers 11 and
12, respectively upon the elimination of 3,5-DCP and the
metal.
In conclusion, we have described a new family of
polyalkynylcarbene complexes (2–5) and
a preliminary
study on their potential synthetic utility. The simple dimeri-
zation reaction not only provides an array of structurally-
diverse and highly-conjugated molecules (6–12) with com-
plete chemo-, regio- and stereoselectivity, but it also makes
feasible the tailored access to other polyunsaturated systems,
particularly the relevant ethane-based push-pull derivatives.
Moreover, this finding makes these metal carbenes excellent
and synthetically useful surrogates for very elusive nonmetal
propargylic carbenes: 1) propynylidene a (metal carbenes 2),
Scheme 5. Mixed diethenyl/polyethynylethenes 11–12 from head-to-tail
dimerization of metal carbene complexes 2 and 4. 3,5-DCP=3,5-
dichloropyridine; TBDMS=(Me3C)Me2Si.
0.5 equiv) as the nucleophile to a THF solution containing the
carbene complex (2) at À808C, with subsequent warming to
room temperature resulted in the stereoselective formation of
the head-to-tail dimerization products (11a–c; ethenyldiethy-
nylethene scaffold (EDEE)) along with small amounts of the
head-to-head regioisomer (head-to-tail/head-to-head ratio: >
20:1for 11a and b; 10:1 for 11c). The major components was
readily isolated in pure form after flash column chromatog-
raphy in 53–84% yields. Under the same reaction conditions,
diynylcarbene 4 afforded the ethenyltriethynylethene scaffold
(ETEE) (12) in 85% yield as a 1:1 E/Z diastereisomeric
mixture.
2) pentadiyn-1-ylidene b (metal carbenes 4), 3) pentadiyn-3-
ylidene d (metal carbenes 3), 4) heptatriyn-1-ylidene c and
heptatriyn-3-ylidene e (metal carbenes 5).[12] The nature of
the molecules produced, as well as the presence of the
removable substituents for synthetic modification, provides a
way to investigate these structures, particularly in the solid
state (applications in materials science, nature of packing,
etc).
Received: April 4, 2008
Revised: May 27, 2008
Published online: July 9, 2008
Although a mechanistic explanation is not obvious at this
stage, a tentative proposal is given in Scheme 6 for the
formation of the different dimeric structures (6–12). Thus,
conjugate addition of tBuOÀ or 3,5-DCP at the electrophilic
Cb of non-heteroatom-stabilized carbenes 2–5 would lead to
the allenyl metallate species I and III. The nature of this
intermediate, anionic I versus zwitterionic III, seems to direct
the process. Thus, conjugate addition of intermediate I by its
propargylic metallate structure to carbene complexes 2–5
Keywords: carbenes · dimerization · enynes · materials science
.
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