Communications
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-
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 6225 –6228