Platinum(II) acetylides in the formal [2+2] cycloaddition- retroelectrocyclization reaction: Organodonor versus metal activation

Article abstract of DOI:10.1002/ejoc.201300300

Formal cycloaddition-retroelectrocyclization (CA-RE) reactions between electron-donor-activated alkynes and electron-poor alkenes yielding cyanobuta-1,3-dienes have recently attracted increasing interest. The transformation has been subjected to a fundame

Full text of DOI:10.1002/ejoc.201300300

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FULL PAPER
Platinum Acetylides
B. H. Tchitchanov, M. Chiu, M. Jordan,
M. Kivala, W. B. Schweizer,
F. Diederich* ................................... 1–13
Platinum(II) Acetylides in the Formal [2+2]
Cycloaddition-Retroelectrocyclization Re-
action: Organodonor Versus Metal Acti-
vation
Donor-activated triple bonds act as sub-
strates for cycloaddition-retroelectrocycli-
zation reactions with TCNE or TCNQ to
give cyanobutadienes. Herein we showcase
the activation ability of two classes of do-
nors: organic amines and platinum(II) cen-
ters directly bound to the acetylenic moie-
ties and compare them directly for the first
time.
Keywords: Cycloaddition
/ Platinum /
Charge transfer / Donor–acceptor systems /
Regioselectivity / Alkynes
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FULL PAPER
DOI: 10.1002/ejoc.201300300
Platinum(II) Acetylides in the Formal [2+2] Cycloaddition-
Retroelectrocyclization Reaction: Organodonor Versus Metal Activation
Boris H. Tchitchanov,[a][‡] Melanie Chiu,[a][‡] Markus Jordan,[a][‡] Milan Kivala,[a]
W. Bernd Schweizer,[a] and François Diederich*[a]
Keywords: Cycloaddition / Platinum / Charge transfer / Donor–acceptor systems / Regioselectivity / Alkynes
Formal cycloaddition-retroelectrocyclization (CA-RE) reac-
tions between electron-donor-activated alkynes and elec-
tron-poor alkenes yielding cyanobuta-1,3-dienes have re-
cently attracted increasing interest. The transformation has
been subjected to a fundamental investigation of the relative
degrees of alkyne activation by organic and metallorganic
donor substituents by using platinum(II) σ-acetylides as
model substrates and studying their behavior towards the
cyano carbons TCNE and TCNQ. Various cyanobutadienes
were obtained in good to excellent yields and four trends
in reactivity were discerned: 1) The presence of an anilino
substituent clearly dominates the regioselectivity of the
TCNQ addition. 2) In the absence of an organodonor, the re-
gioselectivity is inverted. 3) When platinum(II) complexes of
buta-1,3-diynes are used, the addition always takes place at
the triple bond distal to the metal center. 4) In general, trans-
bis-acetylides are more reactive than the corresponding cis
complexes. The structural parameters of the CA-RE adducts
were investigated by X-ray crystallography and their optical
properties by UV/Vis spectroscopy, giving further insights
into the structural trends and degree of intramolecular
charge transfer. These fundamental investigations may
enable the synthesis of new Pt-based molecular dyads in
which the effect of regioselectivity on photoinduced charge
separation can be studied.
Initial reports by Bruce[12] and others[13] on this reaction
focused on platinum or ruthenium σ-acetylides as donors
with TCNQ[12a,12b] or TCNE,[12c] and the transformation
was proposed to proceed by a similar mechanism to the all-
organic systems illustrated in Scheme 1, formally an enyne
metathesis.
We became interested in a direct comparison between the
alkyne-activating effects of organodonors and transition-
metal centers such as the σ-bound PtII. If the alkynes are
functionalized with both types of substituents, which one
has a dominating activating effect and thereby determines
the regioselectivity of the CA-RE reaction? A first step
towards answering this question was made in our recent
study on ferrocenyl- and N,N-dimethylanilino (DMA)-sub-
stituted alkynes and 1,3-diynes, in which we found that the
neutral DMA group was superior to the ferrocene as an
alkyne-activating element for the CA-RE reaction.[14] Upon
protonation of the DMA group, however, the reactivity was
switched and the ferrocene moiety exerted the activating ef-
fect.
Introduction
The formal [2+2] cycloaddition reaction between elec-
tron-poor alkenes and organodonor-activated alkynes fol-
lowed by a retroelectrocyclization of the transient cyclobut-
ene intermediate (CA-RE cascade, Scheme 1) is a versatile
transformation that has been the subject of a growing
number of studies and been used in various applications.[1–4]
It is a “click”-type transformation[5] that is atom-economic,
does not require catalysis, proceeds under mild conditions
in high yields, and exhibits high chemo- and regioselectivity.
Among the best studied olefinic substrates are tetracyano-
ethene (TCNE)[6] and 7,7,8,8-tetracyano-p-quinodimethane
(TCNQ).[7] The transformation yields organodonor-substi-
tuted cyanobuta-1,3-dienes as the products (Scheme 1) that
are nonplanar along the butadiene backbone exhibit a sig-
nificant degree of push–pull conjugation,[8] as evidenced by
their intense, bathochromically shifted intramolecular CT
bands. The CA-RE reaction has been used for the synthesis
of novel molecular architectures[9] and materials featuring
remarkable optoelectronic properties.[10,11]
Herein we report the synthesis of a series of platinum(II)
σ-acetylide complexes bearing weak (phenyl) and strong
(anilines) organodonors on the alkyne moieties, and their
CA-RE reactions with TCNE and TCNQ. The acetylide
complexes are reasonably air-stable and readily synthesized;
they have therefore found widespread application as robust
structural building blocks in molecular and polymeric mate-
rials.[15] Furthermore, their well-defined square-planar co-
[a] Laboratorium für Organische Chemie, ETH Zürich,
Hönggerberg, HCI, 8093 Zürich, Switzerland
Fax: +41-44-6321109
E-mail: diederich@org.chem.ethz.ch
Homepage: http://www.diederich.ethz.ch/
[‡] These authors contributed equally to this work
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300300.
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Scheme 1. General representation of the CA-RE reactions with tetracyanoethene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane
(TCNQ). EDG = electron-donating group.
ordination geometry makes them particularly useful in the lographic analysis, see Figure 2). From previous studies it is
construction of shape-persistent molecular scaffolds.[16] known that the C-atom of the acetylene at the β-position
This study has shown that the regioselectivity of CA-RE relative to the strongest donor substituent attacks the exo-
reactions of organodonor-substituted platinum(II) σ-acet- cyclic dicyanovinyl center, followed by ring closure at the
ylides can be controllably varied depending on the ligands endocyclic quaternary sp2 C-atom to yield the cyclobutene
on the platinum and the configuration of the complexes.
intermediate, which subsequently opens.[7,14,18] In the reac-
tion of 1 with TCNQ, the nucleophilic sp C-atom is in the
β-position with respect to the anilino donor, whereas in the
reaction of 4, it is the other sp C-atom in the β-position
relative to the PtII center and, accordingly, a product of
inverted regioselectivity is obtained.
Results and Discussion
Synthesis of trans-Platinum(II) Bis-acetylides and Their
CA-RE Reactions
This comparison showcases several key reactivity aspects
of platinum acetylides: 1) The bis(phosphane)platinum cen-
ter can act as a competent donor, promoting the CA-RE
reaction of the triple bond attached to it, consistent with
the findings by Bruce and co-workers,[2] 2) in the case of
direct competition between platinum and the anilino donor
on the same acetylene unit, the regioselectivity of TCNQ
addition is determined by the anilino moiety, 3) platinum is
generally a weaker donor, as deduced from the attenuated
reactivity of complex 4 and the exclusive formation of
mono adducts, and 4) the second CA-RE reaction is gen-
erally slower than the first reaction.
We prepared a series of differently substituted trans-plati-
num(II) bis-acetylides by treating the corresponding trans-
bis(phosphane)platinum(II) dichloride with 2 equiv. of alk-
yne and CuI iodide as the catalyst in diisopropylamine
(Table 1).[17]
The symmetric trans-bis-acetylide 1 was treated with
TCNE at ambient temperature for 24 h to afford bis-adduct
2 as the sole product (Scheme 2). In contrast, when the re-
action was conducted with TCNQ as the acceptor, the sec-
ond addition to the triple bond did not occur at all and
only monoadduct 3 was formed. In the product, the di-
cyanoquinodimethane (DCNQ) moiety is adjacent to the
Aiming to validate these principles, we expanded our
N,N-diphenylanilino group (confirmed by X-ray crystallo- substrate studies by varying the number of alkyne units in
graphic analysis, see Figure 2). This regioselectivity con- the platinum complexes. The change from acetylide to 1,3-
trasts that previously observed in the case of trans-bis(tri- diacetylide ligands on the platinum added another dimen-
methylphosphane)bis(1-propynyl)platinum(II), which lacks sion to the regioselectivity of the CA-RE reactions, namely
the aniline donor substituent.[12b] This finding gave rise to the question of which triple bond in the buta-1,3-diynyl
the hypothesis that the electronic nature of the substituents fragments would take part in the transformation. We pre-
on the alkynes as well as the steric bulk of the phosphane pared the trans-bis(buta-1,3-diynyl) complex 7 bearing two
ligands determine the regioselectivity of the reactions.
N,N-dimethylanilino (DMA) substituents and complex 8 in
The relative reactivities of the anilino-substituted com- which the diyne moieties are capped only by phenyl rings
plex 1 and phenyl-substituted bis-acetylide 4 were examined (Scheme 3).[17] The adducts of DMA-activated complex 7
to deduce the influence of the electron-donating N,N-di- with TCNE (9) and TCNQ (10) were readily formed in
phenylanilino groups on the alkyne properties. In the ab- CH2Cl2 at ambient temperature in quantitative yields. The
sence of an anilino donor, compound 4 was much less reac- additions occurred readily at the triple bonds next to the
tive and full conversion was observed only at elevated reac- aniline moieties. These findings were unambiguously con-
tion temperatures. Only single additions leading to adducts firmed by X-ray crystallographic analysis and are in agree-
5 and 6 were observed. Most importantly, the regioselecti- ment with the expectation that the CA-RE reaction occurs
vity of the TCNQ addition to the triple bond was inverted: at the triple bond next to the strongest donor that is also
the DCNQ moiety in adduct 6 is on the carbon atom di- the least sterically hindered activating group. No further ad-
rectly attached to the platinum (confirmed by X-ray crystal- dition products were observed even in the presence of excess
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Table 1. Overview of the trans- and cis-platinum(II) bis-acetylides used in this study.
Scheme 2. Reactions of Pt complexes 1 and 4 with TCNE and TCNQ. Conditions for the synthesis of 2: 1, TCNE (2 equiv.), C2H2Cl4,
25 °C, 24 h. For 3: 1, TCNQ (2 equiv.), C2H2Cl4, 25 °C, 18 h. For 5: 4, TCNE (2 equiv.), C2H2Cl4, 70 °C, 20 h. For 6: 4, TCNQ (2 equiv.),
C2H2Cl4, 80 °C, 23 h.
of the alkene reagents, which reflects the deactivation of the num center was expected to be the stronger directing donor
remaining triple bonds by the newly introduced, strongly center in 8 (Scheme 3). The regioselectivity of the TCNQ
electron-accepting tetracyanobutadiene and cyclohexadi- addition leading to complex 12 is inverted compared with
enyl-expanded tetracyanobutadiene moieties, respectively, 10. This indicates that the platinum center sufficiently acti-
in the adducts.[19]
vates the terminal C-atom of the bound buta-1,3-diynyl
Due to the absence of strong amine donors, the reactions moiety, which reacts in the first step at the exocyclic C-atom
of the bis(4-phenylbuta-1,3-diynyl) complex 8 with TCNE of the dicyanovinyl moiety of TCNQ with the formation of
and TCNQ needed to be carried out at elevated tempera- a zwitterionic intermediate, which subsequently closes to
tures (110 °C) and required longer reaction times to form the cyclobutene.[18] Furthermore, attack of the distal acetyl-
bis-adducts 11 and 12, respectively, in only moderate yields. ene avoids the steric hindrance that would occur if the prox-
Interestingly, X-ray crystallographic analysis of these prod- imal acetylene next to the Pt-center reacts. Nevertheless, the
ucts revealed that the acceptor compounds reacted with the fact that the distal triple bond, and not the one next to the
triple bonds adjacent to the phenyl rings, although the plati- bis(phosphane)platinum(II) center, is involved in the reac-
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Platinum Acetylides in Cycloaddition-Retroelectrocyclizations
Scheme 3. Reactions of trans-bis(buta-1,3-diynyl)platinum(II) complexes 7, 8, 13, and 14 with TCNE and TCNQ. Conditions for the
synthesis of 9: 7, TCNE (4 equiv.), CH2Cl2, 25 °C, 45 min. For 10: 7, TCNQ (2.5 equiv.), CH2Cl2, 25 °C, 3 h. For 11: 8, TCNE (4 equiv.),
C2H2Cl4, 110 °C, 21 h. For 12: 7, TCNQ (5 equiv.), C2H2Cl4, 110 °C, 15 h. For 15: 14, TCNE (5 equiv.), C2H2Cl4, 140 °C, 48 h. For 16:
14, TCNQ (5 equiv.), C2H2Cl4, 140 °C, 48 h. For 17: 13, TCNE (5 equiv.), C2H2Cl4, 110 °C, 24 h. For 18: 13, TCNQ (5 equiv.), C2H2Cl4,
110 °C, 28 h.
tions was surprising and required further investigation. We adducts 15 and 16 (HR-MALDI-MS), but the majority of
therefore investigated 1) why the reactions in trans-bis(buta- the starting material decomposed under the reaction condi-
1,3-diynyl)platinum(II) complexes occur at the triple bond tions. The most interesting question, however, concerned
more distant from the platinum center and 2) whether other the reactions of complex 13 with the two cyano-olefins. We
aspects of the PtII complex structure, namely the configura- were able to isolate and structurally characterize TCNE bis-
tion (cis vs. trans), affect the reactivity pattern of this class adduct 17, which has a structure similar to that of the corre-
of compounds.
sponding DMA-substituted adduct 9. The reaction of 13
To test the influence of the electronic and steric nature with TCNQ, however, led to the formation of an insepa-
of the P-ligands on platinum, we focused our attention on rable mixture of products and the formation of bis-adduct
two additional trans-bis(buta-1,3-diynyl) complexes, 13 and 18 could only be observed by HR-MALDI-MS. This is a
14, which lack DMA activation. Trimethylphosphane, the substantial difference in comparison with acetylide deriva-
ancillary ligand in complex 13, has approximately the same tives 1 and 4, for which the CA-RE reaction with TCNQ
electron-donating properties as triethylphosphane in 8, but stopped after the addition of 1 equiv. of the olefin. On the
has a significantly smaller steric demand (phosphane cone other hand, there is a common trend in the bis(buta-1,3-
angle of 118° for PMe3 vs. 132° for PEt3).[20] On the other diynyl) complexes: The triple bond that takes part in the
hand, triisopropyl phosphite (complex 14) has essentially reaction is the one more distant from the platinum center.
the same cone angle (130°) as PEt3. It is, however, much
These experiments demonstrate that the electron-donat-
less electron-donating. We treated the two complexes with ing properties of the phosphane ligand influence the out-
TCNE and TCNQ. The phosphite complex 14 was substan- come of the CA-RE reaction: Sufficiently electron-donating
tially deactivated and reacted only at high temperature ancillary ligands are necessary for obtaining CA-RE prod-
(140 °C) with the cyano-olefins to produce traces of the bis- ucts. The addition of buta-1,3-diynyl moieties occurs at the
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distal triple bond, presumably due to shielding of the proxi- ethylphosphanyl)ethane (depe), with electronic and steric
mal triple bonds by the alkyl groups on the P ligands. This features similar to those of PEt3 and trans-complex 8.
is consistent with the previous findings of Bruce and co-
Reaction with TCNE or TCNQ took place only in the
cases of bis(buta-1,3-diynyl) complexes 19 and 20, which
bear activating DMA moieties. In sharp contrast, the bis-
ethynyl derivatives 21–23 are completely inert and did not
undergo the CA-RE reaction, even at 140 °C. Contrary to
the trans-complexes, the second CA-RE reaction is faster
than the first; when the cis-complexes were treated with
only 1 equiv. of alkene, mixtures of mono- and bis-adducts
(compounds 24/25, 26/27, 28/29, 30/31, Scheme 4) were ob-
tained. These observations indicate that in the absence of
strong organic donor moieties, the triple bonds in the cis
starting materials are not sufficiently activated to react with
either TCNE or TCNQ, as opposed to the trans-acetylides.
This finding will be discussed further in the comparison of
the crystal structures of starting complexes 23 and 13 below.
workers in related ruthenium diyne complexes.[2a,2b,2e,2f]
Synthesis of cis-Platinum(II) Bis-acetylides and Their
CA-RE Reactions
Lastly, we investigated the behavior of cis-platinum(II)
acetylides in TCNE and TCNQ cycloaddition reactions. We
prepared a series of cis-complexes with different bidentate
ligands (19–23, Scheme 4) by treating the corresponding cis-
dichloridoplatinum(II) complexes with 2 equiv. of the alk-
yne in diisopropylamine in the presence of copper(I) iodide
as the catalyst (Table 1).[17]
They were subsequently subjected to CA-RE reactions
with TCNE and TCNQ under conditions similar to those
employed in the study of the trans-complexes. The starting
complexes were selected to make a direct comparison be-
tween 1) a strongly donating, bulky bidentate ligand, 1,2-
X-ray Crystal Structures
Almost all CA-RE products furnished single crystals
bis(dicyclohexylphosphanyl)ethane (dcpe), as in 20 and 22, suitable for X-ray crystallographic analysis, in most cases
and a less donating, less sterically demanding ligand, 4,4Ј- by slow diffusion of n-hexane into a CH2Cl2 solution of
di-tert-butyl-2,2Ј-bipyridine (tBuBPY), as in complexes 19 the compound. In some cases, low temperatures or slow
and 21, 2) ethynyl and buta-1,3-diynyl ligands, 3) the reac- evaporation of the solvent was necessary to obtain speci-
tivity in the presence or absence of DMA donors, and 4) the mens of crystallographic quality. The crystal structures of
reactivity of the cis-complex 23 bearing a ligand, 1,2-bis(di- starting Pt acetylides 13 and 23 are compared in Figure 1,
Scheme 4. CA-RE reactions of cis-bis(buta-1,3-diynyl)platinum(II) complexes 19 and 20 with TCNE and TCNQ. The mono-adducts 24,
26, 28, and 30 were isolated in low yields when the reactions were carried out with 1 equiv. of the alkene. When 2 equiv. were used,
almost full conversion to the bis-adducts was observed. Conditions for the synthesis of 25: 19, TCNE (2 equiv.), CH2Cl2, 25 °C, 48 h.
For 27: 20, TCNE (2 equiv.), CH2Cl2, 25 °C, 28 h. For 29: 19, TCNQ (2 equiv.), CH2Cl2, 25 °C, 24 h. For 31: 20, TCNQ (2 equiv.),
CH2Cl2, 25 °C, 24 h.
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and Figure 2 shows the crystal structures of the CA-RE ad- elongation in 23 is consistent with the well-known trans-
ducts 2, 5, 6, 9, 10, 12, and 17. Further crystallographic influence due to electron donation from the strong phos-
data are summarized in Table 2 and in the Supporting In- phane ligand in the case of the cis-complexes.[21] However,
formation. The crystal structures demonstrate interesting the C1–C2 triple bond length in 23 is short, even for a cis-
structural trends in the CA-RE products and unambigu- platinum acetylide (with
a typical bond length of
ously confirm the regioselectivity of the transformation.
1.20 Å).[21] Thus, conjugation across the platinum center
seems to be less effective in the cis-complex. A possible ex-
planation for the observed differences in reactivity is the
fact that more extended conjugation in trans-13 could re-
duce the HOMO-LUMO gap in the π-system (unlike 23,
where the two acetylene ligands are in an orthogonal ar-
rangement to each other), leading to easier electron transfer
and thereby facilitating the formation of a charge-transfer
(CT) complex, which is generally accepted as the initial step
of the reaction.[2c,2d]
Comparison of the crystal structures of the CA-RE ad-
ducts yields interesting trends concerning the nature of the
charge transfer in these compounds (Figure 2). All the
products are donor–acceptor chromophores, and the pres-
ence of intramolecular, ground-state CT is verified by a set
of structural parameters derived from X-ray crystallo-
graphic data (Table 2). A good starting point for this analy-
sis is the quinoid character of the aromatic rings δr (defined
in caption of Table 2).[6b,22] Within the subset of anilino-
substituted platinum complexes (9, 10, 2), the highest value
[δr = 0.056(5)] is observed for DMA derivative 9. The anal-
ogous compound 2, however, which lacks the extra acetyl-
ene spacer group between the Pt-center and the tetracya-
nobutadiene unit, exhibits a lower degree of intramolecular
charge transfer [δr = 0.017(6), 0.021(6)] due to competition
with the CT interactions of the tetracyanobutadiene with
the bis(phosphane)platinum(II) center. The TCNQ adduct
10 shows an intermediate value [δr = 0.029(9)], which can
be explained by the increased steric hindrance of the
DCNQ moiety, leading to higher distortion [θ1 = 34.4(16)°],
thus reducing the degree of conjugation between the DMA
donor and the DCNQ acceptor moiety. Further confirm-
Figure 1. ORTEP representation of the molecular structures of
complexes a) 13 and b) 23 in the crystal. Hydrogen atoms and sol-
vent molecules have been removed for clarity. The positions of P2
and its ethyl substituents in 23 are disordered; only one of the posi-
tions is shown (see the Supporting Information for details). Ther-
mal elipsoids are shown at the 50% probability level. Selected bond
lengths [Å]: a) Pt1–C1 1.992(2), C1–C2 1.204(3), C2–C3 1.380(4),
C3–C4 1.202(2); b) Pt1–C1 2.057(7), C1–C2 1.113(9), C11–C12
1.121(9), C2–C3 1.427(9), C12–C13 1.407(10), C3–C4 1.194(9),
C13–C14 1.197(9).
The crystal structures of starting bis(buta-1,3-diynyl) ation for the strong intramolecular CT in 9 is the dimin-
complexes trans-13 and cis-23 suggest that the steric bulk ished bond length alternation in the alkyne spacer, which is
of the ancillary phosphane ligands on Pt in both complexes characterized by a significant shortening of the Pt–C8 bond
prevents the reaction at the proximal acetylene activated by [1.971(6) Å, numbering according to Table 2] compared
the metal center (Figure 1). An interesting difference in with 1.992 Å for a typical PtII–C(sp) bond[23] and C5–C7
bond lengths is observed between the two structures, pre- [1.409(7) Å vs. 1.456 Å],[23] as well as by a lengthening of
sumably due to a trans-influence of the ligands on the plati- the C7–C8 bond [1.223(8) Å vs. 1.188 Å] compared with re-
num atom. In trans-13, the Pt–C bonds are substantially lated platinum alkynyl complexes.[23] This reduced bond
shortened [1.992(2) Å for Pt1–C1 and 1.996(2) Å for Pt1– length alternation indicates that there is a CT interaction
C14, compared to 2.057(7) Å and 2.077(7) Å in 23], and the between the metal center and the adjacent dicyanovinyl ac-
bond-length alternation is less pronounced, with CϵC ceptor. Taking these findings together, the system exhibits
bond lengths of 1.202(4) Å and a single bond length of two independent donor–acceptor fragments that do not
1.380(4) Å (C2–C3). This observation suggests that there is communicate because of the nonplanarity around the C3–
a higher degree of conjugation and delocalization across the C5 single bond, which furthermore is substantially length-
platinum center in trans-complex 13 than in the cis-complex ened [1.505(6) Å].
23. In the latter, the Pt–C bonds are longer, and more pro-
In the absence of the strong DMA donor, the TCNQ
nounced bond-length alternation is observed. The C2–C3 additions in complex 12 show different regioselectivity com-
single bond length is 1.428(4) Å [1.408(4) Å for C12-C13] pared with 10 because the Pt center has become the strong-
and the two acetylene groups differ strongly, with est donor in the system. As a result of the lack of a donor
1.113(9) Å [1.121(9) Å] for the proximal and 1.194(9) Å substituent on the phenyl rings, the CA-RE products 11,
[1.197(9) Å] for the distal CϵC bond. The Pt–C bond 12, and 17 exhibit essentially no quinoid character in these
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Figure 2. ORTEP representation of the molecular structures of the CA-RE adducts a) 2, b) 5, c) 6, d) 9, e) 10, f) 12, and g) 17 in the
crystal. Hydrogen atoms and solvent molecules have been removed for clarity and only the atoms within one asymmetric unit are
numbered. Thermal ellipsoids are shown at the 50% probability level.
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Table 2. Selected structural parameters from the X-ray crystal structures of CA-RE adducts.
Compd. R1
R2
R3
n
θ1 [°][a]
θ2 [°][b]
δr [Å][c]
d(C5–C7) [Å] d(C7–C8) [Å] d(C8–Pt) [Å]
9
10
11
CN
DCNQ
CN
CN
CN
CN
NMe2
NMe2
H
1
1
1
3.04(6)
34.4(16)
38.6(6)
34.6(5)
47.26(7)
22.2(4)
24.7(4)
59.8(7)
51.7(7)
82.2(4)
90.816)
97.0(16)
88.0(6)
54.5(12)
66.1(5)
70.0(5)
76.4(6)
75.1(3)
74.9(3)
56.6(7)
56.0(7)
65.5(4)
65.2(15)
65.9(16)
0.056(5)
0.029(9)
0.007(5)
0.012(5)
0.007(6)
0.010(3)
0.005(3)
0.017(6)
0.021(6)
0.008(4)
0.00(2)
1.409(7)
1.403(11)
1.406(5)
1.395(5)
1.413(7)
1.405(4)
1.399(4)
1.223(8)
1.206(12)
1.210(5)
1.212(5)
1.207(7)
1.219(4)
1.215(4)
1.971(6)
2.000(9)
1.989(4)
1.983(4)
1.978(5)
1.975(3)
1.983(3)
12
17
CN
CN
DCNQ
CN
H
H
1
1
2
CN
CN
NPh2
0
5
6
CN
CN
CN
DCNQ
H
H
0
0
0.06(2)
[a] θ1 is the absolute value of the torsion angle defined by the atoms C1–C2–C3–C4. [b] θ2 is the absolute value of the torsion angle
defined by the atoms C4–C3–C5–C6 (two values are shown in the cases in which the two angles are not symmetry-equivalent). [c] δr =
{[(a+aЈ)/2 – (b+bЈ)/2]+[(c+cЈ)/2 – (b + bЈ)/2]}/2; for benzene, δr = 0.00 Å; for fully quinoid rings, δr = 0.08–0.12 Å.
rings and are twisted substantially out of the plane of the whereas the TCNE adducts of the phenyl-substituted deriv-
adjacent vinyl unit (defined by atoms C2–C3–C4) and thus atives are bright yellow or orange. The electronic absorp-
do not participate in π conjugation. In the cases of the tion spectra of almost all the TCNE and TCNQ adducts
TCNE adducts 11 and 17, more pronounced cumulenic show intense low-energy CT absorption bands in acetoni-
character of the acetylene bridge is observed compared with trile at room temperature (Figure 3). Owing to the presence
the TCNQ adduct 12. In all cases, the buta-1,3-diene unit of strong DMA donors, the CT interactions in the non-
is substantially twisted around the central single bond [θ2 planar D–A chromophores in 9 and 10 are efficient and
from 54.5(12) to 88.0(6)°]. Aiming to gain further insights hence the CT bands are distinctly bathochromically shifted
into the CT chromophore properties of the products, and relative to the bands of the corresponding phenyl deriva-
to confirm the trends observed in the X-ray structures, we tives 11 and 12. Interestingly, the molar extinction coeffi-
analyzed their optical spectra.
cient of the D–A transition of 12 (561 nm) is nearly as large
as that of the corresponding band in 10 (679 nm), although
only the platinum center acts as a donor in the former chro-
mophore (Figure 3). This reveals that, despite being a less
UV/Vis Spectroscopy
The CA-RE adducts of the PtII acetylide complexes pronounced electron donor and having weaker alkyne acti-
formed with TCNQ are usually deeply colored solids, vation ability for the CA-RE reaction compared with
Figure 3. UV/Vis spectra of compounds 9–12 (a) and compounds 2, 3, 5, and 6 (b) in acetonitrile at 25 °C.
Eur. J. Org. Chem. 0000, 0–0
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FULL PAPER
DMA, the bis(phosphane)platinum(II) center can partici- in the case of complexes bearing trimethylphosphane as an
pate in efficient CT from the platinum to the DCNQ moi- ancillary ligand, the triple bond next to the platinum did
ety. This is consistent with the observation of donor acti- not react with TCNE or TCNQ. Reducing the electron-do-
vation of the triple bonds by platinum only in the aniline- nating ability of the substituents on the alkyne or phos-
free acetylide complexes and illustrates the donating ability phane moiety generally leads to attenuated reactivity, as
of the platinum center.
does the switching from trans- to cis-configured complexes.
In contrast to this series of buta-1,3-diyne-derived ad-
These studies provide fundamental insights into the elec-
ducts, the products lacking triple bond spacers 2, 3, 5, 6 tronic preferences and regioselectivity of CA-RE reactions
exhibit smaller extinction coefficients. The reduced donat- by using PtII acetylide complexes as the alkyne reactants.
ing ability of the diphenylanilino group, indicated by the This knowledge may provide the means to access new do-
reduced quinoid character of the aniline 1,4-phenylene ring nor–acceptor chromophores that are interesting and useful
of 2, is further supported by its UV/Vis spectrum, in which in the context of light-harvesting and energy-conversion
the ε value of the CT band at around 500 nm is significantly systems derived from platinum acetylides.[24]
lower than in the related DMA complex 9 (Figure 3). The
unsymmetric TCNQ mono-adducts 3 and 6 exhibit very
similar CT bands even though the diphenylamino donor
groups are missing in 6 and the regioselectivity of the
Experimental Section
Materials and General Methods: Chemicals were purchased from
Acros, Aldrich, Fluka, and TCI, and used as received. 1-[4-(Di-
methylamino)phenyl]-4-trimethylsilylbuta-1,3-diyne,[6b] 1-phenyl-4-
trimethylsilylbuta-1,3-diyne,[6b] 4-ethynyl-N,N-diphenylaniline,[25]
1-phenyl-4-triisopropylsilylbuta-1,3-diyne,[6b] [{P(OiPr)3}2PtCl2],[26]
[(PMe3)2PtCl2],[27] [(depe)PtCl2],[28] [(dcpe)PtCl2],[29] [(tBuBPY)-
PtCl2],[30] as well as acetylides 4,[31] 8,[32] 19,[15b] 20,[33] 21,[15b] and
22[33] were prepared according to or similarly to literature pro-
cedures. All reactions were performed under N2. Column
chromatography (CC) and plug filtrations were carried out with
Silicycle SiliaFlash F60 (particle size 40–63 μm, 230–400 mesh) or
MP neutral Al2O3 and distilled technical solvents. TLC was con-
ducted on aluminium sheets coated with SiO2 60 F254 obtained
from Merck and visualized with a UV lamp (254 or 365 nm). Melt-
ing points were measured in open capillaries with a Büchi Melting
Point B540 apparatus. “Dec.” refers to decomposition. 1H and 13C
NMR spectra were recorded with a Varian Gemini 300 MHz, a
Varian Mercury 300 MHz, a Bruker DRX 400 MHz, or a Bruker
AV 400 MHz spectrometer at 25 °C. The chemical shifts are re-
ported in ppm downfield from SiMe4 with the solvent’s residual
signal as internal reference (1H, 13C) or phosphoric acid (85% in
water) as external reference (31P). Coupling constants (J) are given
in Hz. IR spectra were recorded with a Perkin–Elmer FT1600 spec-
trometer. UV/Vis spectra were recorded with a Varian CARY-5
spectrophotometer. The spectra were recorded in a quartz cuvette
with a pathlength of 1.00 cm at 25 °C. EI-MS spectra were re-
corded with a Micromass AutoSpec-Ultima spectrometer. FT-ICR-
MALDI spectra were recorded with an IonSpec Ultima Fourier
transform spectrometer with [(2E)-3-(4-tert-butylphenyl)-2-meth-
ylprop-2-enylidene]malononitrile (DCTB) or 3-hydroxypyridine-2-
carboxylic acid (3-HPA) as matrix. MALDI-TOF mass spectra
were recorded with a Bruker Daltonics UltraFlex II spectrometer
with DCTB as matrix. ESI mass spectra were recorded with a
Bruker Daltonics maXis spectrometer. Elemental analyses were
performed by the Mikrolabor at the Laboratorium für Organische
Chemie, ETH Zürich, with a LECO CHN/900 instrument.
TCNQ addition is inverted compared to 3. Thus, the strong
absorptions with a λmax above 500 nm have tentatively been
assigned to PtII-to-DCNQ charge transfer.
To corroborate this hypothesis we performed acidifica-
tion and neutralization experiments to clarify the origin of
the CT bands in the UV/Vis spectra. The low-energy CT
absorption bands of two representative CA-RE adducts, 9
and 10, bearing DMA substituents were reversibly
quenched and recovered by protonation with trifluoroacetic
acid (TFA) and deprotonation with triethylamine (TEA),
respectively (see Figure S1 in the Supporting Information).
In contrast, no substantial changes in the UV/Vis spectra
of complexes 11 and 12 were observed upon analogous
treatment (Figure S2), which suggests that the lowest-en-
ergy absorption bands in 9 and 10 can be attributed to the
charge-transfer interaction between the dimethylamino
groups and the acceptor-substituted buta-1,3-diene unit.
In summary, the compounds exhibit two main sets of
intense absorption bands: 1) those caused by direct CT
from the platinum to the acceptor moiety, which are par-
ticularly pronounced in the case of the TCNQ adducts
and 2) bands corresponding to aniline-to-dicyanovinyl or
-DCNQ charge transfer, which are always bathochromically
shifted relative to the metal–acceptor CT band.
Conclusions and Outlook
In this work the CA-RE reactivity patterns of the triple
bonds of platinum acetylides were studied. We have shown
that the presence of organodonor moieties in direct conju-
gation with the triple bonds facilitates the CA-RE process.
The first example of direct competition between two donor
moieties, an organic amine on the one hand and a bis(tri-
alkylphosphane)platinum center on the other, has been
demonstrated and indicates that the aniline substituents are
more efficient activators, which is evident from the regiose-
lectivity of the TCNQ addition reactions as well as the ease
of product formation. The steric demand of the phosphane
ligands is crucial for determining the preferred triple bond
CCDC-924638 (for 2), -924641 (for 5), -924639 (for 6), -924640 (for
9), -924642 (for 10), -924643 (for 11), -924644 (for 12), -924645 (for
13), -924646 (for 17), and -924647 (for 23) contain the crystallo-
graphic supplementary data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Syntheses: Only selected exemplary protocols for the synthesis of
addition in the case of 1,3-diacetylide complexes, as even compounds 7, 9, 10, 12, 25, 27, 29, and 31 are presented here.
10 Eur. J. Org. Chem. 0000, 0–0
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Platinum Acetylides in Cycloaddition-Retroelectrocyclizations
All other synthetic protocols and compound characterizations are
included in the Supporting Information.
crude product subjected to CC (SiO2; 3% EtOAc in CH2Cl2) to
afford 10 (31.2 mg, 99%) as a dark-green metallic solid. Black crys-
tals of 10 suitable for X-ray crystallographic analysis were grown
by layering a CH2Cl2 solution of 10 with n-hexane at –15 °C. Rf =
0.24 (SiO2; 3 % EtOAc in CH2Cl2), m.p. 151–153 °C (dec.). 1H
NMR (300 MHz, CDCl3): δ = 0.87–1.02 (m, 18 H), 1.74–1.92 (m,
12 H), 3.16 (s, 12 H), 6.74 (d, J = 9.1 Hz, 4 H), 7.18 (br. s, 5 H),
7.22 (br. s, 1 H), 7.32 (d, J = 9.0 Hz, 4 H), 7.36 (br. s, 1 H), 7.39
(br. s, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 8.35, 16.36 [t,
1J(P-C) = 17.8 Hz], 40.56, 70.83, 112.53, 112.72, 113.98, 114.91,
115.05, 123.39, 124.21, 125.00, 129.38, 134.55, 134.74, 135.97,
152.44, 152.98, 154.57, 157.56 ppm. 31P NMR (162 MHz, CDCl3):
Bis{4-[4-(dimethylamino)phenyl]buta-1,3-diynyl}bis(triethylphos-
phane)platinum(II) (7): 1-[4-(Dimethylamino)phenyl]-4-trimethyl-
silylbuta-1,3-diyne (84.6 mg, 0.35 mmol) was deprotected in situ
with K2CO3 (110 mg, 0.80 mmol) by stirring in methanol/THF
(1:1, 6 mL) for 2 h at 25 °C. Afterwards, the solution was passed
through a plug of SiO2 (CH2Cl2). N,N-Diisopropylamine (25 mL)
was added to the filtrate and the other solvents were carefully evap-
orated. The mixture was then degassed by sparging with nitrogen
for 30 min, and then [Pt(PEt3)2Cl2] (80 mg, 0.16 mmol) and CuI
(6 mg, 0.03 mmol) were added. After stirring the solution for 65 h
at 25 °C, the solvent was evaporated and the residue subjected to
CC (SiO2; CH2Cl2) to afford 7 (122 mg, 100%) as a yellow solid.
Rf = 0.52 (SiO2; CH2Cl2), m.p. 260–261 °C (dec.). 1H NMR
(300 MHz, CDCl3): δ = 1.13–1.24 (m, 18 H), 2.07–2.19 (m, 12 H),
2.95 (s, 12 H), 6.59 (d, J = 8.5 Hz, 4 H), 7.33 (d, J = 8.9 Hz, 4
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 8.42, 16.37 [t, J(P-C)
= 17.7 Hz], 40.38, 71.40, 76.27, 91.96, 111.07, 111.97, 133.48,
149.71 (10 signals out of 11 expected) ppm. 31P NMR (162 MHz,
δ = 13.59 (s), 13.59 [1J(Pt-P) = 2196 Hz] ppm. IR (ATR): ν = 2919
˜
(w), 2850 (w), 2224 (w), 2198 (m), 2036 (s), 1609 (w), 1578 (m),
1527 (w), 1478 (m), 1404 (w), 1366 (m), 1346 (m), 1286 (m), 1257
(m), 1203 (w), 1165 (s), 1032 (m), 941 (m), 906 (m), 843 (m), 829
(m), 800 (m), 769 (m), 729 (s), 673 (m), 641 (m) cm–1. UV/Vis
(CH3CN): λmax (ε) = 679 (65900), 461 (34900), 344 (45400), 259 nm
(23000 m–1 cm–1). HR-MALDI-MS (3-HPA; neg.): m/z (%) =
1178.4053 (35), 1177.4026 (61), 1176.4007 (100) [M]– (calcd. for
C60H58N10P2195Pt–: 1176.3993), 1175.3988 (92) [M]– (calcd. for
C60H58N10P2194Pt–: 1175.3978), 1174.3961 (49), 1078.2204 (26),
1077.2177 (27), 746.2603 (22), 745.2543 (45), 744.2492 (53),
719.2490 (28), 718.2436 (41), 717.2377 (29).
CDCl3): δ = 12.42 (s), 12.42 (1JP,Pt = 2324 Hz) ppm. IR (ATR): ν
˜
= 2960 (w), 2931 (w), 2877 (w), 2179 (w), 2048 (w), 1602 (s), 1543
(w), 1514 (s), 1478 (w), 1441 (m), 1417 (m), 1352 (s), 1260 (w), 1224
(m), 1188 (s), 1163 (s), 1053 (w), 1034 (s), 1005 (m), 942 (m), 824
(s), 795 (m), 762 (s), 731 (s), 715 (s), 674 (w), 634 (w), 619 (w) cm–1.
UV/Vis (CH2Cl2): λmax (ε) = 270 nm (34400 m–1 cm–1). HR-
MALDI-MS (3-HPA): m/z (%) = 768.3168 (100) [M + H]+ (calcd.
for C36H51N2P2195Pt+: 768.3173), 599.2288 (33) [M – C12H10N +
H]+ (calcd. for C24H51N2P2195Pt+: 599.2284), 569.1674 (48),
430.1399 (42), 286.1721 (47).
Bis{5,5-dicyano-3-[4-(dicyanomethylene)cyclohexa-2,5-dien-1-ylid-
ene]-4-phenylpent-4-en-1-ynyl}bis(triethylphosphane)platinum(II)
(12): Pt complex 8 (28 mg, 0.04 mmol) and TCNQ (42 mg,
0.21 mmol) were dissolved in 1,1,2,2-tetrachloroethane (10 mL)
and stirred at 100 °C for 24 h. The solvent was evaporated and the
residue subjected to CC (SiO2; CH2Cl2 Ǟ2% EtOAc in CH2Cl2)
to afford 12 (10 mg, 23%) as a black solid. Black crystals of 12
suitable for X-ray crystallographic analysis were grown by layering
a CH2Cl2 solution of 12 with n-hexane at –15 °C. Rf = 0.43 (SiO2;
2 % EtOAc in CH2Cl2), m.p. Ͼ300 °C. 1H NMR (400 MHz,
CDCl3): δ = 0.94–1.05 (m, 18 H), 1.74–1.86 (m, 12 H), 7.04 (dd, J
= 9.6, 1.9 Hz, 2 H), 7.14 (dd, J = 9.6, 1.9 Hz, 2 H), 7.31 (dd, J =
9.6, 1.9 Hz, 2 H), 7.50–7.57 (m, 4 H), 7.60–7.67 (m, 2 H), 7.68–
7.75 (m, 6 H) ppm. 13C NMR (100 MHz, CD2Cl2): δ = 8.55, 17.18
[t, 1J(P-C) = 17.8 Hz], 75.00, 85.26, 112.87, 113.25, 114.92, 115.08,
117.11, 125.38, 126.43, 129.94, 130.03, 132.70, 133.78, 134.31,
134.49, 135.19, 137.08, 154.73, 170.32 (21 signals out of 22 ex-
pected) ppm. 31P NMR (162 MHz, CD2Cl2): δ = 14.09 (s), 14.09
Bis{5,5-dicyano-3-(dicyanomethylene)-4-[4-(dimethylamino)phenyl]-
pent-4-en-1-ynyl}bis(triethylphosphane)platinum(II) (9): Pt complex
7 (20 mg, 0.03 mmol) and TCNE (16.7 mg, 0.13 mmol) were dis-
solved in CH2Cl2 (5 mL) and stirred at 25 °C for 45 min. The sol-
vent was evaporated and the residue subjected to CC (SiO2; CH2Cl2
+ 3% EtOAc) to afford 9 (26.4 mg, 99%) as a red solid. Red crys-
tals of 9 suitable for X-ray crystallographic analysis, were grown by
layering a CH2Cl2 solution of 9 with n-hexane. Rf = 0.42 (SiO2; 3%
EtOAc in CH2Cl2), m.p. 272–273 °C (dec.). 1H NMR (300 MHz,
CDCl3): δ = 0.98–1.14 (m, 18 H), 1.88–2.04 (m, 12 H), 3.16 (s, 12
H), 6.70 (d, J = 9.3 Hz, 4 H), 7.78 (d, J = 9.3 Hz, 4 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 8.33, 16.54 [t, J(P-C) = 17.8 Hz],
40.27, 72.98, 88.76, 111.82, 111.93, 112.32, 113.07, 113.62, 114.56,
117.19, 132.34, 153.17, 154.41, 157.21, 163.16 ppm. 31P NMR
(162 MHz, CDCl3 ): δ = 13.47 (s), 13.47 [d, 1 J(P-Pt) =
[1J(Pt-P) = 2236 Hz] ppm. IR (ATR): ν = 2967 (w), 2928 (w), 2231
˜
(w), 2207 (m), 2026 (s), 1596 (m), 1562 (m), 1427 (s), 1345 (m),
1318 (m), 1272 (w), 1189 (s), 1033 (m), 972 (w), 846 (m), 826 (w),
757 (s), 733 (s), 710 (s), 696 (s), 656 (m), 611 (m) cm–1. UV/Vis
(CH3CN): λmax (ε) = 561 (64900), 300 nm (30000 m–1 cm–1). HR-
MALDI-MS (3-HPA; neg.): m/z (%) = 1093.3288 (36), 1092.3229
(57), 1091.3198 (78), 1090.3141 (100), 1089.3108 (76) [M]– (calcd.
for C56H48N8P2195Pt–: 1089.3130), 1088.3076 (40).
2188 Hz] ppm. IR (ATR): ν = 2967 (w), 2930 (w), 2210 (m), 2043
˜
(s), 1603 (s), 1517 (m), 1481 (s), 1437 (s), 1382 (s), 1344 (s), 1305
(m), 1262 (m), 1231 (m), 1210 (s), 1188 (m), 1163 (s), 1063 (m),
1033 (m), 992 (m), 941 (m), 898 (w), 826 (m), 796 (m), 765 (m),
746 (m), 732 (m), 713 (m), 675 (m), 629 (m) cm–1. UV/Vis
(CH2 Cl2 ): λm a x (ε) = 461 (48100), 419 (46400), 337 nm
(28300 m–1 cm–1). HR-ESI-MS (CH2Cl2/CH3OH): m/z (%) =
1028.3463 (11), 1027.3434 (28), 1026.3428 (47), 1025.3403 (100) [M
+ H]+ (calcd. for C36H51N10P2195Pt+: 1025.3421), 1024.3393 (95)
[M + H]+ (calcd. for C48H51N10P2194Pt+: 1024.3419), 1023.3367
(59). C48H50N10P2Pt (1024.02): calcd. C 56.30, H 4.92, N 13.68;
found C 55.95, H 5.26, N 13.02.
cis-Bis{5,5-dicyano-3-(dicyanomethylene)-4-[4-(dimethylamino)phen-
yl]pent-4-en-1-ynyl}(4,4Ј-di-tert-butyl-2,2Ј-bipyridine)platinum(II)
(25): Pt complex 19 (16.0 mg, 0.020 mmol) and TCNE (5.0 mg,
0.040 mmol) were dissolved in CH2Cl2 (0.5 mL) and the mixture
was stirred at 25 °C for 12 h. The solvent was evaporated and the
crude product subjected to CC [SiO2; pentane/EtOAc, 20:80 (v/v)]
to afford 25 (17.9 mg, 85%) as a dark-green metallic solid. Rf =
0.25 [SiO2; pentane/EtOAc, 20:80 (v/v)], m.p. 150 °C (dec.). 1H
Bis(4,4-dicyano-3-{[4-(dicyanomethylene)cyclohexa-2,5-dien-1-ylid- NMR (400 MHz, CD2Cl2): δ = 1.50 (s, 18 H), 3.19 (s, 12 H), 6.78
ene][4-(dimethylamino)phenyl]methyl}but-3-en-1-ynyl)bis(triethyl-
phosphane)platinum(II) (10): Pt complex 7 (20.5 mg, 0.03 mmol)
and TCNQ (14.7 mg, 0.07 mmol) were dissolved in CH2Cl2 (5 mL)
and stirred at 25 °C for 3 h. The solvent was evaporated and the
(d, J = 9.3 Hz, 4 H), 7.68 (d, J = 6.0 Hz, 2 H), 7.84 (d, J = 9.4 Hz,
4 H), 8.12 (s, 2 H), 9.35 (d, J = 6.0 Hz, 2 H) ppm. 13C NMR
(101 MHz, CD2Cl2): δ = 29.94, 40.01, 72.89, 90.74, 101.88, 111.81,
113.38, 114.08, 114.82, 117.34, 119.81, 125.45, 130.88, 132.28,
Eur. J. Org. Chem. 0000, 0–0
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F. Diederich et al.
FULL PAPER
134.96, 151.87, 153.83, 154.43, 155.87, 163.66, 166.11 (21 signals
observed out of 22 expected) ppm. HR-MALDI-MS (3-HPA;
pos.): m/z (%) = 1056.3540 (100) [M]+ (calcd. for C54H45N12195Pt+:
11056.3536).
Acknowledgments
This work was supported by the European Commission (ERC Ad-
vanced Grant, number 246637, OPTELOMAC). B. H. T. is recipi-
ent of a doctoral fellowship from the Stipendienfonds der Schwei-
zerischen Chemischen Industrie (SSCI) and M. C. is recipient of
an ETH Research Fellowship. The authors thank Michael Solar
and Paul Seiler, ETH Zürich, for X-ray crystallographic work, and
Prof. Dr. Paul Pregosin and Dr. Aaron Finke for helpful dis-
cussions.
cis-Bis{5,5-dicyano-3-(dicyanomethylene)-4-[4-(dimethylamino)phen-
yl]pent-4-en-1-ynyl}[1,2-bis(dicyclohexylphosphanyl)ethane]plati-
num(II) (27): Pt complex 20 (20.0 mg, 0.021 mmol) and TCNE
(5.4 mg, 0.042 mmol) were dissolved in CH2Cl2 (0.5 mL) and the
mixture was stirred at 25 °C for 12 h. The solvent was evaporated
and the crude product subjected to CC (SiO2; 1 % EtOAc in
CH2Cl2) to afford 27 (17.9 mg, 85%) as a dark-green metallic solid.
Rf = 0.45 (SiO2; 3% EtOAc in CH2Cl2); m.p. 145–150 °C (dec.).
1H NMR (400 MHz, CD2Cl2): δ = 0.97–1.55 (m, 18 H), 1.57–2.48
(m, 30 H), 3.19 (s, 12 H), 6.79 (d, J = 9.4 Hz, 4 H), 7.91 (d, J =
9.3 Hz, 4 H) ppm. 13C NMR (101 MHz, CD2Cl2): δ = 25.75, 26.5
(m), 28.49, 29.21, 34.74 [d, 1J(P-C) = 31.7 Hz], 40.00, 71.99, 88.64,
111.26, 111.94, 112.05, 112.74, 114.08, 114.83, 117.02, 129.02,
132.43, 153.20, 154.48, 163.19 ppm. 31P NMR (121 MHz, CD2Cl2):
δ = 64.55 (s), 64.55 [d, 1J(P-Pt) = 2242.3 Hz] ppm. HR-MALDI-
MS (3-HPA; pos.): m/z (%) = 1211.499 (100) [M]+ (calcd. for
C62H68N10P2 195Pt+: 1211.479).
[1] a) M. Kivala, F. Diederich, Acc. Chem. Res. 2009, 42, 235–248;
a) S.-i. Kato, F. Diederich, Chem. Commun. 2010, 46, 1994–
2006.
[2] For the organometallic activation of alkynes, see: a) M. I.
Bruce, B. C. Hall, B. D. Kelly, P. J. Low, B. W. Skelton, A. H.
White, J. Chem. Soc., Dalton Trans. 1999, 3719–3728; b) M. I.
Bruce, M. Jevric, C. R. Parker, W. Patalinghug, B. W. Skelton,
A. H. White, N. N. Zaitseva, J. Organomet. Chem. 2008, 693,
2915–2920; c) M. I. Bruce, Aust. J. Chem. 2011, 64, 77–103; d)
M. I. Bruce, A. Burgun, G. Grelaud, C. Lapinte, B. W. Skelton,
N. N. Zaitseva, Aust. J. Chem. 2012, 65, 763–772; e) M. I.
Bruce, S. Büschel, M. L. Cole, N. Scoleri, B. W. Skelton, A. H.
White, N. N. Zaitseva, Inorg. Chim. Acta 2012, 382, 6–12; f)
M. I. Bruce, A. Burgun, G. Grelaud, M. Jevric, B. K. Nichol-
son, B. W. Skelton, A. H. White, N. N. Zaitseva, Z. Anorg. Allg.
Chem. 2011, 637, 1334–1340.
[3] For recent applications of the CA-RE reaction by others, see:
a) A. Leliège, P. Blanchard, T. Rousseau, J. Roncali, Org. Lett.
2011, 13, 3098–3101; b) M. Morimoto, K. Murata, T. Michi-
nobu, Chem. Commun. 2011, 47, 9819–9821; c) S. Niu, G. Ul-
rich, P. Retailleau, R. Ziessel, Org. Lett. 2011, 13, 4996–4999;
d) Y. Washino, T. Michinobu, Macromol. Rapid Commun.
2011, 32, 644–648; e) T. Shoji, J. Higashi, S. Ito, T. Okujima,
M. Yasunami, N. Morita, Org. Biomol. Chem. 2012, 10, 2431–
2438; f) R. Garcia, M. A. Herranz, M. R. Torres, P.-A. Bouit,
J. L. Delgado, J. Calbo, P. M. Viruela, E. Orti, N. Martin, J.
Org. Chem. 2012, 77, 10707–10717; g) S. Chen, Y. Li, C. Liu,
W. Yang, Y. Li, Eur. J. Org. Chem. 2011, 6445–6451; h) W.
Zhou, J. Xu, H. Zheng, X. Yin, Z. Zuo, H. Liu, Y. Li, Adv.
Funct. Mater. 2009, 19, 141–149.
[4] For earlier work, see: a) X. Wu, J. Wu, Y. Liu, A. K.-Y. Jen, J.
Am. Chem. Soc. 1999, 121, 472–473; b) C. Cai, I. Liakatas, M.-
S. Wong, M. Bösch, C. Bosshard, P. Günter, S. Concilio, N.
Tirelli, U. W. Suter, Org. Lett. 1999, 1, 1847–1849; c) H. Ma,
B. Chen, T. Sassa, L. R. Dalton, A. K.-Y. Jen, J. Am. Chem.
Soc. 2001, 123, 986–987; d) A. Galvan-Gonzalez, G. I. Stege-
man, A. K.-Y. Jen, X. Wu, M. Canva, A. C. Kowalczyk, X. Q.
Zhang, H. S. Lackritz, S. Marder, S. Thayumanavan, G. Lev-
ina, J. Opt. Soc. Am. B 2001, 18, 1846–1853; e) J. Luo, H. Ma,
M. Haller, A. K.-Y. Jen, R. R. Barto, Chem. Commun. 2002,
888–889; f) Y. V. Pereverzev, O. V. Prezhdo, L. R. Dalton,
Chem. Phys. Lett. 2003, 373, 207–212; g) V. Mamane, I. Le-
doux-Rak, S. Deveau, J. Zyss, O. Riant, Synthesis 2003, 455–
467; h) Y. Morioka, N. Yoshizawa, J.-i. Nishida, Y. Yamashita,
Chem. Lett. 2004, 33, 1190–1191.
[5] H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. 2001,
113, 2056–2075; Angew. Chem. Int. Ed. 2001, 40, 2004–2021.
[6] a) T. Michinobu, J. C. May, J. H. Lim, C. Boudon, J.-P. Gissel-
brecht, P. Seiler, M. Gross, I. Biaggio, F. Diederich, Chem.
Commun. 2005, 737–739; b) T. Michinobu, C. Boudon, J.-P.
Gisselbrecht, P. Seiler, B. Frank, N. N. P. Moonen, M. Gross,
F. Diederich, Chem. Eur. J. 2006, 12, 1889–1905.
cis-Bis(4,4-dicyano-3-{[4-(dicyanomethylene)cyclohexa-2,5-dien-1-
ylidene][4-(dimethylamino)phenyl]methyl}but-3-en-1-ynyl)(4,4Ј-di-
tert-butyl-2,2Ј-bipyridine)platinum(II) (29): Pt complex 19 (50.0 mg,
0.072.5 mmol) and TCNQ (30.0 mg, 0.145 mmol) were dissolved in
CH2Cl2 (2.5 mL) and stirred at 25 °C for 12 h. The solvent was
evaporated and the crude product subjected to CC (SiO2; CH2Cl2/
EtOAc, 100:0 to 85:15) to afford 29 (55 mg, 75%) as a dark-green
metallic solid. Rf = 0.15 (SiO2; 3% EtOAc in CH2Cl2), m.p. 145–
150 °C (dec.). 1H NMR (400 MHz, CD2Cl2): δ = 1.49 (s, 18 H),
3.23 (s, 12 H), 6.83 (d, J = 9.2 Hz, 4 H), 7.15 (d, J = 9.9 Hz, 4 H),
7.34–7.51 (m, 8 H), 7.55 (dd, J = 6.0, 2.0 Hz, 2 H), 8.09 (d, J =
2.0 Hz, 2 H), 9.21 (d, J = 6.0 Hz, 2 H) ppm. 13C NMR (101 MHz,
CD2Cl2): δ = 29.69, 29.92, 36.01, 40.16, 60.22, 68.66, 91.45, 104.78,
112.22, 112.50, 114.47, 115.37, 119.76, 123.45, 125.14, 128.94,
131.82, 134.83, 151.65, 153.25, 153.50, 154.42, 155.94, 158.05,
166.06 (25 signals observed out of 28 expected) ppm. HR-MALDI-
MS (3-HPA; pos.): m/z (%) = 1208.4163 (100) [M + H]+ (calcd. for
C66H53N12195Pt+: 1208.4158).
cis-Bis(4,4-dicyano-3-{[4-(dicyanomethylene)cyclohexa-2,5-dien-1-
ylidene][4-(dimethylamino)phenyl]methyl}but-3-en-1-ynyl)[1,2-bis-
(dicyclohexylphosphanyl)ethane]platinum (31): Pt complex 20
(20.0 mg, 0.021 mmol) and TCNQ (8.6 mg, 0.042 mmol) were dis-
solved in CH2Cl2 (0.5 mL) and the mixture was stirred at 25 °C for
12 h. The solvent was evaporated and the crude product subjected
to CC (SiO2; 2% EtOAc in CH2Cl2) to afford 31 (17.9 mg, 85%)
as a dark-green metallic solid. Rf = 0.36 (SiO2; 3% EtOAc in
1
CH2Cl2). H NMR (400 MHz, CD2Cl2): δ = 0.92–1.43 (m, 18 H),
1.58–2.11 (m, 32 H), 3.21 (s, 12 H), 6.85 (d, J = 9.2 Hz, 4 H), 7.11–
7.31 (m, 4 H), 7.31–7.43 (m, 4 H), 7.51 ppm (d, J = 9.0 Hz, 4 H).
13C NMR (101 MHz, CD2Cl2): δ = 22.81 [dd, 1J(P-C) = 31.8, 2J(P-
C) = 8.9 Hz], 25.66, 26.29–26.53 (m), 26.58, 28.40, 29.03, 34.67
1
[d, J(P–C) = 31.5 Hz], 40.08, 68.31, 90.16, 112.41, 112.71, 113.31,
113.51, 113.60, 115.38, 122.68, 123.61, 124.25, 128.74, 134.95,
135.19, 136.22, 153.25, 153.37, 154.53, 157.38 ppm. 31P NMR
(121 MHz, CD2Cl2):
δ
=
63.88 (s), 63.88 [d, 1J(Pt-P)
=
=
2248.7 Hz] ppm. HR-MALDI-MS (3-HPA; pos.): m/z (%)
[7] M. Kivala, C. Boudon, J.-P. Gisselbrecht, P. Seiler, M. Gross,
F. Diederich, Chem. Commun. 2007, 4731–4733.
136.5539 (100) [M]+ (calcd. for C74H78N10P2195Pt+: 1363.5539).
[8] S.-i. Kato, M. Kivala, W. B. Schweizer, C. Boudon, J.-P. Gissel-
brecht, F. Diederich, Chem. Eur. J. 2009, 15, 8687–8691.
[9] a) M. Kivala, C. Boudon, J.-P. Gisselbrecht, P. Seiler, M. Gross,
F. Diederich, Angew. Chem. 2007, 119, 6473–6477; Angew.
Supporting Information (see footnote on the first page of this arti-
cle): Further synthetic details, UV/Vis spectra, X-ray data and
NMR spectra.
12
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Job/Unit: O30300
/KAP1
Date: 26-04-13 12:58:10
Pages: 13
Platinum Acetylides in Cycloaddition-Retroelectrocyclizations
Chem. Int. Ed. 2007, 46, 6357–6360; b) F. Silvestri, M. Jordan, [18] For mechanistic studies, see: Y.-L. Wu, P. D. Jarowski, W. B.
K. Howes, M. Kivala, P. Rivera-Fuentes, C. Boudon, J.-P. Gis-
selbrecht, W. B. Schweizer, P. Seiler, M. Chiu, F. Diederich,
Chem. Eur. J. 2011, 17, 6088–6097.
a) C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R.
Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich,
Nat. Photonics 2009, 3, 216–219; b) B. B. Frank, B. Cam-
afort Blanco, S. Jakob, F. Ferroni, S. Pieraccini, A. Ferrarini, C.
Boudon, J.-P. Gisselbrecht, P. Seiler, G. P. Spada, F. Diederich,
Chem. Eur. J. 2009, 15, 9005–9016.
For recent methodological advances, expanding the scope of
the CA-RE reaction, see: A. R. Lacy, A. Vogt, C. Boudon, J.-
P. Gisselbrecht, W. B. Schweizer, F. Diederich, Eur. J. Org.
Chem. 2013, 869–879.
a) H. Masai, K. Sonogashira, N. Hagihara, J. Organomet.
Chem. 1972, 34, 397–404; b) K.-i. Onuma, Y. Kai, N. Yasuoka,
N. Kasai, Bull. Chem. Soc. Jpn. 1975, 48, 1696–1700; c) M. I.
Bruce, J. R. Rodgers, M. R. Snow, A. G. Swincer, J. Chem. Soc.
Chem. Commun. 1981, 271–272.
Schweizer, F. Diederich, Chem. Eur. J. 2010, 16, 202–211.
[19] Buta-1,3-diynediyls end-capped by two strong electron-donat-
ing anilino substituents, on the other hand, undergo double
TCNE addition with the formation of octacyano[4]dendral-
enes, see: B. Breiten, Y.-L. Wu, J.-P. Gisselbrecht, C. Boudon,
M. Griesser, C. Onitsch, G. Gescheidt, W. B. Schweizer, N.
Langer, C. Lennartz, F. Diederich, Chem. Sci. 2011, 2, 88–93.
[20] C. Tolman, Chem. Rev. 1977, 77, 313–348.
[21] a) J. R. Phillips, G. Miller, W. Trogler, Acta Crystallogr. Sect.
C: Cryst. Struct. Commun. 1990, 46, 1648–1650; b) A. Sebald,
C. Stader, B. Wrackmeyer, W. Bensch, J. Organomet. Chem.
1986, 311, 233–242.
[10]
[11]
[12]
[22] a) C. Dehu, F. Meyers, J. L. Brédas, J. Am. Chem. Soc. 1993,
115, 6198–6206; b) J. Beckmann, D. Dakternieks, A. Duthie,
C. Mitchell, M. Schürmann, Aust. J. Chem. 2005, 58, 119–127.
[23] A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Wat-
son, R. Taylor, J. Chem. Soc. Dalton Trans. 1989, S1–S83.
[24] a) S. Chakraborty, T. J. Wadas, H. Hester, R. Schmehl, R. Ei-
senberg, Inorg. Chem. 2005, 44, 6865–6878; b) T. M. Cooper,
D. M. Krein, A. R. Burke, D. G. McLean, J. E. Rogers, J. E.
Slagle, P. A. Fleitz, J. Phys. Chem. A 2006, 110, 4369–4375.
[25] J. Sugiyama, I. Tomita, Eur. J. Org. Chem. 2007, 4651–4653.
[26] S. Anderson, Chem. Eur. J. 2001, 7, 4706–4714.
[27] P. B. Hitchcock, B. Jacobson, A. Pidcock, J. Chem. Soc. Dalton
Trans. 1977, 2038–2042.
[13]
a) K. Onitsuka, S. Takahashi, J. Chem. Soc. Chem. Commun.
1995, 2095–2096; b) K. Onitsuka, N. Ose, F. Ozawa, S. Takah-
ashi, J. Organomet. Chem. 1999, 578, 169–177.
[14] M. Jordan, M. Kivala, C. Boudon, J.-P. Gisselbrecht, W. B.
Schweizer, P. Seiler, F. Diederich, Chem. Asian J. 2010, 6, 396–
401.
[15] a) N. J. Long, C. K. Williams, Angew. Chem. 2003, 115, 2690–
2722; Angew. Chem. Int. Ed. 2003, 42, 2586–2617; b) S. Suzuki,
R. Sugimura, M. Kozaki, K. Keyaki, K. Nozaki, N. Ikeda, K.
Akaiyama, K. Okada, J. Am. Chem. Soc. 2009, 131, 10374–
10375; c) W.-Y. Wong, P. D. Harvey, Macromol. Rapid Com-
mun. 2010, 31, 671–713.
[16] a) F. Diederich, R. Faust, V. Gramlich, P. Seiler, J. Chem. Soc.
Chem. Commun. 1994, 2045–2046; b) R. Faust, F. Diederich,
V. Gramlich, P. Seiler, Chem. Eur. J. 1995, 1, 111–117; c) P.
Siemsen, U. Gubler, C. Bosshard, P. Gunter, F. Diederich,
Chem. Eur. J. 2001, 7, 1333–1341.
[17] K. Campbell, R. McDonald, M. J. Ferguson, R. R. Tykwinski,
J. Organomet. Chem. 2003, 683, 379–387.
[28] S.-Y. Cheung, H.-F. Chow, T. Ngai, X. Wei, Chem. Eur. J. 2009,
15, 2278–2288.
[29] M. Hackett, G. M. Whitesides, J. Am. Chem. Soc. 1988, 110,
1449–1462.
[30] K. D. Hodges, J. V. Rund, Inorg. Chem. 1975, 14, 525–528.
[31] K. Sonogashira, Y. Fujikura, T. Yatake, N. Toyoshima, S. Tak-
ahashi, N. Hagihara, J. Organomet. Chem. 1978, 145, 101–108.
[32] H. Masai, K. Sonogashira, N. Hagihara, J. Organomet. Chem.
1971, 26, 271–276.
[33] C. E. Whittle, J. A. Weinstein, M. W. George, K. S. Schanze,
Eur. J. Inorg. Chem. 2001, 40, 4053–4062.
Received: February 28, 2013
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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