A metal-templated 4 + 2 cycloaddition reaction of an alkyne and a diyne to form a 1,2-aryne

Article abstract of DOI:10.1021/om900578d

Coordination of bis-phosphino diyne substrates to mixed platinum and tungsten metal templates results in a templated 4 + 2 alkyne-diyne cycloaddition reaction to form an aryne intermediate, which abstracts two H atoms to form an arene ring. The aryne inte

Full text of DOI:10.1021/om900578d

4906 Organometallics 2009, 28, 4906–4908
DOI: 10.1021/om900578d
A Metal-Templated 4 + 2 Cycloaddition Reaction of an Alkyne and a
Diyne To Form a 1,2-Aryne
Jordan A. Tsui and Brian T. Sterenberg*
Department of Chemistry and Biochemistry, University of Regina, 3737 Wascana Parkway, Regina,
Saskatchewan, Canada S4S 0A2
Received July 5, 2009
Summary: Coordination of bis-phosphino diyne substrates to
mixed platinum and tungsten metal templates results in a
templated 4 þ 2 alkyne-diyne cycloaddition reaction to form
an aryne intermediate, which abstracts two H atoms to form an
arene ring. The aryne intermediate can also be trapped via a
Diels-Alder reaction with furan.
alkene cycloaddition reactions, including Diels-Alder reac-
tions4 and alkene photodimerization.5 However, applica-
tions to alkyne cycloaddition are limited.6 Previously,
Martin-Redondo et al. showed that coordination of the
phosphine-substituted diyne Ph2PC4PPh2 to PtCl2 frag-
ments effectively templates diyne cycloaddition reactions
to form cyclooctadienediyne and cyclododecatrienetriyne
rings.7 Later, by using W(CO)4 templates, we showed that
the critical parameter controlling cycloaddition is distance
between the diyne terminal carbons (R to P) and that the
Templated reactions are those in which substrates are held
into a specific geometry via interaction with a template such
that they react in a controlled fashion.1 Substrate-template
interactions can involve many types of bonds; examples
include covalent bonds, coordinative bonds to metals,
hydrogen bonds, and π-π interactions.2 Our research is
focused on transition-metal templates, which provide well-
defined geometries, tunable bond lengths, easy substrate-
template complex formation, and straightforward template
removal. Metal templates have been used extensively in the
synthesis of macrocycles, catenanes, and rotaxanes.2,3 We
are using transition metals to template alkene and alkyne
cycloaddition reactions. In these reactions, the template-
substrate interaction is remote from the reactive alkynes. The
cycloadditions are not metal-catalyzed, nor do the alkynes
interact directly with the metals. Such metal templation has
been used to control regioselectivity and stereochemistry of
˚
threshhold for reactivity is about 3.2 A. This carbon-carbon
distance is in turn controlled by the M-P distance.8 The
small Pt-P bond lengths of the PtCl2 complex bring the
diynes into close proximity, inducing reaction. In contrast,
the longer M-P bond lengths in the W(CO)4 complexes hold
the substrates far enough apart that the diynes are unreac-
tive. Here we describe mixed tungsten-platinum template
complexes, in which two diynes are held in close proximity at
one end by PtX2, while the other ends are held further apart
by W(CO)4, and we show how careful choice of the metal
template fragment allows us to control the nature of the
cycloaddition reaction.
The mixed template complexes are formed via sequential
addition of the W(CO)4 precursor followed by the PtX2 pre-
cursor. The compound [{W(CO)4}(Ph2PC4PPh2-κ1P)2] (1) is
first prepared by reaction of [W(CO)4(2-pic)2] with excess
Ph2PC4PPh2.8 In 1, two substrates are coordinated to one
tungsten center, and the other end of each bis-phosphine
dangles. Compound 1was first combined with Pt(CH3)2(COD)
(COD= cyclooctadiene) and reacts to form [{W(CO)4}-
{Pt(CH3)2}(μ-Ph2PC4PPh2)2] (2) (Scheme 1). This complex
was not expected to be reactive toward cycloaddition, on the
basis of the fact that the templated complex with two Pt(CH3)2
templates is unreactive.7 This prediction was borne out, and 2
does not react, even when heated. An ORTEP diagram of 2 is
shown in Figure 1.
*To whom correspondence should be addressed. E-mail: brian.
sterenberg@uregina.ca.
€
(1) (a) Hoss, R.; Vogtle, F. Angew. Chem., Int. Ed. 1994, 33, 375.
(b) Anderson, S.; Anderson, H. L.; Sanders, J. K. M. Acc. Chem. Res. 1993,
26, 469. (c) MacGillivray, L. R.; Papaefstathiou, G. S.; Friscic, T.; Varshney,
D. B.; Hamilton, T. D. Top. Curr. Chem. 2004, 284, 201.
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K. C.-F.; Liu, Y.; Stoddart, J. F. Top. Curr. Chem. 2005, 249, 203.
(3) (a) Laughrey, Z. R.; Gibb, B. C. Top. Curr. Chem. 2005, 249, 67.
(b) Crowley, J. D.; Goldup, S. M.; Lee, A.-L.; Leigh, D. A.; McBurney, R. T.
Chem. Soc. Rev. 2009, 38, 1530.
(4) (a) Leung, P.-H. Acc. Chem. Res. 2004, 37, 169. (b) Yeo, W.-C.;
Chen, S.; Tan, G.-K.; Leung, P.-H. J. Organomet. Chem. 2007, 692, 2539.
(d) Tan, K.-W.; Liu, F.; Li, Y.; Tan, G.-K.; Leung, P.-H. J. Organomet. Chem.
2006, 691, 4753. (e) Yeo, W.-C.; Tan, G.-K.; Koh, L. L.; Leung, P.-H. Eur. J.
Inorg. Chem. 2005, 2005, 4723.
(5) (a) Papaefstathiou, G. S.; Zhong, Z.; Geng, L.; MacGillivray,
L. R. J. Am. Chem. Soc. 2004, 126, 9158. (b) Toh, N. L.; Nagarathinam, M.;
Vittal, J. J. Angew. Chem., Int. Ed. 2005, 44, 2237. (c) Chu, Q.; Swenson,
D. C.; MacGillivray, L. Angew. Chem., Int. Ed. 2005, 44, 3569.
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(e) Papaefstathiou, G. S.; Georgiev, I. G.; Friscic, T.; MacGillivray, L. R.
Chem. Commun. 2005, 3974.
Reaction of 1 with PtCl2(COD) also leads to the rapid
formation of the mixed template complex [{W(CO)4}-
{PtCl2}(μ-Ph2PC4PPh2)2] (3), which has been characterized
spectroscopically.9 In CH2Cl2 solution, 3 starts turning black
within minutes and decomposes within 24 h to a complex
mixture. However, if 3 is formed in THF rather than CH2Cl2,
(6) (a) Carty, A. J.; Taylor, N. J.; Johnson, D. K. J. Am. Chem. Soc.
1979, 101, 5422. (b) Johnson, D. K.; Rukachaisirikul, T.; Sun, Y.; Taylor,
N. J.; Canty, A. J.; Carty, A. J. Inorg. Chem. 1993, 32, 5544. (c) García, A.;
(7) Martin-Redondo, M. P.; Scoles, L.; Sterenberg, B. T.; Udachin,
K. A.; Carty, A. J. J. Am. Chem. Soc. 2005, 127, 5038.
(8) Tsui, J. A.; Bolton, T. M.; Sterenberg, B. T. Can. J. Chem. 2009,
87, 197.
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Gomez, J.; Lalinde, E.; Moreno, M. T. Organometallics 2006, 25, 3926.
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(d) Charmant, J. P. H.; Fornies, J.; Gomez, J.; Lalinde, E.; Moreno, M. T.;
Orpen, A. G.; Solano, S. Angew. Chem., Int. Ed. 1999, 38, 3058. (e) Ara, I.;
Fornies, J.; García, A.; Gomez, J.; Lalinde, E.; Moreno, M. T. Chem. Eur. J.
2002, 8, 3698. (f) Berenguer, J. R.; Bernechea, M.; Fornies, J.; Garcia, A.;
Lalinde, E.; Moreno, M. T. Inorg. Chem. 2004, 43, 8185.
(9) Spectroscopic data for 3: 1H NMR δ 7.0-8.0 (m, Ph); 31P NMR δ
1.5 (s, 1JWP = 241 Hz, WP), -10.8 (s, 1JPtP = 3710 Hz, PtP); IR (cast,
cm-1) ν(CtC) 2089 w, ν(CO) 2024 s, 1930 s, 1904 vs.
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r
pubs.acs.org/Organometallics
Published on Web 08/11/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 28, No. 17, 2009 4907
Scheme 1
Figure 1. ORTEP diagram showing the crystal structure of
compound 2. Hydrogen atoms and cocrystallized solvent have
˚
been omitted. Selected distances (A) and angles (deg): C5-
C6 = 1.192(6), C6-C7 = 1.390(6), C7-C8 = 1.192(6), P1-
Pt = 2.282(1), P2-Pt = 2.281(1), P3-W = 2.493(1), P4-W =
2.496(1), C5-C9 = 3.358, C8-C12 = 3.549; P2-Pt-P1 =
97.30(4), P3-W-P4 = 93.03(3).
one major product, [{W(CO)4}{PtCl2}{μ-C6H2(PPh2)3-
(CCPPh2)}] (4), can be isolated in 92% yield (Scheme 1
and Figure 2). The compound consists of a cis-W(CO)4P2
unit and a cis-PtCl2P2 unit bridged by a phosphine-substi-
tuted phenylacetylene. The arene ring is substituted in the
1-, 2-, and 5-positions by PPh2 units and by CtCPPh2 in the
4-position. The PPh2 units in the 1- and 2-positions are
coordinated to platinum, while the 5-PPh2 and CtCPPh2
phosphines coordinate to tungsten. All distances and angles
are as expected, with the exception of alkyne angles of
151.0(7) and 157.0(8)° (C11 and C12 in Figure 2), which
indicate significant strain in the ring formed by coordination
of the ligand to tungsten.
We propose that the arene ring in 4 results from cycloaddi-
tion of three of four alkynes, formally a 4 þ 2 cycloaddition of
an alkyne with a diyne (a dehydro-Diels-Alder reaction).10
This cycloaddition reaction leads to a 1,2-aryne intermediate,
which abstracts two hydrogen atoms. If the reaction is carried
out in d8-THF, the deuterium NMR spectrum of the product
shows two broad peaks at δ 7.87 and 7.6, supporting THF as
the source for the hydrogen atoms in 4. The corresponding
peaks in the 1H spectrum are obscured by the diphenylpho-
sphino hydrogen resonances.
Further evidence for the proposed aryne intermediate was
obtained by carrying out the reaction in furan, which is
known to trap benzyne intermediates via a Diels-Alder
reaction.11 Compound 3 can be formed by dissolving 1 and
PtCl2(COD) in furan and can be identified by 31P NMR in
furan. If the resulting solution is stirred for 24 h, the new
product 5 is formed, which is the Diels-Alder adduct of the
proposed benzyne intermediate with the furan (Scheme 1). In
contrast to the reaction in THF, formation of the furan
adduct is essentially quantitative by 31P NMR, and minor
side products are not observed. The structure of 5 has been
confirmed by X-ray crystallography. An ORTEP diagram is
shown in Figure 3.
Figure 2. ORTEP diagram showing the crystal structure of
compound 4. Cocrystallized solvent and hydrogen atoms,
except for those on the newly formed phenyl ring, have been
˚
omitted. Selected distances (A) and angles (deg): Pt-P1 =
2.210(2), Pt-P2 = 2.210(2), W-P3 = 2.598(2), W-P4 =
2.512(2), C11-C12=1.201(1); P1-Pt-P2=88.32(7), P3-W-
P4 = 87.39(7), P4-C12-C11 = 151.0(7), C12-C11-C10 =
157.0(8).
cycloaddition of an alkyne and a diyne to form a 1,2-aryne is
very rare.10,12 1,2-Aryne rings are more typically formed via
elimination reactions from appropriately substituted arenes.13
The first clear example of a [4 þ 2] diyne-alkyne cycloaddition
was reported as recently as 1997, when the pyrolysis of 1,3,8-
nonatriyne was shown to lead to indane via a 1,2-benzyne
intermediate.12 The intermediacy of a 1,2-benzyne was proven
using labeling studies. Ueda has also described reactions in
which tetraacetylenes cyclize via intramolecular [4 þ 2] diyne-
alkyne additions to form fluorene derivatives.14 In this case,
the intermediacy of the 1,2-aryne was demonstrated by char-
acterization of their Diels-Alder adducts with anthracene.
Diyne-alkyne cycloaddition is also thought to be involved in
the pyrolysis of acetylene to form benzene (the Berthelot
reaction).15 The reverse reaction, a retro-Diels-Alder reaction
of 1,2-benzyne to form acetylene and butadiyne, has been
observed in the gas phase by mass spectrometry.16
(12) Bradley, A. Z.; Johnson, R. P. J. Am. Chem. Soc. 1997, 119, 9917.
(13) Wenk, H. H.; Winkler, M.; Sander, W. Angew. Chem., Int. Ed.
2003, 42, 502.
(14) (a) Miyawaki, K.; Kawano, T.; Ueda, I. Tetrahedron Lett. 1998,
39, 6923. (b) Miyawaki, K.; Suzuki, R.; Kawano, T.; Ueda, I. Tetrahedron
Lett. 1997, 38, 3943. (c) Ueda, I.; Sakurai, Y.; Kawano, T.; Wada, Y.; Futai,
M. Tetrahedron Lett. 1999, 40, 319.
(15) Fields, E. K.; Meyerson, S. Tetrahedron Lett. 1967, 6, 571.
(16) Zhang, X.; Maccarone, A. T.; Nimlos, M. R.; Kato, S.; Bierbaum,
V. M.; Ellison, G. B. J. Chem. Phys. 2007, 126, 044312.
Unlike the well-known 4 þ 2 cycloaddition of a diene with
an alkene or an alkyne (the Diels-Alder reaction), 4 þ 2
€
(10) Wessig, P.; Muller, G. Chem. Rev. 2008, 108, 2051.
(11) Kitamura, T.; Todaka, M.; Fujiwar, Y. Org. Synth. 2002,
78, 104.

4908 Organometallics, Vol. 28, No. 17, 2009
Tsui and Sterenberg
The reactions described here, along with those described by
Ueda, are also remarkable in that they occur at room tempera-
ture. A concerted 4 þ 2alkyne-diyne cycloaddition is expected
to have a large activation barrier as a result of the large
distortion required for the diyne to reach the transition state.11
The facile room-temperature reactivity suggests that the reac-
tion is not concerted but stepwise. The fact that the cycloaddi-
tion reaction is induced by holding only one end of the diynes in
close proximity is consistent with a reaction initiated by σ-bond
formation between the R-carbons on the Pt end.
In addition to its theoretical interest, the reaction des-
cribed here has potential synthetic utility in the formation of
complex and highly functionalized ring systems. The aryne
formed has a sufficient lifetime to react intermolecularly and
thus can potentially undergo a wide variety of addition and
cycloaddition reactions, allowing other functional groups or
rings to be added. The unreacted fourth alkyne may also be
susceptible to facile addition reactions because of its signifi-
cant ring strain. We are currently exploring these reactions.
In summary, we have demonstrated an unambiguous
example of a rare 4 þ 2 cycloaddition of a diyne with an
alkyne to form a 1,2-aryne. This is the first example of a
templated 4 þ 2 alkyne-diyne cycloaddition between two
separate substrates. We have also shown that, through care-
ful choice of metal template size, we can control the nature of
the cycloaddition reaction occurring between diyne sub-
strates in metal templated reactions by controlling the
relative geometries of the two reactive substrates.
Figure 3. ORTEP diagram showing the crystal structure of
compound 5. Hydrogen atoms and cocrystallized solvent have
been omitted. Selected distances and angles: Pt-P1=2.2074(8),
Pt-P2 = 2.2139(8), W-P3 = 2.5444(8), W-P4 = 2.5078(8),
C11-C12 = 1.201(4); P1-Pt-P2 = 87.95(3), P3-W-P4 =
88.02(3), P4-C12-C11=149.4(3), C12-C11-C10 = 155.8(3).
In the reaction described here, we have used metal co-
ordination to induce the [4 þ 2] alkyne-diyne cycloaddition
between the two diyne substrates. The observed reactivity is a
direct result of the geometry imposed on the diyne substrates
by the Pt and W templates. Although it was impossible to a
obtain crystal structure of the template complex 3 prior to
cycloaddition, the structure of the unreactive Pt(CH3)2
analogue 2 (Figure 1) can be used as a model. The smaller
Pt radius results in shorter M-P bonds and thus holds the Pt
˚
ends of the diynes in close proximity (3.358 A). The larger
W radius means that the W ends of the diynes are further
˚
apart at 3.549 A. Both ends are well above the reactivity
Acknowledgment. This work was financially supported
by the NSERC (Discovery Grant to B.T.S. and USRA to
J.A.T.) and the University of Regina. We also thank Bob
McDonald and Mike Ferguson (University of Alberta)
for X-ray data collection.
8
threshold of 3.2 A, consistent with the unreactivity of this
˚
complex. In the W(CO)4-PtCl2 templated complex 3, the
weaker trans influence of Cl versus CH3 leads to shorter
Pt-P bonds, which in turn bring the Pt ends of the diynes
˚
closer than 3.2 A, leading to reaction. The greater distance at
Supporting Information Available: Text, a table, and CIF files
giving full experimental procedures, spectroscopic characteriza-
tion data, and X-ray crystallographic data for 2-4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
the W end of the diynes prevents formation of the cyclooc-
tadienediyne ring observed in the homometallic bis-PtCl2
complex and instead results in cycloaddition of three of the
four triple bonds, leading to the 1,2-aryne intermediate.