Double cyclization/aryl migration across an alkyne bond enabled by organoboryl and diarylplatinum groups

Article abstract of DOI:10.1002/anie.201201781

Synergy between metals: Diarylacetylenes 1, containing both a boryl (BMes₂) group and a diarylplatinum group, undergo a transformation involving a double metallocyclization/aryl migration to give tetracycles 2 in high diastereoselectivity (see scheme). This remarkable transformation is enabled by the synergistic interplay of the boryl and the diarylplatinum group. Copyright

Full text of DOI:10.1002/anie.201201781

Angewandte
Chemie
DOI: 10.1002/anie.201201781
Cascade Cyclization
Double Cyclization/Aryl Migration Across an Alkyne Bond Enabled
by Organoboryl and Diarylplatinum Groups**
Christina Sun, Zachary M. Hudson, Leanne D. Chen, and Suning Wang*
Organoboron compounds exhibit rich chemical reactivity,
associated with the electron-accepting nature of the boron
center, and this reactivity can lead to fascinating transforma-
tions and applications.[1–4] In particular, molecules that con-
tain both Lewis acidic boranes and Lewis basic amines or
phosphines have unusual reactivity, which has been used in
the activation of molecular hydrogen, alkenes, and
alkynes.[5–7] In a recent report, Yamaguchi and co-workers
demonstrated that such a Lewis acid/base pair, specifically the
boryl group, BMes2 (Mes = 2,4,6-trimethylphenyl), and
a phosphino group, can act synergistically to promote
a double intramolecular cyclization of p-conjugated systems,
containing an internal alkyne, to give a tetracyclic product
(Scheme 1).[5b] This reactivity has been exploited in a highly
effective strategy for preparing a new class of zwitterionic
p-conjugated materials that have unusual photonic and
electronic properties.[5] Stephan and co-workers have shown
that the intermolecular addition of a phosphine and B(C6F5)3
to an alkyne can also occur readily.[6a,d]
to the BMes2/alkyne scaffold. Furthermore, L1 is known for
its unusual reactivity: it undergoes facile hydration of the
internal alkyne at ambient temperature, facilitated by the
cooperation of the borane and the pyridine groups.[9] We
found that the coordination of L1 to an electron-rich PtII
center initiates a reaction sequence involving a spontaneous
double cyclization of the p system, two aryl migrations, and
À
a reduction of the internal alkyne to a C C single bond
(compound 3a and 3b, Scheme 2). The details of this
remarkable transformation are reported herein.
Scheme 2. Spontaneous transformation of 2a and 2b involving two
aryl migrations and two cyclizations.
The mixing of equimolar amounts of L1 and
[Pt(p-RC6H4)2(dmso)2][10] in either CH2Cl2 or toluene at
ambient temperature gave complexes 2a–c, which were
isolated in near quantitative yield and were fully character-
ized by 1H NMR spectroscopy and elemental analysis (see the
Supporting information). Compounds 2a–c are air-stable
light-yellow solids, however, upon prolonged standing in
solution under N2, using CH2Cl2 or toluene as solvent, 2a and
2b undergo gradual transformation into compounds 3a and
3b, respectively; 2c remains unchanged. 3a was isolated as
block-shaped yellow crystals, and its structure was deter-
mined by single-crystal X-ray diffraction analysis (see the
Supporting Information).[11]
Scheme 1. An intramolecular cyclization of a p-conjugated system
involving an internal alkyne, a phosphine, and a borane.
Inspired by this previous work, we wanted to determine if
similar cooperative reactivity, between an electron-rich metal
center, such as PtII, and a BMes2 group, could be used to
construct metallocyclic p skeletons. We are interested in
boron-containing PtII metallocyclic compounds because the
presence of a BMes2 group, for example, can greatly enhance
the phosphorescence efficiency or the anion-sensing ability of
PtII complexes.[8] For this study, we designed ligand L1, which
has a suitable handle for coordination of transition metal ions
The crystal structure of 3a reveals important information
about the transformation (Figure 1). Firstly, one mesityl
group from the boron center and one phenyl group from
the PtII center have migrated to the C(2) and C(1) atoms,
respectively. In addition, new s bonds have been formed
between the boron atom and the C(2) atom, and between the
PtII center and the C(1) atom. As a consequence of this bond
[*] C. Sun, Z. M. Hudson, L. D. Chen, Prof. Dr. S. Wang
Department of Chemistry, Queen’s University
Kingston, ON K7L 3N6 (Canada)
E-mail: suning.wang@chem.queensu.ca
[**] We gratefully acknowledge the Natural Sciences and Engineering
Council of Canada for financial support. We thank Prof. Nicholas
Mosey for his assistance in DFT computations.
À
formation and migration, the C(1) C(2) triple bond is
À
transformed into a single bond, with a C C bond length of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201781.
1.627(4) ꢀ. Furthermore, the migrated aryl groups adopt
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and 2b into 3b, was observed within a few days in either
CD2Cl2 or C6D6, although minor impurities associated with
decomposition of the product or the presence of side
reactions were observed after prolonged standing of the
solution. NMR studies confirmed that for 2a and 2b, the
desired products 3a and 3b, respectively, are the dominant
products. Interestingly, we observed that the transformation
of 2b into 3b proceeded much faster than that of 2a into 3a.
For example, in CD2Cl2, whereas 37% conversion of 2b into
3b was observed after a period of seven days, only 18%
conversion of 2a into 3a was observed after the same period
of time. Longer reaction times led to higher conversions for
both compounds, for example, up to 56% conversion of 2a
into 3a can be observed after 4 weeks at ambient temper-
ature. Conversion can be accelerated by warming the reaction
mixture: when a solution of 2a in C6D6 was maintained at
458C, a 50% conversion into 3a was observed after a reaction
time of one day (see the Supporting Information). In contrast
to the facile transformation of 2a and 2b, compound 2c, as
a solution in CD2Cl2, which was maintained at ambient
temperature, did not undergo any transformation within 17
days. These results suggest that the unusual transformation is
promoted by the presence electron-rich aryl groups on the PtII
center.
Figure 1. Crystal structure of compound 3a. Thermal ellipsoids at
drawn at 50% probability and hydrogen atoms are omitted for clarity.
Important bond lengths [ꢀ] and angles [8]: Pt(1)–C(1) 2.120(3), Pt(1)–
C(3) 2.025(3), Pt(1)–N(1) 2.113(2), Pt(1)–S(1) 2.3026(9), B(1)–C(2)
1.656(4), B(1)–C(10) 1.528(5), B(1)-C(17) 1.587(5), C(1)-C(2) 1.627(4),
C(1)-C(9) 1.514(4), C(1)–C(31) 1.529(4), C(2)–C(26) 1.515(4), C(2)–
C(37) 1.558(4); C(3)-Pt(1)-N(1) 174.04(10), C(3)-Pt(1)-S(1) 177.69(8),
C(2)-B(1)-C(10) 106.0(3), C(2)-B(1)-C(17) 127.1(3), C(10)-B(1)-C(17)
125.1(3).
To determine the roles of both the boryl and the diaryl-
platinum group in this unusual transformation, we synthe-
sized control compounds 2d and 2e and examined their
reactivity (Figure 2). In comparison to the initially studied
compounds 2a and 2b, compound 2d lacks the BMes2 group,
whereas compound 2e contains a considerably more electron-
deficient PtII center (PtCl2). The structure of 2d was
determined by single-crystal X-ray diffraction analysis
(Figure 2). The distances between the PtII center and the
two carbon atoms of the alkyne moiety were 3.230(6) and
3.834(6) ꢀ, and are similar to those of the computationally
optimized structure of 2a. However, when a solution of
compound 2d in CD2Cl2 was allowed to stand at ambient
temperature, no reaction occurred after 10 days, as deter-
mined by 1H NMR analysis, thus indicating that the presence
of the boron acceptor is required for a reaction between the
a syn relationship in all crystals examined, and form
significant p–p interactions with each other. Similar p–p
interactions are also evident between the C(3) and the C(9)
phenyl rings, as well as between the pyridyl and the C(17)
mesityl ring. The C(1) and C(2) atoms both have distorted
tetrahedral geometries, whereas the PtII center has a square
À
planar geometry, in which the two Pt C bonds are cis to each
other. The boron center has a typical trigonal planar geometry
À
and there is considerable B C bond length variation, which is
attributable to the steric environment of the molecule.
In the transformation of 2a into 3a, the alkyne carbon
atoms, C(1) and C(2), become stereogenic centers, and both
ꢀ
enantiomers of 3a are found in the unit cell (space group P1).
Also, the magnetically equivalent methyl groups at the ortho
positions of the mesityl groups of 2a transform into two
chemically inequivalent pairs of diastereotopic methyl groups
in 3a; these methyl groups together with the methyl groups at
the para position, correspond to the six well-resolved methyl
1
peaks in the H NMR spectrum of 3a (see the Supporting
Information). Compound 3a is air-stable in the solid state,
despite the presence of a three-coordinate boron center
bearing only one bulky mesityl group. NMR spectroscopic
studies confirmed that compound 3b, an air-stable yellow
powder, is an analogue of 3a (see the Supporting Informa-
tion). In non-halogenated solvents, such as benzene or
toluene, 3a and 3b are stable for days under air. However,
in CH2Cl2, slow degradation of both compounds was observed
when the solutions were exposed to air for a period of several
days.
To establish whether compounds 3a and 3b are the major
products from the transformation of the corresponding
starting materials, 2a and 2b, we monitored the reaction by
1H NMR spectroscopy at ambient temperature (see the
Supporting Information). Conversion of both 2a into 3a,
Figure 2. The structures of 2d and 2e (left) and the crystal structure of
compound 2d. Thermal ellipsoids are drawn at 50% probability and
hydrogen atoms are omitted for clarity.
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alkyne and the diarylplatinum center to occur. Similarly, 2e
does not undergo any transformation under the same
conditions; this result indicates that the presence of the
electron-rich diarylplatinum group is necessary for the trans-
formation to occur. These results suggest that both of the aryl
migrations and cyclizations for the formation of 3a and 3b
require the synergistic action of the diarylplatinum and boryl
groups.
calculated Pt···C distances of 2.91 and 3.41 ꢀ; the correspond-
ing distances in 2a are 3.33 and 3.99 ꢀ, respectively. This close
proximity would activate the alkyne, thus promoting both the
migration of a phenyl group from the PtII center to the
p system and the cyclization upon formation of a bond
between the p system and the boron center. We believe that
the electron-deficient BMes2 group and the electron-rich
diphenylplatinum group thus act in synergy to promote this
first step. DFT calculations support the hypothesis that the
rate-determining step of the transformation of 2a into 3a
involves the simultaneous formation of bonds between both
the platinum and boron centers and the alkyne moiety, and
the migration of the phenyl ring from the PtII center. This is
consistent with the lack of reactivity of compound 2c because
it contains a relatively electron-poor diarylplatinum group,
which would engender the aforementioned step with a rela-
tively high activation barrier. Re-association of dimethyl
sulfoxide would give species B, a key zwitterionic intermedi-
ate, which was identified by DFT calculations. In this species
B, the electron-rich boryl group and the electrophilic PtII
center enable a second migration step[15]—a sigmatropic
rearrangement involving the transfer of a boron-bound
mesityl group to the proximate alkenyl carbon atom with
Alkynyltriorganylborates with the general formula
ꢀ
Li[R’-C C-BR3] undergo 1,2-migration reactions, in which
upon electrophilic attack at the b-carbon atom of the triple
bond, one R group migrates from the boron center to the
a carbon atom of the alkyne.[12a–g] However, the substrates for
such migration reactions are typically terminal alkynes. The
ꢀ
intermolecular addition of boranes to M-C C-R’ (M = main
group or transition metal) have also been previously descri-
bed.[12h–i] More recently, Erker and co-workers have shown
that a 1,2-alkyl and a 1,2-aryl migration reaction involving
a B(CH3)(C6F5)2 and a B(C6F5)3 group, respectively, and an
internal alkyne are also possible.[13] In contrast, a 1,4-migra-
tion of an aryl group from a boron center to a carbon center is
highly unusual. To the best of our knowledge, the work
described herein contains the first example of this type of
migration, which is probably facilitated by the initial cycliza-
tion of the molecular backbone.
À
concomitant formation of a Pt C s bond, thus yielding the
final product 3a.
Based on the well-established mechanistic features of PtII
facilitated alkyl migrations to alkyne bonds[14] as well as the
phosphine-borane cascade cyclization reported by Yamaguchi
and co-workers,[5] we propose a plausible mechanism for the
transformation of 2a into 3a (and 2b into 3b) (Scheme 3).
The proposed mechanism is supported by density functional
theory (DFT) calculations (see the Supporting Information).
The direct precursor for the transformation of 2a into 3a is
species A, which is formed by dissociation of dimethyl
sulfoxide from the PtII center. In A, the diphenylplatinum
unit is much closer in space to the alkyne bond, with
This mechanism also explains the diastereoselectivity of
the transformation of 2a into 3a. As the PtII center in B blocks
one face of the alkene, only the boron-bound mesityl group
positioned near the other face of the alkene can migrate. This
migration results in a final product wherein the two aryl
groups that have undergone migration are syn to each other.
Computational data also show that product 3a is more stable
than the starting material 2a by approximately 36.8 kJmolÀ1.
In summary, we have demonstrated an unusual intra-
molecular reaction of 2a and its analogue 2b; this reaction
involves two aryl migrations and two cyclizations and results
in the formation of a unique bicyclic organometallic scaffold.
Furthermore, the complementary electronic properties of the
electron-deficient borane and electron-rich PtII centers were
found to play a key role in this unprecedented transformation.
Future work will involve the application of this strategy to the
preparation of other polycyclic frameworks, as well as the
incorporation of other electron-rich metal centers into boron-
containing organometallic scaffolds.
Received: March 5, 2012
Published online: && &&, &&&&
Keywords: alkynes · aryl migration · boron · cyclizations ·
.
platinum
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Communications
Cascade Cyclization
C. Sun, Z. M. Hudson, L. D. Chen,
S. Wang*
&&&& — &&&&
Double Cyclization/Aryl Migration Across
an Alkyne Bond Enabled by Organoboryl
and Diarylplatinum Groups
Synergy between metals: Diarylacety-
lenes 1, containing both a boryl (BMes2)
group and a diarylplatinum group,
undergo a transformation involving
a double metallocyclization/aryl migra-
tion to give tetracycles 2 in high diaste-
reoselectivity (see scheme). This remark-
able transformation is enabled by the
synergistic interplay of the boryl and the
diarylplatinum group.
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