Macrocycle Formation by Cooperative Selection at a Double-Sited Frustrated Lewis Pair

Article abstract of DOI:10.1021/acs.organomet.9b00002

Hydroboration of the 4,5-bis(allyl,mesityl)phosphanyl xanthene derivative 2 with Piers' borane [HB(C₆F₅)₂] gave the bis-P/B frustrated Lewis pair 3. In this situation, the two symmetry-equivalent P/B FLP units featured markedly different reaction modes. With resorcinol, they formed the 20-membered macrocycle 11 by a sequence of -OH deprotonation/borane-induced tautomerization. With aryl acetylenes, the 18-membered macrocyclic products 15 (three examples) were formed in a unique sequence of carbon-carbon coupling reactions.

Full text of DOI:10.1021/acs.organomet.9b00002

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Macrocycle Formation by Cooperative Selection at a Double-Sited
Frustrated Lewis Pair
Long Wang,† Dongsheng Deng,†,‡ Karel Skoch, Constantin G. Daniliuc, Gerald Kehr,
̌
and Gerhard Erker*
̈
̈
̈
Organisch-Chemisches Institut der Universitat Munster, Corrensstr. 40, 48149 Munster, Germany
S
* Supporting Information
ABSTRACT: Hydroboration of the 4,5-bis(allyl,mesityl)-
phosphanyl xanthene derivative 2 with Piers’ borane [HB-
(C6F5)2] gave the bis-P/B frustrated Lewis pair 3. In this
situation, the two symmetry-equivalent P/B FLP units featured
markedly different reaction modes. With resorcinol, they
formed the 20-membered macrocycle 11 by a sequence of
−OH deprotonation/borane-induced tautomerization. With
aryl acetylenes, the 18-membered macrocyclic products 15
(three examples) were formed in a unique sequence of
carbon−carbon coupling reactions.
favored that otherwise would probably not have been observed
INTRODUCTION
■
under other circumstances at all. Our observation may direct
us toward using unusual methodical ways of finding new
reactions. Two representative examples of this development
are described in this account.
Cooperativity is a powerful principle in biology and biological
chemistry.1 Many carrier proteins and enzymes contain two or
more identical active sites, which may mutually influence their
chemical behavior upon addition of reagents. If the reagent has
been added to one site, then there are three options for the
reactivity features of the second site: (a) the sites react
independentlytheir behavior is additive; (b) the addition of
the reagent to the second site is enhanced by the occupation of
the first site (or the equilibrium constant is higher)this
marks cooperative behavior; (c) alternatively, the second site
reacts slower. This is then termed anticooperative behavior (or
marks negative cooperativity) (see Figure 1).2
RESULTS AND DISCUSSION
■
Formation of the Double-Sited FLP System and Its
Conventional Reaction with a Conjugated Ynone. We
chose to prepare a double-sited P/B FLP system at the 9,9-
dimethyl-9H-xanthene backbone.4 We started our synthesis by
metalation of 4,5-diiodo-9,9-dimethyl-9H-xanthene with n-
butyl lithium in ether at −78 °C followed by two-fold
phosphanylation with mesityl-PCl2.5 Subsequent reaction with
2 molar equiv of allyl magnesium chloride gave compound 2 as
a white solid (Scheme 1). From the workup procedure, we
obtained the rac-diastereomer 2 pure in 40% yield (δ31P:
−31.3). The X-ray crystal structure analysis as well as the
NMR spectra identified it as the C2-symmetric isomer (for
details, see the Supporting Information). Compound 2 was
then reacted with Piers’ borane [HB(C6F5)2] (2 equiv, 2 h,
rt).6 Workup gave the two-fold hydroboration product 3,
which we isolated as a white solid in 90% yield. Single crystals
of compound 3 that were suitable for the X-ray crystal
structure analysis were obtained from dichloromethane/
pentane by the diffusion method.
Figure 1. Cooperativity in biological systems.
We have now designed and prepared a completely artificial
system for which such cooperative/anticooperative effects
should be expected in its reactions. We chose frustrated Lewis
pair (FLP) chemistry3 for that purpose. Our system (see
below) contained a pair of identical FLP units attached at a
rigid organic backbone. Indeed, this led to cooperative
behavior in some reactions, as expected, but surprisingly
went beyond that in others: in some reactions, we recorded the
consequences of an apparently new reaction principle, namely,
“cooperative selection”, between activated substrates at the pair
of active sites which has caused reaction pathways to become
The structure (Figure 2) shows near to (but not
crystallographic) C2-molecular symmetry. Each arene ring of
the 9H-xanthene framework now bears a trimethylene-bridged
P/B pair. We note that the boron atom of each unit binds to
the phosphorus Lewis base [B1−P1 2.066(6), B2−P2
Received: January 2, 2019
© XXXX American Chemical Society
A
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rearrangement to give the pair of nine-membered heterocyclic
units in product 6 (Scheme 1). We assume that the reaction
proceeded via the mono-addition product 5 as an intermediate
(see the Supporting Information for details, including the
characterization of compound 6 by X-ray diffraction).7
Scheme 1. Formation of the Double-Sited P/B FLP 3 and
Its Reaction with the Conjugated Ynone 4
Reaction of the Bis-FLP 3 with Resorcinol and with 1-
Alkynes: Unusual Cooperative Behavior. We next reacted
the in situ generated double-sited FLP 3 with 1 equiv of
resorcinol 7 or with 2 molar equiv of a small series of terminal
aryl alkynes 8. With terminal acetylenes, FLPs feature two
prevailing competing reaction pathways, namely, cleavage of
the terminal (sp)C−H bond in an acid/base reaction8 or 1,2-
addition of the FLP components to the carbon−carbon triple
bond.9 With Brønsted acidic ArO-H derivatives, P/B FLPs
usually undergo protonation at phosphorus and formation of
the respective boron phenolates. One might, therefore, have
expected the formation of the conventional symmetric
products 9 and 10, respectively, from the reactions of the
double-sited FLP 3 with these reagents (Scheme 2).
Scheme 2. Expected Reactions of the Two-Sited FLP 3 with
Resorcinol or Two Phenyl Acetylene Equivalents
Figure 2. View of the molecular structure of the double-sited
frustrated P/B pair rac-3 (thermal ellipsoids are shown at 30%
probability; substituents at P1/2 and B1/2 are omitted for clarity
except their ipso-carbon atoms). Selected bond lengths (Å) and
angles (deg): P1−B1 2.066(6), P2−B2 2.071(6), P1−C11 1.814(5),
P2−C51 1.826(5), ΣP1CCC 326.6, ΣP2CCC 326.0, ΣB1CCC 336.8,
ΣB2CCC 334.0, C12−C11−P1−B1−145.3(5), C52−C51−P2−B2−
179.1(5), B1−P1···P2−B2 −82.5, C21−P1···P2−C61 125.9.
However, both of these reactions took a different course.
The Brønsted acidic resorcinol reacted cleanly with 3 in a 1:1
molar ratio to give the macrocyclic product 11, which we
isolated as a white solid in 69% yield (Scheme 3). The X-ray
crystal structure analysis showed that the boron atoms of both
2.071(6) Å]. Compound 3 has retained its rac-PR,PR
configuration. Consequently, compound 3 shows a single set
of NMR data of the two symmetry-equivalent halves of the
molecule in solution and a single resonance of the geminal
dimethyl moiety at the backbone (CD2Cl2, 299 K): it shows a
single 31P NMR resonance at δ 26.5 and a 11B NMR feature at
δ −5.7. Due to the phosphorus chirality, the C6F5 groups of
each (C6F5)2B unit are diastereotopic and there is hindered
rotation around the B−C6F5 vectors at 258 K. Consequently,
we observe a total of 10 19F NMR signals. Compound 3 show
six 1H NMR signals of the hydrogen atoms of the trimethylene
bridge (for details, see the Supporting Information).
Scheme 3. Reaction of the Twin FLP 3 with Resorcinol
We first reacted the two-fold P/B FLP system 3 with the
bifunctional conjugated ynone reagent 4. The system 3 took
up 2 molar equiv of 4. Each five-membered P/B unit
underwent 1,4-addition to the ynone unit followed by enolate
B
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the FLP units had become bonded to the resorcinol oxygen
atoms (Figure 3). One of the former resorcinol OH protons is
Scheme 4. Formation of the Macrocycles 15 by the Reaction
of Bis-FLP 3 with Aryl Acetylenes
Figure 3. Molecular structure of the 20-membered macrocyclic
product 11 (thermal ellipsoids are shown at 15% probability;
substituents at P1/2 and B1/2 are omitted for clarity except for the
arene ipso-carbon atoms). Selected bond lengths (Å) and angles
(deg): P1···P2 4.466, P1−C11 1.799(5), P2−C51 1.823(6), B1−O2
1.528(7), B2−O3 1.547(7), O2−C91 1.277(7), O3−C93 1.295(6),
C91−C92 1.402(8), C92−C93 1.408(8), C93−C94 1.462(8), C94−
C95 1.401(8), C95−C96 1.391(8), C91−C96 1.459(8), ∑P1CCC
337.3, ∑P2CCC 315.8, B1−O2···O3−B2 −3.7.
found at the phosphorus atom P1 (to generate a phosphonium
unit), whereas the other has been shifted in a boron Lewis-
acid-induced dearomatization reaction10 to the arene carbon
atom C94 to generate a cyclohexadienone-type moiety
(delocalized in the β-diketonate subunit). We found a bent
conformation of the newly formed 20-membered macrocycle
with the resorcinol-derived subunit oriented toward the side of
the xanthene oxygen atom.
In solution (CD2Cl2, 299 K), we found a ca. 40:60 mixture
of two isomers. We assume that they are persistent conformers
that probably exhibit the resorcinol-derived unit oriented
either toward the xanthene oxygen (as found in the crystal) or
pointing away from it [31P NMR: δ(major/minor) −14.1/−
13.9 (PH+), δ −37.5/−37.0 (P)]. The 19F NMR spectrum
shows four separate o-C6F5 signals of the pairs of
diastereotopic C6F5 groups at the two boron atom for each
of the isomers. The CH2 group of the cyclohexadienone-
derived backbone shows up at δ 3.19 and 2.37 (major isomer,
minor isomer δ 3.37/3.30; both with 2JHH = 26.4 Hz) in the 1H
NMR spectrum of compound 11 (for further details, see the
Supporting Information).
Figure 4. Molecular structure of the macrocyclic phenylacetylene
coupling product 15a (thermal ellipsoids are shown at 30%
probability; substituents at P1/2 and B1/2 are omitted for clarity).
Selected bond lengths (Å) and angles (deg): P1···P2 4.540, P1−C11
1.788(4), P2−C51 1.847(4), B1−C3 1.652(6), B1−C91 1.659(9),
B2−C92 1.553(6), C91−C92 1.366(6), C92−C93 1.515(6), C93−
C94 1.342(6), C94−C9 1.497(6), ∑P1CCC 338.6, ∑P2CCC 315.0,
∑B1C3C31C41 326.4, ∑B2CCC 359.5, B1−C91−C92−B2 175.1(4),
C92−C93−C94−C9 177.2(4).
with rearrangement and involvement of both the P/B FLP
units. We find the phosphorus atom P1 protonated. Its
trimethylene linker still has the boron atom B1 attached. This
(C6F5)2B moiety has now the −C91(Ph)C92 group bonded
to it, which originates from one phenylacetylene building
block. Carbon atom C92 bears the B(C6F5)2 substituent. It has
become detached from its trimethylene chain, and it is found
to be planar-tricoordinate. The pair of phenylacetylene
building blocks has become head to tail coupled. Con-
sequently, carbon atom C92 is found attached to the
−C93(Ph)C(94)H− unit, which is then found connected
to the remaining trimethylene unit (C9−C7) which binds back
it to the tricoordinate phosphorus atom P2. The newly formed
conjugated diene unit (C91−C94) inside the resulting 18-
membered macrocycle deviates markedly from coplanarity (θ
C91−C92−C93−C94−72.9(6)°).
The reaction of the in situ generated bis-FLP 3 with phenyl
acetylene (3 days, rt) also took an unexpected course. We
isolated the unconventional 2:1 addition product 15a as a
white solid in 71% yield (Scheme 4). Single crystals suitable for
the X-ray crystal structure analysis (Figure 4) were obtained
from pentane/dichloromethane at rt by the diffusion method.
It showed that two phenylacetylene units had been coupled
C
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In solution (CD2Cl2, 299 K), compound 15a shows six
Scheme 5. Potential Rationalization of the Formation of
Macrocycles 15 by Unconventional Alkyne Carbon−Carbon
Coupling at the Two-Sited FLP 3
1
separate H NMR CH signals and two methyl signals of the
1
xanthene-derived framework. It features a broad H NMR [P]
H phosphonium doublet at δ 8.02 (1JPH = 479.6 Hz) and a
corresponding 31P NMR doublet at δ −12.3 in addition to the
P singlet at δ −32.3. We have monitored two very different 11B
NMR features, namely, a very broad signal of the planar-
tricoordinate boron center at δ 61.0 and a rather sharp signal at
δ −9.7 assigned to the tetracoordinate borate boron atom. The
spectral information on the remaining cyclic moiety is rather
complicated (see the Supporting Information for details), but
we have located the CH 1H NMR resonance of the new 1,3-
butadiene unit at δ 6.14 as a triplet (3JHH = 7.1 Hz). We note
that compound 15a shows strongly temperature-dependent
NMR spectra. Upon decreasing the temperature, we observed
decoalescence to a double set of signals in most of the spectra
(see the Supporting Information for the depicted spectra). We
assume that this behavior originates from “freezing” a
conformational equilibrium at low temperature on the
respective NMR time scale. In view of the almost orthogonal
geometry of the newly formed conjugated diene unit that we
have observed in the solid-state structure of compound 15a, it
may be assumed that rotational “inversion” of the conforma-
tional chirality of this unit might be counted responsible for
this observation.
The reaction of the in situ generated FLP 3 with p-
tolylacetylene proceeded analogously (rt, 24 h, dichloro-
methane), and we isolated the 18-membered heterocyclic
macrocycle 15b as a white solid in 66% yield. It was
characterized by C,H elemental analysis, spectroscopy, and
X-ray diffraction. It shows structural (θ C91−C92−C93−C94
−73.2(8)°) and NMR features [31P: δ −12.2 (1JPH ∼ 480 Hz),
δ −32.2; 11B: δ 61.0 (very broad), δ −9.8] similar to those of
15a (for details including the depicted spectra and a view of
the molecular structure, see the Supporting Information). Like
15a, compound 15b also shows dynamic NMR spectra and
features two probable conformational isomers at low temper-
ature.
The reaction of compound 3 with p-anisyl acetylene was
complete after 24 h at rt in dichloromethane. Crystallization
from pentane/dichloromethane gave compound 15c as white
solid that we isolated in 71% yield. The X-ray crystal structure
analysis shows the framework of the newly formed 18-
membered macrocycle that was formed by carbon−carbon
coupling of the two aryl acetylene units at the pair of P/B FLP
functionalities. Like in 15a and 15b, the newly formed dienyl
unit in 15c is markedly rotated from conjugation (θ C91−
C92−C93−C94 79.2(5)°). In solution, compound 15c shows
NMR data similar to those of its congeners 15a,b (see the
Supporting Information for details).
The formation of the macrocyclic alkyne coupling products
15 requires some rearrangement steps on the way. The
formation of compound 15 can potentially be rationalized by
means of a rather short reaction sequence as it is schematically
depicted in Scheme 5, although this pathway is not proven as
yet. It starts with a conventional deprotonation/addition of the
C−H acidic alkyne8 by the FLP phosphane in a typical FLP
reaction to generate the intermediate 12. We have actually
observed this type of an intermediate in the special case of the
reaction of 3 with o-tolyl acetylene (see the Supporting
Information for details). We then might assume that the
carbon−carbon coupling of the B-trimethylene unit of the
adjacent P/B FLP unit takes place with a terminal alkyne
equivalent. As seen from the composition of the final products
15, this might proceed selectively by a trans-1,2-carboboration
process,11 as depicted schematically by the respective assumed
transition state geometry 13≠. We might assume that this step
then is followed by a 1,1-carboboration at the stage of the
intermediate 14 (with either boryl or aryl migration)12 to
directly give the observed products 15.
CONCLUSIONS
■
The reaction of the double-sited FLP 3 with the ynone 4
follows a route that is conceptually equivalent to the behavior
of the biological systems, as schematically depicted in Figure 1:
both the intramolecular trimethylene-bridged FLPs react in an
identical way, giving the same medium-sized ring product by
1,4-P/B FLP addition, but maybe with different individual
reaction rates. The reaction of 3 with resorcinol has a different
quality. It is known that phenolic O−H groups can be
deprotonated by the FLP phosphanyl unit, and strongly
electrophilic boranes were shown to induce tautomerization
reactions.7,10 The special quality of the formation of compound
11 is that these two very different reactions have become
specifically coupled in this cooperative situation. It seems that
B−O bonding on one side has caused the tautomerization
reaction to become favored over a second deprotonation
reaction on the other side (or vice versa). It means that a
specific activation at one site not only induces a change in rate
of a reaction at the other site, but leads to favoring a different
reaction type out of a possible manifold of competing
reactions.
The situation has become even more extreme in the case of
the acetylene coupling/rearrangement processes that we have
observed to take place at the double-sited FLP 3: There is
increasing evidence that boranes can effectively couple
acetylene units. Usually this was observed in intramolecular
cases where bis-alkynyl-containing substrates underwent
sequences of 1,1-carboboration reactions, leading to hetero-
cyclic or arene annulation products.13 Recently, we observed
an intermolecular halogenoborane-induced alkyne oligomeri-
zation reaction.14 The alkyne/alkyne coupling reaction
D
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observed here is of a new quality. Here, each of the P/B FLP
units activated an alkyne equivalent apparently in a different
but highly specific way (possibly in an equilibrium situation),
and then the two resulting units underwent a specific
combined action to form the observed carbon−carbon coupled
products, here, the 18-membered macrocycles 15. In other
words, the neighboring FLP units each have activated an
alkyne unit in some way, and then one of these intermediate
system has apparently selected another differently activated
alkynyl-derived species at the other FLP moiety for a coupling
reaction that had become favored in that specific situation.
This “cooperative selection” bears a new quality in the
cooperative/anticooperative manifold: we not only change
the rates (or equilibrium constants) of the subreactions in this
situation but also induce a fit that leads to a selection of
reaction pathways that can lead to completely new reactions,
which cannot be observed in other “conventional” situations.
We will see how far this reaction principle will lead us. We
hope that we can use it for finding synthetically useful new
reaction types by applying such cooperative selection features
of combinations of activating functionalities in the future.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00002.
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Full experimental procedures and characterization data
(PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: erker@uni-muenster.de.
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ORCID
̌
Karel Skoch: 0000-0001-6406-989X
Gerald Kehr: 0000-0002-5196-2491
Gerhard Erker: 0000-0003-2488-3699
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†L.W. and D.D. contributed equally.
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Notes
The authors declare no competing financial interest.
‡On leave from Henan Key Laboratory of Function-Oriented
Porous Materials, College of Chemistry and Chemical
Engineering, Luoyang Normal University, Luoyang 471934,
P.R. China.
ACKNOWLEDGMENTS
■
Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged. K.S. thanks the DAAD for a
̌
postdoctoral stipend. D.D. thanks the China Scholarship
Council for a stipend.
E
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DOI: 10.1021/acs.organomet.9b00002
Organometallics XXXX, XXX, XXX−XXX