Rapid access to substituted 2-naphthyne intermediates: Via the benzannulation of halogenated silylalkynes

Article abstract of DOI:10.1039/c7sc01625e

Aryne intermediates are versatile and important reactive intermediates for natural product and polymer synthesis. 2-Naphthynes are relatively unexplored because few methods provide precursors to these intermediates, especially for those bearing additional substituents. Here we report a general synthetic strategy to access 2-naphthyne precursors through an Asao-Yamamoto benzannulation of ortho-(phenylethynyl)benzaldehydes with halo-silylalkynes. This transformation provides 2-halo-3-silylnaphthalenes with complete regioselectivity. These naphthalene products undergo desilylation/dehalogenation in the presence of F^- to generate the corresponding 2-naphthyne intermediate, as evidenced by furan trapping experiments. When these 2-naphthynes are generated in the presence of a copper catalyst, ortho-naphthalene oligomers, trinaphthalene, or binaphthalene products are formed selectively by varying the catalyst loading and reaction temperature. The efficiency, mild conditions, and versatility of the naphthalene products and naphthyne intermediates will provide efficient access to many new functional aromatic systems.

Full text of DOI:10.1039/c7sc01625e

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Rapid Access to Substituted 2-Naphthyne Intermediates via
the Benzannulation of Halogenated Silylalkynes
Samuel J. Heina,b, Dan Lehnherrb, and William R. Dichtela,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Aryne intermediates are versatile and important reactive intermediates for natural product and polymer synthesis.
2-Naphthynes are relatively unexplored because few methods provide precursors to these intermediates, especially for
those bearing additional substituents. Here we report a general synthetic strategy to access 2-naphthyne precursors through
an Asao-Yamamoto benzannulation of ortho-(phenylethynyl)benzaldehydes with halo-silylalkynes. This transformation
provides 2-halo-3-silylnaphthalenes with complete regioselectivity. These naphthalene products undergo
desilylation/dehalogenation in the presence of F– to generate the corresponding 2-naphthyne intermediate, as evidenced
by furan trapping experiments. When these 2-naphthynes are generated in the presence of a copper catalyst, ortho-
naphthalene oligomers, trinaphthalene, or binaphthalene products are formed selectively by varying the catalyst loading
and reaction temperature. The efficiency, mild conditions, and versatility of the naphthalene products and naphthyne
intermediates will provide efficient access to many new functional aromatic systems.
www.rsc.org/
ortho-Arynes are reactive intermediates generated through the benzaldehydes and diarylalkynes, is capable of modifying
elimination of adjacent functional groups from an aromatic conjugated polymer backbones with high efficiency12 and is
system.1 These species have a rich history in physical and regioselective for alkynes with electronically different aryl
synthetic organic chemistry since their existence was first substituents.13 The scope of these benzannulation reactions has
postulated by Stoermer and Kalhert.2 Transformations involving since been expanded to silyl- and haloalkynes, enabling the
nucleophilic additions, bond insertions, pericyclic reactions, and synthesis of sterically hindered14 and polyheterohalogenated
multicomponent coupling reactions involving arynes have been naphthalenes.15 Here we demonstrate a new variant of the
employed for the synthesis of natural products and polycyclic Asao-Yamamoto
aromatic hydrocarbons.3 Although early arynes were first
generated using strong bases with limited functional group
compatibility, Kobayashi reported that ortho-silylaryl triflates
generate arynes efficiently in the presence of fluoride ions.4 The
mild conditions and excellent functional group tolerance of this
approach has led to ortho-silylaryl triflates becoming popular
aryne precursors. Yet few ortho-silylaryl triflates are
commercially available, and their synthesis is often laborious,
despite the recent development of cycloaddition-based
strategies by Harrity5 and aryl C–H bond silylations by Daugulis.6
Arynes derived from other aromatic systems7,8 enable the
synthesis of more complex structures, yet access to
appropriately substituted precursors is even more limited.
Wong9 and Maly10 reported substituted 2-naphthyne
precursors, but the seven step syntheses of each method limit
general adoption (Scheme 1a–b). We recently noted that the
Asao-Yamamoto benzannulation11, which was known to
benzannulation
that
provides
provide
2,3-diarylnaphthalenes
from
o-phenylethynyl-
Scheme 1 Synthesis of substituted 2-naphthyne precursors
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2-silyl-3-halonaphthalenes as single regioisomers (Scheme 1c). 70–93% whereas those for benzaldehydes bearingVtiewwoArptichleeOnnolilniec
These compounds generate 2-naphthyne intermediates under esters (1g) and two alkyl groups (1h)DOwI:e1r0e.106339/Ca7nSdC015692%5E,
mild conditions, which were trapped by furan, oligomerized, respectively. Benzaldehydes bearing stronger electron donating
and found to undergo cyclodimerization or cyclotrimerization to groups, such as dimethoxy benzaldehyde 1i, did not undergo
bi-and trinaphthalenes, respectfully. Several examples benzannulation, and a mixture of aldehyde decomposition
featuring
substituted
o-phenylethynylbenzaldehydes products and unreacted alkyne was obtained.
demonstrate the generality of this approach to access
substituted 2-naphthyne intermediates.
The benzannulation reactions of 2a and 2b (Fig. 1) provide
substituted naphthalene products as single regioisomers when
We evaluated the scope and regioselectivity of the non-pseudosymmetric benzaldehydes (e.g., 1c
, 1d) are
benzannulation reaction using substituted benzaldehydes and employed. Our previous studies of diarylalkynes and
silylhaloalkynes (Fig. 1). Previously, a Cu(OTf)2 catalyst with haloarylalkynes indicated that regioselectivity arises from the
added CF3CO2H had been employed for the benzannulation of ability of the alkyne substituents to preferentially stabilize
substituted acetylenes, but these conditions favour developing positive charge at one of the alkyne carbons.13,15 For
protodesilylation of the trimethylsilyl (TMS) substituent.14 ZnCl2 example, when brominated benzaldehyde 1b undergoes
is a milder catalyst that does not require acidic additives.16 benzannulation with an aryl-haloalkyne, using either Cu(OTf)2
Naphthalene products 3a
–
j
were obtained as single or ZnCl2 catalysts, the syn-regioisomer with respect to the
regioisomers, often in good to excellent yield. The reaction was iodine and bromine positions is obtained (Fig. 2a). The opposite
more efficient for iodoalkynes than bromoalkynes, as shown for regioisomer is obtained for silylhaloalkyne substrates; 1b reacts
benzaldehyde 1b with 2a (X = I, 93% yield) and 2b (X = Br, 18% with 2a and 2b to provide the corresponding anti-regioisomers
yield). Likewise, the reaction of 1c with 2a and 2b provided 3d 3b and 3c, respectively. These outcomes were confirmed by
(93%) and 3e (42%). Unsubstituted and halogenated single crystal X-ray crystallography of 3c (Fig. 2a), compared to
benzaldehydes (1a–f) performed well with yields between
Fig. 2 a) Haloarylalkynes and halosilylalkynes provide opposite regioselectivity in
benzannulation reactions, as demonstrated by x-ray crystallography.17 Ellipsoids set to
50 % probability level for 3c. b) Rationale for the regioselectivity of each reaction. The
silicon substituent stabilizes developing positive charge on the carbon adjacent to the
halogen, which makes the observed regioselectivity consistent with other
benzannulation reactions.
Fig. 1. Synthesis of 2-naphthyne precursors from substituted 2-
(phenylethynyl)benzaldehydes All yields are isolated yields. Reaction Conditions: Alkyne
(2a–c , 0.10 M in 1,2-DCE); benzaldehyde (1a, 2.0 equiv or 1b–i,1.3 equiv); ZnCl2 (0.20
equiv).
2 | Chem. Sci., 2017, XX, X-X
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Scheme 2. Conversion of 3c to 4
the syn-regioisomer for phenylbromoalkyne. An alternate
mechanism in which the metal is not bound to the
benzopyrylium intermediate is also plausible, and transition
state energies of those structures also support the observed
regioselectivity (see ESI).
Furthermore, we had previously accessed a library of
polyheterohalogenated
naphthalenes
through
the
benzannulation of arylhaloalkynes. Although this approach was
used to prepare more than twenty polyheterohalogenated
naphthalenes as single regioisomers, it was limited to
2-arylnaphthalene
derivatives.15
Silylhaloalkyne
benzannulations eliminate this restriction, and instead provide
2-trimethylsilyl groups that are easily removed or further
transformed. For example, treatment of 3c with ICl afforded
Fig 3. DFT calculated transition-states using B3LYP/6-31G(d) potentially responsible for
the regioselectivity outcome in the benzannulation of: (A,B) silylhaloalkynes & (C,D)
arylhaloalkynes, along with their relative electronic energies and bond forming
interatomic distances. Element coloring scheme: C = silver, H = white, O = light red, Zn =
blue, Cl = dark green, F = light green, S = yellow, Br = dark red, Cu = bronze.
polyheterohalogenated naphthalene
(Scheme 2). The TMS group incorporated into the naphthalene
products serves as versatile handle for further
4 in quantitative yield
a
functionalization, as C(aryl)–Si bonds are readily transformed
into I, Br, Cl, H, OH, CF3, Me, and others, highlighting the vast
chemical space available through this method.19
The 2-silyl-3-iodonaphthalenes serve as efficient precursors
of 2-naphthyne intermediates in the presence of F– at room
temperature, as demonstrated by furan trapping
experiments.20–22 Unsubstituted (3a), 6-bromo- (3b), 6,7-
dichloro- (3h), and 6,7-dibutyl-substituted (3j) 2-naphthynes
that obtained for a typical arylhaloalkyne. The regiochemical
outcome is insensitive to substitution patterns on the
benzaldehyde, as demonstrated for the benzannulations of
monofluorinated benzaldehyde regioisomers 1c and 1d with 2a
.
Each reaction provides a single fluoronaphthalene regioisomer,
whose structures were assigned by 19F and 2D NMR
spectroscopy (see ESI). Finally, halogenation of the silylalkyne is
also essential for regioselectivity, as the benzannulation
reaction of trimethylsilylacetylene (2c) and 1b provides a nearly
1:1 ratio of the two regioisomers. The regiochemical outcome
of these ZnCl2-catalyzed benzannulations with silylarylalkynes
are congruent with those using silylhaloalkynes. This
observation supports our hypothesis that the preferred
regiochemical outcome is based on going through the more
stabilized regioisomeric cation, in this case the cation stabilized
by both the beta-silyl effect and formation of a benzylic cation
(see ESI).
provided [2.2.1]oxabicyclic alkenes (5a–d) in good to excellent
isolated yields (68–86 %, Scheme 3). These structures are of
interest as precursors of poly(ortho-phenylene)s, iptycenes,
acenes, and other aromatic structures.23-31 Given the availability
of many substituted benzaldehyde cycloaddition partners,
these
findings
demonstrate
that
benzannulation
The
reversed
regioselectivity
of
halosilylalkyne
benzannulations likely originates from the combined ability of
the silicon and halogen substituents to stabilize developing
positive charge on the alkyne carbon beta to the silicon atom.18
In contrast, when arylhaloalkynes are benzannulated, the
aromatic ring stabilizes a developing positive charge on the
alkyne carbon alpha to the ring (Fig. 2b). A DFT model of the
proposed transition states leading to either the syn- or anti-
regioisomer correctly predicts the reversal in regioselectivity
observed experimentally between silylhaloalkynes and
arylhaloalkynes (Fig. 3). The B3LYP/6-31G(d) calculated ZPE-
corrected electronic energies of regioisomeric transition states
predict a 2.0 kcal/mol preference for the anti-regioisomer in the
case of silylhaloalkyne 2b compared to 5.6 kcal/mol in favor of
Scheme 3. Aryne generation and trapping with furan
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Fig. 4 (A) Synthesis of oligo(ortho-naphthalene) 6 and cyclic compounds 7 and 8. All yields are isolated yields. Reaction conditions: 3j (0.1 mM in THF), CsF (2 equiv), and 18-crown-6
(4 equiv). (B) SEC traces for the copper mediated oligomerization and cyclization of 3j with 0.5 mol % CuCN at room temperature (green), 5 mol % CuCN at room temperature (blue),
and 5 mol % CuCN at 60 °C (red). Molecular weight distributions were determined by calibration with polystyrene standards. (C) Single crystal X-ray structure of 8 showing bond C-C
distances around the aromatic rings in Å. Hydrogens are omitted for clarity, thermal ellipsoids shown at the 50% probability level. (D) Solid state packing arrangement of 8 illustrating
the interplanar distances between arenes.
chemistry provides rapid entry to many substituted 2- provided
naphthyne intermediates. (19%). Size-exclusion chromatography (SEC) of
6
as an oligomeric mixture in modest isolated yield
(Fig. 3)
6
The synthesis of oligo- and poly(o-arylene)s represents a indicated a monomodal molecular weight distribution with Mn
long-standing synthetic challenge. Oligomers have been = 1900 g/mol compared to polystyrene standards, with tailing
prepared through stepwise cross-coupling strategies32,33 or to low molecular weight, corresponding to desilylated and/or
cycloaddition approaches34 and adopt preferred or exclusive dehalogenated naphthalene side products. MALDI-TOF mass
helical conformations. Formal or direct aryne polymerizations spectrometry (see ESI) showed peaks with spacings consistent
have provided the first polymers with moderate to high degrees with the expected dibutylnaphthalene repeat unit and
of polymerization (DP: 20–100).23,35 We explored the CuCN- suggested that these species had well-defined nitrile end group
mediated polymerization of 3j (Fig. 4a) under conditions on one end of the oligomer chain. These results confirm the
adapted from Uchiyama’s pioneering study.35 Naphthalene expected reactivity of 3j under reported aryne polymerization
derivative 3j (0.235 mM) was polymerized in THF at 25 °C in the conditions, despite the formation of low molecular weight
presence of CsF (2 equiv), CuCN (0.05 equiv). n-BuLi (0.10 equiv) species. It may ultimately prove possible to achieve higher
and 18-crown-6 (4 equiv). The addition of n-BuLi to CuCN yields and DP by optimizing the rate of 2-naphthyne generation.
generates a Lipshultz-type cuprate that is thought to be the However, our efforts to explore the role of reaction conditions,
active catalytic species in this reaction.35,36 These conditions such as increased catalyst loading and temperatures, provided
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unexpected and previously undescribed cyclooligomerization silylnaphthalenes with high regioselectivity. The addition of a
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DOI: 10.1039/C7SC01625E
products in synthetically useful yields (Fig. 4a).
fluoride anion generates 2-naphthyne reactive intermediates,
During our attempts to obtain higher molecular weight which were trapped as the [2.2.1]oxabicyclic alkene. These
polymers, we found the product distribution drastically trapped arynes are themselves synthetically useful building
changed when the CuCN loading was increased to 5 mol % and blocks.23-31 A copper mediated aryne polymerization afforded
the reaction run at room temperature. Analysis of the GPC trace low molecular weight oligomers, however when higher catalyst
of the precipitated product revealed a significant shift in the loadings are used the [2+2+2] and [2+2] cycloaddition products
retention time of the oligomeric peak reflective of shorter were observed in synthetically useful yields. These results show
oligomers (Fig. 4b). A sharp peak emerged at significantly longer the utility of a copper-catalyzed benzannulation reaction for
retention times, suggesting a well-defined, low molecular preparing to synthesize substituted tri- and binaphthalenes that
1
weight compound. The H and 13C NMR spectra indicated the are otherwise not easily prepared or derivatized. We anticipate
formation of a single naphthyne species with only two aromatic that the control of the halogenation pattern in conjunction with
1H resonances and five aromatic carbon shifts, suggesting the the diversity of available aryne reactions will enable rapid
formation of a cyclic compound with a high degree of symmetry access to diverse and unique polycyclic conjugated aromatic
(see ESI figures S40–41). Finally, high-resolution mass architectures,
spectrometry of the isolated product after purification by poly(arylene)s, among others.
chromatography identified it as the cyclic
hexabutyltrinaphthylene , which was isolated in 49 % yield (Fig.
including
functionalized
acenes
and
7
Acknowledgements
4a). This extended form of triphenylene is of interest for discotic
liquid crystals and as a potential monomer for covalent organic
frameworks (COFs).37
When the above reaction is run at elevated temperature, a
different molecular product dominates, as indicated by a shift
to longer retention time in the GPC trace of the crude reaction
This work was supported by the Beckman Young Investigator
Program of the Arnold and Mabel Beckman Foundation. This
work made use of the Cornell Center for Materials Research
Shared Facilities which are supported through the NSF MRSEC
program (DMR-1120296) and the IMSERC at Northwestern
University, which has received support from the NSF (CHE-
0923236, CHE-1048773); Soft and Hybrid Nanotechnology
Experimental (SHyNE) Resource (NSF NNCI-1542205); the State
of Illinois and International Institute for Nanotechnology (IIN).
mixture (Fig 4b). We identified this species as the
1
tetrabutylbinaphthalene
8
through H and 13C NMR (ESI Fig.
S42–43) spectroscopy and high-resolution mass spectrometry.
Single-crystal X-ray crystallography unambiguously identified
the structure as tetrabutylbinaphthalene product
The solid-state arrangement of features cofacial
8
(Fig. 4c).
8
p-stacking
with an unusually long interplanar distance of 3.61 Å, compared
to the typical 3.4 Å distance of acenes (Fig 4d).38 This
arrangement arises from the butyl side chains, whose
conformation places their termini perpendicular to the arene
plane, preventing closer cofacial packing. [N]phenylenes are
traditionally challenging synthetic targets. Vollhardt and
coworkers have prepared many of these compounds through a
cobalt-catalyzed [2+2+2] cycloaddition of phenylene
ethynylenes,39 and new synthetic strategies have been reported
by Swager,40 Xia,41 and Bunz.42 These compounds consist of
alternating aromatic rings fused cyclobutadienes with strong
anti-aromatic character. The X-ray crystal structure showed a
nearly planar geometry with torsion angles of 1.56° and 0.55°
around the phenylene link. Analysis of the bond lengths
confirms localization of the π-bonds around the 4-membered
ring to offset the antiaromatic character of the
cyclobutadienoid with alternating 1.44 Å endocyclic and 1.35 Å
exocyclic bond distances. Bonds linking the acenoid segments
have a larger single bond character with a bond length of 1.50
Å (Fig. 4c).43 UV-vis absorption and photoemission spectra of
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