A simple Hückel model-driven strategy to overcome electronic barriers to retro-Brook silylation relevant to aryne and bisaryne precursor synthesis

Article abstract of DOI:10.1039/d0cc08283j

ortho-Silylaryl triflate precursors (oSATs) have been responsible for many recent advances in aryne chemistry and are most commonly accessed from the corresponding 2-bromophenol. A retro-BrookO- toC-silyl transfer is a key step in this synthesis but not all aromatic species are amenable to the transformation, with no functionalized bisbenzyneoSATs reported. Simple Hückel models are presented which show that the calculated aromaticity at the brominated position is an accurate predictor of successful retro-Brook reaction, validated synthetically by a new success and a predicted failure. From this, the synthesis of a novel difunctionalized bisaryne precursor has been tested, requiring different approaches to install the twoC-silyl groups. The first successful use of a disubstitutedo-silylaryl sulfonate bisbenzyne precursor in Diels-Alder reactions is then shown.

Full text of DOI:10.1039/d0cc08283j

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A simple Huckel model-driven strategy to
¨
overcome electronic barriers to retro-Brook
silylation relevant to aryne and bisaryne precursor
Cite this: Chem. Commun., 2021,
57, 1663
Received 21st December 2020,
Accepted 13th January 2021
synthesis†
Edward A. Neal, *‡ A. Yannic R. Werling ‡ and Christopher R. Jones
*
DOI: 10.1039/d0cc08283j
rsc.li/chemcomm
ortho-Silylaryl triflate precursors (oSATs) have been responsible for silylation of 2-bromophenol 1 with hexamethyldisilazane (HMDS)
many recent advances in aryne chemistry and are most commonly (Scheme 1a).4 Subsequent halogen-metal exchange then triggered
accessed from the corresponding 2-bromophenol. A retro-Brook instantaneous retro-Brook O- to C-silyl transfer (3 to 4) prior to
O- to C-silyl transfer is a key step in this synthesis but not all triflation.
aromatic species are amenable to the transformation, with no
Whilst this synthetic strategy permits access to many aryne and
functionalized bisbenzyne oSATs reported. Simple Huckel models heteroaryne precursors6 with wide functional group tolerance, it is
¨
are presented which show that the calculated aromaticity at the not universal. In some instances, longer routes are needed, such as
brominated position is an accurate predictor of successful retro-Brook Kobayashi’s original approach3a via C,O-disilylation, O-desilylation,
reaction, validated synthetically by a new success and a predicted then triflation.7 This method was used by Duong in 2003 to first
failure. From this, the synthesis of a novel difunctionalized bisaryne produce 1,4-bisbenzyne oSAT precursor 85b before the successful
5c
ˇ
precursor has been tested, requiring different approaches to install the retro-Brook optimization by Pavlicek et al. in 2015 (Scheme 1b).
two C-silyl groups. The first successful use of a disubstituted o-silylaryl 1,4-Bisbenzynes from 8 have appeared in around 15 papers,5,8 as
sulfonate bisbenzyne precursor in Diels–Alder reactions is then shown. well as a recent trisaryne oSAT from its triphenylene trimer,5d yet no
additionally substituted examples of oSAT bisbenzynes have been
Arynes are a class of highly reactive electron-deficient inter- developed.
mediate derived from an arene or heteroarene with two vacant
positions, usually ortho to each other. The reactions of arynes
generally involve the introduction or extension of a p-system
and have been extensively reviewed.1 These transformations
are particularly useful in the pharmaceuticals industry and in
p-extended polyaromatic hydrocarbon (PAH) material produc-
tion for sensing, energy generation and other applications.1a,c
Classical methods of aryne generation typically required the
use of strong bases, sodium metal, pyrophoric tert-butyllithium
or potentially explosive ortho-diazocarboxylate precursors, as
well as more extreme temperatures. In contrast, ortho-silylaryl
triflate precursors (oSATs) operate under milder reaction condi-
tions and possess a more attractive safety profile,2a which has led
to their widespread uptake.1,2b–d Originally developed by Kobayashi
in 1983,3a o-(trimethylsilyl)phenyl triflate 5 reliably affords benzyne
upon treatment with either fluoride or carbonate base.3b In 2002
˜
Pena et al. improved the synthesis of oSAT 5 through the initial
School of Biological and Chemical Sciences, Queen Mary University of London,
Mile End Road, London, UK. E-mail: e.a.neal@qmul.ac.uk, c.jones@qmul.ac.uk
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc08283j
Scheme 1 (a) Synthesis of oSAT benzyne precursor
5
from
2-bromophenol 1.4 (b) Approaches to unsubstituted bisbenzyne oSAT
precursor 85a–c (c) This work: novel route towards challenging disubstituted
bisbenzyne precursors based on simple Huckel aromaticity models.
¨
‡ These authors contributed equally to this work.
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The different methods of formation of bis/trisarynes have novel retro-Brook application proved successful (70% yield;
been reviewed by Shi, et al. and usually require more forceful ESI,† S3).
conditions than oSATs, thus limiting functional group
We next considered the extended aromatic precursors
tolerance.1d This presents unrealized potential for the rapid 13o–q, with naphthalene derivative 13o11k found to possess a
generation of functionalized PAHs from bisbenzyne precursors. high aromatic charge (+0.076). Interestingly, [5]helicene 13p11l
In this work, we demonstrate that an electronic barrier is also successfully undergoes retro-Brook rearrangement but
responsible for the failure of the retro-Brook step during the displays a significantly lower charge (+0.003). As Hu¨ckel approx-
formation of certain oSATs and that a fast and accessible imations presume a planar aromatic system, twisted systems
Hu¨ckel calculation provides a reliable indication of success. such as 13p appear to represent a limitation of our model. By
We then validate this model by experimentally confirming contrast, the planar 9,10-phenanthryne precursor intermediate
three predicted outcomes: (1) a successful new approach to a 13q has a near-zero aromatic charge (+0.001), which suggests
known benzyne oSAT precursor; (2) the failure of the key step the retro-Brook reaction to be disfavoured;9 indeed, the synth-
towards a 9,10-phenanthryne oSAT precursor;9 and (3) the esis of the corresponding phenanthryne oSAT precursor
˜
development of a synthetic route to a novel difunctionalized described by Pena et al. in 2000 did not involve this
bisaryne precursor 12, wherein only one of the two C-silyation transformation.13a According to Clar’s Rule,14 the two peripheral
steps proceeds via a retro-Brook rearrangement (Scheme 1c).
rings in phenanthrene are more aromatic than the central ring,
We hypothesized that o-bromophenyl silyl ethers, such as 2, allowing the 9,10-bond to be functionalized in a manner similar
should undergo halogen-metal exchange then retro-Brook rear- to non-aromatic double bonds.15 Even when conjugated alkenes
rangement if an aromatic charge is present at the brominated have been exposed to conditions analogous to the retro-Brook
position. To this end, Hu¨ckel aromatic charges were calculated reaction, halogen retention was preferred over halogen-metal
on MM2 energy-minimized models of a wide range of bro- exchange.16
moaryl silyl ether intermediates (2 and 13a–q) towards reported
or potential oSAT aryne precursors (Fig. 1).10
With a view to accessing novel functionalized bisbenzyne
oSATs we decided to apply our Hu¨ckel model to test the viability
As a benchmark, the intermediate (2) towards unsubstituted of
a retro-Brook-based synthetic approach towards para-
benzyne oSAT 5 has an aromatic charge of +0.056 (ESI,† S2).3a dimethylated bisxylyne oSAT 12. As only small positive induc-
The sign of the charge was not important, as 1,3-benzodioxole tive effects are present, the steric encumbrance common to all
derivative 13a, reported to undergo successful retro-Brook pendant substituents can be tested for, while minimizing
reaction,6a was calculated at ꢀ0.029. A range of literature electronic effects specific to each. Investigations started by
examples 13c–13m7,11a–i were found to possess charges comparable modelling the unfunctionalized bisbenzyne oSAT precursor 7
to benchmark 2, with just 4,5-difluoro 13e11a showing a significantly that was accessed via retro-Brook rearrangement by Pavlicek
ˇ
lower value (+0.005). In light of this difference, it is noteworthy that et al.5c The MM2 energy-minimized models of post-O-silylation
several mechanisms for halogen-metal exchange have been intermediates 13r and 13s confirmed that aromatic character
proposed.12 The model predicted that 3,6-dimethoxy 13n (ꢀ0.011) was maintained in both brominated positions (Fig. 2).
should also undergo retro-Brook rearrangement to the known aryne Although calculations on the xylyne precursor 13t17 revealed
n
precursor 13z, originally accessed via C-deprotonation with BuLi suitable aromatic character (+0.026), significant deactivation
and direct C-silylation of the 2,5-dimethoxyphenol.11j Pleasingly, this was found for O-silylated bisxylyne prototype intermediates 13u
and 13v. Intriguingly, if the first silyl transfer were to be forced,
or silylation achieved via other means, then potential
intermediates 13w and 10 for the second stage both have a
significant Hu¨ckel charge (+0.032). This would suggest that the
second silyl transfer should be possible by retro-Brook. Else-
where, sizeable aromatic charges were calculated for 1,3-diyne
Fig. 1 Calculated Huckel aromatic charges at brominated positions in
Fig. 2 Huckel aromatic charges on brominated positions in halogen-
¨
¨
halogen-metal exchange intermediates for the synthesis of a range of
aryne precursors (MM2 in PerkinElmer Chem3D).10
metal exchange intermediates for a range of literature bisbenzyne
precursors and prototype models. (MM2 in PerkinElmer Chem3D)10
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precursors 13x (ꢀ0.019 and +0.015) and 13y (ꢀ0.023), in agree-
ment with the reported retro-Brook syntheses.11m,n
Encouraged by the ability of the Hu¨ckel model to predict
successful retro-Brook syntheses, we experimentally investi-
gated the two outliers: 9,10-phenanthryne and bisxylyne. The
˜
phenanthryne precursor 15 was first reported by Pena et al.,
starting from 9-bromo-10-phenanthrol 14 and prepared using a
C,O-disilylation method.13a We successfully repeated this on a
gram scale, using an in situ solvent-exchange step to remove
impurities and increase the yield by 8% (Scheme 2a).13
To test our aromaticity model, attempts were made to form
oSAT 15 from 14 using HMDS and a retro-Brook reaction, which –
although reported once4 – failed in line with our predictions
(ESI,† S5).9 To provide sufficient material for these tests, an
improved synthesis of 14 from phenanthrene 16 was developed
(Scheme 2b). Interestingly, the bromination of 16 at C-9 and C-
10 would not usually be possible at an aromatic carbon without
Lewis acid catalyst.15 Our updated synthesis of 15 has: (a) full
contemporary purification and characterization; (b) replaces
toxic CCl4 with CHCl3; (c) a 76% yield of 19 from 18 (cf. 48%); and
(d) a 34% yield of 15 at gram scale over five steps from 16 (ESI,†
S4).13a,18 Finally, a quantitative Diels–Alder reaction between 15
and furan was successfully repeated to form adduct 20 (Scheme 2
inset), validating its suitability as an aryne precursor (ESI,† S5).19
Attention now turned to the synthesis of 1,4-bisxylyne oSAT
prototype 12a. Our model suggests that the first prospective
O- to C-silyl transfer in intermediate 13v is disfavoured, whereas
a second C-silyl group could be installed via a retro-Brook
reaction using intermediate 13w or 10. To probe this predic-
tion, 2,5-dimethylbenzoquinone 21 was reductively brominated
to form 2,5-dibromo-3,6-dimethylhydroquinone 9 (Scheme 3a;
ESI,† S6). Attempts to produce bisxylyne oSAT prototype 12a
directly from 9 and HMDS via a retro-Brook reaction afforded no
product, only crude mixtures highly prone to oxidation (ESI,†
S7). Even when forcing conditions were used to access O-silyl
intermediate 13v, subsequent sodiation5a to initiate retro-Brook
silyl transfer gave an insoluble mixture with no target.
Scheme 3 (a) Reductive bromination of 2,5-dimethyl-1,4-benzoquinone
21 with subsequent failed attempts to form a bisxylyne oSAT prototype
12a; (b) abridged putative mechanism of the formation of 12a (K) from 9
(A). Full scheme in ESI,† S9.
Subsequent retro-Brook reaction and triflation would lead to
oSAT 12a (Scheme 3b; ESI,† S8–9). Reaction aliquots were
analysed at each stage by NMR (ESI,† S14) and GC/MS (ESI,†
S18). The O-silylations (Phase 1) succeeded, although disilylated
13v (C) was observed after the first step and C-lithiated D from
halogen-metal exchange after the second, due to excess nBuLi.
Crucially, no retro-Brook reactions were observed with D
despite the excess lithiation, as confirmed by a separate test on
isolated 13v (ESI,† S10). Following the first C-silylation step
(Phase 2), C,C0-dilithiated DD was present despite overnight
stirring at room temperature with chlorosilane. This suggests a
significant barrier to intermolecular C-silylation ortho to
O-silylated species precluding a first retro-Brook (Fig. 3, top).
Phenolic signals in the 1H NMR spectrum confirm that the
large triply-silylated species must be C,C0,O-isomer G (11),
rather than F. Hence, a retro-Brook reaction on the second
side is successful, in line with our mechanistic model. After
Having verified the fates of intermediates 9 and 13v, a
synthetic route towards 12a was developed based on our model.
Initial O,O0-disilylation and C-silylation of 9, as for phenanthryne
oSAT 15, was proposed to access intermediate aryl silane 10.
n
solvent exchange and further BuLi, without further silylation,
the next aliquot contains only G (11), F and tetrasilylated H
Scheme 2 Synthesis of 9,10-phenanthryne precursor 15: our reported
improvements in yield and scale for (a) the final step; (b) the conversion of
phenanthrene to precursor starting material 9-bromo-10-phenanthrol 14.
This does not involve a retro-Brook step.
Fig. 3 Partial stack plot of GC/MS aliquots from the formation of bisxylyne
oSAT prototype 12a (K). Letters correspond to Scheme 3b.
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the bisaryne proceeded successfully in 13% yield (Scheme 4).
In summary, we have developed a quick and facile MM2/
Hu¨ckel modelling strategy to test the success of retro-Brook O-
to C-silyl transfer relevant to aryne precursor synthesis. A novel
application (13z) and an expected failure (15) were validated
experimentally. Next, a route towards 1,4-bisxylyne precursors
12a/b was developed with a retro-Brook step for just one
C-silylation. This is the first C,C0-disubstituted o-silylaryl sulfo-
nate bisbenzyne precursor: with appropriate substituents,
many previously elusive bisaryne precursors may now be
accessible.
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the National Mass Spectrometry Facility at Swansea University.
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