Facile access to boryltetralins and borylnaphthalenes via a cycloaddition using o-quinodimethanes

Article abstract of DOI:10.1039/c002949a

Borylalkenes are found to serve as efficient dienophiles in a cycloaddition reaction with o-quinodimethanes, giving diverse boryltetralins, which are convertible into borylnaphthalenes via an oxidative aromatization.

Full text of DOI:10.1039/c002949a

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Facile access to boryltetralins and borylnaphthalenes via a cycloaddition
using o-quinodimethanesw
Hiroto Yoshida,* Masashi Mukae and Joji Ohshita
Received 11th February 2010, Accepted 28th May 2010
First published as an Advance Article on the web 25th June 2010
DOI: 10.1039/c002949a
Borylalkenes are found to serve as efficient dienophiles in a
cycloaddition reaction with o-quinodimethanes, giving diverse
boryltetralins, which are convertible into borylnaphthalenes via
an oxidative aromatization.
Table 1 [4+2] Cycloaddition between o-QDM and borylalkenesa
Organoboron compounds constitute one of the most important
reagents in synthetic organic chemistry1 especially for carbon–
carbon bond forming processes through Suzuki–Miyaura
coupling,2 Petasis reaction,3 transition metal-catalysed
1,2- and 1,4-additions,4 etc. Whilst organoboron compounds
of multifarious structures are accessible by a reaction of
Grignard reagents (or organolithiums) with boron electro-
philes, catalytic borylation of C–X (X: halogen or pseudo-
halogen)5 or C–H bonds,6 and cycloaddition of borylalkynes,7
the search for new synthetic methods which provide access to
hitherto unprecedented classes of organoboron compounds is
still of great significance.8
Entry
R
Time/h
Yield (%)b
3
1
2
3
4
5
6
7
8
9
10
Ph (2a)
1-Naphthyl (2b)
4-PhC6H4 (2c)
4-n-BuC6H4 (2d)
2,5-(MeO)2C6H3 (2e)
4-CF3C6H4 (2f)
4-BrC6H4 (2g)
4-(HCRC)C6H4 (2h)
4-(n-HexCRC)C6H4 (2i)
2-Thienyl (2j)
13
9
12
12.5
8
6.5
9
9
87
81
75
65
80
78
82
81
79
49
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
12
11
3aj
Owing to salient reactivity at an exo-diene moiety,
o-quinodimethanes (o-QDMs) have been utilized as a potent
four-carbon unit in [4+2] cycloaddition reaction (Diels–Alder
reaction).9 Although the cycloaddition enables diverse tetralin
derivatives to be synthesized in a straightforward manner, the
potential versatility of the reaction remains to be exploited,
because most of the dienophiles so far employed have been
those having carbonyl or cyano substituents as electron-
withdrawing groups. Herein we disclose that introduction of
a boryl moiety into alkenes enhances the dienophilicity of the
alkenes,10 leading to facile formation of boryltetralins of
structural diversity via the [4+2] cycloaddition with o-QDMs.
Furthermore, aromatization of the resulting tetralins provides
a convenient approach to borylnaphthalenes (Scheme 1).11
We first conducted the reaction of simple o-QDM, generated
in situ from 2-[(trimethylsilyl)methyl]benzyl phenyl carbonate
(1a) and a fluoride ion,12,13 with (E)-b-borylstyrene (2a) in 1,4-
dioxane at 100 1C, and observed that the cycloaddition
product, 3-phenyl-2-boryltetralin (3aa), was formed in 87%
yield (entry 1, Table 1). The configuration between phenyl and
11
10.5
54
3ak
(2k)
n-Hexyl (2l)
Cyclopropyl (2m)
12
13
11
13
32
19
3al
3am
a
The reaction was carried out in dioxane (1 mL) at 100 1C using 1a
(0.2 mmol), KF (0.33 mmol) and 18-crown-6
(0.3 mmol),
(0.33 mmol). Isolated yield based on 2.
2
b
boryl substituents in 3aa has proven to be anti, which verifies
that the (E)-stereochemistry of the dienophile is retained
throughout the reaction. In addition, borylalkenes bearing
naphthalene (2b) or biphenyl (2c) moieties could also participate
in the reaction, giving the respective cycloadducts (3ab or 3ac)
in high yields (entries 2 and 3). The reaction of electron-rich or
electron-deficient borylstyrenes (2d–2f) took place smoothly,
irrespective of their electronic characters (entries 4–6), and
furthermore, a carbon–bromine bond in the dienophile (2g)
remained intact through the reaction (entry 7). Dienophilicity
of a borylalkene moiety was far superior to that of an ethynyl
moiety, and thus, the products (3ah or 3ai) were formed solely
in the reaction of 2h or 2i (entries 8 and 9). A heteroaromatic
thienyl group in 2j and an acetal moiety in 2k were compatible
with the cycloaddition (entries 10 and 11), whereas the reaction
of aliphatic borylalkenes (2l or 2m) became sluggish (entries 12
and 13). The reactivity of 2a toward o-QDM has proven to be
comparable to that of methyl cinnamate, whereas the reaction
of b-bromostyrene did not afford the cycloadduct despite the
inductive electron-withdrawing property of the bromine atom,
demonstrating that the boryl substituent actually increases the
Scheme 1 Synthesis of boryltetralins and borylnaphthalenes.
Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, Higashi-Hiroshima 739-8527, Japan.
E-mail: yhiroto@hiroshima-u.ac.jp; Fax: +81-82-424-5494;
Tel: +81-82-424-7724
w Electronic supplementary information (ESI) available: Experimental
procedure including spectroscopic and analytical data. See DOI:
10.1039/c002949a
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Scheme 2 Comparison of dienophilicity in the reaction with o-QDM.
dienophilicity of the carbon–carbon double bond attached
with its vacant p-orbital (Scheme 2).
The boryltetralins were found to be convertible into
borylnaphthalenes 4 via an oxidative aromatization by use of
NBS/MeONa/AIBN (Method A) or DDQ (Method B)
(Scheme 3). For example, 2-boryl-3-phenylnaphthalene (4aa)
was synthesised from 3aa in 89% yield according to Method
A. In addition, a variety of substituted borylnaphthalenes,
which would be hardly accessible by conventional methods,
could be straightforwardly prepared through the cycloaddition/
aromatization sequence.
We finally investigated the cycloaddition of substituted
o-QDMs (Scheme 4). The reaction of a-phenyl-o-QDM (from
1b) with 2a occurred with high regioselectivity to give 3ba as
the major product (93% selectivity among four possible
isomers), in which two phenyl groups are adjoining in cis
configuration. The observed regio- and stereoselectivities can
be rationally explained by the following two factors: exo
approach of the bulky B(pin) moiety which orients the
a-phenyl group to a remote position, and a secondary orbital
Scheme 4 Reaction using substituted o-QDMs.
interaction between C2 of the o-QDM and the phenyl group of
2a (Fig. 1).14 a-(1-Naphthyl)-o-QDM (from 1c) reacted with
2a in a similar manner to afford 3ca selectively, whereas the
reaction of o-QDM having a biphenyl (from 1d) or a naphthalene
framework (from 1e) furnished a mixture of two regioisomers,
which shows that a substituent on an aromatic backbone of
o-QDM exerted a little effect on the regioselectivity. The same
Scheme
3 Synthesis of borylnaphthalenes via an oxidative
aromatization.
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Notes and references
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Scheme 5 Reaction using a catechol ester.
regio- and stereoselectivities as those with 2a were observed in
the reaction using a catechol ester of (E)-styrylboronic acid
(2n), albeit at the cost of a decreased yield of the cycloadduct
(Scheme 5). As depicted in eqn (1), the reaction of borylethene
2o with a-phenyl-o-QDM proceeded with lower regio- and
stereoselectivities as compared with those in the reaction of 2a,
which implies that the secondary orbital interaction (Fig. 1)
considerably affects the regio- and stereochemical outcome in
the present cycloaddition.14 The multisubstituted boryltetralins
thus obtained from substituted o-QDMs could be readily
aromatized to produce highly p-assembled borylnaphthalenes
according to the above procedure.
8 We have recently reported on the platinum-catalysed diboration of
arynes, which gives 1,2-diborylarenes: H. Yoshida, K. Okada,
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ð1Þ
11 For recent examples of new synthetic transformations using
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and A. Kunai, Org. Lett., 2006, 8, 4157; (b) R. Kuwano and T. Shige,
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In conclusion, we have disclosed that the [4+2] cycloaddition
between o-QDMs and borylalkenes offered facile access to
boryltetralins of structural diversity, which were further
transformable into variously substituted borylnaphthalenes
via oxidative aromatization. Moreover, a high level of
regio- and stereoselectivities was observed in the cycloaddition
of a-arylated o-QDMs, which can be attributed to the steric
bulkiness of the B(pin) moiety and the secondary orbital
interaction. Further studies on synthetic applications of the
cycloaddition/aromatization sequence to other substrates are
in progress.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 5253–5255 | 5255