Rhodium catalysed chemo- and stereoselective arylative and alkenylative cyclisation reactions of unsymmetric diynes containing a terminal alkyne moiety with organoboronic acids

Article abstract of DOI:10.1039/b926553h

Unsymmetric diynes possessing a terminal alkyne moiety reacted with organoboronic acids both chemo- and stereoselectively to afford arylated or alkenylated exocyclic dienes by catalysis from the [Rh(cod)OCH₃] ₂ complex. The use of a polar protic solvent, e.g. CH₃OH is required for the success of the process under mild conditions.

Full text of DOI:10.1039/b926553h

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Rhodium catalysed chemo- and stereoselective arylative and alkenylative
cyclisation reactions of unsymmetric diynes containing a terminal alkyne
moiety with organoboronic acids†‡
a
a
a
a
b
¨
¨
Levent Artok,* Melih Kus¸, Bag˘dagu¨l N. Urer, Gu¨ls¸ah Tu¨rkmen and Ozge Aksın-Artok
Received 22nd December 2009, Accepted 11th February 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b926553h
Unsymmetric diynes possessing a terminal alkyne moiety reacted with organoboronic acids both
chemo- and stereoselectively to afford arylated or alkenylated exocyclic dienes by catalysis from the
[Rh(cod)OCH3]2 complex. The use of a polar protic solvent, e.g. CH3OH is required for the success of
the process under mild conditions.
moiety is tethered to another unsaturated functionality positioned
in 1,6- or 1,7-arrangements.
Introduction
In 2001, Hayashi et al. reported the first Rh-catalysed cis-1,2-
addition of arylboronic acids to internal alkynes.1 Since then,
numerous studies have been performed on rhodium catalysed
cascade reactions of alkynes bearing an electrophilic moiety
at an appropriate position with organoborons as a strategy
for constructing arylative cyclised products, offering sequential
formation of multiple C–C bonds in one pot.2,3
The reaction is generally believed to be initiated via the
regioselective 1,2-addition of organorhodium species, generated
through transmetalation between organoboron and rhodium(I),4
across a carbon–carbon multiple bond. This addition is facilitated
through the coordination of the rhodium species across two
electrophilic reactive parts of the substrate and hence, requires
milder conditions compared to that of simple alkynes.3g Then, the
resulting organorhodium species adds to the other intramolecular
group, thus constructing a cyclic product. Interestingly, however,
the process has not yet been explored for acceptors with two or
more electrophilic functionalities possessing a terminal alkyne
site.5 This may be due either to the high propensity of terminal
alkynes towards oligomerisation reactions or to their lower
reactivity.6
There is only one example in the literature that demonstrated the
results of the rhodium catalysed reactions of 1,5-enynes possessing
a terminal alkyne moiety with organoboronic acids, in which the
intermolecular carborhodation process proceeded in a geminal
fashion through the formation of rhodium vinylidene species
to provide 1-substituted cyclopentene products.7 However, we
have determined in this study that a chemo- and stereoselective
arylation or alkenylation of a terminal alkyne is possible without
the involvement of vinylidene species provided that the alkynyl
Results and discussion
Unsymmetric diynes with a terminal alkyne moiety were the
substrates mainly employed in this study to verify the relative
preference of the carborhodation process between internal and
terminal alkyne sites. We initially attempted a reaction with an
unsymmetric 1,6-diyne 1a and phenylboronic acid 2a mixture
(1 : 1.2) in the presence of [Rh(cod)OH]2 (3% Rh, cod = cycloocta-
1,5-diene) in a dioxane–H2O (40 : 1) mixture at room temperature,
which was derived from the method employed for the arylative
cyclisation of internal 1,6-diynes by Murakami et al. (at a Rh
concentration of 6% and with an arylboronic acid to diyne ratio
of 3 : 1).3o Surprisingly, these experimental conditions failed to
produce any product and the starting material was recovered
(Table 1, entry 1). Diyne 1a was also completely unreactive
under the conditions that generated [Rh]–OH species in situ
from the [Rh(cod)Cl]2 complex in tetrahydrofuran (THF), 1,2-
dimethoxyethane (DME), and dioxane solvents (Table 1, entries
2–4).8 Nevertheless, the increase in the amount of added water
provided the formation of an arylative exocylic conjugated diene
product 3aa, albeit at a low yield (Table 1, entry 5); the application
of more forcible reaction conditions (60 ◦C and with 6% Rh)
was also unsuccessful, resulting in a complex mixture (Table 1,
entry 6).
Gratifyingly, 1a and 2a were effectively coupled in a chemo-
and stereoselective manner with complete conversion within just
1 h, in the presence of the [Rh(cod)OCH3]2 complex (3% Rh),
and in a CH3OH–H2O (40 : 1) solvent system, thus providing 3aa
in an isolated yield of 73% (Table 1, entry 7). The aryl group
was incorporated exclusively into the terminal alkyne site and the
configurations of the exocyclic double bonds were assigned by an
NOE study.9,10
Also similar results were obtained in a dry CH3OH solvent and
under conditions when [Rh]–OCH3 was likewise generated in situ
by the use of the [Rh(cod)Cl]2–KOH combination in a CH3OH–
H2O solvent system (Table 1, entries 8 and 9).11 The beneficial effect
of water became noticeable only when using a lower concentration
of the [Rh(cod)Cl]2 (0.8% Rh) complex, though lower yields were
aDepartment of Chemistry, Faculty of Science, Izmir Institute of Technology,
Urla, 35430, Izmir, Turkey. E-mail: leventartok@iyte.edu.tr; Fax: (+)90-
232-7507509
bOrganic Chemistry II, Dortmund University of Technology, Otto-Hahn-
Strasse 6, D-44227, Dortmund, Germany
† Dedicated to Prof. Masakatsu Nomura on the occasion of his 70th
birthday
‡ Electronic supplementary information (ESI) available: Compound char-
acterization data, and 1H and 13C NMR spectra of compounds. See
DOI: 10.1039/b926553h
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Table 1 Rhodium-catalysed reaction of diethyl 2-(but-2-ynyl)-2-(prop-2-ynyl)malonate (1a) with phenylboronic acid (2a): optimisation study
Entry
Rh complex
Solvent
Additive (0.1 mmol)
Conversion (%)a
Yield (%)a
1
2
3
[Rh(cod)OH]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)OH]2
[Rh(cod)Cl]2
[Rh(cod)OCH3]2
[Rh(cod)OCH3]2
[Rh(cod)Cl]2
[Rh(cod)OCH3]2
[Rh(cod)OCH3]2
[Rh(cod)OCH3]2
[Rh(cod)OCH3]2
[Rh(nbd)Cl]2
Dioxane
THF
DME
—
0
0
0
0
0
0
0
0
KOH
KOH
KOH
—
KOH
—
—
KOH
—
—
—
—
KOH
—
4
Dioxane
Dioxane
Dioxane
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
Dioxane
5b
46
100
100
100
100
36
78
58
100
77
47
100
100
46
15
20
77 (73)
73
73
31
49
15
(80)
37
16
51
73
15
6c
7d
8e
9
10e,f
11f
12g
13h
14
15
16
17e
18b
[Rh(cod)Cl]2
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OCH3]2
—
—
—
a Calculated by 1H NMR relative to an internal standard (1,3,5-trimethoxybenzene). Isolated yields are given in parentheses. b Contained 0.2 mL of
water. Reaction time is 17 h. c Contained 6◦% of Rh and 0.2 mL of water and performed at 60 ◦C for 4 h. d Reaction time is 1 h. e Without added water.
f Contained 0.8% of Rh. g Performed at 10 C for 14 h. h Contained 0.6 mmol of PhB(OH)2.
obtained in both cases (Table 1, entries 10 and 11). Lowering the
reaction temperature to 10 ◦C greatly reduced the activity of the
[Rh(cod)OCH3]2–CH3OH system (Table 1, entry 12). Although a
somewhat higher yield for 3aa (80%) was possible with the use
of three equivalents of 2a (Table 1, entry 13), a preferred ratio of
1 : 1.2 (1 : 2) was chosen for the rest of the study in order to limit
the use of the expensive arylboronic acid reagent.
Interestingly, the analogous [Rh(nbd)Cl]2 complex (nbd =
norbornadiene) proved less effective in basic CH3OH solution
(Table 1, entry 14), indicating that cod is a better ligand partner
than nbd for the catalytic activity of the rhodium species in this
reaction.
To better comprehend whether the superior activity
of the [Rh(cod)OCH3]2–CH3OH system compared to the
[Rh(cod)OH]2–dioxane system is due to the different anionic
ligand partner type on the rhodium complex or to the effect of
the solvent type, several arylative cyclisation experiments were also
carried out in methanol with [Rh(cod)Cl]2 and [Rh(cod)OH]2 com-
plexes, and in dioxane with [Rh(cod)OCH3]2. The [Rh(cod)Cl]2
complex displayed only slight activity in the CH3OH–H2O solvent
in the absence of a base, producing 3aa in 16% yield (Table 1,
entry 15). On the other hand, conversion of diyne 1a was
complete in the presence of the [Rh(cod)OH]2 complex under
identical conditions, giving rise to 3aa in 51 and 73% yields in
the moist and dry methanol solvents, respectively (Table 1, entries
16 and 17), contrasting with the performance of the complex
[Rh(cod)OCH3]2 in these solvent systems. However, the fact that
the [Rh(cod)OCH3]2 complex also showed low activity in a dioxane
solvent indicates that the reaction can proceed effectively only in
a polar protic solvent, such as CH3OH (Table 1, entry 18).
Fig. 1 shows the 1H NMR spectra of CDCl3 solutions of
the rhodium complexes [Rh(cod)Cl]2 and [Rh(cod)OH]2 (ª0.031
M) observed in the presence of ª0.31 M concentration of
CH3OH. The [Rh(cod)Cl]2 complex seemed stable during neutral
CH3OH treatment as no structural changes could be assigned
1
by the H NMR study. Nevertheless, the presence of a signal at
2.67 ppm, which is assigned to the rhodium attached methoxo
ligand,11 within the 1H NMR spectrum of [Rh(cod)OH]2–CH3OH
combination indicates that a partial conversion of [Rh(cod)OH]2
to [Rh(cod)OCH3]2 complex can take place in the presence of
CH3OH even in the absence of a base.
The rhodium complex [Rh(cod)OCH3]2 was treated in a
dioxane–H2O (40 : 1) mixture under an Ar atmosphere in the
absence of any other reagent. The NMR analyses (in CDCl3) of
the rhodium sample recovered from the dioxane–H2O solution
by evaporation demonstrated the presence of the [Rh(cod)OH]2
complex only. These results suggest that the effective exchange of
anionic ligands with the alkoxo ligand is needed to render higher
activity to the rhodium complex, while the possible complexation
of the CH3OH solvent with a metal centre and/or a strong
hydrogen bond formation between solvent and –OCH3 and –OH
ligands, as shown previously, may also affect catalytic activity of
rhodium at various stages of the reaction cycle.6a,12
The [Rh(cod)OCH3]2–CH3OH combination also proved rela-
tively more amenable to the arylative cyclisation of a methyl
substituted internal symmetric diyne 4; the reaction of 4 with
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Table 2 Rhodium catalysed reaction of diethyl 2,2-di(but-2-ynyl)malonate (4) with phenylboronic acid (2a)
Entry
Rh complex
Solvent
Time/h
Conversion (%)a
Yield (%)a
1
2
[Rh(cod)OCH3]2
[Rh(cod)OH]2
CH3OH
Dioxane
0.5
5
100
82
96 (75)
74
a Calculated by GC. Isolated yields are given in parentheses.
substrate 4 was incomplete when the reaction was performed in
dioxane in the presence of the [Rh(cod)OH]2 complex (Table 2,
entry 2). However, it would also be interesting to note that the
conversion of substrate 4 was less than 10% when reacted with 1.2
equivalents of 2a in dioxane, in the presence of [Rh(cod)OH]2, and
at room temperature.
The optimal method that is established for the arylative
cyclisation of diynes in this study, which made use of the complex
[Rh(cod)OCH3]2 (3% Rh) and CH3OH–H2O solvent system, also
operated well with an enyne reagent (6) which possesses an
unsubstituted alkyne functionality, affording the cyclative product
7a in an isolated yield of 80% (Table 3, entry 1). In contrast, the use
of the [Rh(cod)OH]2–dioxane–H2O system was highly detrimental
for the conversion of 6 under analogous reaction conditions
(Table 3, entry 2). Nevertheless, following Hayashi’s experimental
conditions, which involved the use of a higher Rh concentration
(6%) and a higher temperature (60 ◦C) while generating [Rh]–OH
in situ, led to the formation of 7a in 69% yield.3e
Having determined the effective conditions, an array of substi-
tuted boronic acids was then subjected to the reactions with diyne
1a. The reaction tolerated both electron withdrawing and donating
groups on the m- and p-positions (2b–g), and o-tolylboronic acid
(2h), giving rise to good yields of the corresponding products 3ab–
ah (61–74%, Table 4, entries 1–7). An electron-poor disubstituted
phenylboronic acid 2i, a heterocyclic boronic acid 2j, and an
alkenylboronic acid 2k were all suitable components for the
reaction, yielding related cyclised products 3ai–ak in the range
of 68–74% yield (Table 4, entries 8–10).
The scope of unsymmetric diyne substrates was also surveyed
with different tether types and substituent groups on one of the
alkyne termini. The unsymmetric 1,6-diynes having a malonate-
based tether with –C2H5 and –Si(CH3)3 substituents at one of the
alkynyl groups (1b,c), and primary and tertiary 1,6-diynols (1d,e)
(Table 5, entries 1–4) were all applicable substrates that converted
in a chemo- and stereoselective manner to the corresponding
1,2-dialkylidenecyclopentane products in good yields. It must
be noted that in a previous report, rhodium-catalysed arylative
cyclisation reactions of internal unsymmetrical 1,6-diynes with
CH3/CH2OR substituents worked with opposite chemoselectiv-
ity and involved an arylation step primarily on the CH2OR-
substituted site, which is probably directed by the coordination
of the Rh species with the propargylic oxygen atom.3o In contrast,
rhodoarylation is under selective steric control for the reactions of
Fig. 1 1H NMR spectra of [Rh(cod)Cl]2 (a) and [Rh(cod)OH]2 (b)
observed in the presence of methanol in CDCl3 (Rh complex = 0.031 M,
CH3OH = 0.31 M).
three equivalents of 2a proceeded to completion at 30 ◦C in 30 min
to afford a high yield of the corresponding product 5a with this
combination (Table 2, entry 1), whereas the conversion of the
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Table 3 Rhodium catalysed reaction of (E)-4,4-diethyl 1-methyl hept-1-en-6-yne-1,4,4-tricarboxylate (6) with phenylboronic acid (2a)
Entry
Rh complex
Solvent
Time/h
Conversion (%)a
Yield (%)a
1
[Rh(cod)OCH3]2
[Rh(cod)OH]2
[Rh(cod)Cl]2–KOH (0.3 eq)
CH3OH
Dioxane
Dioxane
2
15
5
100
27
100
(80)
10
69
2
3b
1
a Calculated by H NMR relative to an internal standard (1,3,5-trimethoxybenzene). Isolated yields are given in parentheses. b Contained 6% Rh and
0.2 mL of water and performed at 60 ◦C.
diynes 1d and 1e, resulting in the arylation at the terminal alkyne
site.
A modest yield was obtained from the reaction of a symmetric
terminal 1,6-diyne (Table 5, entry 5). Both oxygen 1g and
sulfonamide 1h tethered diynes can also be used as substrates
(Table 5, entries 6 and 7).
The reaction of 1,7-diyne also proceeded with high chemoselec-
tivity yielding a 6-membered cyclisation product 3ia together with
a small amount of its stereoisomer with an unassigned structure
in the ratio of 15 : 1 in an overall yield of 36% (Table 5, entry
8). Unfortunately, diynes 1j and 1k, substituted with phenyl and
ester groups, respectively, produced complex mixtures of various
arylative cyclised products.
reaction of a 1 : 1 mixture of 9 and 10 containing three equivalents
of 2a in the presence of 3% rhodium yielded the same result as their
corresponding individual reactions. In this last case the alkyne 9
was consumed to give a complex mixture and 10 was recovered
unreacted. These results imply that rhodium catalysed arylation
of a terminal alkyne has to be facilitated through the interaction
of another intramolecular functional group with the metal centre
under the established conditions.
Conclusion
We have shown in this article that unsymmetric diynes with
a terminal alkyne functionality can undergo Rh(I)-catalyzed
arylative cyclisation in both a chemo- and stereoselective manner
with organoborons to yield exocyclic diene products. The arylation
took place selectively at the terminal alkyne site. The reaction
worked effectively with the [Rh(cod)OCH3]2 complex in a CH3OH
solvent system at room temperature, while the [Rh(cod)OH]2–
dioxane system, which was reported to be a sufficient combination
for analogous reactions with internal malonate-tethered diynes,3o
was observed to be ineffective with diyne 1.
Vinylidene formation, which is a common intermediate in
various rhodium catalysis of terminal alkynes,7,13 is unlikely in
our case when considering the product profile. Likewise, mindful
that the reaction of diyne 1a with a phenylboronate ester 8a in
dry CD3OD produced 3aa-d1 with introduction of deuterium only
at the vinylic position, which suggests the following mechanisms:
coordination of organorhodium to the unsaturated carbon-carbon
bonds (I) triggers vicinal addition across the unhindered terminal
alkyne (II), followed by subsequent intra-carborhodation onto the
next unsaturated carbon-carbon bond providing a vinyl rhodium
species (IV) (Scheme 1). Alternatively, the intermediate IV may
arise via oxidative cyclisation of PhRh(I) with the diyne 1a to
afford a rhodacyclopentadiene intermediate (III) and following
reductive elimination step.2a,14 Protonolysis with methanol at the
last step produces product 3 and regenerates the catalytically active
Rh(I) species.
Experimental
General
The products were analysed by GC and GC-MS (Varian Star
3400CX/Saturn 2000 or HP 6890/5973N) and isolated by column
chromatography. High-resolution mass spectral analyses were
performed at the Dortmund University of Technology Mass
Spectrometry Laboratory on a Thermo Electron system. NMR
spectra were recorded on a Varian VnmrJ 400 spectrometer or a
Bruker DRX 400 spectrometer. Infrared spectra were obtained
using Perkin–Elmer Spectrum 100 by ATR method with neat
samples. Methanol was dried over Mg turnings and stored
on molecular sieve 3A under Ar. THF, DME, and dioxane
were distilled from benzophenone-ketyl under argon prior to
use.
An analogous reaction of monoalkyne 9 with a terminal alkyne
moiety failed, however, to donate any adduct product but instead
led to its full consumption giving rise to a complex mixture,6 while
the corresponding internal alkyne 10 was entirely unreactive under
these conditions and unchanged (Scheme 2). Comparatively, the
Diynes 1a,15,16 1b,15,16 1c,15,16 1e,17 1f,16 1g,18 and 1k,17 enyne 6,3e
monoalkynes 9 and 10,3e and rhodium complexes [Rh(cod)Cl]2,19
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Table 4 Rhodium catalysed reaction of diethyl 2-(but-2-ynyl)-2-(prop-2-ynyl)malonate (1a) with various organoboronic acids (2)a
Entry
1
Boronic Acid
Time/h
1
Product
Isolated Yield (%)
68
2
3
4
5
6
7
1
72
61
72
70
66
74
2
2
2
2
4.5
8
1
74
9
2
2
68
80
10
a 1a (0.2 mmol), 2 (0.24 mmol), [Rh(cod)OCH3]2 (3% Rh), CH3OH (2 mL), H2O (50 mL), RT.
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Table 5 Rhodium-catalysed reaction of diynes having a terminal alkyne terminus (1) with phenylboronic acid (2a)a
Entry
1
Diyne
Time/h
4
Product
Isolated Yield (%)
68
2
3
2
2
61
44
4
2
62
5
6
2
2
44
62
7
8
2
2
56
36
a 1 (0.2 mmol), 2a (0.24 mmol), [Rh(cod)OCH3]2 (3% Rh), CH3OH (2 mL), H2O (50 mL), RT.
11
11
[Rh(cod)OH]2 and [Rh(cod)OCH3]2 were synthesised based
on the literature. Diyne 1h was synthesised via deprotonation
of a terminal alkyl group of N-(prop-2-ynyl)-N-tosylprop-2-
yn-1-amine20 with BuLi and following treatment with CH3I in
THF. Diyne 1i was synthesised via sequential alkylation of diethyl
malonate with 4-bromo-1-butyne,21 and propargyl bromide.15,16
Diyne 1d was synthesised by the alkylation of diethyl propargyl-
malonate with 2-(4-chloro-but-2-ynyloxy)-tetrahydropyran and
following deprotection of the hydroxyl group.17,22 Diyne 1j was
synthesised via phenylation of diethyl propargylmalonate by the
Sonagashira coupling method followed by alkylation with 4-
bromo-1-butyne.21,23
General procedure for the reaction of 1,6-diynes with
organoboronic acids
To a Schlenk tube was added diyne (0.2 mmol), boronic acid
(0.24 mmol), [Rh(cod)OMe]2 (3% Rh), methanol (2 mL), and wa-
ter (50 mL). The reaction mixture was stirred at room temperature
under Ar until the substrate disappeared as monitored by TLC or
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Scheme 1 The arylative cyclisation reaction of 1,6-diyne 1a with phenylboronic acid ester 8a in CD3OD and proposed mechanisms.
Scheme 2 Rhodium-catalysed reaction of diethyl 2-methyl-2-(prop-2-ynyl)malonate (9) and diethyl 2-(but-2-ynyl)-2-methylmalonate (10) with 2a.
GC. The solvent was evaporated and the residue was purified by
flash chromatography (hexane–EtOAc) affording the product. All
the products appeared colourless or as light-yellow oil.
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Acknowledgements
The support of The National Boron Research Institute (BOREN-
2006-14-C, 13-09), and IZTECH (2007-IYTE-14) to this work
and the postdoctoral support of The Scientific and Technological
Research Council of Turkey (TUBITAK) to G. Tu¨rkmen is
gratefully acknowledged. We thank Ms. N. Meszaros of Dortmund
¨
University of Technology for HRMS analyses, Mrs. I. Ozc¸elik for
the measurement of NMR spectra, Dr R. Eanes for proofreading.
Notes and references
4 T. Hayashi, M. Takahashi, Y. Takaya and M. Ogasawara, J. Am. Chem.
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5 During the preparation of this manuscript, a Rh-catalysed reaction of
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allenol structure has been reported. The rhodoarylation process seems
to be facilitated by coordination of the rhodium species with the oxygen
functionality of the alkyne substrate: T. Miura, M. Shimada, P. de
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2 Related reviews: (a) T. Miura and M. Murakami, Chem. Commun.,
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9
.
10 The 1,2-dialkylidenecycloalkanes synthesised in this study are not
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