A Study of Polarization and Directing Effects of Unsymmetrical Alkynes Using Regioselective Pd-Catalyzed Bromoallylation

Article abstract of DOI:10.1002/ejoc.201901476

Herein is disclosed the first comprehensive study of factors that affect the regioselectivity of PdBr₂(PhCN)₂-catalyzed bromoallylation of unsymmetrically substituted internal alkynes. The study was performed on a wide array of electronically and structurally diverse alkynes with aryl-aryl, aryl-ferrocenyl, and aryl-alkyl substitutions. The regioselective formation of bromoallylation products was mostly driven by the polarization of the triple bond in aryl-aryl and aryl-ferrocenyl-substituted ethynes. On the other hand, directing effect, which arises from the presence of a directing group in the side-chain of aryl-alkyl-substituted alkynes, was the dominating factor that determined the regioselectivity of these reactions.

Full text of DOI:10.1002/ejoc.201901476

A Journal of
Accepted Article
Title: A Study of Polarization and Directing Effects of Unsymmetrical
Alkynes Using Regioselective Pd-catalyzed Bromoallylation
Authors: Nikola Topolovčan, Shuto Hara, Ivana Císařová, Zdeněk
Tošner, and Martin Kotora
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901476
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901476
Supported by

10.1002/ejoc.201901476
European Journal of Organic Chemistry
A Study of Polarization and Directing Effects of Unsymmetrical Alkynes Using
Regioselective Pd-catalyzed Bromoallylation
Nikola Topolovčan,[a] Shuto Hara,[a,b] Ivana Císařová,[c] Zdeněk Tošner[d] and Martin
Kotora*[a]
[a]
Dr. N. Topolovčan, S. Hara, Prof. M. Kotora
Department of Organic Chemistry, Faculty of Science
Charles University
Hlavova 8, 128 43 Praha 2, Czech Republic
E-mail: kotora@natur.cuni.cz
Homepage: http://orgchem.natur.cuni.cz/_kotora
S. Hara
[b]
Visiting student from: Okayama University,
3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan. Current address: NOF
Corporation, Yebisu garden place tower, 20-3, Ebisu 4-chome, Shibuya-ku, Tokyo,
150-6019, Japan
[b]
[d]
Dr. I. Císařová
Department of Inorganic Chemistry, Faculty of Science
Charles University
Hlavova 8, 128 43 Praha 2, Czech Republic
Dr. Z. Tošner
NMR Laboratory, Faculty of Science
Charles University
Hlavova 8, 128 43 Praha 2, Czech Republic
Supporting information for this article is given via a link at the end of the document.
1
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10.1002/ejoc.201901476
European Journal of Organic Chemistry
Abstract: Herein is disclosed the first comprehensive study of factors that affect the
regioselectivity of PdBr2(PhCN)2―catalyzed bromoallylation of unsymmetrically substituted
internal alkynes. The study was performed on a wide array of electronically and structurally
diverse alkynes with aryl-aryl, aryl-ferrocenyl, and aryl-alkyl substitutions. The regioselective
formation of bromoallylation products was mostly driven by the polarization of the triple bond
in aryl-aryl and aryl-ferrocenyl substituted ethynes. On the other hand, directing effect, which
arises from the presence of a directing group in the side-chain of aryl-alkyl substituted alkynes,
was the dominating factor that determined the regioselectivity of these reactions.
Introduction
Regioselective functionalization of multiple bonds such as C-C double and triple bonds is
influenced by several different factors, including steric, electronic, and directing effects caused
by adjacent substituents. Unfortunately, in many cases it is challenging, if not impossible to
determine their respective contributions. The polarization of the triple bond has been proposed
to be the main factor affecting preferential formation of one regioisomer in many cases of triple
bond functionalizations (vide infra);[1] however, the majority of these reports lack any detailed
discussion. On the other hand, para-substituted diarylethynes constitute a class of compounds
where steric factors are negligible (or similar) allowing electronic effects to be investigated. In
2005, an elegant study by Gevorgyan et al. on the deduction of electronic polarization of a triple
[2,3]
bond using 13C NMR
showed that regioselectivity of Pd-catalyzed hydrostannation of
variously substituted diarylethynes is driven by the polarization of the triple bond. The
effect of the polarization of the triple bond on regioselective functionalization of internal
alkynes was also studied by Helaja et al. in a Pauson-Khand reaction[4] and by Alami et
al. in Pd-catalyzed hydrostannation of Z
-enynols.[5] A slightly different approach was taken
by Bennett et al. who used electronic factors based on frontier orbitals to rationalize the
observed regioselectivities in the insertion of unsymmetrical alkynes into Ni-C bonds.[6]
Electronic control of regioselectivity was proposed for Rh- and Pd-catalyzed hydroarylations
of propargyl amines.[7] In general, these precedents indicate that the polarization of the triple
bond drives the observed regioselectivity. On the other hand, primary control of regioselective
functionalization of the triple bond by steric effects was proposed as well. Representative
examples include the work of Larock and Cacchi et al. on carbopalladations of unsymmetrically
substituted diynes.[8]
PdCl2-Catalyzed haloallylation of internal alkynes is a reaction of significant synthetic
interest and importance and represents a facile approach towards functionalized haloalkenes. In
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a series of publications, Kaneda et al. demonstrated that a catalytic amount of PdX2 is effective
in bringing about a stereoselective haloallylation of a variety of terminal and internal alkynes
with allyl halides, resulting in the exclusive formation of cis-1-halo-1,4-dienes.[9] The presence
of the carbon-halogen bond in the formed products allows for further transformations as it was
well exemplified in the Stille[10] and Suzuki[11] cross-couplings, Wacker-Tsuji oxidation,
Sonogashira reaction,[12] and in the Ni-promoted formation of cyclopentenones.[13]
These findings encouraged further expansion of the methodology, which was
successfully applied in the haloallylation of aromatic ynol ethers, ynamides, alkynyl ketones,
and sulfones, resulting in the synthesis of the corresponding α-haloenol ethers,[14]
polysubstituted enamides,[15] β-halo alkenyl ketones, and sulfones,[16] respectively.
Haloallylation was also one of the key steps in syntheses of natural compounds such as (–)-
haterumalide and (–)-oocydin A.[17]
Recently, we have reported a three-step procedure for the synthesis of a variety of 1,2-
disubstituted cyclopentadienes based on Pd-catalyzed haloallylation of internal alkynes.[18]
During the course of this study, it was observed that PdBr2(PhCN)2―catalyzed bromoallylation
of various unsymmetrically substituted internal alkynes proceeded in the syn manner giving the
cis regioisomers (with respect to the relative position of the bromine atom and allyl group) with
good to modest regioselectivity. Nevertheless, the formation of the opposite cis regioisomer
was also observed in several cases. These findings sparked our interest on factors dictating the
observed regioselectivity. A generally accepted mechanism, proposed by Kaneda,[9a,d] suggests
the following: syn halopalladation, that is, the addition of halogen and palladium atoms as
components of the catalyst across the triple bond of the alkyne, is the first step,[19,20] which is
followed by migration of the allyl halide into the C―Pd bond of the formed vinylpalladium
intermediate (Scheme 1). In the last step of the catalytic cycle, β-halide elimination occurs,
which results in the formation of the cis halodiene, with simultaneous regeneration of the
catalytically active Pd species.
It should be mentioned that other haloallylation reactions have been reported as well.
For example, PdX2-catalyzed haloallylation of alkynes in the presence of allyl alcohols and
CuX2 salts in aqueous media or ionic liquids may give either cis or trans halodienes.[21] Even
though the halopalladation of the triple bond is considered the first step of the proposed reaction
mechanism, the subsequent steps differ from the one depicted in Scheme 1.
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10.1002/ejoc.201901476
European Journal of Organic Chemistry
Scheme 1. Proposed mechanism for the bromoallylation of internal alkynes.
Taking into the account the proposed catalytic cycle that leads to the predominant
formation of cis 1-halo-1,4-dienes, it is reasonable to assume that the regioselectivity of
the reaction is defined in the first step. In this respect, we hypothesized that a preferential
formation of one of the two possible cis regioisomers is affected by the polarization of
the triple bond of the unsymmetrically substituted alkynes. By allocating the charges on
the triple bond carbon atoms of unsymmetrical internal alkynes, it is easy to see that the
halide (the bromide) would attack the carbon atom possessing a lower electron density
(an electrophilic carbon atom) whereas the palladium center should form a bond with
the carbon atom with a higher electron density (a nucleophilic carbon atom) (Figure 1).
Figure 1. Influence of the triple bond polarization on the regioselectivity of the
halopalladation step.
Results and Discussion
Easy access to electronically unsymmetrical diarylethynes makes them attractive model
substrates to test the proposed hypothesis. Moreover, the electron density around sp-carbon
atoms in phenylethynes is largely affected by the nature and position of the substituent on the
aromatic ring,[22] meaning that alteration of substituents would allow smooth control over the
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direction of polarization of the triple bond. On top of that, a comparison of the relative position
13
of signals in C NMR spectra constitutes a simple method for identification of nucleophilic
and electrophilic character of the corresponding alkyne carbon atoms.[2] In this respect, herein,
we focus primarily on the para-functionalized diarylethynes in order to avoid steric influence
of ortho- and para-positioned substituents on the regioselectivity. Initially, a series of para-
substituted diarylethynes 1 was prepared to determine the polarization of the carbon atoms of
the triple bonds. Moreover, the chemical shift difference of the respective signals of the sp-
carbon atoms were calculated assuming that higher  ppm would translate into higher
regioselectivity (Table 1).
Bromoallylation of unsymetrical diarylethynes. To test these proposals, Pd-catalyzed
bromoallylation of the above mentioned diarylethynes was carried out under previously used
reaction conditions (Table 1).[18] At first, bromoallylation of 1-(4-cyanophenyl)-2-phenylethyne
1a afforded a mixture of two regioisomers, 2a and 3a, in a 2:1 ratio with a combined isolated
yield of 48% (Entry 1). On the other hand, a single regioisomer 2b was obtained upon
bromoallylation of 1-(4-methoxyphenyl)-2-(4-trifluoromethylphenyl)ethyne 1b, which possess
a "push-pull" system consisting of the trifluoromethyl and methoxy groups (Entry 2). Reaction
with 1-(4-trifluoromethylphenyl)-2-phenylethyne 1c gave two products: 2c and 3c in a 1:1 ratio
(Entry 3). Again, two regioisomers, 2d and 3d, were obtained by bromoallylation of 1-(4-
bromophenyl)-2-phenylethyne 1d in a 2.5:1 ratio and 79% isolated yield (Entry 4). On the other
hand, high regioselectivity was observed for 1-(4-methoxyphenyl)-2-phenylethyne 1e and 1-
(4-methylphenyl)-2-phenylethyne 1f bearing the electron-donating methoxy or methyl groups.
Their bromoallylation yielded regioisomers 3e and 3f in 77% and 82% isolated yields,
respectively (Entries 5 and 6). Finally, regioisomers 2g and 3g were obtained by
bromoallylation of 1-(biphen-4-yl)-2-phenylethyne 1g in a 2.5:1 ratio (Entry 7). In general,
higher regioselectivity and yields were observed with diarylethynes possessing electron-
donating groups (perhaps by a combination of resonance and inductive effects). In the cases
where the mixtures of two regioisomers were obtained in unequal ratios (Entries 1, 4, and 7),
the major regioisomer was always the one whose formation is directed by polarization of the
triple bond. Intriguingly, bromoallylation of 1-(pentadeuterophenyl)-2-phenylethyne 1h with
the Δδ of 0.06 ppm provided exclusively regioisomer 2h (Entry 8). Although it could be
assumed that the larger chemical shift difference should reflect higher regioselectivity that trend
was not observed. The obtained data summarized in Table 1 clearly indicates correlation
between the direction of polarization and regioselectivity of bromoallylation reactions, while
5
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the strength of polarization of the triple bond expressed as the “polarization effect” (Δδ)[2] does
not parallel observed regioselectivity ratios.
The case of 1-(4-methoxyphenyl)-2-(4-trifluoromethylphenyl)-ethyne 1b, which has a
large Δδ value (5.09 ppm), deserves a closer look. Since this compound has been used as a
substrate in various carbometallations under different reaction conditions, we decided to
compare our results with the published ones. The polarization product was obtained
predominantly or solely in the following cases: a) Rh-catalyzed C-C and C-O coupling of
quinoline N
-oxides with alkynes (two regioisomers in a 1.5:1 ratio);[23] b) Rh-catalyzed C-
H activations of 3-aryl-dihydrooxazoles (a single regioisomer was formed),[24] of sultams (two
regioisomers in a 2:1 ratio),[25] of 2-phenylimidazolino[1,2-a]pyridines (two regioisomers in a
2:1 ratio),[26] of 2-aminopyridines (two regioisomers in a 2.2:1 ratio),[27] and of -unsaturated
ketoximes (two regioisomers in a 7:3 ratio);[28] c) Co-catalyzed C-H activation of -
unsaturated imines proceeding via hydrometallation (two regioisomers in a 8.1:1 ratio);[29] d)
Pd-catalyzed oxidative dearomatization of naphthols with alkynes (three regioisomers in a 4:3:1
ratio),[30] e) Ni-catalyzed C-C bond cleavage (two regioisomers in a 55:45 ratio),[31] and f) Ni-
catalyzed C‐S bond cleavage (two regioisomers in a 2:1 ratio). It should be noted that additional
examples of reactions with other unsymmetrically substituted diarylethynes were previously
reported and proceeded according to the polarization of the triple bond.[32]
It should be also noted that there have been reports of reactions where the major products
were not those expected to be formed according to the polarization of the triple bond. As typical
examples may serve the Pd-catalyzed C‐H activation of ortho-aminoanilines (two regioisomers
in a 1:2.4 ratio),[33] catalytic boracupration,[34] Pd-catalyzed spirocyclization of aryl halides,[35]
Rh-catalyzed C―H activation of benzamides[36] and alkenyl sulfonamides,[37] Ir-catalyzed
C―H activation of ketimines,[38] and in Ni-catalyzed C―S bond activation of
thiocarbamates.[39]
The above mentioned data as well as our results indicate that there is a general trend
suggesting that the regioselective addition across the polarized triple bond could be
predominantly controlled by the electron distribution, resulting in the formation of polarization
products. On the other hand, taking into account the differences in reaction conditions, type of
catalyst, and mechanism between the provided examples, the observed results suggest that the
regioselectivity cannot be attributed only to the polarization in a common substrate, but rather
should be treated as a combination of distinct factors such, as the electron distribution around
the metal center, steric effects of the respective ligand environments, and the nature of the
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migrating group. Nevertheless, the recognized close correlation, in most of the cases, between
the regioselectivity and polarization of a triple bond may serve as a springboard for further
exploratory studies of this interesting and important topic.
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Table 1. Bromoallylation of alkynes 1.
Entry
1
Alkyne 1
Δδ/ppm
Products
2
3
2:3 ratio[b] Yield (%)[c]
1a
1b
1c
1d
6.07
2a
3a
2:1
48
82
36
79
2[d]
8.09
3.79
2.20
2b
2c
2d
3b >20:1[e]
3
3c
1:1
4
3d
2.5:1
8
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5
6
7
8
1e
1f
1.30
0.84
0.77
0.06
2e
2f
3e >20:1[e]
77
82
54
79
3f
>20:1[e]
2.5:1
1g
1h
2g
2h
3g
3h >20:1[e]
[a] A mixture of PdCl2(PhCN)2 and allyl bromide was initially stirred for 10 min. at 0 °C. During that period, halogen exchange between catalyst
and allyl bromide took place, which resulted in the formation of PdBr2(PhCN)2. [b] 1H NMR ratio determined by analysis of the crude mixture.
[c] Isolated yields of analytically pure compounds. [d] This result was reported in ref. 18. [e] The other regioisomer was not detected (according
to 1H NMR).
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Bromoallylation of ferrocenylarylethynes. Then our attention turned to the internal alkynes
possessing the ferrocenyl group. It was assumed that the ferrocenyl component of an internal
alkyne would have substantial influence on the direction of polarization of the triple bond, since
the ferrocenyl group possesses electron-donating ability in comparison with the phenyl
group.[40] To test this assumption, four differently substituted alkynes 4 were prepared for the
study of the bromoallylation. The presence of the ferrocenyl group steers polarization in one
direction regardless of the other substituent (Table 2). As a consequence, a higher electron
density is located around the sp-carbon atom at the -position to the ferrocenyl group, while a
partially positive charge is assigned to the carbon atom bonded to the ferrocenyl moiety. Even
though the electron-withdrawing or electron-donating properties of the substituents on the
aromatic ring do not affect the direction of polarization, large differences in sp-carbon Δδ values
in 13C NMR were observed. The presence of the electron-withdrawing trifluoromethyl and
methoxycarbonyl groups is exhibited by higher values of Δδ: 6.85 and 6.29 ppm, respectively.
On the other hand, the presence of the phenyl and para-tolyl groups significantly reduces these
values to 2.54 ppm and 1.51 ppm, respectively. With respect to the observed polarization, it can
be expected that products of bromoallylation of these alkynes would have the bromine atom
positioned at the -position to the ferrocenyl group, while the allyl moiety would be placed at
the β-position. Bromoallylation reactions, indeed, provided bromodienes 5 with the proposed
regioselectivity (Table 2). Specifically, only one regioisomer 5 was formed upon
bromoallylation of 4b and 4d, while bromoallylation of 4a and 4c yielded a mixture of
regioisomers 5a and 6a, and 5c and 6c in 2:1 ratios. The structures of 5b and 5c were confirmed
by a single crystal X-ray diffraction analysis (Figures SI-35 and SI-36). Regioisomers 5 were
obtained as major products, despite the polarization. The observed regioselectvity was
rationalized by the steric effect as a main contributing factor. It is reasonable to assume that a
larger steric hindrance resulting from an interaction between the ferrocenyl group and the large
Pd atom compared to the bromine atom might influence the direction of the halopalladation
step. A single crystal X-ray diffraction analysis of 5d unambiguously confirmed its structure
(Figure 2).
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Table 2. Bromoallylation of ferrocenylalkynes 4.
Entry
1
Alkyne 1
Δδ/ppm
Products
2
3
2:3 ratio[a] Yield (%)[b]
4a
6.85
5a
6a
2:1
48
2
3
4b
4c
4d
6.29
2.54
1.51
5b
5c
5d
6b >20:1[c]
82
36
79
6c
2:1
4
6d >20:1[c]
1
[a] H NMR ratio determined by analysis of the crude mixture. [b] Isolated yields of analytically pure compounds. [c] The other regioisomer
was not detected (according to 1H NMR).
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Figure 2. PLATON drawing of 5d. Displacement ellipsoid are drawn on 30% probability
level.
Bromoallylation of arylalkylethynes with a pendant chelating group. Controlling the
regioselectivity of the bromoallylation of internal alkynes independently of polarization of the
triple bond would allow a design of a synthetically useful approach towards regio- and
stereodefined halodienes. Undoubtedly, it would represent a valuable tool for accessing
appropriate building blocks suitable for specific transformations. In this respect, we turned our
attention to the factors that could either overcome or attenuate the effect of polarization on the
regioselectivity of bromoallylation reactions. Again, we assumed that the control over the
halopalladation step would be reflected in the overall regioselectivity of the process. One way
to direct approach of a Pd-catalyst to the triple bond is to introduce a chelating group in the side
chain of the substrate. In this respect, the influence of chelation to direct the regioselectivity of
transition-metal-catalyzed transformations has been observed before, e.g. in the regioselective
Rh- and Pd-catalyzed carbometallation of alkynyl pyridines and other heterocycles,[41] Pd-
catalyzed carbometallation of alkynes possessing various amide directing groups,[42] Pd-
catalyzed anti hydrochlorination of alkynes bearing pendant directing groups such as amides
and amines,[43] and Pd-catalyzed carbonylation.[44] Halopalladations of alkynes with pendant
heteroatom groups are also worth mentioning. [45]
The presence of a heteroatom possessing electron lone pairs could result in coordination
to the palladium atom, which would consequentially lead, after the halopalladation step, to the
vinylpalladium intermediate A (Scheme 2). Then, insertion of allyl bromide into the Pd‐C bond
followed by β-halide elimination should give the directed bromoallyltion product. On the other
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hand, if the direction of polarization would be the main factor controlling the course of the
halopalladation step, then it should provide the vinylpalladium intermediate B.
Scheme 2. Directing vs polarization effects on bromoallylation.
As far as polarization of the triple bonds in 7 is concerned, partially negative charges
based on 13C NMR spectral data were assigned to α-carbon atoms and partially positive charges
to β-carbons. To verify our hypothesis regarding the directing effect, bromoallylation of several
aryl-alkyl substituted internal alkynes was explored. Depending on the prevailing factor, two
types of products can be expected: a) directed bromoallylation through chelation products 8
with the allyl group attached to β-carbon atom and b) polarization products 9 where the bromine
atom is positioned at the β-carbon (Table 3). First, bromoallylation of the TBS-protected alkyne
7a under the standard conditions gave a mixture of regioisomers 8a and 9a in 66% isolated
combined yield and in a 2:1 ratio (Entry 1). In a similar fashion, but with higher regioselectivity,
the Bn-protected alkyne 7b provided regioisomers 8b and 9b in 53% isolated yield and in a 6:1
ratio (Entry 2). Next, bromoallylation of alkyne 7c gave rise exclusively to product 8c with the
allyl group connected to the β-carbon atom. Although 8c was isolated in a rather modest yield
of 33%, which could be attributed to isolation problems of the product from the reaction
mixture, the other possible regioisomer 9c was not detected (Entry 3). The same result, with
respect to regioselectivity, was obtained in the reaction of alkyne 7d. The reaction provided
regioisomer 8d in 66% isolated yield (Entry 4). The bromoallylation of alkyne 7e gave again a
mixture of regioisomers 8e and 9e in combined isolated yield of in 40% and in a 3:1 ratio (Entry
5).
In general, in all these examples, the bromodienes in which the allyl group is positioned
to the β-carbon atom (more electrophilic carbon atom) were formed as the sole products or as
the major regioisomers. These results imply the dominating directing effect via chelation over
the polarization of the triple bond on the regioselectivity of bromoallylation of this type of
unsymmetrical alkynes. Interestingly enough, the higher regioselectivity was observed in the
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bromoallylation of 7c and 7d compared with 7a and 7b. In the former case, the halopalladation
as the first step of the catalytic cycle would proceed via a five-membered chelating ring, while
a six-membered ring would be formed in halopalladation of 7a and 7b. The effect of the
chelating ring size in the formation of the intermediate is not clear at the moment.
Finally, to assess the influence of the directing effect via chelation, alkyne 7f bearing
the n-butyl side chain was prepared. It was expected that the direction of the polarization, which
is the same as in 7a-7e, in combination with the absence of the chelating group, should result
in the predominant formation of the polarization product. Indeed, the bromoallylation reaction
afforded two regioisomers 8f and 9f in a 1:2 ratio. These results confirm our hypothesis about
directing effect via chelation to a Lewis basic group as the key factor controlling the
regioselectivity of these reactions
.
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Table 3. Bromoallylation of alkynes 7.
Entry
1
Alkyne 7
Δδ/ppm
Products
8
9
2:3 ratio[b] Yield (%)[c]
7a
7b
7c
7d
9.20
8a
9a
2:1
6:1
66
53
33
66
2
3
4
8.71
5.64
5.24
8b
8c
8d
9b
9c >20:1[c]
9d >20:1[c]
5
7e
4.74
8e
9e
3:1
40
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6
7f
9.90
8f
9f
1:2
47
1
[a] H NMR ratio determined by analysis of the crude mixture. [b] Isolated yields of analytically pure compounds. [c] The other regioisomer
was not detected (according to 1H NMR).
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Conclusions
In conclusion, we have shown that the unequal electron distribution around the sp-
carbon atoms of unsymmetrically substituted alkynes dictates the regioselectivity of several
reactions. First, regioselective Pd-catalyzed bromoallylation of non-equivalent internal alkynes
was controlled by the polarization of the triple bond and proceeded in primarily to give the
polarization products (with one exception). Second, we have demonstrated that the presence of
a directing group, such as oxygen containing groups, in the pendant side-chain of
unsymmetrically substituted alkynes can alter the regioselectivity, overriding the polarization
of the alkyne. Third, the regioselectivity of several previously reported catalytic reactions
involving alkyne 1b was compared with our system. It was concluded that, in most cases, the
major products were the expected polarization products. On the other hand, our results as well
as those obtained by others , indicate that the level of regioselectivity does not correlate with
the values of the chemical shift difference of the respective sp-carbon atoms, indicating that
other previously unraveled effects might be involved in the control of regioselectivity.
Nonetheless, it may be concluded that the polarization of the triple bond has, in many cases, a
strong effect and may help to predict the predominat regioisomer.
Experimental Section
Electronic Supplementary Information (ESI) available: Experimental details and spectral data.
CCDC 1858621, 1858622, 1858623, 1858624, 1858625 contain the supplementary
crystallographic data for this paper.
Acknowledgments
We thank the Czech Science Foundation (Grant number 18-17823S) for financial support.
Keywords: bromoallylation • catalysis • directing effect • polarization • regioselectivity
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10.1002/ejoc.201901476
European Journal of Organic Chemistry
Abstract
Key topic: Regioselective functionalization
Nikola Topolovčan, Shuto Hara, Ivana Císařová, Zdeněk Tošner and Martin Kotora*
Page No. – Page No.
A Study of Polarization and Directing Effects of Unsymmetrical Alkynes using
Regioselective Pd-catalyzed Bromoallylation
Pd-catalyzed bromoallylation of unsymmetrical alkynes represents a simple approach towards
functionalized olefins. The regioselectivity of this process is influenced by the direction of the
polarization of the triple bond as well as by directing effects of a coordinating group in the
pendant chain.
21
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