Divergent Elementoboration: 1,3-Haloboration versus 1,1-Carboboration of Propargyl Esters

Article abstract of DOI:10.1002/chem.201801493

This work showcases the 1,3-haloboration reaction of alkynes in which boron and chlorine add to propargyl systems in a proposed sequential oxazoliumborate formation with subsequent ring-opening and chloride migration. In addition, the functionalization of

Full text of DOI:10.1002/chem.201801493

DOI: 10.1002/chem.201801493
Communication
&
Boranes
Divergent Elementoboration: 1,3-Haloboration versus
1,1-Carboboration of Propargyl Esters
Lewis C. Wilkins+,[a] Yashar Soltani+,[a] James R. Lawson,[a] Ben Slater,[b] and
Rebecca L. Melen*[a]
Abstract: This work showcases the 1,3-haloboration reac-
tion of alkynes in which boron and chlorine add to prop-
argyl systems in a proposed sequential oxazoliumborate
formation with subsequent ring-opening and chloride mi-
gration. In addition, the functionalization of these prop-
argyl esters with dimethyl groups in the propargylic posi-
tion leads to stark differences in reactivity whereby a
formal 1,1-carboboration prevails to give the 2,2-dichloro-
3,4-dihydrodioxaborinine products as an intramolecular
chelate. Density functional theory calculations are used to
rationalize the distinct carboboration and haloboration
pathways. Significantly, this method represents a metal-
Scheme 1. Background and overview to this work.
free route to highly functionalized compounds in a single
step to give structurally complex products.
ly through a 1,1- or 1,2-haloboration to yield the correspond-
ing halovinylboronic ester (Scheme 1, top). The formation of
these species has been generated through the use of simple
haloboranes such as BX3 (X=Cl, Br), or borocations developed
by Ingleson et al. In the case of borocations, stereoselective
control is observed to give predominantly the syn-addition
product.[11] Interestingly, a similar study showcased a sequential
alkyne addition to affect a formal 1,4-haloboration whereby
phenylacetylene undergoes a 1,2-addition when exposed to
[LutBCl2][AlCl4] which, upon addition of various 1-trimethylsilyl-
alkynes, undergoes a subsequent 1,2-carboboration to yield
the diene product.[11d] All such haloboration reactions are con-
venient synthetic protocols to append functional groups to
olefins, specifically in the formation of tri- and tetra-substituted
alkenes through subsequent cross-coupling reactions of the
boronic ester.[12] Another aspect of the reaction outlined within
is the installation of a pendant alkyl chloride (Scheme 1,
bottom), which has countless uses within organic chemistry
from reactions with acetylides, alkoxides, and cyanates, as well
as Grignard chemistry.
The activation of carbon–carbon double and triple bonds by
main group compounds has been a staple motif in the synthe-
sis of a plethora of new element–carbon bonds such as CÀC,[1]
CÀH,[2] CÀN,[3] CÀB,[4] and CÀO[5] bonds amongst many others.[6]
Seminal work by Wrackmeyer et al. showcased a powerful
methodology using trivalent boranes in conjunction with “acti-
vated” alkynes, that is, M-CꢀC-R, where M=Si, Ge, Sn, Pb inter
alia.[7] In these early cases, 1,1-carboboration reactions were
observed whereby a 1,2-alkyl/aryl shift occurs between the
distal and proximal carbons of the alkyne with the concomi-
tant 1,2-shift of the R group from boron to carbon (Scheme 1,
top). Further to this, Erker has demonstrated extensive use of
the carboboration mechanism to affect a number of complex
rearrangement processes such as benzannulations[8] and cycli-
zations,[9] amongst others.[10]
More recent work in such elementoboration reactions are
seen through the synthetically useful haloboration reaction
whereby a halogen, predominantly chlorine, is installed typical-
Although a significant amount of research has focused on
the sterically encumbered, strong Lewis acid, B(C6F5)3, as well
as others of a similar nature,[13] other commercially available
boranes have seemingly been absent from recent studies.
Hence, this work aims to reinvigorate the use of such boranes
in a range of synthetically imperative transformations.
[a] Dr. L. C. Wilkins,+ Y. Soltani,+ Dr. J. R. Lawson, Dr. R. L. Melen
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT, Cymru/Wales (UK)
E-mail: MelenR@cardiff.ac.uk
[b] Prof. B. Slater
Herein, we show how subtle adaptations to the alkyne start-
ing material can dramatically alter the reactivity with the
borane reagent to give the stereoselective trans-product of a
formal 1,3-haloboration, or alternatively a complex 1,1-carbo-
boration mechanism to yield a stable dichlorodihydrodioxa-
borinine heterocycle all in very good to excellent conversions.
Department of Chemistry, University College London
20 Gordon Street, London WC1H 0AJ (UK)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
https://doi.org/10.1002/chem.201801493.
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Importantly, these reagents are then well positioned
to undergo further functionalization such as cross-
couplings[14] or allylations.[15]
Initial investigations of the commercially available
PhBCl2 used the model substrate 1a in a 1:1 stoi-
chiometric ratio to yield a single product in near
quantitative yields as observed using in situ multinu-
clear NMR spectroscopy. Detailed NMR spectroscopy
(HSQC, HMBC) revealed the proposed structure of
3a, which interestingly is the product of a formal
1,3-haloboration reaction. These encouraging initial
results then led us to expand the substrate scope to
a series of phenyl substituted propargyl esters
(Figure 1, Scheme 2). It was observed that in most
cases the target haloboration product could be
clearly identified with conversions greater than 95%
at ambient temperature with reaction times of 8 h
(3a), 18 h (3b,c), and 48 h (3d).
Fortunately, the storage of a saturated CH2Cl2/
hexane solution of 3c at À408C produced a crop of
crystals suitable for X-ray diffraction. The structure
was unambiguously determined to be indeed the
product of a formal 1,3-haloboration agreeing with
spectroscopic analyses (Figure 2). Metrics of the
solid-state structure are as expected with the stereo-
chemical conformation being determined as the
trans product. Earlier work by Erker showcased the
Figure 2. Stacked in situ spectra for the reaction between propargyl ester 1a or 1aD and
PhBCl2 to yield a) 3a (1H); b) 3aD (1H); c) 3a (2H); d) 3aD (2H).
ability of vinylboranes to undergo photoinduced interconver-
sion between the E/Z conformers upon exposure to UV
light;[16] thus, it was hoped similar reactivity could be observed
here to yield the intramolecular chelate. However, no such spe-
cies could be detected in the 11B NMR post-irradiation, leaving
the spectra identical to that of the non-irradiated product 3.
Conducting the reaction between PhBCl2 and 1a in a varia-
tion of solvents ([D6]benzene, [D8]toluene, CH2Cl2, C6H5Cl) ap-
peared to make no difference in reactivity with all showing
almost quantitative conversion within 9 hours. Additionally, tri-
alling other boron reagents such as BCl3 as well as the boro-
cation [PhClB(2-DMAP)][AlCl4][11a] were unsuccessful with a mix-
ture of products prevailing as observed in the resultant multi-
nuclear NMR spectra.
Figure 1. Propargyl ester substrates used in this work.
To gain some insight into the proposed mechanism, isotopic
labeling studies were performed. The terminal position of the
alkyne was deuterated selectively using an amine appended
resin (WA50) in accordance with the literature.[17] Tracking the
reaction progress using both 1H and 2H NMR spectroscopy
whilst comparing the in situ data of the protic versus deuterat-
ed compounds shed light on the fate of the terminal alkynyl
hydrogen atom and hence the reaction mechanism (see
1
below, Scheme 4). A H resonance at d=6.6 ppm is observed
in 3a for the proton on the carbon adjacent to boron, which is
evidently absent in 3aD (Figure 2). Additionally, following the
2H NMR spectra of the reactions using 1a and 1aD clearly
shows the alkyne resonance at d=2.5 ppm diminishing in in-
tensity with the commensurate appearance of the previously
identified new resonance at d=6.6 ppm.
Scheme 2. Reaction between PhBCl2 and 1 to give 1,3-haloboration products
3. Values are given as in situ NMR conversions. Solid-state structure of com-
pound 3c, C: grey, H: white, N: blue, O: red, B: yellow-green, Cl: green. Ther-
mal ellipsoids shown at 50% probability (inset).
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Further derivatization of the starting materials to include
methyl groups in the propargylic position was undertaken to
yield compounds 2a–2c (Figure 1). Upon exposure of 2 to a
stoichiometric amount of PhBCl2, new resonances in the 1H
and 11B NMR spectra were noted after 8 h at 458C, which, inter-
estingly, were not consistent with the 1,3-haloboration prod-
ucts 3 from reagents 1. Indeed, a broad singlet resonance was
tion, which indeed determined the molecular structure to be
the product of a formal 1,1-carboboration reaction (Figure 3).
Of particular note is the regioselectivity of this reaction with
the product predominating as the selective transfer of the aryl
group over the chloride fragment.[18] This migration pattern
could be confirmed once again through detailed 2D NMR
spectroscopy to affirm the molecular connectivity revealed in
the solid-state structure (see the Supporting Information).
When comparing the divergent elementoboration observed
here, it is proposed that the inclusion of non-H groups in the
propargylic position must play a critical role in which pathway
is undertaken in this reaction. Mechanistically we propose that
an initial 1,2-trans-oxyboration step occurs to yield the zwitter-
ionic dioxolium borate.[19] During the formation of this 5-mem-
bered dioxolium intermediate (I, Scheme 4), when simple hy-
drogen atoms occupy the R2 position, the formation of 3 is
slightly more favorable compared to when methyl groups are
included in the R2 position. Conversely, if more bulky methyl
groups are included, then the chloride migration pathway is
less favored over 1,2-aryl group migration resulting in the gen-
eration the intramolecular chelate 4. These experimental find-
ings are supported through in silico studies (see below and
the Supporting Information). Additionally, when using com-
pound 5, which features a combination of H and Me in the
propargyl position, a more complex transformation is observed
when monitoring the reaction coordinate over time. Analyzing
1
observed in the H NMR spectrum at ca. d=3.8 ppm alongside
a sharp singlet resonance in the 11B NMR spectrum at ca. d=
8 ppm indicating the formation of a chelating dioxaborinine
type structure as seen in Scheme 3. This was further expound-
ed by means of the 13C NMR spectra with the presence of a
new sp3 carbon adjacent to boron presenting a resonance at
ca. d=40 ppm versus 120 ppm for the adjacent sp2 carbon in
3. Storing 4a–c as a saturated CH2Cl2/hexane solution pro-
duced a number of colorless crystals suitable for X-ray diffrac-
1
the in situ H and 11B NMR spectra suggests that, after initial
combination of PhBCl2 with 5 in a 1:1.2 ratio, the haloboration
product 6 prevails, as observed by the characteristic broad sin-
glet at d=6.35 ppm alongside the formation of a resonance at
Scheme 3. Reaction between PhBCl2 and 2 to give 1,1-carboboration prod-
ucts 4. Yields are given as isolated yields.
Figure 3. Solid-state structure of compounds 4a–c, C: grey, H: white, O: red, B: yellow-green, Cl: green, F: pink. Thermal ellipsoids shown at 50% probability.
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Scheme 4. Proposed mechanism for the divergent elementoboration of 1
and 2 using PhBCl2.
ca. d=5.2 ppm for the proposed vinyl and methylene protons,
respectively (see the Supporting Information). Over time, these
resonances reduce in intensity giving way to a new broad sin-
glet at d=3.36 ppm, consistent with the generation of the
proton in the adjacent to the borane in the chelating struc-
ture 7. In addition, new resonances appear for the newly
formed vinyl proton quartet at d=5.45 ppm, and the methyl
doublet at d=1.90 ppm. This assertion is bolstered when ob-
serving the in situ 11B NMR spectra over time whereby the ex-
pected singlet at about d=55.1 ppm forms after 1 h at ambi-
ent temperature, which reduces in intensity over time yielding
another singlet resonance at ca. d=9.1 ppm, again indicating
the reversible formation of 6 en route to 7 (Scheme 5).
Figure 4. Free energy diagram comparing the mechanism of the halobora-
tion (red) and carboboration (blue) reactions yielding products 3 and 4 re-
spectively.
tion (R2 =Me) is strongly thermodynamically preferred over
haloboration (R2 =H). Intriguingly, however, we found that the
hypothetical carboboration product (see the Supporting Infor-
mation, Table S1) for R2 =H is >30 kcalmolÀ1 more stable than
the R2 =H haloboration product obtained experimentally, rais-
ing the question: why does the carboboration reaction not
occur for R2 =H?
After 1,2-trans-oxyboration and formation of the intermedi-
ate dioxolium intermediate I, two pathways are available:
1) chloride migration that results in a metastable haloboration
product or 2) phenyl migration that yields the thermodynamic-
ally favored carboboration product. The carboboration reaction
of 1a is disfavored for two reasons: first, following the kineti-
cally favored pathway, chloride migration occurs yielding the
haloboration product and reversion back to the dioxolium I is
Scheme 5. Conversion of 5 to 6 and 7 via a proposed reversible 1,3-halobo-
strongly hindered by
a
high reverse barrier (DGgas =
ration or 1,2-carboboration mechanism.
+28.9 kcalmolÀ1) from the product. Second, in order to yield
the carboboration product from I, a prohibitively large barrier
for phenyl migration (DEgas = +37.20 kcalmolÀ1) must be over-
come (see the Supporting Information, sections 3.2.3 and
3.2.4). For comparison, the phenyl migration barrier for com-
pounds 2a (R2 =Me) is 9 kcalmolÀ1 lower than for 1a (R2 =H).
Hence, for 1a (R2 =H), the kinetically favored haloboration
product is formed. Conversely, the haloboration product of 2a
(R2 =Me) has a comparatively low reverse barrier from the
product (DGgas = +23.3 kcalmolÀ1) and also a low barrier for
phenyl migration (DEgas = +28.2 kcalmolÀ1) in comparison to
that for 1a (R2 =H). Hence, any haloboration product formed
To shed light on the divergent reactivity realized in this
work, DFT calculations were performed. The reaction pathways
for haloboration (upper) and carboboration (lower) are dis-
played in Figure 4. Pleasingly, the formation of products 3 and
4 proceeds in line with the proposed Scheme 4 via the key di-
oxolium intermediate I. Once the intermediate I is formed,
chloride migration is the transition state for the haloboration
reaction and phenyl migration is the rate-determining transi-
tion state for the carboboration reaction. It is clear from
Figure 4, upon comparison of the pathways, that carbobora-
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> +70 kcalmolÀ1 barrier (DGsol) to revert to I.
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Intriguingly, in line with the observed reversible behavior of
the mono-methylate 5 to form haloboration product 6 and
then the carboboration product 7 (see Scheme 5), calculations
show a reverse barrier from the product of DGgas = +25.8 kcal
molÀ1, which is intermediate between those found for R2 =H
and R2 =Me and an intermediate barrier for phenyl migration
of DEgas = +29.2 kcalmolÀ1. Hence, according to mechanistic
and energetic considerations, the mono-methylate should
form a more kinetically stable haloboration product than bi-
methylate and is therefore more likely to be isolable under
comparable reaction conditions, exactly in line with our find-
ings. The phenyl migration barrier is lower for R2 =Me, H than
R2 =H, so the carboboration product is more likely to be isola-
ble, again in agreement with our observations in Scheme 5.
These findings show that 3 and 6 are more kinetically stable
than the haloborated product of 2a (R2 =Me), explaining why
both 3 and 6 are observed. The relatively low barriers for
phenyl migration to form 4 and 7 with the strong thermody-
namic driving force explain why the carboboration products
occur for R2 =Me and R2 =Me, H. Overall, these calculations
reveal a remarkably subtle interplay of kinetic and thermody-
namic factors that are acutely sensitive to the R2 groups and
which cause profoundly different reaction products.
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In summary, this work has shown both the formal 1,1-carbo-
boration as well as formal 1,3-haloboration of alkynes can
occur through simple tuning of the alkyne starting material
being used. All of these multi-step reactions proceed cleanly
with high conversions and yields being noted in a one-pot,
atom efficient manner, garnering synthetically useful and func-
tionally diverse compounds for further reactivity. In depth com-
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Acknowledgements
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R.L.M. would like to acknowledge the EPSRC (EP/N02320X/1)
for funding. B.S. wishes to acknowledge ARCHER UK National
Supercomputing Service for support of this work and access
through EPSRC grant (EP/L000202).
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkynes · boranes · boron · haloboration · Lewis
Manuscript received: March 23, 2018
acid
Version of record online: && &&, 0000
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COMMUNICATION
&
Boranes
L. C. Wilkins, Y. Soltani, J. R. Lawson,
B. Slater, R. L. Melen*
&& – &&
Divergent Elementoboration: 1,3-
Haloboration versus 1,1-
Carboboration of Propargyl Esters
The 1,3-haloboration reaction of al-
kynes are described within whereby
boron and chlorine add to propargyl
systems in a proposed sequential trans-
oxyboration with subsequent ring-open-
ing and chloride migration. In addition,
the simple derivatization of these prop-
argyl esters with dimethyl groups in the
propargylic position leads to a formal
1,1-carboboration to give the 2,2-di-
chloro-3,4-dihydrodioxaborinine prod-
ucts as an intramolecular chelate. This
method represents a metal-free route to
highly functionalized compounds in a
single atom-economic step to give
structurally diverse products.
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