The Propargyl Rearrangement to Functionalised Allyl-Boron and Borocation Compounds

Article abstract of DOI:10.1002/chem.201602719

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon–carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes [PhB(C

Full text of DOI:10.1002/chem.201602719

DOI: 10.1002/chem.201602719
Full Paper
&
Boranes
The Propargyl Rearrangement to Functionalised Allyl-Boron and
Borocation Compounds
Lewis C. Wilkins,[a] James R. Lawson,[a] Philipp Wieneke,[b] Frank Rominger,[b]
A. Stephen K. Hashmi,[b, c] Max M. Hansmann,[b] and Rebecca L. Melen*[a]
Abstract: A diverse range of Lewis acidic alkyl, vinyl and aryl
boranes and borenium compounds that are capable of new
carbon–carbon bond formation through selective migratory
group transfer have been synthesised. Utilising a series of
heteroleptic boranes [PhB(C6F5)2 (1), PhCH2CH2B(C6F5)2 (2),
and E-B(C6F5)2(C6F5)C=C(I)R (R=Ph 3a, nBu 3b)] and boreni-
um cations [phenylquinolatoborenium cation ([QOBPh]
[AlCl4], 4)], it has been shown that these boron-based com-
pounds are capable of producing novel allyl- boron and
boronium compounds through complex rearrangement re-
actions with various propargyl esters and carbamates. These
reactions yield highly functionalised, synthetically useful
boron substituted organic compounds with substantial mo-
lecular complexity in a one-pot reaction.
groups).[9] More recently, work by Erker et al. has furthered this
research by applying such transformations to ‘normal’ or unac-
tivated terminal and internal alkynes with B(C6F5)3, generating
vinyl boranes.[8,10]
Introduction
For many years the chemistry of boron compounds has played
an important role in synthetic organic and inorganic chemistry,
reflected by the award of several Nobel prizes in chemistry.
The use of boronic acids in industry for Suzuki cross-coupling
reactions is critical for the preparation of a range of com-
pounds, such as poly-olefins and styrene derivatives, and is
used for the synthesis of pharmaceuticals and speciality chemi-
cals,[1] whereas a range of boron hydrides have applications in
reduction and hydroboration reactions.[2] In addition, the inher-
ent Lewis acidity of electron deficient Group 13 compounds
has led to their broad application as Lewis acid catalysts.[3,4] Re-
lated to this, Lewis acidic boranes, such as B(C6F5)3,[5] have
found widespread applications in catalysis (such as in frustrat-
ed Lewis pair (FLP) hydrogenation[6] and hydrosilylation[7] reac-
tions) as well as in 1,1-carboboration reactions.[8] Traditionally,
‘activated’ alkynes were employed in such synthetic methodol-
ogies, for example, in the systems introduced by Wrackmeyer
et al. whereby acetylide moieties were functionalised with
heavier Group 14 metals (i.e., silyl, germyl, stannyl or plumbyl
Borocations have recently emerged as comparable alterna-
tives to B(C6F5)3, whereby the Lewis acid possesses a formal
positive charge, enhancing the Lewis acidity of the boron
centre.[11] It has been shown previously that a variety of these
three-coordinate borocations (borenium cations) have been
employed in borylation[12] reactions including haloboration[13]
and 1,n-carboboration.[14,15] The latter involve the addition of
R-[B] to a p-bond, and present a selective migratory aptitude
of the R-group from boron to carbon.[14] In addition, borenium
cations have recently been found to act as the Lewis-acid com-
ponent of FLPs for hydrogenation reactions and can offer su-
perior catalytic activity compared to neutral borane Lewis
acids.[16]
In this context, our recent studies have probed how B(C6F5)3
can mimic established precious metal (AuI) catalysts in intra-
molecular alkyne activation[17] in the metal-free, catalytic syn-
thesis of oxazoles.[18] Although these studies reveal that
B(C6F5)3 is a powerful reagent to facilitate organic transforma-
tions, we have also shown that B(C6F5)3 exhibits a propensity
to undergo C6F5 group migration (as also observed in 1,1-car-
boboration reactions by Erker inter alia),[8] which has led to
a family of allylboron compounds that are themselves extreme-
ly versatile reagents in organic synthesis.[19] Traditional routes
to allylation reagents rely upon the hydroboration of alkenes,
transition-metal-catalysed borylation of allylic compounds or
the addition of reactive s-block metal allyl reagents to a borinic
or boric ester.[20] However, our approach, using the propargyl
rearrangement, allows a unique method for generating allyl-
boron reagents that demonstrates an atom-economic solution.
Currently however, our approach has been limited to the use
of the homoleptic highly Lewis-acidic borane B(C6F5)3, which
[a] L. C. Wilkins, Dr. J. R. Lawson, Dr. R. L. Melen
School of Chemistry, Main Building
Cardiff University
Cardiff, CF10 3AT, Cymru/Wales (UK)
E-mail: MelenR@cardiff.ac.uk
[b] P. Wieneke, F. Rominger, Prof. Dr. A. S. K. Hashmi, Dr. M. M. Hansmann
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[c] Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University (KAU)
Jeddah 21589 (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602719.
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severely confines the scope of these reactions to the genera-
tion of allyl boron compounds with incorporation of C6F5-
groups only. Approaches to expand the library of reagents are
critical to fully exploit this potential reactivity pathway in or-
ganic synthesis. In this work, we reveal a novel methodology
for the formation of a series of densely functionalised allylbor-
on compounds with synthetic ease, in which we incorporate
a variety of synthetically useful, transferable groups other than
from the 1,2-hydroboration of styrene using Piers’ borane,
HB(C6F5)2, using literature procedures.[24,26]
Novel 1,1-carboboration reactions of iodoalkynes, R-CꢀC-I
(R=Ph, nBu) using B(C6F5)3 were found to proceed rapidly with
excellent selectivity, yielding the iodoalkenylborane products
(3a,b) within 10 min at room temperature in quantitative
yields in CDCl3 (Scheme 2; for experimental details, see the
Supporting Information). Importantly, the incorporation of
a halogen into the borane offers a functional handle for subse-
quent reactivity. Indeed, the products of these reactions are
vinyl iodides, which are instrumental in new CÀC bond-form-
ing reactions, showing particular promise with a breadth of
specialties from routine laboratory protocols to the production
of natural products and drug molecules (e.g., through Sonoga-
shira or Suzuki coupling or through the Heck reaction).[27] Nota-
bly, phenyl iodoalkyne (Ph-CꢀC-I) reacted with B(C6F5)3 to pro-
duce the product with very high stereoselectivity (ca. 98:2 E/Z)
of the product 3a as determined by in situ multinuclear NMR
spectroscopy, whereas the butyl derivative (nBu-CꢀC-I) was
slightly less selective, giving a mixture of E/Z isomers of the
product 3b in an approximate 80:20 ratio. This high selectivity
for one isomer is in contrast to that observed previously with
“normal” terminal alkynes, that is, alkyl/aryl substituents, where
an approximate 1:1 ratio of E/Z isomers is formed.[25] This 1,1-
carboboration product was further evidenced when observing
the 19F NMR spectra; both 3a and 3b produced resonances for
the fluorine atoms in the ortho (d=ca. À127.0 (4F), À137.5
(2F) ppm), para (d=ca. À144.5 (2F), À152.6 (1F) ppm) and
meta (d=ca. À160.5 (6F) ppm) positions, clearly showing the
migration of one perfluorophenyl fragment. Minor artefacts
observed in the 19F NMR spectra of 3b can be assigned to the
minor stereoisomer formed during the rearrangement step.
Supporting 11B NMR spectra show a single broad resonance for
3a at d=57.9 ppm, with 3b showing a similar broad major
peak at d=58.4 ppm consistent with other 1,1-carboboration
products.[8] Attempts to obtain the single stereoisomer from
the E/Z mixture of 3b through irradiation with UV light (HPK
125, pyrex filter)[25] proved unfruitful, with a composite spec-
trum still remaining post exposure.
’
simply ‘C6F5 units. Thus, the elucidation of the diverse chemis-
try involved with the newly established propargyl rearrange-
ment reaction through the use of a series of heteroleptic bor-
anes and borenium compounds is explored here (Scheme 1).
Scheme 1. This work: tandem synthesis of BÀC bonds to novel boranes and
borocations.
Results and Discussion
The starting point of this project was the preparation of a li-
brary of electrophilic boron compounds that are by nature
Lewis acidic but also contain
a “transferable” R-group
(Scheme 2).[10b,21] To maintain high Lewis acidity at boron,
which is important in the propargyl rearrangement, com-
pounds of the type RB(C6F5)2 and borocations were selected as
target boron Lewis acids. The boron Lewis acid derivative
PhB(C6F5)2 (1)[22] was prepared from salt metathesis of PhBCl2
and LiC6F5,[23] whereas PhCH2CH2B(C6F5)2 (2)[24,25] was prepared
By introducing a formal positive charge at boron, the neces-
sity to employ fluorinated substituents to retain a highly
Lewis-acidic centre is unnecessary. Capitalising on this, we syn-
thesised borenium compound 4 ([QOBPh][AlCl4]) from the re-
action of TMS-protected 8-hydroxychinoline (QOH) with
PhBCl2, followed by addition of AlCl3. Compound 4 has recent-
ly been reported by Ingleson with successful employment in
carboboration reactions with terminal alkynes, featuring exclu-
sive transfer of the phenyl group.[14]
The borane and borenium compounds synthesised above
were then tested as alkyl, vinyl and aryl transfer reagents in
the propargyl rearrangement reaction, which to date has only
been investigated using B(C6F5)3 (Scheme 3).[19] The heterolep-
tic boranes (1–3) synthesised here would be expected to un-
dergo selective transfer of the more electron-rich substituent,
as seen in 1,1-carboboration reactions reported by Erker and
Ingleson inter alia.[8,10,14] A series of propargylic substrates (5a–
h) were synthesised, including propargyl esters and carba-
Scheme 2. Highly electrophilic borane and borenium Lewis acids.
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Scheme 3. Rearrangement products (8a–f) from vinyl group transfer. Insert
(9) shows minor product from C6F5-migration. Yields indicated are isolated
yields.
Figure 1. The propargyl rearrangement with boranes and borenium cations
(top), and propargyl substrates tested (bottom).
Similar results were observed when substituting the phenyl
group for an ethylbenzene group (-CH2CH2Ph). The reaction of
borane 2 with 5 f also gave unselective R-group transfer, indi-
cating that the lability of the alkyl group is similar to that of
the C6F5-fragment in these reactions (in contrast to previous re-
ports).[8,14] Although this transformation proved unselective by
NMR spectroscopy, giving an alkyl vs. C6F5 transfer ratio of
64:36, single crystals of the product (6) in which the ethylben-
zene group had migrated were isolated and characterised by
single-crystal X-ray diffraction (Figure 2).
Although the reactions with heteroleptic boranes 1 and 2
proved unselective, they provide proof of principle that R-
groups other than C6F5 may be transferred in the propargyl re-
arrangement to give novel allylboranes. It should be noted
that additional reactions were conducted in which 5 f was re-
acted with Piers’ Borane [HB(C6F5)2]. In this instance rapid and
clean 1,2-hydroboration of the terminal alkyne unit occurred
within a few minutes, yielding the corresponding vinyl borane
(7), indicating that the hydroboration reaction is much faster
than the propargyl rearrangement.
Figure 2. Solid-state structure of 6. C: black, H: white, O: red, B: yellow-
green, F: pink.
It was found that iodoalkenylboranes (3) underwent more
selective reactions, showing that the migratory aptitude of the
vinyl group appears to be greater than that of C6F5. Addition
of propargyl substrates (5e–h) to the in situ generated 1,1-car-
boboration products (3a,b) in CDCl3 showed that preferential
migration of the vinyl group occurs over that of the C6F5-
group to form the highly functionalised compounds 8a–f
(Scheme 3). The reactions were monitored by in situ multinu-
clear spectroscopy (1H, 11B, 19F) in which it was observed that
the reaction had generally reached completion within 18 h at
room temperature to give the products 8. Evidence of the al-
mates (Figure 1). Initially, PhB(C6F5)2 (1) was reacted with the
propargyl ester 5 f in a 1:1 stoichiometric ratio. Unfortunately,
the desired migration of the more electron rich phenyl group
over the pentafluorophenyl group was found to not occur ex-
clusively, instead giving a mixture that mainly consisted of
three products resulting from unselective aryl group transfer
(See supporting information).
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lylboron product was clearly seen in the 11B NMR spectra by
the generation of a broad singlet at dꢁ2 ppm, indicative of
these intramolecularly chelated dioxaborinine-type struc-
tures.[19] Further evidence was obtained from the 19F NMR spec-
tra, which showed inequivalence of the fluorine atoms of the
C6F5-groups. The steric strain induced through the juxtaposi-
tion of the perfluorophenyl rings and the highly substituted
vinyl group results in the lack of free rotation of the C6F5-rings,
affording ten peaks in the 19F NMR instead of the expected six
signals. This results in four resonances for the ortho fluorine
atoms (d=ca. À130 to À140 ppm (6F)), three peaks for the
para fluorine atoms (d=ca. À150 to À160 ppm, (3F)) and
three resonances for the meta fluorine atoms (d=ca. À160 to
À165 ppm (6F)).
Other characteristic features of these reactions include a sin-
glet at d=ca. 5 ppm in the 1H NMR spectrum, which is as-
signed to the CÀH adjacent to boron, previously the alkynyl
proton. Secondly, the quartet seen at dꢁ5.5 ppm is indicative
of the vinyl proton formed as a result of the rearrangement
process. Additionally, in the case of the product of the reaction
between 3 and the carbamate precursor (5h) to yield 8 f, the
characteristic exovinyl protons are clearly visible as two dou-
blets at d=5.00 and 4.72 ppm, respectively, in the 1H NMR
spectrum.
The products of these reactions were isolated by low-tem-
perature crystallisation and the solid-state structures of 8a–
c
and 8e–f were determined by X-ray crystallography
(Figure 3). Importantly, these structures reveal the vinyliodide
to be the E-isomer in all cases, thus elucidating the major
isomer of the borane 3 as being the E-isomer. In addition to
this, the isolated products 8a–f present a chiral centre adja-
cent to boron as a result of the rearrangement step. This ap-
pears to be formed, as anticipated, in a racemic mixture of
both R and S enantiomers, as confirmed by X-ray crystallo-
graphic studies. In one case, we were able to isolate a few
crystals of the minor product (9) of the reaction in which C6F5
migration occurs from the reaction of 5e with 3a, which was
confirmed by X-ray diffraction analysis (Figure 3).
Figure 3. Solid-state structure of 8a–c, 8e–f and 9 (top to bottom). C: black,
An entirely different rearrangement reaction was observed
in the absence of the methyl group in the propargylic position
(R2) of the esters 5a–d (Scheme 4). When the reactions of 5a–
d (R2 =H) with borane 3a were monitored by multinuclear
NMR spectroscopy, a similar chemical shift to that seen with
esters 5e–g (R2 =Me) was observed in the 11B NMR spectra at
approximately 2 ppm, indicating a dihydrodioxaborinine struc-
ture. However, the absence of the exo-vinylic protons in the
1H NMR spectrum prompted further investigation. X-ray crystal-
lographic studies of the reaction of the p-NO2 substituted ester
5d with the vinyl borane 3a unambiguously determined that
the products of the reaction were a dimeric bis(perfluorophe-
nyl)boranyl ester (10a) and the (perfluorphenylethynyl)ben-
zene fragment (11) (Figure 4).
H: white, O: red, B: yellow-green, F: pink, I: brown, N: blue.
The mechanism for this reaction (Scheme 4) is presumed to
proceed through activation of the propargyl position of the
substrate 5 by coordination of the Lewis-acidic borane to the
ester functionality of 5. This promotes transfer of the iodide to
the propargylic position, yielding propargyliodide, (perfluoro-
Scheme 4. Alternative reactivity of propargyl esters 5a–d with vinyl boranes
(3).
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tion, which allows for an intramolecular halide shift upon coor-
dination of the borane to the carbonyl.
Finally, to stimulate exclusive and selective transfer of one R-
group, we turned our attention to borocations, specifically the
boreniums recently reported by Ingleson as transfer re-
agents.[14] The 8-hydroxychinoline (QOH) derived borenium re-
agent 4 was selected for the propargyl rearrangement owing
to the intramolecularly coordinating nature of the quinolato
ligand, which promotes exclusive transfer of the phenyl group
during the reaction. Indeed, such quinolatoborenium salts
have been reported by Ingleson in carboboration reactions,
which demonstrated selective transfer of phenyl and (5-hex-
yl)thienyl groups.[14] Upon treating the propargyl substrates 5 f
with borenium (4) in CD2Cl2, it was seen via in situ NMR spec-
troscopy that the expected migration of the phenyl group had
occurred to give the rearranged allylboronium product 12. As
a result of this transfer step, two diasteroisomers of the prod-
uct are formed in a 5:1 ratio of (R,R):(R,S) owing to the genera-
tion of a chiral boron centered spiro-complex (Scheme 6). A
preparative scale reaction permitted single crystals of the
major (R,R) diastereoisomer to be isolated, which were suitable
for X-ray diffraction, confirming the assigned connectivity
(Figure 5).
Figure 4. Solid-state structure of 10a, 10b and 11 (top to bottom). C: black,
H: white O: red, B: yellow-green, F: pink, N: blue.
phenylethynyl)benzene (11) and a dimeric bis(perfluorophe-
nyl)boranyl ester 10, which could be isolated and characterised
by X-ray diffraction (Figure 4) and observed using multinuclear
NMR spectroscopy (see the Supporting Information). To test
this mechanism, we reacted benzyl benzoate with the borane
3a (Scheme 5). As expected, the products of the rearrange-
ment could be isolated by recrystallisation and confirmed by
Scheme 6. Propargyl rearrangements with borenium cations (4). [a] Conver-
sion to both diasteroisomers 12 and 12’ determined by in situ NMR spec-
troscopy.
1
X-ray diffraction (10b, 11, Figure 4) or by H NMR spectroscopy
(benzyl iodide; see the Supporting Information).The divergence
in this reactivity from that observed with 5e–g is assumed to
arise from the lack of steric obstruction at the propargylic posi-
Conclusion
In conclusion, this work has shown that a range of Lewis-acidic
alkyl, vinyl and aryl boranes and borenium compounds are ca-
pable of new carbon–carbon and carbon–boron bond forma-
tion through migratory group transfer. A diverse series of het-
eroleptic boranes and borenium compounds have greatly ex-
panded the formation of allyl- boron and borenium com-
pounds beyond our initial reports. This demonstrates the fact
that these Lewis-acidic boron-based compounds can lead to
step-change developments in the synthetic methodology, af-
fording novel boron and boronium compounds through com-
Scheme 5. Reaction of 3a with benzyl benzoate. [a] Conversion determined
by in situ NMR spectroscopy.
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Figure 5. Solid-state structure of 12a. C: black, H: white O: red, N: blue, B:
yellow-green, Al: grey, Cl: aquamarine.
plex rearrangement reactions yielding boron substituted or-
ganic compounds with substantial molecular complexity in
a one-pot reaction. The reactions of the boranes and boroca-
tions with propargyl esters results in allylboron and allylboroni-
um species through p-activation of the alkyne followed by:
1) a 5-exo-oxoboration reaction yielding a vinyl borate and
2) subsequent ring opening with concurrent 1,2-C6F5 transfer
from boron to carbon. In the case of PhB(C6F5)2 (1) and
PhCH2CH2B(C6F5)2 (2), the migration reactions show reduced se-
lectivity; however, with the vinyl boranes (3) and borocations
(4), the migratory aptitude of the vinyl or phenyl group respec-
tively is highly favoured. The methodology shown here pro-
vides a strong basis for the relative ease at which a vast library
of allylboron and allylboronium compounds can be synthes-
ised, and their widespread applications in a diverse range of
fields. Indeed, these reactions lead to a novel set of allylboron
and allylboronium compounds that will be of significant use to
the synthetic chemist. The construction of CÀC bonds through
allylation and crotylation reactions have gained a prominent
position as tools in organic chemistry and natural product syn-
thesis. In the case of the selective vinyl halide migration, yet
more complex functionality can be easily installed giving the
products a multitude of reactive handles for subsequent trans-
formations.
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Acknowledgements
R.L.M. would like to acknowledge the EPSRC (grant number
EP/N02320X/1) for funding. M.M.H. is grateful to the Fonds der
Chemischen Industrie for a Chemiefonds scholarship and the
Studienstiftung des deutschen Volkes.
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Keywords: allylboron
carboboration · main group
·
borenium
·
borocation
·
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Received: June 8, 2016
Published online on && &&, 0000
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FULL PAPER
A diverse range of Lewis acidic alkyl,
vinyl and aryl boranes and borenium
compounds that are capable of new
carbon–carbon bond formation through
selective migratory group transfer have
been synthesised. Utilising a series of
heteroleptic boranes as well as boreni-
um cations, synthetic pathways to
highly functionalised, synthetically
useful boron-substituted organic com-
pounds with substantial molecular com-
plexity have been achieved in a one-pot
reaction.
&
Boranes
L. C. Wilkins, J. R. Lawson, P. Wieneke,
F. Rominger, A. S. K. Hashmi,
M. M. Hansmann, R. L. Melen*
&& – &&
The Propargyl Rearrangement to
Functionalised Allyl-Boron and
Borocation Compounds
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&
Chem. Eur. J. 2016, 22, 1 – 8
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!