1,1-Carboboration

Article abstract of DOI:10.1039/c1cc15628d

The use of very electrophilic boranes RB(C₆F₅) ₂ widens the scope of the 1,1-carboboration reaction substantially. Simple terminal alkynes HCCR undergo this reaction with the RB(C ₆F₅)₂ reagents rapidly under mild conditions to give high yields of very useful new alkenylborane products. Even internal alkynes RCCR undergo 1,1-carboboration with the RB(C₆F ₅)₂ reagents to provide a novel way of carbon-carbon σ-bond activation. Variants of these reactions involving phosphorus substituted alkynes and more complex bisalkynyl main group and transition metal substrates give rise to the formation of very interesting functionalized metallacyclic products upon treatment with RB(C₆F₅) ₂ reagents by means of reaction sequences involving selective 1,1-carboboration steps.

Full text of DOI:10.1039/c1cc15628d

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FEATURE ARTICLE
1,1-Carboboration
Gerald Kehr* and Gerhard Erker*
Received 12th September 2011, Accepted 24th October 2011
DOI: 10.1039/c1cc15628d
The use of very electrophilic boranes RB(C6F5)2 widens the scope of the 1,1-carboboration
reaction substantially. Simple terminal alkynes HCRCR undergo this reaction with the
RB(C6F5)2 reagents rapidly under mild conditions to give high yields of very useful new
alkenylborane products. Even internal alkynes RCRCR undergo 1,1-carboboration with the
RB(C6F5)2 reagents to provide a novel way of carbon–carbon s-bond activation. Variants of
these reactions involving phosphorus substituted alkynes and more complex bisalkynyl main
group and transition metal substrates give rise to the formation of very interesting functionalized
metallacyclic products upon treatment with RB(C6F5)2 reagents by means of reaction sequences
involving selective 1,1-carboboration steps.
boron substituent at an olefinic C(sp2) center which makes
Introduction
the system attractive for metal catalyzed cross coupling reactions.1
1,1-Carboboration has been an attractive way of synthesizing
various types of alkenylboranes.2,3 Topologically, the 1,1-carbo-
boration reaction is reminiscent of the reversal of the Fritsch–
Buttenberg–Wiechell (FBW) reaction,4 where formally an
in situ generated vinylidene type intermediate is stabilized by
rearrangement to the corresponding alkyne (see Scheme 1).
Alkenylboranes are interesting reagents. They are boron Lewis
acids and they feature a planar tricoordinate electrophilic
Organisch-Chemisches Institut der Universitat Munster, Corrensstr.
¨
40, 48149 Munster, Germany. E-mail: erker@uni-muenster.de,
¨
kehrald@uni-muenster.de
¨
Gerald Kehr obtained his
doctoral degree at the Univer-
sity of Bayreuth under the
guidance of Professor Bernd
Wrackmeyer. After postdoctoral
research with Prof. Clement
Sanchez at the University
‘‘Pierre et Marie Curie’’ (Paris,
France) and Prof. Rudolph
Willem at the Free University
of Brussels (Begium), he
Gerhard Erker did his doctoral
work with Prof. Wolfgang R.
Roth at the Ruhr-Universitat
¨
Bochum and was a postdoc
with Prof. Maitland Jones,
Jr. at Princeton University.
After his habilitation he joined
the Max-Planck-Institut fur
¨
Kohlenforschung in Mulheim
¨
a.d. Ruhr as a Heisenberg
fellow and then became
C3-Professor at the Univer-
sitat Wurzburg. Since 1990
a
became a permanent staff
member in the Organisch-
¨
¨
Chemischen Institut at Munster.
¨
His responsibilities in the
research group of Prof. Erker
he is a full Professor at the
Organisch-Chemisches Institut
Gerald Kehr
Gerhard Erker
at the Universitat Munster.
¨
¨
include, inter alia, applied state of the art NMR analyses.
His current research interests are in carboboration and FLP
chemistry. Dr Gerald Kehr was fellow of the ‘‘Studienstiftung
des deutschen Volkes’’ and is an author of more than 220
publications and patents.
Prof. Erker’s research interests are in Organometallic Chemistry
and in catalysis. He has made major contributions to the chemistry
of the Group 4 metallocenes, including their utilization in olefin
polymerization catalysis. His current major research interests are
in frustrated Lewis pair chemistry and carboboration. Prof. Erker
is the first recipient of the Krupp Award (1984) and he has recently
received the Adolf-von-Baeyer-Award (2009) and the Eugen and
Ilse Seibold Award (2011). He was the President of the German
Chemical Society, GDCh in the years 2000 and 2001 and a member
of the Senate of the Deutsche Forschungsgemeinschaft. Besides
science Gerhard Erker loves art and music.
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Scheme 1
The 1,1-carboboration formally resembles an insertion of a
vinylidene isomer of an alkyne into a B–C bond of a borane,
although mechanistically the reaction of suitable alkynes with
boranes takes a very different mechanistic course to arrive at
the 1,1-carboboration products 1.
Scheme 2
Typically, the 1,1-carboboration of such ‘activated’ alkynes
with trialkylborane required specific reaction conditions depend-
ing on the substituents at the acetylene unit; e.g. ethynyltri-
methylsilane reacted with triethylborane at 120 1C in an
autoclave whereas the corresponding reaction of ethynyltri-
methylstannane was carried out at À20 1C.5b Triallylborane is
more reactive and undergoes the respective 1,1-carboboration
reactions more easily.12,13
Examples of the 1,1-carboboration reactions have long been
known. However, this reaction seems so far not to have been
used on a large scale for a general synthesis of alkenylboranes.
This is probably due to the fact that the previously described
examples (with few exceptions) mostly employed ‘activated’
alkynes, i.e. substrates where R1 or R2 were silyl, germyl,
stannyl or plumbyl5–8 groups, respectively, or transition metal9
containing substituents. These substituents have a high propen-
sity for undergoing the required 1,2-migration along the alkynyl
backbone, but were not overly useful synthetically. From recent
work by us and others it has now become apparent that the use
of the strongly electrophilic R–B(C6F5)2 type borane reagents
greatly facilitates the 1,1-carboboration reaction. With the use
of these reagents a great variety of simple 1-alkynes (and even of
internal alkynes) just bearing typical organic substituents have
undergone the 1,1-carboboration reaction cleanly and rapidly.
This new development will probably result in some revival of
this interesting reaction type that proceeds with a remarkable
combination of addition and rearrangement processes. In this
feature article some recent developments are described.
The reactions of dialkynyl group 14 metal substrates 5 with
the borane reagents (e.g. BEt3) were shown to provide very
interesting and useful entries to important heterocyclic metal
containing systems. The reaction sequences are apparently
initiated by the usual 1,1-carboboration reaction at one of the
acetylene units to give intermediate 6 (see Scheme 3). This then
has a choice to follow one of the three principally competing
pathways. 1,2-Carboboration may lead to 1-bora-3-metalla-
cyclopent-4-enes 7.13a–c Intramolecular alkynyl abstraction leads
to equilibration with 4a, which then has two choices of sub-
sequent 1,1-carboboration to yield 1-bora-4-metallacyclohexa-
2,5-dienes 8 (1,1-alkylboration)14 or boryl-substituted metalla-
cyclopenta-2,4-dienes 9 (1,1-alkenylboration).15
Wrackmeyer et al. were able to isolate examples of the
crucial intermediates 4a derived from each a corresponding
1,1-Carboboration of ‘activated’ alkynes: the
Wrackmeyer reaction
It had early on been demonstrated that trialkyl(alkynyl)borate
salts 2 react readily with a variety of electrophilic silicon
or tin halide reagents to form the respective alkenylborane
products 1.5b,10,11 In the course of this reaction an alkyl group
R is shifted from boron to the same carbon atom to which the
remaining –BR2 group is attached (see Scheme 2). Wrackmeyer
et al. later showed with a great number of examples that
‘activated’ alkynes 3, i.e. acetylenic substrates that were bearing
a suitable potential migrating substituent at the C(sp) carbon
atom, would undergo a very similar reaction when treated
with a trialkylborane reagent. Typical migrating groups in
the ‘‘Wrackmeyer’’ 1,1-carboboration reaction are main group
metal centered substituents such as –SiR3, –GeR3, –SnR3 or
even –PbR3 and a few selected transition metal containing
groups –MLn (M = Pt or Ti).5–9 Mechanistically, the Wrackmeyer
reaction proceeds by means of a two step reaction pathway
involving an alkynyl abstraction/rearrangement sequence
(see Scheme 2). Variants of intermediate 4 were isolated in
some special cases (see below) and even characterized by X-ray
diffraction.
Scheme 3
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Scheme 4
Scheme 6
Scheme 5
lead (4b(Pb))14d,16 and tin (4c(Sn))14h,k,15h,j,17 system and charac-
terize them by X-ray crystal structure analyses (see Scheme 4).
Wrackmeyer 1,1-carboboration reactions starting from group
14 metal diacetylide starting materials have been very useful for
preparing a great variety of boryl-substituted metallacyclopenta-
diene systems. Typical examples are shown in Scheme 5, includ-
ing rare examples of transition metal derived 1,1-carboboration
products [9d(Ti)9e and 9e(Pt)9a–d].
The B-C6F5 effect
Tris(pentafluorophenyl)borane [B(C6F5)3 (10a)]18 is a strongly
Lewis acidic borane. It has been extensively employed as
an activator component in homogenous Ziegler–Natta olefin
polymerization catalysis by making use of its ability to very
effectively abstract methyl anion equivalents from e.g. Zr or Ti
Fig. 1 Molecular structure of the –B(C6F5)2 substituted silole deri-
vative 14a.
compound 14a is formed by a sequence of 1,1-carboboration
steps similar to the reactions depicted in Scheme 3 (see
above).22
+
centres to generate the active Cp2M–CH3 cation of these
single site catalysts.19 B(C6F5)3 is known as a superb general
Lewis acid catalyst e.g. in hydrosilylation reactions of ketones.20
Lately B(C6F5)3 has been extensively used as a Lewis acid
component in frustrated Lewis pair chemistry.21
We have also treated the dialkynylsilanes 13a (R2 = Ph)
and 13b (R2 = CH3) with ‘‘Piers’ borane’’ [HB(C6F5)2) (10b)23].
The reaction takes place smoothly at room temperature to give
the silole products 15a,b in good yields. We assume a similar
sequence of abstraction/rearrangement reactions as discussed
above, only that here an initial 1,1-hydroboration reaction must
be assumed (see Scheme 7). Products 15a,b undergo a photo-
chemical rearrangement of their framework to give 16a,b.
Compound 16b (R2 = CH3) was characterized by X-ray diffrac-
tion (see Fig. 2).24 Compound 14a was analogously isomerized
photolytically to give product 17a (see Scheme 6).24a The photo-
chemical rearrangement of these silole derivatives can formally
be regarded as di-p-rearrangements (see Scheme 8).25
Tris(pentafluorophenyl)borane is very effectively able to
undergo 1,1-carboboration reactions with e.g. ‘activated’ alkynes.
The respective products are similar to the typical ‘‘Wrackmeyer
products’’ but they are usually formed under very mild reac-
tion conditions. The reactions of the dialkynylsilanes 11 with
B(C6F5)3 are typical examples. Treatment of bis(phenylethynyl)-
dimethylsilane (11a) with B(C6F5) (10a) gave the boryl-
substituted silole product 12a at room temperature (see
Scheme 6). Similarly, silole 14a that bears six very bulky sub-
stituents at its perimeter was analogously obtained by simply
stirring the bis(alkynyl)silane starting material 13a with B(C6F5)3
(10a) at room temperature. Product 14a was isolated and
characterized by X-ray diffraction (see Fig. 1). We assume that
Actually we detected the formation of the borylated silacyclo-
butene products 18a,b in the reactions of 13a,b with HB(C6F5)2
at low temperature. Their formation was probably reversible;
warming the reaction mixture to room temperature eventually
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Scheme 8
Scheme 7
resulted in the formation of the previously observed silole
derivatives 15a,b (see Scheme 7).24b
In the case of the reaction of the dialkynyl silanes 11a,b with
HB(C6F5)2 (10b) we observed the formation of the stable four-
membered ring products Z-19a,b (see Scheme 9). We assume
that the silacyclobutene products were formed by a sequence
comprising a conventional cis-1,2-hydroboration reaction
followed by a 1,1-carboboration.26 Subsequent photolysis
resulted in an efficient Z- to E-isomerization at the exocyclic
carbon–carbon double bond leading to photostationary equili-
brium mixtures that were highly enriched in the respective
E-isomers [E-/Z-19a (R1 = Ph): 10 : 1; E-/Z-19b (R1 = tBu):
3.5 : 1]. Compound E-19a was characterized by X-ray diffrac-
tion (see Fig. 3).24b
Scheme 9
shown to exhibit a very unsymmetrical Zr(p-acetylene) bonding
mode [Zr–C1: d 2.513(5) A, Zr–C2: 2.841(5) A, C1–C2: 1.209(7) A].
Warming a solution of 21a to 20 1C resulted in a 1,1-
hydroboration reaction. This should actually give the reactive
intermediate 22 (see Scheme 10) which we did not observe, but
we observed its alkynyl shifted isomer 23a instead by NMR
spectroscopy [e.g. 11B NMR: d À25]. Further warming even-
tually resulted in the formation of the carbon–carbon coupling
product 24a. Compound 24a (M = Zr) was characterized by
X-ray diffraction (see Fig. 5). It is formally to be described as a
We used the HB(C6F5)2 reagent (10b) for initiating similar
coupling reactions of bis(alkynyl)group 4 metallocenes. This gave
some very interesting results. The reaction of the bis(alkynyl)-
zirconocene complex 20a (M = Zr) with HB(C6F5)2 was rapid at
À40 1C. We spectroscopically identified the obtained reaction
product as the initial alkynyl abstraction product 21a [1H NMR:
d À0.65 (1 : 1 : 1 : 1 quartet, B–H), 11B: d À27]. Compound 21a
was also characterized by X-ray diffraction (see Fig. 4) and
Fig. 2 A view of the molecular structure of compound 16b (R2 = Me).
1842 Chem. Commun., 2012, 48, 1839–1850
Fig. 3 Molecular structure of compound E-19a (R1 = Ph).
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Scheme 11
resonance hybrid between a metallacycloallene s-complex
structure and a metallocene (Z4-enyne) complex. The hafnocene
system shows the analogous overall reaction mode (i.e. 20b +
10b - 24b). The result of a DFT analysis has confirmed a
suitable description of this type of compounds as a metalla-
cycloallenoid exhibiting a pronounced ligand to metal s-inter-
action inside the five-membered ring system.27 This places
compounds 24 alongside Rosenthal’s metallacyclocumulenes
2528 and Suzuki’s metallacyclopentynes 2629 (see Scheme 11).
Fig. 4 A projection of the molecular geometry of complex 21a.
1,1-Carboboration of ‘non-activated’ 1-alkynes: a
conceptual alternative to alkyne hydroboration
The ‘‘Wrackmeyer’’ carboboration proceeds well with ‘activated’
alkynes that contain the metal containing substituents mentioned
above as potential migrating groups. There are only very few
exceptions for this behaviour when ordinary trialkylborane
reagents are employed. In principle, hydrogen should be a
suitable migrating group2a,e,30 in 1,1-carboboration reactions
but apparently a general realization of such a reaction requires
a more electrophilic borane reagent. This is actually well met
with the B(C6F5)3 and RB(C6F5)2 reagents. The reaction of
phenylacetylene (27a) with B(C6F5)3 (10a) is a typical example.
Simple stirring of a mixture of the two reagents in a common
solvent for some minutes resulted in the formation of a near to
equimolar mixture of the E-/Z-28a 1,1-carboboration products.
Crystallization gave a sample that was highly enriched in the
E-isomer. Subsequent photolysis resulted in a very efficient
E- to Z-isomerization to give a highly Z-28a enriched sample.
The Z-28a isomer, obtained from the 1,1-carboboration reac-
tion of phenylacetylene with B(C6F5)3 followed by photo-
chemical isomerization, was characterized by X-ray diffraction
(see Fig. 6).31 The reactions of the differently substituted
1-alkynes 27b–e with B(C6F5)3 proceeded analogously. In each
case a high yield of the respective E-/Z-28(b–e) mixture of 1,1-
carboboration products was obtained after a short reaction
time under ambient conditions. Subsequent photolysis gave
highly Z-28 enriched samples (see Scheme 12).32 The 1,1-
carboboration reaction of 1-alkynes apparently is a good
method of synthesizing specifically substituted alkenylborane
products.
Scheme 10
The 1,1-carboboration of e.g. 1-pentyne (27c) with the alkylbis-
(pentafluorophenyl)borane reagents RB(C6F5)2 10c (R = –CH3)
and 10d (R = –CH2CH2Ph), respectively, proceeded remarkably
selective. Both these 1,1-carboboration reactions went to comple-
tion within hours at room temperature. In both cases we only
observed migration of the alkyl group from boron to carbon
to form the respective 1,1-carboboration products 29a,b. These
were obtained as E-/Z-mixtures. After photolysis we were able
to isolate the Z-29a,b isomers in good yields (see Scheme 13).
Fig. 5 A view of the molecular structure of the zirconacycloallenoid
system 24a.
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Scheme 14
Fig. 6 Molecular structure of the 1,1-carboboration product Z-28a.
Scheme 12
Fig. 7 Molecular structure of the 1,1-carboboration product E-31b
(R = Me).
Scheme 13
The alkenylboranes obtained by the 1,1-carboboration reactions
of the strongly electrophilic RB(C6F5)2 boranes (10) are inter-
esting Lewis acids in themselves. Some have been employed as
Lewis acid components for the generation of new advanced
frustrated Lewis pairs, e.g. for utilization in catalytic hydrogenation
processes.34 The alkenylboranes were also shown to be suit-
able trivalent borane reagents for utilization in Pd-catalyzed
Suzuki–Miyaura type carbon–carbon coupling reactions.32
The system Z-28c is a typical example. The Pd-catalyzed
coupling reaction with phenyl iodide gave the tri-substituted
olefinic product Z-32c in >80% yield. Similarly the –O-TMS
containing 1,1-carboboration products E-31c and E-31d were
coupled to directly give the O-deprotected products E-32c and
Z-32d, respectively, in good yields (see Scheme 15). The overall
reaction sequence can also be carried out as a one-pot procedure,
starting from the respective 1-alkyne without specific isolation of
1,1-carboboration products to eventually give the C–C coupling
products in good yields. The reaction sequence starting by
treatment of the alkyne 27e with MeB(C6F5)2 (10c) to eventually
yield E-33 is a typical example.
These examples demonstrate that the new 1,1-carboboration
reaction of 1-alkynes may serve in some cases as a specifically
selective conceptual alternative of the ubiquitous alkyne hydro-
boration reaction.
We can also selectively address the synthesis of E-trisubsti-
tuted olefin isomers by the 1,1-carboboration method.32,33
Typically, the 1-alkynes 30 featuring ether substituents as part
of their framework undergo facile 1,1-carboboration reactions
under ambient conditions (B2 h) to yield the respective E-/Z-
carboboration product mixtures 31 (see Scheme 14). However,
in these cases subsequent photolysis resulted in the preferred
formation of the E-31 isomers that feature an internal oxygen
to boron coordination. Similarly, MeB(C6F5)2 (10c) reacted at
elevated temperature (75 1C) with the –OTMS substituted
alkyne 30d by selective transfer of the methyl group from
boron to carbon to give E-/Z-31d. Again, subsequent photo-
lysis gave a good yield of E-31d (see Scheme 14). Examples
of the E-31 products were characterized by X-ray diffraction
(see Fig. 7).
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Fig. 8 Molecular structure of compound 38.
Scheme 15
Scheme 17
Frustrated Lewis Pairs (FLPs) can undergo specific reac-
tions with 1-alkynes depending on the basicity/nucleophilicity
of the Lewis base component. Typically, 1,2-diethynylbenzene
(34) reacts by deprotonation/alkynyltransfer with the PtBu3/
B(C6F5)3 FLP to give salt 35 whereas the combination with
a weaker triarylphosphine Lewis base leads to cooperative
1,2-FLP addition to give the zwitterion 36 (see Scheme 16).
With the still weaker P(C6F5)3 Lewis base we see a shifting of
the preferred reaction pathway to the formation of product 37,
which is apparently formed by a sequence of 1,1-carbo-
boration followed by cooperative 1,2-FLP addition of the
newly formed alkenylborane/P(C6F5)3 Lewis pair to the remaining
Fig. 9 A view of the molecular structure of compound 40b.
carbon–carbon triple bond.35 Such a situation of closely
competing 1,1-carboboration and 1,2-FLP addition is frequently
observed in frustrated Lewis pair chemistry. The formation of
the eight-membered heterocycle 38 from 1,6-heptadiyne and
the B(C6F5)3/P(o-tolyl)3 pair is another typical example (see
Scheme 16 and Fig. 8).36
1,1-Carboboration reactions can actually be used for the
preparation of alkenylen-bridged intramolecular FLPs. Typical
examples are the reactions of the phosphinyl substituted
alkynes 39a,b with B(C6F5)3. In these cases the 1,1-carboboration
requires heating. At ca. 80 1C these systems smoothly undergo
the –PPh2 migration coupled with C6F5 shift from boron to
carbon to yield the 1,1-carboboration products 40a,b (see
Scheme 17 and Fig. 9).37 Actually, the alkynylphosphine 39c
[(mesityl)2P–CRC-(p-tolyl)] reacts readily with B(C6F5)3
at ambient temperature to give the phosphirenium–borate
Scheme 16
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Fig. 11 A view of the molecular structure of the B(C6F5)2 insertion
product 44c.
Fig. 10 Molecular geometry of the phosphirenium–borate zwitterions 41.
Scheme 18
zwitterion 41. Heating of 41 eventually leads to the formation of
the respective B/P-system 40c. Compound 41 was also charac-
terized by X-ray diffraction (see Fig. 10).38
Scheme 19
This work shows that these advanced 1,1-carboboration
reactions provide an attractive alternative to some of the
conventional preparative routes to systems 40, which follow
the pathway characterized by the early work by Koster,
Binger and Balueva10,11 (see Schemes 2 and 18), which here
involves treatment of an alkenylborate salt 42 with a Ph2P–X
electrophile.
cleaved and the B(C6F5)2 moiety inserted. A detailed study43
indicated that the sequence started by RB(C6F5)2 Lewis acid
addition (to give 45) followed by a rapid 1,5-H shift. In the
resulting intermediate 46 the organyl substituent R at the boron
atom can migrate to the adjacent carbon atom to generate 47
followed by ring-carbon migration back to the boron center
to give the ring expansion products 44 (see Scheme 19). In
principle this sequence is a rare example of an equivalent of a
1,1-carboboration reaction of a CQC double bond.
The 1,1-carboboration reaction can directly be used for carbon–
carbon s-bond activation. We treated the internal alkyne 4-octyne
with the Lewis acid B(C6F5)3 (10a). It required forcing conditions,
but at 110 1C a clean 1,1-carboboration reaction ensued
leading to the alkenylborane product 48.43 In the course of
this reaction a strong C(sp)–C(sp3) carbon–carbon s-bond
was broken and consequently one of the n-propyl substituents
was shifted across the CRC triple bond (see Scheme 20).
Thus, it seems that the 1,1-carboboration reaction of internal
acetylenes may represent an interesting novel way of cleaving
a strong non-activated carbon–carbon single bond in a well
defined way to actually yield a useful product (see below).
This is not a singular example. We found that the diaryl-
acetylenes 49a (R = H) and 49b (R = CH3) also underwent
the 1,1-carboboration reaction, admittedly under rather forcing
conditions (see Scheme 21) to yield the respective alkenylborane
products 50 that each feature a geminal pair of aryl groups at
Carbon–carbon r-bond activation
There is much known about the activation of the strong H–H
s-bond.39 Also numerous methods have successfully been
developed to activate C–H bonds.40 Although the carbon–
carbon s-bonds are thermodynamically much weaker, C–C
s-bond activation41 is still in its infancy and far away from
being a thoroughly established part of synthetic organic or
organometallic chemistry. 1,1-Carboboration chemistry might
turn out to provide a useful tool to contribute to the search
and development of suitable activation procedures and proto-
cols for activating some strong carbon–carbon s-bonds.
We recently found a remarkable carbon–carbon bond cleavage
process to take place upon treatment of some amino dihydro-
pentalene systems 43 with the RB(C6F5)2 boranes 10c (R =
CH3) or 10d (R = CH2CH2Ph). The reaction proceeded slowly
at room temperature. Both products 44c and 44d were isolated
and eventually characterized by X-ray diffraction (see Fig. 11).42
Formally, the strong CQC double bond of the ‘‘left’’ carbocyclic
five-membered ring of the dihydropentalene framework was
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Scheme 20
Scheme 22
was cleaved to allow the 1,2-migration of the phenyl substituent
to take place during the 1,1-carboboration reaction sequence.
As usual the –B(C6F5)2 substituent and the remaining –C6F5
group, that had migrated from boron to carbon, were found
bonded at the other former acetylenic carbon center that now
completed the central C(sp2)–C(sp2) carbon–carbon double
bond.44
The 1,1-carboboration reaction of the diarylalkynes is quite
chemoselective. Treatment of bis-(p-tolyl)acetylene 49b with
CH3B(C6F5)2 (10c) at 125 1C eventually gave product 50c,
which was formed by selective migration of the methyl sub-
stituent from boron to carbon during the 1,1-carboboration
reaction (see Scheme 21). A related acetylene C–C bond
activation process was recently described by Piers et al. who
showed that diphenylacetylene reacted with the Lewis acidic
borole derivative 51 to form the 1,1-carboboration product 52
with rupture of a strong C(sp)–C(sp2) s-bond.30b
We have begun to use the very bulky alkenylboranes as
Lewis acid components in frustrated Lewis pair chemistry.
These products were also used as trivalent alkenylborane com-
ponents for Pd-catalyzed cross-coupling reactions. System 48
Scheme 21
the rehybridized distal C(sp2) carbon center. The X-ray crystal
structure analysis of product 50a (R = H) (see Fig. 12) shows
that during this process one of the strong C(sp)–C(sp2) bonds
Fig. 12 Molecular structure of the C–C bond activation product 50a.
Fig. 13 Molecular structure of compound 53c.
Chem. Commun., 2012, 48, 1839–1850 1847
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gave 53a upon treatment with phenylbromide under typical
Suzuki–Miyaura conditions (see Scheme 22). The alkenyl-
borane 50c gave product 53c when coupled with phenyliodide
under similar conditions. Product 53c was characterized by
X-ray diffraction (see Fig. 13), showing the geminal pair of
p-tolyl groups at C1 (originating from the vicinal p-tolyl
substituents at the diarylacetylene starting material), the
methyl group at the olefinic carbon atom C2 (which originates
from the methylborane reagent 10c) and the phenyl substi-
tuent that was introduced by the Pd-catalyzed cross-coupling
reaction.44
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Conclusions and outlook
The 1,1-carboboration reaction has come a long way from
early work involving the treatment of alkynylborate anions
with various electrophilic reagents through the ‘‘Wrackmeyer’’
reaction of activated alkynes all the way to the facile 1,1-
carboboration sequences of 1-alkynes with the RB(C6F5)2
reagents that proceed easily under very mild conditions to
give a variety of useful alkenylboranes and related systems.
The boron Lewis acid B(C6F5)3 is commercially available and
a variety of RB(C6F5)2 reagents can be made following well
established literature pathways. These are good prerequisites
from which the modern variants of the 1,1-carboboration
reaction will be developed further and will become increas-
ingly useful. In combination with some simple photochemistry
the 1,1-carboboration reaction of 1-alkynes may become the
method of choice as a chemo- and regioselective alternative to
the ubiquitous hydroboration reaction of alkynes. It is becom-
ing increasingly useful for the preparation of functionalized
highly substituted heterocycles by one pot reaction sequences
starting from readily available bisalkynyl starting materials.
Remarkably, the 1,1-carboboration reaction may provide
novel entries to carbon–carbon s-bond activation chemistry,
an area that will probably find increasing attention in the years
to come. It seems that the 1,1-carboboration reaction has taken
some remarkable development from its early stages toward an
interesting and quite promising future.
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Acknowledgements
G.E. thanks his group of very talented co-workers who
contributed to this field over the past years. We all had a lot
of fun working together, learning about the fascinating
development that the 1,1-carboboration reaction took in the
recent years. Generous financial support from the Deutsche
Forschungsgemeinschaft, the Alexander von Humboldt-
Stiftung and the Fonds der Chemischen Industrie is gratefully
acknowledged.
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c
1850 Chem. Commun., 2012, 48, 1839–1850
This journal is The Royal Society of Chemistry 2012