Indene formation upon borane-induced cyclization of arylallenes, 1,1-carboboration, and retro-hydroboration

Article abstract of DOI:10.1039/d1cc01750k

We herein report the reaction of arylallenes with tris(pentafluorophenyl)borane that yields pentafluorophenyl substituted indenes. The tris(pentafluorophenyl)borane induces the cyclization of the allene and transfers a pentafluorophenyl ring in the course of this reaction. A Hammett plot analysis and DFT computations indicate a 1,1-carboboration to be the C-C bond-forming step.

Full text of DOI:10.1039/d1cc01750k

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Indene formation upon borane-induced
cyclization of arylallenes, 1,1-carboboration, and
Cite this: DOI: 10.1039/d1cc01750k
retro-hydroboration†
Received 2nd April 2021,
Accepted 29th April 2021
Max Hasenbeck,a Felix Wech,a Arthur Averdunk,a Jonathan Beckerb and
a
Urs Gellrich
*
DOI: 10.1039/d1cc01750k
rsc.li/chemcomm
We herein report the reaction of arylallenes with tris(pentafluo- Piers’ borane induced dimerization of the allene, were detected
rophenyl)borane that yields pentafluorophenyl substituted indenes. (for details, see the ESI†). The detection of 3 gives a direct hint
The tris(pentafluorophenyl)borane induces the cyclization of the allene to the intermediate formation of Piers’ borane HB(C6F5)2 in the
and transfers a pentafluorophenyl ring in the course of this reaction. course of the reaction.9 Accordingly, the reaction constitutes a
A Hammett plot analysis and DFT computations indicate a 1,1-carbo- formal exchange of a pentafluorophenyl group of the B(C6F5)3
boration to be the C–C bond-forming step.
with hydrogen. The reaction was subsequently performed on a
preparative scale and the indene 2 was isolated and fully
In 2010, Erker and co-workers reported that the reaction of the characterized. The structural assignment is further supported
strongly Lewis acidic tris(pentafluorophenyl)borane (B(C6F5)3) by SCXRD (Scheme 2). We then investigated the scope of this
with terminal alkynes leads to a 1,1-carboboration of the transformation by reacting different aryl allenes with B(C6F5)3.
alkyne.1 The substrate scope of the 1,1-carboboration was As the hydroboration observed in the initial experiment con-
extended to phosphinyl substituted alkynes, yielding alkenyl- sumes some of the allene, these reactions were performed with
bridged frustrated Lewis pairs.2 Furthermore, the 1, two equivalents of the allene. Under these conditions, 2 was
1-carboboration of internal alkynes with concomitant C–C isolated in 68% yield with respect to B(C6F5)3, the limiting
cleavage was demonstrated.3,4 More recently, Melen et al. reagent (Scheme 3). The same reaction with one equivalent
reported the 1,2-carboboration of allenyl ketones.5
phenylallene and norbornene as sacrificial olefine to capture
We found now that the reaction of B(C6F5)3 with phenylal- Piers’ borane yielded 2 in 58% yield. The benzindene 4 is
lene 1 leads to the formation of the pentafluorophenyl- obtained in 65% yield from the reaction of 1-naphthylallene
substituted indene 2 (Scheme 1).6 Our initial finding was that with B(C6F5)3. However, it was isolated as a mixture of isomers
the addition of 1 to a solution of B(C6F5)3 in dichloromethane- that differ regarding the position of the benzylic double bond.
d2 (DCM-d2) leads to the formation of indene 2 in 48% Likewise, the reaction of p-tolylallene and p-(isopropyl)pheny-
NMR-yield within 45 minutes (Scheme 2).
lallene gives the indenes 5 and 6 as a mixture of regioisomers.
Hence, the B(C6F5)3 induces a ring closure and transfers a
pentafluorophenyl ring to the allene.7 As a side product of this
reaction, we observed the allylborane 3 that is likely formed by
hydroboration of the phenylallene 1.8 Besides, minor amounts
of a vinylborane, originating from the hydroboration of the
internal double bond, and a 2-boryl-hexa-1,5-dien, formed by
a
¨
¨
Institut fur Organische Chemie, Justus-Liebig-Universitat Gießen,
Heinrich-Buff-Ring 17, 35392 Gießen, Germany.
E-mail: urs.gellrich@org.chemie.uni-giessen.de
b
¨
¨
Institut fur Analytische und Anorganische Chemie, Justus-Liebig-Universitat
Gießen, Heinrich-Buff-Ring 17, Gießen 35392, Germany
† Electronic supplementary information (ESI) available: Experimental and com-
putational details, spectroscopic and crystallographic data, copies of NMR
spectra. CCDC 2059168. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/d1cc01750k
Scheme 1 The context of this work: the 1,1-carboboration of alkynes
described by Erker and the borane-induced ring closure with concomitant
aryl group transfer described herein.
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Scheme 2 Formation of the pentafluorophenyl-substituted indene
2
upon the reaction of 1 with B(C6F5)3. aYields determined by qNMR with
1,3,5-trimethoxybenzene as internal standard. The inset shows the mole-
cular structure of 2 derived from SCXRD (50% probability ellipsoids, all
hydrogen atoms are omitted for clarity).
Fig. 1 Hammett plot analysis of the B(C6F5)3- mediated formation of
indenes from aryl allenes. Note that only the major isomer is shown in
the scheme.
Scheme 4 Formation of the methyl indene 9 upon reaction of p-(iso-
propyl)phenylallene with MeB(C6F5)2.
The mechanism of the B(C6F5)3-mediated indene formation
was further investigated computationally by DFT at the revDSD-
PBEP86-D4/def2-QZVPP//PBEh-3c level of theory (Fig. 2).11–13 As
zwitterionic intermediates are likely involved in the reaction,
the SMD solvent model for DCM was used for the structure
optimizations and the single point computations.14 We assume
Scheme 3 Scope of the B(C6F5)3 mediated formation of indenes from
allenes. Yields of products isolated by column chromatography are given.
(a) With one equivalent phenylallene and one equivalent norbornene.
(b) Reaction temperature 0 1C. (c) Oil bath temperature 60 1C.
A better yield was obtained when the reactions with these that the reaction commences with the addition of B(C6F5)3 to
substrates were run at 0 1C, presumably because of side the p-system of 1. This step yields the zwitterion INT-1. The
reactions at r.t. In contrast, the reactions with p-(chloro)pheny- positive charge in this intermediate is stabilized by allyl and
lallene and p-(fluoro)phenylallene required an elevated reaction benzyl resonance. An intramolecular Friedel–Crafts alkylation
temperature to give 7 and 8 in moderate yields. These findings via TS-2 closes the five-membered ring of the indene core. We
indicate that electron-rich arylallenes undergo a faster cycliza- further assume that an intermolecular proton shift leads to a
tion whereas electron-withdrawing substituents slow down the re-aromatization and the intermediate INT-3 (Scheme 5). How-
reaction.
ever, all attempts to optimize the structure of INT-3 resulted in
The effect of the substituents on the phenyl ring on the a pentafluorophenyl transfer and lead to INT-4. Relaxed
reaction rate was further assessed by a Hammett analysis potential energy surface scans further showed that the penta-
(Fig. 1).10 The negative slope of the Hammett plot and the fluorophenyl transfer is a barrierless process. This finding
rho value of À3.9 Æ 0.7 indicate that in the rate-determining agrees with computational studies of the 1,1-carboboration of
transition state of this reaction positive charge is built up in the alkynes by Erker, Grimme, and co-workers that showed that
benzylic position of the arylallene.
The increased reactivity of electron-rich allenes was used for moiety, the pentafluorophenyl transfer is barrierless.
a cyclization with a less Lewis acidic borane. The reaction of
A retro-hydroboration, i.e. the liberation of HB(C6F5)2 from
p-(isopropyl)phenylallene with MeB(C6F5)2 leads to cyclization Int-4, forms product 2. According to the computations, this step
of the allene and transfer of the methyl group (Scheme 4). requires only a moderate activation energy of 16.7 kcal molÀ1
once a carbocation is formed in a-position to the B(C6F5)3
.
However, the formation of methylindene 9 required a pro- Notably, the retro-hydroboration is computed to be exergonic
longed reaction time.
by 2.2 kcal molÀ1. According to the computations, TS-1, the
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Fig. 2 Gibbs free energies of the B(C6F5)3-mediated indene formation computed at the revDSD-PBEP86-D4/def2-QZVPP//PBEh-3c level of theory.
The SMD solvent model for DCM was used for the structure optimizations and the single point computations. The inset shows the optimized structure of
TS-1.
We thank the German research foundation (DFG) for finan-
cial support (GE 3117/1-1).
Conflicts of interest
There are no conflicts to declare.
Notes and references
¨
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putations are in favorable agreement with the Hammett analy-
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arylallenes with B(C6F5)3 leads to pentafluorophenyl substi-
tuted indenes. A plausible mechanistic scenario consists of a
B(C6F5)3-induced cyclization, a pentafluorophenyl transfer, and
a retro-hydroboration. This reaction is a rare example of a
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herein might inspire the development of synthetic methods
that rely on 1,1-carboboration and retro-hydroboration
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