A Boratafulvene

Article abstract of DOI:10.1002/anie.202107968

Structurally authenticated free B-alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron-bound exocyclic methyl group in a B-methyl borole yields the first 5-boratafulvene anion—an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The pK_a value of the exocyclic B?CH₃ in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti-aromatic moiety. The non-aromatic boratafulvene reacts as a C-centered nucleophile with the mild electrophile Me₃SnCl to give a stannylmethyl borole, regenerating the anti-aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora-Wittig-type reaction generating a borole oxide.

Full text of DOI:10.1002/anie.202107968

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A Boratafulvene
Tobias Heitkemper, Leonard Naß, and Christian P. Sindlinger*
Dedicated to Gerhard E. Herberich
Abstract: Structurally authenticated free B-alkyl boroles are
presented and electronic implications of alkyl substitution were
assessed. Deprotonation of a boron-bound exocyclic methyl
group in a B-methyl borole yields the first 5-boratafulvene
anion—an isomer to boratabenzene. Boratafulvene was struc-
turally characterized and its electronic structure probed by
ꢀ
DFT calculations. The pKa value of the exocyclic B CH3 in
Scheme 1. Examples for fulvenes and heterofulvenes.
a set of boroles was computationally approximated and
confirmed a pronounced acidic character caused by the boron
atom embedded in an anti-aromatic moiety. The non-aromatic
boratafulvene reacts as a C-centered nucleophile with the mild
electrophile Me3SnCl to give a stannylmethyl borole, regener-
ating the anti-aromaticity. As nucleophilic synthons for
boroles, boratafulvenes thus open an entirely new avenue for
synthetic strategies toward this highly reactive class of hetero-
cycles. Boratafulvene reacts as a methylene transfer reagent in
a bora-Wittig-type reaction generating a borole oxide.
We now present an anionic 5-boratafulvene, accessed by
deprotonation of B-methyl 1H-boroles, with an exocyclic
methylene group as a new entry into heterofulvene chemistry
(Scheme 1). 1H-Boroles are unsaturated five-membered
boron heterocycles with four cyclic conjugate p-electrons
and reveal (weakly) anti-aromatic properties.[7] This results in
high reactivity of the butadiene and pronounced Lewis acidity
of the organoborane moiety.
Introduction
Results and Discussion
Replacement of carbon by isoelectronic heteroelement
fragments in classic hydrocarbons has long been a fruitful
Only few substitution patterns that sufficiently stabilize
free boroles have been reported and our group has recently
established reliable protocols towards 1-chloro-2,5-(TMS)2-
borole (A).[7b,8] When A was treated with ethereal methyl
Grignard solutions, 1-methylborole 1 is formed and is
obtained in ca. 80% crystalline yield as a brightly orange
solid. Boron-bound alkyl groups in free boroles are rare:
(PhC)4BCH3, prepared by Sn/B exchange from (PhC)4SnMe2
and MeBX2, is the only derivative described in the literatur-
synthetic challenge which led to fundamental structural
[1]
ꢀ
motifs of E/C E bonding interactions. A plethora of
molecules and materials with altered properties resulted
from these efforts, particularly when more electropositive
boron atoms are introduced.[2] Among classic hydrocarbon
molecules, the parent (penta)fulvene is a reactive isomer of
benzene featuring an unsaturated five-membered ring with
a “cross-conjugate” exocyclic methylene group.[3] Compounds
conventionally also considered heterofulvenes usually feature
exocyclic electronegative oxygen or imine nitrogen atoms
(Scheme 1).[4] Heterofulvenes with endocyclic heteroatoms,
except for ubiquitous N-atom containing rings as in dipyrro-
methene-based compounds, are much scarcer and often
transient.[5] Erker and Nçth reported on borata-(di)benzoful-
[6]
=
vene derivatives with exocyclic BR2 moieties.
[*] T. Heitkemper, L. Naß, Dr. C. P. Sindlinger
Institut fꢁr Anorganische Chemie
Georg-August-Universitꢂt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: christian.sindlinger@chemie.uni-goettingen.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202107968.
Figure 1. ORTEP plot of the molecular structure of 1. Atomic displace-
ment parameters are drawn at 50% probability level. Hydrogen atoms
except for C1-bound H are omitted for the sake of clarity. Selected
bond lengths [ꢀ]: B1–C1 1.559(2), B1–C2 1.595(2), C2–C3 1.357(2),
C3–C4 1.539(2), C4–C5 1.354(2), C5–B1 1.587(2), C2–Si2 1.871(2),
C5–Si1 1.866(2).
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published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
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e.[7b,c] Related B-alkyl (di)benzoboroles,[9] and Me-borole
derivatives, sufficiently stabilized in transition metal com-
plexes or as base adducts, are documented.[10] We were able to
structurally characterize 1 and its molecular structure is
depicted in Figure 1.
Localized single and double bonds are found within the
central borole ring in 1 as to be expected for an anti-aromatic
system. The exocyclic methyl group is slightly bent out of the
C6F5); (F3CC)4BMe 6.7]. Methyl boranes with comparably
inductively active vinyl substituents, yet lacking the cyclic
conjugation, reveal significantly higher computational pKa
(vinyl: 28.8; 1-silyl-2-phenylvinyl: 32.2) than the respective
boroles, clearly pointing at the remarkable acidity enhance-
ment which results from the thermodynamic incentive that is
the removal of anti-aromaticity upon deprotonation. Com-
parison with predicted pKa of five-membered B-methyl 2- or
3-borolene (32.6; 31.7) also advocates against ring-strain
ꢀ
borole plane by ca. 78 (H3C-B-(CbCb)centroid ca. 1738) with a B
ꢀ
ꢀ
CH3 bond length of 1.559(2) ꢀ in the typical range of B
effects to account for the increased C H acidity in methyl-
C(sp3) single bonds. The characteristic p!p* transition of
borole-based frontier orbitals is found at lmax = 458 nm,
slightly blue shifted to comparable B-aryl derivatives (lmax
ꢁ 475 nm).[7b,8d] As to be expected for tri-coordinate boron,
the 11B NMR resonance is found lowfield-shifted at 80.0 ppm
and the B-methyl group resonates at 1.32 ppm (1H) and
11.9 ppm (13C).
borole compared to acyclic divinyl derivatives. Compared to
parent methyl borole (HC)4BMe, benzannulation as in 1-
boraindene (21.8) or 9-borafluorene (23.7) increasingly
ꢀ
reduces the C H acidity, presumably due to reduced anti-
aromatic character. We propose this (computational) acidity
assessment to be a useful measure for anti-aromaticity-driven
reactivity enhancement in boroles. Previous approaches to
quantify this effect include shifts in CN-stretching modes of
nitril adducts to boroles,[7b] and computational measures such
as aromatic stabilization energy (ASE) or NICS.[16]
a-CH acidity of boranes is known.[6a,11] However, suitable
diorgano alkyl boranes R2B(CHR’2) and conditions that allow
selective deprotonation are scarce.[6b] Successful deprotona-
tion of Ar2BCH3 with suitable amides to yield borataalkenes
is restricted to sterically demanding aryl groups (such as
mesityl) and bases that prevent from adduct formation and
quaternation at the boron atom.[11b,12] This route granted
access to the yet sole example of a structurally characterized
borataalkene with an unsupported terminal methylene unit in
[Mes2BCH2]ꢀ.[11b] Erker proposed intermediate formation of
borataalkenes by tautomerization in an indane-bridged
FLP.[11a]
As a further example, Herberich reported twofold de-
protonation of endocyclic a-CH in 1-amino-3-borolene to
yield the Hꢁckel-aromatic borole dianion.[13] We reasoned
that the exocyclic methyl protons of 1 bound to a Lewis-acidic
boron atom, which is embedded in an anti-aromatically
destabilized borole moiety, might reveal an increased acidic
character that would facilitate deprotonation. Along the lines
of a recent computational approach by Erker and coworkers
to estimate pKa values for a-CH bonds in boranes (with
pKa(CpH) = 18.0 as a reference),[14] we found the pKa of
1 with polar DMSO solvent model to be 22.6, slightly higher
than, for example, (C6F5)2BCH3 (18.7, Figure 2).[11a,15] Nota-
bly, a series of substituted B-methyl boroles were probed and
all revealed general, significantly increased acidity along with
strong dependency on the substituents [(MeC)4BMe 24.8;
(PhC)4BMe 18.8; (HC)4BMe 18.6; (PhFC)4BMe 11.1 (PhF =
Successful deprotonation of 1 (Scheme 2) and reliable
isolation of boratafulvene anion 2 are very sensitive to base
and solvation conditions. In benzene, treatment with LiTMP
(TMP = 2,2’,6,6’-tetramethylpiperidine) leads to decomposi-
tion and intractable mixtures, while in [D8]THF immediate
clean conversion with LiTMP is indicated by NMR monitor-
ing. However, isolation attempts fail as, again, intractable
mixtures form. Treatment of orange solutions of 1 in toluene
with K[N(SiMe3)2] for 18 h yields a sparingly soluble yellow
solid. This crude solid contained boratafulvene 2 and varying
Scheme 2. Synthesis and deprotonation of methyl borole 1.
Figure 2. Computationally approximated pKa values of various B-methyl boranes (black dashes), B-methyl boroles (red squares) and B-
methyl(di)benzoborole (blue circles). RI-BP86-D3BJ/def2TZVPP with a solvent model for DMSO and exp. pKa =18.0 as reference.[11a]
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amounts (0 to ca.30%) of two side-products, of which one was
identified as the colorless amide adduct B (Scheme 2, see SI
for structure).[17] Computational assessment (BP86/
def2TZVPP and benzene solvation model, see SI) of the
reaction indeed reveals the adduct formation to be more
exergonic (ꢀ28.8 kcalmolꢀ1) than the deprotonation reaction
(ꢀ15.7 kcalmolꢀ1). The Lewis-acidic boron atom in 1 is
significantly less sterically shielded than in previous cases of
Scheme 3. NRT comparison of fulvene versus boratafulvene.
ꢀ
ꢀ
successful R2B Me deprotonation (as in Mes2B CH3 with
pKa = 29.0), where adduct formation is sterically impaired. In
that respect, the successful deprotonation of 1 likely benefits
dominant contributions (65%) of the more electronegative
carbon atom. NRT analysis further suggests very similar
resonance structure contributions compared to fulvene
(Scheme 3).[22] Despite similar contributions of a cationic
ꢀ
from its increased C H acidity (pKa = 22.6).
After work-up, boratafulvene 2[K(thf)2] can be isolated in
small to moderate yields from THF by fractional crystalliza-
tion. These crystals have been probed several times by X-ray
diffraction but only provided poor data which only allowed
identification of the connectivity pattern as a coordination
polymer of {2[K(thf)2}1 (see SI). Adding 18-crown-6, 2a
(2[K(18-crown-6)]) can be reliably isolated from toluene as
crystalline material suitable for X-ray diffraction in moderate
yields (ca. 44%). The molecular structure of 2a is shown in
Figure 3.[18,19] The structure reveals B C contacts to a crown-
=
Figure 4. Electrostatic potential maps of parent hydrogen-substituted
a) fulvene, b) boratafulvene, c) N-heterocyclic olefin, d) NHC-support-
ed borole anion at an isolevel of 0.006.
exocyclic CH2 group attached to an aromatic, dianionic
borole ring, the dominant polar double bond resonance
structure results in an inverted directionality of the dipole
moment in boratafulvenes compared to fulvenes. The electro-
static potential surface map of boratafulvene thus reveals
=
a nucleophilic site along the B C bond and in that respect,
boratafulvene more resembles the profile of N-heterocyclic
olefins (Figure 4), which have evolved as versatile C-nucle-
ophilic ligands in coordination chemistry.[23] Respectively, (N-
heterocyclic) borataalkenes were most recently shown to be
suitable ligands.[12b,24]
Fulvene is a high energy isomer of benzene (by ca.
33 kcalmolꢀ1) and accordingly, parent boratafulvene is
31 kcalmolꢀ1 higher in energy than boratabenzene, of which
substitution and transition-metal complex derivatives are
known.[25,26]
We treated 2 with Me3SnCl as a mild electrophile. As
observed for borataalkenes before,[11c,e] 2 reacts as a carbon-
centered nucleophile to selectively give the stannaneopentyl
borole 3 (Scheme 4a). Remarkably, in the course of this
reaction the unfavorable anti-aromatic character within the
borole ring is regained from a non-aromatic precursor, as
supported by a characteristic NICSzz profile of 3 (see
Supporting Information).[27] However, on the basis of the
NICS(1)zz value, the anti-aromatic character in 3 (13.0) is
significantly reduced compared to 1 (24.8). Thus, borataful-
venes allow for the synthesis of functionalized alkyl-substi-
tuted free boroles with the borole fragment being introduced
Figure 3. ORTEP of the molecular structure of boratafulvene 2[K(18-
crown-6)] (2a). ADP are drawn at 50% probability. Non-methylene
hydrogen atoms, disorder in tBu groups, and lattice toluene are
omitted for clarity. Selected bond lengths [ꢀ]: B1–C1 1.457(2), B1–C2
1.602(2), C2–C3 1.367(2), C3–C4 1.501(2), C4–C5 1.370(2), C5–B1
1.601(2), C1–K1 3.210(2), K1–B1 3.322(2).
ꢀ
ether-coordinated K-cation. Compared to 1, the B1 C1 bond
lies in the borole plane and is significantly shortened to
=
1.457(2) ꢀ as to be expected for authentic C B in borataal-
kenes (C B in [Mes2B CH2] : 1.444(8) ꢀ).[1,11b,20] The
11B NMR resonance is found at 40.3 ppm highfield-shifted
relative to 1 indicating involvement of the previously empty
ꢀ
=
=
=
boron p-orbital in a B C p-bond. The methylene signals are
found at 4.48 ppm (1H) and 96.1 ppm (13C).
Anion 2 is isoelectronic to (penta)fulvene and thus a rare
case of a heterofulvene with an endocyclic heteroatom. Due
to resonance stabilization of a negative charge in aromatic
cyclopentadienyl moieties, fulvenes are polar molecules with
=
the dipole moment aligned along the polar exocyclic C C
double bond.[21] NBO analysis of boratafulvene reveals
ꢀ
similar polarities for both, the B C s- and p-bonds with
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The 11B NMR resonance of 3 is found at 68.7 ppm, which
is considerably highfield-shifted compared to other free 2,5-
disilylboroles.[8d] The B-bound methylene group resonates at
2.27 ppm (1H) and 24.9 ppm (13C). 1JSn C coupling amounts to
ꢀ
335 Hz for the CH3 groups (almost identical to the coupling in
SnMe4) but only 46 Hz for the CH2 group. This points at
ꢀ
reduced s-orbital contributions involved in the C2 Sn1
bond.[29] Indeed, NBO calculations suggest sp3 hybridization
for the Sn-atom and the methyl C-atoms attached but only
a fairly small s-orbital contribution (10%) of methylene C-
ꢀ
atom to the C2 Sn bond, thus reducing the Fermi contact (see
SI for further details).
Scheme 4. Reactivities of Boratafulvene 2.
A further spectroscopic feature of stannaneopentyl borole
3 is its bright yellow color. The characteristic p!p* transition
in boroles is observed at lmax ꢁ 424 nm, notably blue-shifted
compared to its related methyl derivative 1 (458 nm). This
correlates with an increased HOMO/LUMO gap (2.04 eV in
3; 1.81 eV in 1) that mainly results from an energetically
elevated LUMO level.[25] NBO analysis of 3 suggests a classic
Lewis structure as depicted in Scheme 4, however second-
order perturbation theory (SOPT) calculations suggest sig-
as a nucleophilic reagent. It is important to stress that this new
synthetic avenue can be very valuable as the known routes for
the synthesis of highly reactive, anti-aromatic free boroles,
particularly the rare B-alkyl-substituted derivatives, are very
limited. A distinct difference to formally related NHC-
supported borole anions (or rather B-imidazolium substituted
ꢀ
borole dianions), featuring a B Ccarbene single bond, must be
ꢀ
noted as those are reported to react as one-electron reduc-
tants or boron-centered nucleophiles, yet for the latter not
generating free boroles.[10f,28] This differences of a p-acidic
formal methylene unit attached to the boron atom in borole
anions compared to a dominantly s-donating N-heterocyclic
carbene is reflected in their electrostatic potential maps
(Figure 4d).
nificant hyperconjugation of the C Sn s-bond into the empty
p-orbital of the boron atom (17.8 kcalmolꢀ1).[22] When this
hyperconjugation is probed computationally for a series of
boranes, the exceptional Lewis-acidic character of anti-
aromatic boroles becomes apparent (Scheme 5). Acyclic
boranes R2B(CH2SnMe3) including those with electron-with-
drawing substituents such as C6F5 groups reveal smaller
respective hyperconjugation interaction energies from SOPT.
Only CF3 groups render boranes comparably Lewis-acidic to
rival the hyperconjugation predicted in the parent borole
(HC)4B(CH2SnMe3) in accordance with the pKa approxima-
tions. Reduced interaction in 3 compared to the parent borole
may stem from steric hindrance preventing from smaller B-C-
Sn angles.
The molecular structure of stannaneopentyl borole 3 is
depicted in Figure 5. The -CH2SnMe3 group is notably bent
out of the borole plane by ca. 168 (C2-B1-(C4/C5)centroid ca.
1648). The single and double bond lengths within the ring are
as to be expected but the exocyclic B1–C2 distance is
relatively short (1.496(7) ꢀ) ranging between the single bond
in 1 (ca. 1.56 ꢀ) and the double bond in 2 (ca. 1.46 ꢀ). The
ꢀ
methylene Sn bond is notably elongated compared to the
ꢀ
other Sn CH3 contacts and the B-C-Sn angle is found at
103(1)8.
ꢀ
Scheme 5. Sn C hyperconjugation interactions with boron p-orbital
from SOPT.
We further probed the reactivity of the boratafulvene
anion 2 towards benzophenone as a model carbonyl com-
pound and monitored the reaction by NMR spectroscopy
(Scheme 4b). After several days at room temperature, clean
conversion to 1,1-diphenylethylene and a new borole species,
borole oxide 4 (the molecular structure of its [K(18-crown-
6)]+ salt is shown in Figure 6) was observed, indicating that 2
serves as a methylene transfer reagent in a borata-Wittig-type
reaction to form alkenes.
Such reactions were reported previously for borataal-
kenes and carbonyls.[11c,12a,15,30] Preliminary analysis of the
reaction mixtures by NMR spectroscopy indicates the for-
mation of an oxaboretane intermediate C as the dominant
species of a mixture after a few hours.[15] The methylene CH2
Figure 5. ORTEP of the molecular structure of stannaneopentyl borole
3. ADP are drawn at 50% probability. Non-methylene hydrogen atoms
and disorder are omitted for clarity. Selected bond lengths [ꢀ]: B1–C2
1.496(7), B1–C3 1.597(6), C3–C4 1.346(8), C4–C5 1.525(6), C5–C6
1.354(5), C6–B1 1.617(9), C2–Sn1 2.215(3); Sn1–CH3 2.073(5),
2.148(6), 2.180(9).
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