Angewandte
Chemie
DOI: 10.1002/anie.201410562
Boron Compounds
Fine-Tuning the Nucleophilic Reactivities of Boron Ate Complexes
Derived from Aryl and Heteroaryl Boronic Esters**
Guillaume Berionni, Artem I. Leonov, Peter Mayer, Armin R. Ofial, and Herbert Mayr*
Dedicated to Dr. Klaus Rçmer on the occasion of his 75th birthday
Abstract: Boron ate complexes derived from thienyl and furyl
boronic esters and aryllithium compounds have been isolated
and characterized by X-ray crystallography. Products and
mechanisms of their reactions with carbenium and iminium
ions have been analyzed. Kinetics of these reactions were
monitored by UV/Vis spectroscopy, and the influence of the
aryl substituents, the diol ligands (pinacol, ethylene glycol,
neopentyl glycol, catechol), and the counterions on the
nucleophilic reactivity of the boron ate complexes were
examined. A Hammett correlation confirmed the polar
nature of their reactions with benzhydrylium ions, and the
correlation lgk(208C) = sN(E + N) was employed to determine
the nucleophilicities of the boron ate complexes and to
compare them with those of other borates and boronates. The
neopentyl and ethylene glycol derivatives were found to be 104
times more reactive than the pinacol and catechol derivatives.
Scheme 1. Reactions of boron ate complexes (BACs) with electro-
philes.
B
oron ate complexes (BACs), which are readily available by
treatment of boronic esters R-B(OR)2 with organolithium
compounds, are air-stable, functional-group-tolerant nucleo-
philes,[1] which undergo various noncatalyzed reactions with
electrophiles.[2–5] Their treatment with trifluoroacetic anhy-
dride yields intermediate borinic esters which undergo
stereoselective allylation reactions with aldehydes, ketones,
and imines, when allylboronates are used as precursors.[2]
Following the principle of the Zweifel olefination[3]
(Scheme 1, reaction 1), Aggarwal et al. recently reported
that treatment of BACs with suitable electrophiles led to
the stereospecific coupling of arenes with secondary alkyl
groups (Scheme 1, reaction 2).[4]
in this way with a broad range of carbon-, oxygen- and
nitrogen-centered electrophiles, including carbenium and
iminium ions.[5]
Borate salts M+ arylBX3 with electron-rich arene rings,
ꢀ
on the other hand, are known to react with electrophiles at the
ipso position of the aryl ring.[6] We have recently demon-
strated that the nucleophilicities of thiophenes, furans, and
ꢀ
indoles bearing boron substituents, such as -B(OR)3ꢀ, -BF3
,
-B(pin), and -B(MIDA) (MIDA = N-methyliminodiacetate),
span a reactivity range of ten orders of magnitude.[7] We now
report that BACs with two aryl rings behave similarly and
describe their isolation and X-ray structures as well as the
determination of their nucleophilicities by the benzhydrylium
methodology.[8]
In contrast, BACs with less-electron-rich arenes
(Scheme 1, reaction 3) were attacked by electrophiles at the
boron-substituted sp3-hybridized carbon, and the coupling
products R-E were formed with a high degree of inversion.[5]
Chiral BACs, derived from secondary boronic esters, reacted
Following a procedure by Brown,[1a,b] we added the thienyl
boronic esters 1a–d to ethereal solutions of the trifluorome-
thylated aryllithium compounds 2 and 2’ and then removed
the solvent by evaporation to produce the lithium BACs (3–
4)-Li+ as colorless crystalline solids (Scheme 2). The
11B NMR spectra of all BACs show a sharp resonance at d
+ 2 to + 10 ppm that is characteristic of an anionic tetracoor-
dinated boron atom derived from boronic esters.[1] No
decomposition of these BACs in CD3CN solutions was
observed by 1H NMR spectroscopy over several days at
208C.[9]
As shown in Scheme 3, addition of thienyl- and furyl-
lithium derivatives 5 and 5’ to the pinacol boronates 6 and 7
produced the respective BACs (8–14)-Li+ in good yields. The
tetraethylammonium salt 8-Et4N+ was obtained by cation
metathesis from 8-Li+ with Et4N+Clꢀ.
[*] Dr. G. Berionni, Dipl.-Chem. A. I. Leonov, Dr. P. Mayer,
Dr. A. R. Ofial, Prof. Dr. H. Mayr
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13, 81377 Mꢁnchen (Germany)
E-mail: Herbert.Mayr@cup.uni-muenchen.de
[**] We thank the Deutsche Forschungsgemeinschaft (SFB 749, project
B1) for financial support as well as Prof. Dr. Paul Knochel, Dr. Peter
Byrne, Kohei Moriya, Elsa Follet (all from LMU), Kathryn Feeney
(Bristol, UK), Dr. Sami Lakhdar (Caen, France), and Prof. Dr.
FranÅois Terrier (Versailles, France) for helpful discussions.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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