Angewandte
Chemie
treating selected aryliodonium zwitterions with CNÀ, AcOÀ,
and MeCN as synthons for the COOH, OH, and NH3 groups,
respectively (Table 4). Reactions of iodonium 6a[NEt4] and
11a-p with [NEt4]+CNÀ at 558C gave the corresponding
nitriles 24[2NEt4] and 25-p[NEt4] in good yields, thus
opening access to such derivatives of the [closo-B10H10]2À
cluster and simplifying the preparation of 25-p[NEt4][23]
(Table 4, entries 1 and 2). In contrast, a similar cyanation of
12 gave the 8-iodo derivative [8-I-3,3’-Co(1,2-C2B9H10)(1’,2’-
C2B9H11)]À (28)[24] as the sole product, instead of the expected
nitrile (Table 4, entry 5). Similarly, [closo-1-CB11H11-12-I]À
(29-p) was cleanly formed in a reaction of the iodonium
zwitterion 11a-p with C6H13MgBr (Table 4, entry 6). A
reaction of the bisphenylodonium zwitterion 7 with 1 equiv-
alent of the acetate anion at 608C gave the unsymmetrically
substituted 1-acetoxy derivative 27[NEt4], which was isolated
in good yield (Table 4, entry 4).
two mechanisms are common for aryl carboranyl[25] and diaryl
iodonium salts,[26] whereas the third mechanism is typical for
dinitrogen zwitterions.[13]
The addition–reductive elimination mechanism (I) oper-
ates at lower temperatures for charged nucleophiles, such as
CNÀ and AcOÀ, which form trigonal-bipyramidal 10-I-3
intermediates (e.g. 32-p in Scheme 5), and for clusters 1–4 is
generally highly selective for substitution at the B atom. In
the transition state, presumably of tetragonal-pyramidal
geometry, the nucleophile migrates to the boron atom and
is reductively eliminated, thus relieving the steric strain.[25a]
The reaction of 12 (Ar= MeOC6H4) with CNÀ, however,
proceeds differently, presumably by a mechanism involving
single-electron transfer (SET) from the cyanide to the
À
iodonium center, followed by homolysis of the I Ar bond
in the 9-I-2 intermediate (II),[25c] as evident from the
formation of 28 and the presence of significant amounts of
anisole in the reaction mixture. Similarly, the iodonium
zwitterion 11a-p undergoes efficient SET from a Grignard
reagent to form 29-p exclusively (Table 4 and Scheme 5).
Finally, substitution on the cluster with a weak nucleo-
phile (e.g. MeCN, thian, thioformamide) proceeds presum-
ably through the boronium ylide mechanism (III; for
example, 33-p in Scheme 5), which requires higher temper-
atures and is occasionally complicated by the competing
Thermolysis of a mixture of isomeric acids 8c (Table 1,
entry 5) in MeCN at 658C selectively gave the nitrilium
zwitterion 30-m, which, upon isolation on a SiO2 column,
underwent hydrolysis to the acetamido derivative 31-m
(Scheme 4). The latter cluster was hydrolyzed with aqueous
À
insertion of the ylide into the C H bond of ArI. Consistent
with this mechanism are products of these thermal reactions,
for example, the efficient formation of N-pyridinium deriv-
atives and ArI. Further support for the involvement of
boronium ylides is provided by MP2//DFT calculations, which
Scheme 4. Synthesis of the 6-ammonium zwitterion 26-m.
À
show generally low DG298 values for heterolysis of the I B
bond in these zwitterions (e.g. 17.4 kcalmolÀ1 for 11a-p versus
31.4 kcalmolÀ1 for [9-PhI-m-C2B10H11]+ in MeCN dielectric
medium).[18]
HCl to the 6-ammonium derivative [closo-1-CB9H8-COOH-
6-NH3] (26-m), which was isolated in 54% overall yield
(Table 4, entry 3). This procedure represents a significant
simplification of the preparation of this previously reported
amino acid.[14]
Analysis of the data in Tables 2–4 indicates that the
iodonium zwitterions react with nucleophiles according to
one of three mechanisms: I) addition–reductive elimination
via a tricoordinated iodine species (the 10-I-3 intermediate),
II) single-electron transfer followed by cleavage of the I Ar
bond in the 9-I-2 intermediate, or III) heterolysis of the I B
bond and formation of a boronium ylide, which is trapped
with a nucleophile, as shown for 11a-p in Scheme 5. The first
In conclusion, we have demonstrated a simple, conven-
ient, and general two-step method for the practical prepara-
tion of a broad spectrum of closo-borate derivatives via
aryliodonium zwitterions. The presented method provides
unprecedented access to functionalized closo-borates and
new classes of pharmaceuticals and materials, such as liquid
crystals, with tailored properties. The presented reactions
demonstrate the synthetic versatility of the closo-borate
aryliodonium zwitterions and suggest their rich chemistry in
reactions with other nucleophiles, palladium-catalyzed cou-
pling processes, and photochemistry commonly used with
diaryliodonium and arylcarboranyliodonium[25] salts.[26] In this
respect, the presented method provides a new and versatile
À
À
tool for the construction of B X bonds in closo-boranes.[8] We
À
are currently expanding the scope of these reactions, inves-
tigating their mechanisms, and exploring them for the
preparation of new materials.
Keywords: closo-borates · cluster compounds ·
functionalization · iodonium zwitterions · synthetic methods
Scheme 5. Three possible pathways for the reaction of the phenyl-
iodonium zwitterion 11a-p with nucleophiles.
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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