Functionalization of closo-borates via iodonium zwitterions

Article abstract of DOI:10.1002/anie.201411858

A simple method for the functionalization of closo-borates [closo-B₁₀H₁₀]^2- (1), [closo-1-CB₉H₁₀]^- (2), [closo-B₁₂H₁₂]^2- (3), [closo-1-CB₁₁H

Full text of DOI:10.1002/anie.201411858

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201411858
German Edition: DOI: 10.1002/ange.201411858
Boron Cluster Anions
Functionalization of closo-Borates via Iodonium Zwitterions**
´
Piotr Kaszynski* and Bryan Ringstrand
Dedicated to Professor Russell Grimes on the occasion of his 80th birthday
Abstract: A simple method for the functionalization of closo-
borates [closo-B10H10]2À (1), [closo-1-CB9H10]À (2), [closo-
B12H12]2À (3), [closo-1-CB11H12]À (4), and [3,3’-Co(1,2-
C2B9H11)2]À (5) is described. Treatment of the anions and
their derivatives with ArI(OAc)2 gave aryliodonium zwitter-
ions, which were sufficiently stable for chromatographic
purification. The reactions of these zwitterions with nucleo-
philes provided facile access to pyridinium, sulfonium, thiol,
carbonitrile, acetoxy, and amino derivatives. The synthetic
results are augmented by mechanistic considerations.
capture therapy (BNCT) technology.[6a,7] In spite of favorable
properties and a great deal of previously discovered chemis-
try,[8] controlled functionalization of the boron atoms and the
range of accessible functional groups in 1–5 still remain
problematic.
The regiospecific functionalization of boron atoms in
closo-borates typically relies on a leaving group L that serves
as a synthetic handle for the introduction of other function-
alities (Scheme 2).[9] One synthetically important group is the
c
loso-Borates, such as 1–4 (Scheme 1),[1] and metallaborates
(e.g. 5)[2] are becoming increasingly important for the devel-
opment of new materials and pharmacophores as a result of
their steric and electronic properties, low metabolic reactivity,
Scheme 2. Functionalization of closo-borates. X=B (n=2), X=C
(n=1).
iodo group (L = I), introduced by direct iodination. However,
iodination of the clusters often leads to ionic regioisomers
that are difficult to separate,[4b] and subsequent synthetic
transformations of iodo derivatives are limited to palladium-
catalyzed coupling reactions.[10] Another versatile group is the
+
dinitrogen substituent (L = N2 ); unfortunately, stable and
À
Scheme 1. closo-Borates used in this study. Spheres represent a C H
synthetically useful examples of such zwitterions are limited
to the apical derivatives of 10-vertex closo-borates 1 and 2.
Dinitrogen derivatives of the [closo-B10H10]2À anion (1) are
obtained by a direct diazotization/reduction sequence[11] or by
an elegant diazo-transfer process[4e,12] in moderate yields. In
contrast, apical dinitrogen derivatives of the [closo-1-
CB9H10]À anion (2) are obtained through the diazotization
of amines.[13] The dinitrogen group can be readily replaced
with a nucleophile in a SN1-type process involving a boronium
ylide by heating in the reagent/solvent. Such reactions permit
the efficient introduction of mercapto, sulfido, acetamido,
amino, carbonyl, and N-pyridinyl functionalities or their
equivalents at the apical positions of the 10-vertex
anions.[4e,11–13] Unfortunately, dinitrogen zwitterions of equa-
torial derivatives of the 10-vertex clusters 1 and 2 and 12-
vertex analogues 3 and 4 have extremely limited use owing to
their insufficient stability.[4b,14]
À
fragment, and all other vertices are B H fragments.
hydrophobic properties, and highly delocalized negative
charges.[3] For example, they have been used as structural
elements of polar and ionic liquid crystals,[4] proposed as
elements of nanoscale construction sets,[5] investigated as
pharmacophores,[6] and are components of boron neutron
´
[*] Prof. Dr. P. Kaszynski, Dr. B. Ringstrand
Department of Chemistry, Vanderbilt University
Nashville, TN 37235 (USA)
E-mail: piotr.kaszynski@vanderbilt.edu
´
Prof. Dr. P. Kaszynski
Department of Chemistry, Middle Tennessee University
Murfreesboro, TN 37123 (USA)
´
Prof. Dr. P. Kaszynski
´
Faculty of Chemistry, University of Łꢀdz
We now report that aryliodonium zwitterions (L = ArI+)
provide a general synthetic handle for the 10- and 12-vertex
closo-borates 1–4, and for metallacarborates, such as 5. The
preparation of the aryliodonium zwitterions was straightfor-
ward, and they reacted with a wide range of nucleophiles
(Scheme 2). Initially, we focused on [closo-B10H9-1-IPh]À
(6a),[15] the only reported aryliodonium zwitterion of
´
Tamka 12, 91-403 Łꢀdz (Poland)
[**] Support for this project has been provided by the National Science
Foundation (grant DMR-1207585). We thank Middle Tennessee
State University, TN (USA) for the use of a laboratory for synthetic
work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411858.
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a closo-borate.[16] Attempts to reproduce the original syn-
thesis[15] (1a[2NH4], PhIO, H2O/CH3CN, steam bath) were
unsuccessful; at best, the product was obtained in impure
form in low yield. By a systematic approach, we discovered
that the treatment of a solution of 1a[2NH4] in aqueous
acetic acid (70%) with solid PhI(OAc)2 (1 equiv) at 08C in
the presence of [NEt4]+ immediately resulted in the formation
of a white precipitate, from which 6a[NEt4] was obtained in
46% yield by SiO2 chromatography (Table 1, entry 1). During
the preparation of 6a[NEt4], the 1,10-bisphenyliodonium
zwitterion 7 was isolated as a minor component (7%).
However, the same reaction conducted with 2 equivalents of
PhI(OAc)2 or PhI(OH)(OTs) (Ts = p-toluenesulfonyl) gave 7
as the sole product in 82% yield (Table 1, entry 2). A similar
reaction of the 1-quinuclidinium derivative 1b[NMe4][4e] with
PhI(OAc)2 in MeCN gave the 1-phenyliodonium derivative
6b in 59% yield (Table 1, entry 3).
Analysis of the reaction mixture revealed that PhI+ is fully
selective for the apical position of the [closo-B10H10]2À cluster,
and in this respect is similar to ArN2 .
+ [12] The regioselectivity
of the phenyliodination was inves-
tigated for the [closo-1-CB9H10]À
anion (2a), which usually exhibits
strong preference for equatorial
electrophilic substitution.[17] Thus,
the treatment of 2a[Cs] with PhI-
(OAc)2 (1 equiv) in aqueous AcOH
gave a 2:5 mixture of 8a-p and 8a-
m in 93% yield (Table 1, entry 4).
This ratio of isomers is the highest
ever observed for the electrophilic
substitution of 2a. Unfortunately,
these regioisomers could not be
separated by chromatography or
crystallization. Similar results were
observed for the phenyliodination
of the [closo-1-CB9H9-1-COOH]À
anion (2c), for which a 1:2 mixture
of 8c-p and 8c-m was obtained in
95% yield (Table 1, entry 5).
Table 1: Synthesis of iodonium zwitterions 6–12.[a]
Entry
Anion
Product(s) and yield
1
6
7
1
2
3
a, R=H
a, R=H
b, R=+Quin[b]
46% [NEt4]
0% [NEt4]
59%
7%
82%
–
2
8-p
8-m
4
5
a, R=H
c, R=COOH
93%[c]
The phenyliodination reaction
27%[d]
95%[c]
–
was extended to [closo-B12H12]2À. In
aqueous AcOH, the treatment of
3a[2Na] with PhI(OAc)2 (2 equiv)
gave no product, whereas in aque-
ous trifluoroacetic acid (TFA),
a precipitate of the bis-zwitterions
3
9-p
9-m
10-p
10-m
6
7
8
9
a, R=H,
Ar=Ph
Ar=4-MeOC6H4
Ar=Ph
69%[c]
–
–
9
was obtained in 69% yield
(Table 1, entry 6). The product,
presumably mixture of two
12%
51%[c]
34%
d, R=+SMe2,
d, R=+SMe2,
–
–
77%[c]
a
Ar=4-MeOC6H4
13%
39%
regioisomers, was only sparingly
soluble in typical solvents. To
remedy this problem, we added
a
methoxy substituent to PhI-
(OAc)2. The subsequent reaction
of [closo-B12H12]2À 2Na+ (3a[2Na])
with 4-MeOC6H4I(OAc)2 gave
a 2:7 mixture of regioisomers 9-p
and 9-m (Ar= 4-MeOC6H4) in
51% yield (Table 1, entry 7) as
a microcrystalline solid that was
soluble in typical solvents. The
individual isomers 9-p and 9-m
could be separated chromatograph-
ically, although with mass loss
owing to their limited thermal sta-
bility in solution.
4
11-p
81%
81%
11-m
11%
8%
10
11
a, R=H
e, R=C5H11
5
12
12
13
Ar=Ph, 65%
Ar=4-MeOC6H4, 51%
[a] Typical conditions: The closo-borane (1.0 mmol) in 70% aqueous AcOH or CF3COOH (TFA; 12 mL)
was treated with ArI(OAc)2 (1.05 or 2.1 mmol) at 08C, and after 1–2 h the product was filtered from the
mixture. [b] Quin=4-pentylquinuclidine; CH3CN was used as the solvent. [c] Mixture of two isomers.
[d] Yield of the isolated product after thermolysis of the isomeric mixture.
The [closo-B12H12]2À anion sub-
stituted with a +SMe2 group [closo-
2
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B12H11-1-SMe2]À [NEt4]+ (3d[NEt4]) also reacted with PhI-
(OAc)2 in a 5:2 mixture of TFA/CH2Cl2 to give two
regioisomers, 10d-p and 10d-m, which were obtained in
a 1:3 ratio and isolated in 77% yield (Table 1, entry 8). Their
chromatographic separation was hindered by relatively low
solubility, which was improved by the use of 4-MeOC6H4I-
(OAc)2 as the reagent, and the individual regioisomers 10d-p
and 10d-m (Ar= 4-MeOC6H4) were then readily isolated
(Table 1, entry 9).
suffered 47% decomposition in the same time period. The
aryliodonium derivatives of [3,3’-Co(1,2-C2B9H11)2]À were
moderately unstable, whereas derivatives of the [closo-1-
CB11H12]À anion and the 10-phenyliodonium derivatives of
[closo-1-CB9H10]À and [closo-B10H10]2À showed no decompo-
sition under ambient conditions over several weeks.[18] In all
experiments, PhI or 4-MeOC6H4I was identified as the major
organic product. Full kinetic analysis and single-crystal XRD
data for selected iodonium zwitterions will be reported
elsewhere. We used the difference in stability of the phenyl-
iodonium regioisomers of [closo-1-CB9H10]À to prepare the
pure carboxylic acid [closo-1-CB9H8-1-COOH-10-IPh] (8c-p)
by thermolysis of the isomeric mixture 8c in MeCN at 558C,
followed by simple chromatographic separation of 8c-p from
decomposition products of 8c-m (Table 1, entry 5).
The reaction of [closo-1-CB11H12]À (4a[Cs]) with PhI-
(OAc)2 was successful only in the presence of TFA. Thus,
zwitterions 11a-p and 11a-m were isolated in 81 and 11%
yield, respectively, by chromatography (Table 1, entry 10).
Similar results were obtained for the 1-pentyl derivative
4e[Cs] (Table 1, entry 11). Finally, the anion [3,3’-Co(1,2-
C2B9H11)2]À (5) was aryliodinated with PhI(OAc)2 and 4-
MeOC6H4I(OAc)2 in aqueous TFA to give the 8-aryliodo-
nium species 12 as the sole product in good yield (Table 1,
entry 12 and 13).
The aryliodonium zwitterions appeared to be stable in the
solid state, but in solution their stability varied depending on
the cluster and regioisomer. Thus, 1H NMR spectroscopic
analysis of solutions in CD3CN demonstrated that derivatives
of the [closo-B12H12]2À anion were least thermally stable and
that the 1,7-substituted isomer decomposed faster than the
1,12-substituted analogue: 10d-m underwent 76% decompo-
sition after 2 days at ambient temperature, whereas 10d-p
Investigation of the reactivity of aryliodonium zwitterions
demonstrated their synthetic value for the preparation of
functionalized boron clusters. Examples of such reactions
with pyridines, sulfur nucleophiles, and other nucleophiles are
shown in Tables 2–4. Reactions of aryliodonium zwitterions
with pyridine and its 4-alkoxy derivatives at 60–908C gave the
expected pyridinium products 13–19 of nucleophilic replace-
ment of the iodoarene in good to excellent yields (Table 2).
The yields observed for 13–16, 18, and 19 are comparable to
or better than those observed when the analogous dinitrogen
derivatives were used.[4b,e,12,19] In particular, the preparation of
acid 16-p (Table 2, entry 5), an important intermediate for the
Table 2: Reactions of selected iodonium zwitterions with pyridines.[a]
Entry
1
Zwitterion
Nucleophile
pyridine
Product(s) and yield
6a[NEt4]
13[NEt4], 74%[12]
14, 93%[4e]
2
6b
4-C7H15OC5H4N
3
4
7
7
4-MeOC5H4N
4-C7H15OC5H4N
15[1], R=Me, 57%
15[7], R=C7H15, 79%[19]
5
6
8c-p
4-C7H15OC5H4N
4-MeOC5H4N
16-p, 90%
10d[b]
17-p, 10%
17-m, 30%
7
8
11a-p
11e-p
4-MeOC5H4N
4-MeOC5H4N
18-p, R=H, 87%[4b]
19-p, R=C5H11, 61%[4b]
[a] Typical conditions: The iodonium zwitterion (1.0 mmol) and appropriate pyridine derivative (2 mL) were stirred at 65–1008C. [b] Mixture of
regioisomers.
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synthesis of liquid crystals, was carried out with a fivefold
increase in yield as compared to the previous method with
Table 3, entry 3), along with a product presumably derived
from cage arylation with PhI. The same sulfonium derivative,
22-p, was cleanly obtained in 73% yield by the alkylative
cyclization of 21-p with 1,5-dibromopentane under hydrolytic
conditions (Scheme 3).
a N2 handle and in fewer steps.[4h] Thermolysis of the
+
regioisomeric mixture 10d in 4-methoxypyridine, followed by
chromatographic separation, gave pure fluorescent pyridi-
nium isomers 17-p and 17-m, rare examples of such deriva-
tives[20] of the [closo-B12H12]2À cluster (Table 2, entry 6).
Selected aryliodonium zwitterions were treated with two
types of sulfur species: N,N-dimethylthioformamide, a syn-
thon for the SH group, and thian (Table 3). Thus, reactions of
phenyliodonium derivatives 11a-p and 12 with N,N-dime-
thylthioformamide gave the corresponding protected mer-
captans 21-p and 23, the first such derivatives of the [closo-1-
CB11H12]À cluster and the cobalt dicarbollide,[21] in good yields
(Table 3, entries 2 and 4). Reactions of aryliodonium zwitter-
ions with thian permitted direct substitution of the sulfonium
fragment on the boron cage. Thus, thermolysis of a regioiso-
meric mixture of iodonium 8c in thian at 908C, followed by
esterification with CH2N2, gave a mixture of sulfonium esters
20, from which only a small amount of the 10-sulfonium ester
20-p was isolated by recrystallization. Ester 20-p, the parent
compound of a class of polar liquid crystals,[4c] was obtained in
higher yield (81%) from the isomerically pure carboxylic acid
8c-p (Table 3, entry 1). This result again represents a signifi-
cant improvement over the previous method.[22] A similar
reaction of the phenyliodonium species 11a-p with thian gave
the sulfonium derivative 22-p in moderate yield (30%;
Finally, the regiospecific introduction of three other
functional groups to closo-borates was demonstrated by
Scheme 3. Preparation of the sulfonium zwitterion 22-p.
Table 4: Reactions of selected iodonium zwitterions with other nucleo-
philes.[a]
Entry
1
Zwitterion
Nucleophile
[NEt4]+ CNÀ
Product and yield
6a[NEt4]
24[2NEt4], 71%
Table 3: Reactions of selected iodonium zwitterions with sulfur nuc-
leophiles.[a]
2
3
11a-p
8c-m
7
[NEt4]+ CNÀ
25-p[NEt4], 65%[23]
Entry
1
Zwitterion
Nucleophile
Product and yield
MeCN
8c-p
(CH2)5S (thian)
26-m, 54%[b][14]
20-p, 81%[b]
4
5
6
[NEt4]+ [AcO]À
[NEt4]+ CNÀ
C6H13MgBr
2
3
11a-p
11a-p
Me2NCHS
27[NEt4], 60%[c]
21-p, 71%
22-p, 30%
12
Ar=4-MeOC6H4
(CH2)5S (thian)
28[NEt4], 73%[24]
11a-p
4
12
Me2NCHS
29-p[NEt4], 83%[10a]
23, 55%
[a] Typical conditions: The aryliodonium zwitterion (1.0 mmol) was
treated with the reagent (1.1 mmol) in solution. [b] After hydrolysis of the
nitrilium zwitterion. [c] A 73% yield of the product was calculated on the
basis of recovered 7.
[a] Typical conditions: The iodonium zwitterion (1.0 mmol) and the
appropriate reagent (2 mL) were stirred at 55–1008C. [b] The product
was isolated after treatment with CH2N2.[22]
4
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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.
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[3] a) R. N. Grimes, Carboranes, 2nd ed., Academic Press, Boston,
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[13] B. Ringstrand, P. Kaszynski, V. G. Young, Jr., Z. Janousek, Inorg.
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[16] Aryl(carboranyl)iodonium cations have been investigated
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Received: December 9, 2014
Revised: March 4, 2015
[11] W. H. Knoth, J. Am. Chem. Soc. 1966, 88, 935 – 939.
Published online: && &&, &&&&
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Boron Cluster Anions
Openo to change: The simple preparation
of iodonium zwitterions of closo-borates
and their reactions with nucleophiles (see
scheme) provided access to a broad
spectrum of cluster derivatives with
potential as new classes of pharmaceut-
icals and materials with tailored proper-
ties. The presented reactions demon-
strate the synthetic versatility of closo-
borate aryliodonium zwitterions.
´
P. Kaszynski,*
B. Ringstrand
&&&& — &&&&
Functionalization of closo-Borates via
Iodonium Zwitterions
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!