Unique radical dearomatization and two-electron reduction of a redox-active ligand

Article abstract of DOI:10.1021/ja307050r

The syntheses and characterizations of [Li₄][(1,2-di-(tert- butyl)-dpp-BIAN)₂] (7), (1,2-di-(tert-butyl)-dpp-BIAN) (8), and (1-(tert-butyl)-2-OH-dpp-BIAN) (9) are described. Compound 7 was formed via a radical dearomatization, two-electron reduction pathway that was accompanied by vicinal di-tert-butylation of the BIAN ligand backbone. Oxidation of 7 afforded a dearomatized vicinal di-tert-butyl substituted BIAN ligand (8). An analogous dearomatized vicinal tert-butyl-hydroxy substituted BIAN ligand (9) was also isolated in the course of mechanistic studies related to the formation of 7.

Full text of DOI:10.1021/ja307050r

Communication
pubs.acs.org/JACS
Unique Radical Dearomatization and Two-Electron Reduction of a
Redox-Active Ligand
Daniel A. Evans and Alan H. Cowley*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
multifunctional activity with respect to the addition of
ABSTRACT: The syntheses and characterizations of
[Li4][(1,2-di-(tert-butyl)-dpp-BIAN)2] (7), (1,2-di-(tert-
butyl)-dpp-BIAN) (8), and (1-(tert-butyl)-2-OH-dpp-
BIAN) (9) are described. Compound 7 was formed via a
radical dearomatization, two-electron reduction pathway
that was accompanied by vicinal di-tert-butylation of the
BIAN ligand backbone. Oxidation of 7 afforded a
dearomatized vicinal di-tert-butyl substituted BIAN ligand
(8). An analogous dearomatized vicinal tert-butyl-hydroxy
substituted BIAN ligand (9) was also isolated in the course
of mechanistic studies related to the formation of 7.
electrons, we became interested in exploring the patterns of
reactivity of the dpp-BIAN ligand with a variety of nucleophiles.
A survey of the literature revealed only one previous report
of the dearomatization of a dpp-BIAN molecule.5 In this work,
Hill et al.5 demonstrated that treatment of dpp-BIAN with the
calcium salt of the bulky nucleophile [CH(SiMe3)2]− resulted
in dearomatization of one of the naphthalene rings and
formation of the chiral amido-imine calcium complex 5
depicted in Figure 1.
From the standpoint of the naphthalene moiety, there is an
obvious structural parallel between compounds 1 and 5, from
which it can be inferred that the latter is also formed by a
nucleophilic dearomatization pathway. On the other hand,
there is an interesting contrast with the work of Fedushkin et
al.6 who demonstrated that the dpp-BIAN ligand reacts with
the less bulky nucleophile n-BuLi to afford a different chiral
amido-imine compound, namely 6. In this case the aromaticity
of the naphthalene moiety is preserved as illustrated in Figure 1.
Curious about the consequences of further increases in the
steric bulk of the attacking nucleophile, we decided to
investigate the reaction of dpp-BIAN with t-BuLi in hexanes
solution at ambient temperature (Scheme 1). Initially, the
reaction mixture developed a dark blue color. However, over a
period of approximately 10 min this solution rapidly assumed a
dark purple hue. Concentration of the latter solution, followed
by recrystallization from benzene, afforded a crop of purple
crystals of 7 in 87% yield. A single-crystal X-ray diffraction
earomatization represents a fundamentally important
D
process in the contexts of both organic1 and organo-
metallic chemistry.2 Such dearomatization processes can be
employed to overcome the inherent stabilities of aromatic
systems, thus permitting access to further functionalization.1 A
well-known example features the nucleophilic dearomatization
of naphthalene with tert-butyllithium which results, after
aqueous workup, in the formation of a mixture of compounds
1, 2, and 3 in an overall yield of 17% (Figure 1).3 In the case of
Scheme 1. Dearomatization Reactions of dpp-BIAN with t-
BuLi in Hexanes Solution
Figure 1. Functionalization of naphthalene and dpp-BIAN.
naphthalene, products 1−3 were formed as a result of the
nucleophilic attack of a tert-butyl anion on the aromatic system
which causes a partial loss of aromaticity.
The dpp-BIAN ligand (4) (dpp = 2,6-diisopropylphenyl;
BIAN = bis(imino)acenaphthene) can be regarded as the
product of fusing a 1,4-diaza-1,3-butadiene (R-DAB) moiety to
a naphthalene ring. As first recognized by Fedushkin et al.,4 this
ligand will undergo sequential addition of four electrons when
treated with sodium metal in Et2O or THF solution. Given this
Received: July 18, 2012
Published: September 13, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
study revealed the identity of 7 to be that of the interesting
complex, [Li4][(1,2-di-(tert-butyl)-dpp-BIAN)2] (Figure 2).
process. The diimino functionality of the BIAN ligand was
reduced by two electrons, thereby affording a diamido moiety
as evidenced by inspection of the metrical parameters for the
N−C−C−N fragment of 7 (Table 1). The observed
dearomatization is a consequence of vicinal di-tert-butylation
of the naphthalene backbone of the BIAN ligand. As a result of
this disubstitution, aromaticity is completely absent in the
C(6)−C(8)−C(9)−C(10) and C(50)−C(52)−C(53)−C(54)
fragments. Note, however, that double bonding is retained
between the C(10)−C(11) and C(54)−C(55) linkages (Table
1). Vicinal di-tert-butylation of the naphthalene backbone also
created two chiral centers on each BIAN ligand, namely at
C(8), C(9), C(52), and C(53). However, the formation of 7
occurred with high stereoselectivity and only the trans
stereoisomer was isolated.
Exposure of a hexanes solution of 7 to ambient air resulted in
an immediate color change from purple to red. Concentration
of the red hexanes solution resulted in the isolation of the red
crystalline product 8 in virtually quantitative yield (Figure 4).
Figure 2. ORTEP diagram of 7 with thermal ellipsoids shown at 50%
probability. All hydrogen atoms, one molecule of benzene, and all aryl
isopropyl groups have been removed for clarity.
Figure 3. ORTEP diagram of the Li−N core and appended BIAN
ligand fragments of 7. Thermal ellipsoids shown at 50% probability.
Figure 4. ORTEP diagram of 8 with thermal ellipsoids shown at 50%
probability. All hydrogen atoms have been removed for clarity.
The core geometry of 7 (Figure 3) features a markedly
distorted antiprismatic structure consisting of alternating
lithium and nitrogen atoms which, in turn, are bonded to the
N−C−C−N moieties of the BIAN substituents. The core
structure of 7 is similar to that reported by Fedushkin et al. in
which a silyl-substituted BIAN ligand was reduced by 2 equiv of
lithium metal.7 The overall coordination geometry of 7 is
strikingly asymmetric in the sense that each lithium and
nitrogen atom resides in a unique bonding environment. Three
faces of the antiprism comprise fairly regular parallelograms.
However, the remaining three faces are less symmetrical,
especially the Li(4)−N(1)−Li(1)−N(3) face which is partic-
ularly distorted. This deformed face resulted in the long Li(1)−
N(1) bond distance of 2.887(7) Å which exceeds the range of
1.977(6)−2.265(7) Å that was determined for the remaining
Li−N bonds. Short contacts between each lithium atom and
the C(1)−C(12) and C(45)−C(56) double bonds are also
evident, the contact distances for which range from 2.160(6) to
2.667(5) Å.
The structure of 8 was determined by single-crystal X-ray
diffraction. From the standpoint of the BIAN moieties, the only
significant difference between the structures of 7 and 8 relates
to the N−C−C−N fragments of each compound (Table 1). As
evident from Table 1, the N−C−C−N fragment of 8
underwent a two-electron oxidation, thereby converting the
diamido moiety to a diimino functionality. The formation of
the oxidized diimine of 8 is related to that reported by
Fedushkin et al. in which a hydrolyzed diamine product is
readily oxidized to the cooresponding diimine.8
The unanticipated features of the foregoing reactions
stimulated our interest in the mechanisms of formation of 7
and 8. A plausible mechanistic pathway for the formation of 7
involves two successive independent nucleophilic dearomatiza-
tion steps. Such a mode of nucleophilic attack could result in a
structure akin to that observed in the case of 7. Had this
reaction proceeded via two successive nucleophilic dearomati-
zation processes, a monoalkylated species similar to that
described by Hill et al.5 should have been isolable. However,
despite the use of a 1:1 stoichiometric ratio of reactants, a
monoalkylated derivative was not observed. Instead, this
The structure of 7 is completed by the attachment of two
BIAN moieties to the opposite faces of the antiprism. During
the formation of 7, the BIAN ligand underwent naphthalene
backbone dearomatization and a two-electron reduction
Table 1. Selected Bond Distances for 7, 8, and 9
bond
7
8
9
C(1)−C(12) C(45)−C(56)
C(1)−N(1) C(45)−N(3)
C(12)−N(2) C(56)−N(4)
C(10)−C(11) C(54)−C(55)
1.401(3) Å 1.394(3) Å
1.402(3) Å 1.405(3) Å
1.430(3) Å 1.405(3) Å
1.347(3) Å 1.347(4) Å
1.521(3) Å
1.280(3) Å
1.277(3) Å
1.338(3) Å
1.519(2) Å
1.278(2) Å
1.274(2) Å
1.340(2) Å
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Journal of the American Chemical Society
Communication
stoichiometric ratio only resulted in a diminished yield of the
disubstituted compound 8.
In an effort to isolate a monoalkylated species, the
intermediate dark blue colored solution referred to previously
was investigated (Scheme 1). Interestingly, hydroylsis of the
reaction mixture at the dark blue stage afforded a different dark
purple colored species, the color of which changed to dark red
over a period of 24 h. A red-orange precipitate was extracted
from the dark red hexanes solution and recrystallized from
toluene to afford red-orange crystalline 9 in a modest yield of
34%. Surprisingly, compound 9 is not the anticipated mono-
tert-butyl-dihydro product that would have resulted from a
single nucleophilic dearomatization process followed by
hydrolysis. A single-crystal X-ray diffraction study of 9 revealed
the structure to be that of a mono-tert-butyl-hydroxy analogue
of 8 (Figure 5). The metrical parameters for 9 are almost
identical with those of 8. Akin to the structures of 7 and 8, only
the trans stereoisomer was isolated.
Figure 6. X-band (9.87 GHz) EPR spectrum of the radical
intermediate in toluene solution at ambient temperature: (a)
experimental spectrum with proposed radical structure; (b) simulated
spectrum [AN = 4.67 and 4.20 G, ALi = 2.00 and 1.90 G, line width =
1.20 G].
Figure 5. ORTEP diagram of 9 with thermal ellipsoids shown at 50%
probability. All hydrogen atoms and a disordered molecule of toluene
have been removed for clarity.
occurs in distinct steps. This assertion is supported by the
isolation of the tert-butyl-hydroxy product 9 that was generated
by hydrolysis of the reaction mixture at the intermediate radical
stage. Furthermore, a single tert-butyl group must be present on
the proposed structure to break the C2v symmetry of the BIAN
ligand thereby resulting in nonequivalent nitrogen and lithium
nuclei.
The foregoing radical dearomatization reactions are highly
regio- and stereoselective. Without exception, trans disubstitu-
tion takes place at C(8) and C(9) (Figures 2, 4, and 5).
Furthermore, this transformation is independent of the quantity
of t-BuLi employed. The use of either a deficiency or an excess
of t-BuLi always results in the formation of the vicinal di-tert-
butyl substituted product thus proving that disubstitution is
thermodynamically favored.
In conclusion, we have demonstrated the first example of a
radical dearomatization, two-electron reduction of a BIAN
ligand with t-BuLi, the formation of a Li−N antiprismatic
complex flanked by dearomatized redox-active BIAN ligands,
and the first examples of vicinal di-tert-butyl and tert-butyl-
hydroxy dearomatized BIAN ligands. To the best of our
knowledge, the foregoing radical dearomatization reactions
involving t-BuLi are unprecedented in the realm of BIAN ligand
chemistry.
Given the foregoing results, it was concluded that the
proposed mechanism for the formation of 9 is inconsistent with
that anticipated for a typical nucleophilic dearomatization
process.1 It was therefore hypothesized that the mechanism of
formation of 7, 8, and 9 involves a free radical pathway. Support
for this proposal was provided by EPR monitoring of the
reaction of dpp-BIAN with t-BuLi in toluene solution at
ambient temperature. Toluene, which affords a dark green
radical intermediate species, was chosen as the EPR solvent due
to the poorly defined signal that was detected in hexanes
solution. The EPR spectrum displayed in Figure 6 was recorded
at the initial dark green color stage of the reaction, thus
confirming the postulated free radical mechanism. The EPR
spectrum (g = 2.003286, AN = 4.67 and 4.20 G, ALi = 2.0 and
1.9 G) exhibited a 15 lined spectrum due to hyperfine coupling
of the unpaired electron with two nonequivalent 14N nuclei (I =
7
1, natural abundance 99.6%) and two nonequivalent Li nuclei
(I = 3/2, natural abundance 92.58%). The hyperfine coupling
of the radical intermediate to the N−C−C−N fragment of
BIAN is typical of that for a dpp-BIAN radical anion.9
Overall, the details concerning the proposed mechanisms of
formation of 7, 8, and 9 can be understood on the basis of the
radical intermediate that was detected by EPR spectroscopy in
conjunction with the X-ray crystallographic data for 7, 8, and 9.
The proposed structure for this radical intermediate (Figure 6)
features a single tert-butyl group on the naphthalene backbone
and two lithium atoms in the N−C−C−N fragment of BIAN,
where the unpaired electron exhibits hyperfine coupling. The
aforementioned hyperfine structure along with the formation of
9 implies that tert-butylation of the naphthalene backbone
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental data, spectroscopic characterization, and
single-crystal X-ray crystallographic data are provided. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
cowley@mail.utexas.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Robert A. Welch Foundation
(Grant No. F-0003) is gratefully acknowledged.
■
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