Alkali-metal mediated reactivity of a diaminobromoborane: Mono- and bis-borylation of naphthalene versus boryl lithium or hydroborane formation

Article abstract of DOI:10.1039/c2cc31758c

Reaction of lithium with PDABBr [PDA = C₆H₄-1,2- (NTripp)₂, Tripp = 2,4,6-Prⁱ₃C ₆H₂] and naphthalene afforded 2- and 2,6-borylated naphthalenes; conversely, use of high-sodium lithium (0.5% Na) afforded the lithium boryl [(PDAB)Li(THF)₂]; this work establishes that main group reagents can achieve selective borylations of fused polycyclic aromatics under mild conditions in good yields.

Full text of DOI:10.1039/c2cc31758c

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COMMUNICATION
Alkali-metal mediated reactivity of a diaminobromoborane: mono- and
bis-borylation of naphthalene versus boryl lithium or hydroborane
formationwz
Sarah Robinson, Jonathan McMaster, William Lewis, Alexander J. Blake and
Stephen T. Liddle*
Received 9th March 2012, Accepted 2nd April 2012
DOI: 10.1039/c2cc31758c
Reaction of lithium with PDABBr [PDA = C6H4-1,2-(NTripp)2,
Tripp = 2,4,6-Pri3C6H2] and naphthalene afforded 2- and
2,6-borylated naphthalenes; conversely, use of high-sodium
lithium (0.5% Na) afforded the lithium boryl [(PDAB)Li(THF)2];
this work establishes that main group reagents can achieve
selective borylations of fused polycyclic aromatics under mild
conditions in good yields.
are expensive,7 which has led to the search for complementary
stoichiometric main group reagents that will provide new
synthetic methods.3a Of pertinence to this point is the report
by Mulvey of alkali metal-mediated 2- and 2,6-zincation of
naphthalene which could easily be converted into boryl derivatives,
although elevated temperatures were required to effect double
metallation.8 Thus, the selective functionalisation and borylation
of polycyclic aromatic hydrocarbons under mild conditions
represents a significant synthetic hurdle to overcome without
invoking the use of expensive metal complexes. Herein, we
report our preliminary results in this area, namely the control-
lable switching of the reactivity of a sterically demanding
diaminobromoborane in one pot to afford mono- (2-borylated)
and bis- (2,6-diborylated) naphthalenes, or a boryl lithium, or a
hydroborane. The reactivity is completely controlled by the nature
of the alkali metal and the regioselective borylated products are
notable for establishing the formation of thermodynamic, rather
than kinetic, isomers under mild reaction conditions.
The direct and selective functionalisation of hydrocarbon C–H
bonds is an enduring challenge in synthetic chemistry.1 Significant
progress has been made in the conversion of aliphatic C–H bonds
into C–C, CQC, and C–O bonds and aromatic C–H bonds into
C–X bonds (X = C, N, O, halide).2 In contrast, however,
although C–B bonds are rendered highly attractive by virtue of
their extensive utility in synthesis, only since 1993 has their metal
catalysed preparation from C–H groups been developed.3
A
number of transition metal boryl compounds are active in boryla-
tion chemistry, but arguably iridium complexes are the most
effective. For example, Marder showed that a mixture of
[{Ir(OMe)(COD)}2], 4,40-di-tert-butyl-2,20-bipyridine, and (Bpin)2
(pin = OCMe2CMe2O) could borylate naphthalene, pyrene, and
perylene.4 For naphthalene, which represents the simplest and
most important fused-ring aromatic hydrocarbon, mono- and
bis-borylation at the 2- and 2,6- and 2,7-positions, respectively, was
observed. This is notable because electrophilic functionalisation at
the 1- and 2-positions of naphthalene can be kinetically or
thermodynamically controlled, respectively,5 but regioselective
functionalisation is difficult to achieve without directing groups
or high-steric loadings.6
Treatment of the 1,2-phenylenediamine C6H4-1,2-(NH-
2,4,6-Pri3C6H2)2 (PDAH2), which has been fully characterised
and was prepared in 62% yield by a standard palladium-
catalysed cross coupling of C6H6-1,2-Br2 and NH2-2,4,6-
Pri3C6H2,9 with calcium hydride and boron tribromide
afforded, after work-up, multi-gram quantities of PDABBr
(1) as a cream powder in 44% yield.10 The characterisation
data for 1 fully support its formulation. In particular, the
11B NMR spectrum of 1 exhibits a broad singlet at 24.6 ppm
and the X-ray crystal structure reveals a planar, three-coordinate
boron which is coordinated to a terminal bromide and the two
amine centres of the PDA diamine scaffold. In contrast to
boron trihalide compounds, 1 is air- and moisture stable;
hexane solutions of 1 left under a static atmosphere of air
are stable indefinitely and 1 is recovered unchanged if a hexane
solution of 1 is allowed to evaporate from an open flask. The
stability of 1 demonstrates the steric protection afforded to the
boron centre and is germane in terms of the ease of use of this
compound for synthetic applications. With compound 1 in
hand we explored its reactivity with lithium and potassium.
We observed dramatically different, and switchable, reactivity
which is mediated by the nature of the alkali metal and this is
summarised in Scheme 1.
Harsh reaction conditions are often required to effect transi-
tion metal-mediated borylations,3 and some metal catalysts
School of Chemistry, University of Nottingham, University Park,
Nottingham, NG7 2RD, UK.
E-mail: stephen.liddle@nottingham.ac.uk; Fax: +44 115 951 3563;
Tel: +44 115 846 7167
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Full experi-
mental, crystallographic, and computational details for 1–5. CCDC
862667–862671. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc31758c
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5769–5771 5769

2,6- and 2,7-isomers was observed in essentially equal quantities.4
However, by using 1 the formation of the 2,7-isomer of 3 is
completely suppressed and the crystalline yield of 34% for 3
compares well to yields of 10 and 12% for 2,6- and 2,7-
(Bpin)2C10H6. Finally, 3 can be isolated in good yield by adjusting
the reaction stoichiometry and the bulky PDA group imparts
crystallinity enabling straightforward separation of 2 and 3.
The structures of 2 and 3 were determined by X-ray diffraction
and are illustrated in Fig. 1.10 The structural data confirm the
mono- and bis-borylations of naphthalene and 3 straddles an
inversion centre commensurate with its 2,6-functionalisation.
Interestingly, in both 2 and 3 the naphthalene ring is coplanar
with respect to the PDAB ring suggesting some conjugation
between the formally vacant 2p-orbital on boron and the filled
carbon 2p-orbital of the ipso-carbon(s) of the naphthalene rings.
Additionally, in 2 and 3 the 1,3-hydrogens (and symmetry-
related 5,7-hydrogens for 3) are aligned towards the centroids
of the Tripp rings with distances of 2.63 and 2.81 A and 2.79 and
2.91 A, respectively, which may provide a further stabilising
contribution towards the co-planar arrangements. The B–N and
B–C distances for 2 and 3 are unexceptional.11
The assembly of 2 and 3 from relatively simple main group
reagents raises the question as to the reaction mechanism.
Clearly the steric demands of 1 play a part, but the role of the
lithium in this reaction is intriguing. Unfortunately, we were
not able to definitively establish the fate of the eliminated
hydrogen atoms.12 However, lithium bromide was eliminated
which suggests that HBr elimination is unlikely. We tentatively
suggest that the lithium reduces the naphthalene ring and
eliminates lithium bromide; the resulting PDAB+ cation could
then electrophilically attack the electron rich naphthanalide
ring, which is then poised to eject Hꢁ, ultimately forming
dihydrogen before further reduction occurs. Such an electro-
philic attack would be notable as, in contrast to Friedel Crafts
chemistry, intermolecular arene borylation by electrophilic
substitution is rare and ether cleavage-side reactions, which
are not observed with 1, are common with strong boron
electrophiles.7,13 Although the formation of PDAB+ under
reducing conditions may seem counterintuitive, the reduction
is clearly slow (see below). Where radical or borylene groups
are generated in the presence of naphthalene either no reaction
occurs, the boron centres insert into C–H bonds (rather
than substituting them), or [2+1] cycloaddition reactions are
observed.14,15 Thus the reductive reactivity of 1 adds to a
complex palate of reactivity which is emerging in boryl/
borylenoid chemistry. The experimental observation of emerald
green solutions is consistent with the formation of the naphtha-
nalide radical anion, but given that a radical is converted to a
diamagnetic compound little could be gleaned from solution
studies. This reactivity suggests that the electron transfer could
be relatively slow with pure lithium, thus enabling the formation of
2 and 3 instead of a boryl lithium complex of which there are only
a small number.16 In order to test our hypothesis we reacted 1 with
high-sodium lithium powder (0.5% Na) as this is a more reactive
source of lithium than pure lithium.
Scheme 1 Synthesis of 2–5. Reagents and conditions: (i) lithium
powder, THF, ꢀ10 1C, 6 h, –LiBr; (ii) high-sodium content lithium
powder (0.5% Na), THF, ꢀ10 1C, 6 h, –LiBr; (iii) potassium, DME,
ꢀ78 or ꢀ10 1C or room temperature, –KBr.
Carried out under mild conditions (ꢀ10 1C, 6 h), the reaction of
lithium powder with naphthalene and 1 in a 1 : 1 ratio under argon
afforded a mixture of (PDAB)-2-C10H7 (2) and (PDAB)2-2,6-
C10H6 (3). Compounds 2 and 3 are easily separated by fractional
crystallisation and were isolated as colourless crystals in average
yields of 46 and 34%, respectively.10 When the reaction under
these conditions is carried out in a 1 : 2 ratio 3 is isolated in high
yield (82% av.) yield with only trace (4–6%) amounts of 2 formed.
The 1H NMR spectrum of 2 exhibits four broad multiplets
(2 : 3 : 1 : 1 ratio) attributable to naphthalene protons. The
13C NMR spectrum of 2 exhibits ten naphthalene carbon
resonances. Together these data are indicative of the now
asymmetric mono-borylated naphthalene unit. The 11B NMR
resonance at 27.3 ppm is shifted only slightly compared to 1 and
is consistent with retention of a trigonal planar boron centre in 2.
For symmetric 3, the 1H NMR spectrum reveals only three
naphthalene resonances and the 11B NMR spectrum shows a
singlet at 25.8 ppm. The NMR data support the 2-borylated and
2,6-diborylated assignments for 2 and 3 and this was confirmed
by X-ray crystallographic studies (Fig. 1).
The 2- and 2,6-regioselectivity observed in 2 and 3 is
noteworthy. The thermodynamic (2,6) and not the kinetic
(1,5) positions are functionalised even though the reaction is
carried out under mild conditions which favour kinetic
products and this is likely due to the steric demands of the
PDA group. The crystalline yield of 46% for 2 is comparable to a
49% yield for the iridium catalysed preparation of 2-(Bpin)C10H7.4
Furthermore, the second borylation step could occur at the 6- or
7-positions after the initial 2-borylation and this was observed in the
iridium catalysed synthesis of (Bpin)2C10H6 where formation of the
Accordingly, treatment of 1 with high-sodium lithium
under essentially identical reaction conditions to that which
produced 2 and 3 afforded, after work-up, colourless crystals
of the new boryl lithium complex [(PDAB)Li(THF)2] (4)
c
5770 Chem. Commun., 2012, 48, 5769–5771
This journal is The Royal Society of Chemistry 2012

Fig. 1 Molecular structures of (a) 2, (b) 3, and (c) 4 with selective atom labelling and hydrogen atoms omitted for clarity. Selected bond lengths
(A): (a) B(1)–C(37) 1.581(3), B(1)–N(1) 1.441(3), B(1)–N(2) 1.438(3); (b) B(1)–C(39) 1.573(4), B(1)–N(1) 1.443(4), B(1)–N(2) 1.441(4); (c)
B(1)–Li(1) 2.285(4), B(1)–N(1) 1.483(3), B(1)–N(2) 1.482(3), Li(1)–O(1) 1.919(4), Li(1)–O(2) 1.952(4).
in 85% (av.) yield. The spectroscopic and analytical data for 4
7
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are consistent with its formulation and the 11B and Li NMR
spectra reveal broad singlets at 51.0 and 0.6 ppm, respectively.10
The crystal structure of 4 is shown in Fig. 1 and reveals a trigonal
planar lithium centre coordinated by two THF molecules and
the boryl PDAB ligand. The B–Li bond length was found to be
2.285(4) A which falls in the range of known B–Li bond
distances.16
The combined yield of 2 and 3 from the lithium reaction is
80% and the high-sodium lithium reaction produces 4 in 85%
yield. The switchable nature of this reaction is therefore clear-cut,
clearly mediated by the nature of the lithium metal employed,
and entirely controllable. To further confirm this we reacted
1 with the more reactive alkali metal potassium and isolated the
hydroborane compound PDABH (5) with an average yield
of 82%.10 Presumably a boryl potassium complex is formed
transiently, but the putative B–K bond, which would be expected
to be more ionic than the B–Li bond in 4, must be too reactive
and thus abstracts a proton from solvent to give 5.
10 See the Supporting Informationz for full details.
11 From a search of the Cambridge Structural Database (CSD v1.13,
date: 15/12/2011): F. H. Allen, Acta Crystallogr., Sect. B: Struct.
Sci., 2002, 58, 380.
12 Attempts to trap the headspace of the reactions or to analyse the
LiBr byproduct for LiH proved inconclusive.
To summarise, this study establishes that alkali metals can be
employed to modulate the reactivity of a diaminobromoborane. On
one hand regioselective mono- and bis-borylation of naphthalene
can be effected, or alternatively boryl lithium or hydroborane
compounds can be prepared.
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and I. B. Sivaev, Organometallics, 2010, 29, 5366.
We thank the Royal Society, EPSRC, ERC, and the University
of Nottingham for generously supporting this work.
14 (a) P. Bissinger, H. Braunschweig, K. Kraft and T. Kupfer, Angew.
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15 The reactivity of 1 contrasts to less sterically demanding diamino-
haloboranes which react with alkali metals to afford diborane(4)
compounds, see: L. Weber, H. B. Wartig, H.-G. Stammler and
B. Neumann, Z. Anorg. Allg. Chem., 2001, 627, 2663.
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Notes and references
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5769–5771 5771