Highly nucleophilic dipropanolamine chelated boron reagents for aryl-transmetallation to iron complexes

Article abstract of DOI:10.1039/c5dt03835a

New aryl- and heteroarylboronate esters chelated by dipropanolamine are synthesised directly from boronic acids. The corresponding anionic borates are readily accessible by deprotonation and demonstrate an increase in hydrocarbyl nucleophilicity in comparison to other common borates. The new borates proved competent for magnesium or zinc additive-free, direct boron-to-iron hydrocarbyl transmetallations with well-defined iron(ii) (pre)catalysts. The application of the new borate reagents in representative Csp²-Csp³ cross-coupling led to almost exclusive homocoupling unless coupling is performed in the presence of a zinc additive.

Full text of DOI:10.1039/c5dt03835a

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Highly nucleophilic dipropanolamine chelated
boron reagents for aryl-transmetallation to iron
complexes†
Cite this: Dalton Trans., 2015, 44,
20577
Jay J. Dunsford, Ewan R. Clark and Michael J. Ingleson*
New aryl- and heteroarylboronate esters chelated by dipropanolamine are synthesised directly from
boronic acids. The corresponding anionic borates are readily accessible by deprotonation and demon-
strate an increase in hydrocarbyl nucleophilicity in comparison to other common borates. The new
borates proved competent for magnesium or zinc additive-free, direct boron-to-iron hydrocarbyl trans-
metallations with well-defined iron(II) (pre)catalysts. The application of the new borate reagents in repre-
sentative Csp2–Csp3 cross-coupling led to almost exclusive homocoupling unless coupling is performed
in the presence of a zinc additive.
Received 1st October 2015,
Accepted 26th October 2015
DOI: 10.1039/c5dt03835a
www.rsc.org/dalton
additives such as MgBr2, ZnCl2 or Zn(Ar)2 are required for
good heterocoupling yields in the Fe catalysed SM reaction,
Introduction
Nucleophilic organoboron compounds are crucial in modern with additive free systems providing inferior heterocoupling
synthesis, particularly in transition metal catalysed carbon– outcomes.14 These additives are proposed to facilitate hydro-
carbon bond formation such as the synthetically ubiquitous carbyl transmetallation, although their specific role(s) in this
Suzuki–Miyaura (SM) reaction.1–3 In the SM reaction the trans- process have yet to be defined. Hydrocarbyl transmetallation
fer of a hydrocarbyl group from boron to a transition metal is from boron-to-iron16 employing modular boronate reagents
an essential step and one that has been extensively studied for that are readily accessible, particularly from the widely avail-
palladium systems.4 Extending the SM reaction from palla- able boronic acids, and are activated for transmetallation
dium catalysis to base metal catalysts, with a particular empha- using simple bases (e.g., Group I M[OR] salts) would be attrac-
sis on iron is important to reduce our current reliance upon tive for iron catalysed SM protocols.17 Herein, we describe our
expensive and toxic palladium catalysts.5,6 The unique reaction efforts toward this goal, principally through the design of
pathways accessible to iron catalysis6–10 (both one and two highly nucleophilic borate reagents and their application in
electron manifolds) also represent a considerable driving force boron-to-iron hydrocarbyl transmetallation.
for the advancement of this area. Recent advances in iron cata-
lysed SM type cross-coupling have been significant, particu-
larly in the employment of anionic borate nucleophiles,
[B(Ar)4]−,11,12 [RB(pin)(tBu)]− (pin = pinacolato, R = aryl/
alkenyl)13,14 and [R′BR3]− (R′ = Alkyl or Aryl).15 However, in
contrast to Pd catalysed SM, where ArB(OR)2 and a range of
bases in aqueous/ether media readily effect boron to Pd trans-
metallation, transmetallation to iron is more challenging and
analogous conditions do not effect hydrocarbyl transfer.
Instead boron to iron transmetallation requires organometallic
Results and discussion
Our recent investigations on boron-to-iron hydrocarbyl trans-
metallation, employing [Fe(NHC)2X2] (NHC = N-heterocyclic
carbene, X = Cl or OMe) systems with arylborate nucleophiles
afforded key observations of significance to our current investi-
gation.18 Specifically, that alkoxide transfer from arylborates of
the general formulae, [ArB(eg)(OMe)]− (eg = ethylene glycolato)
occurs in preference to aryl transfer (Fig. 1). Considering this,
we set out to design a new class of highly nucleophilic borates
that incorporate tethered, trianionic ligands and are accessible
from boronic acids and Group I M[OR] salts, with the inten-
tion of facilitating hydrocarbyl transfer in preference to alkox-
ide transfer (Fig. 1).
t
activating agents (e.g. BuLi or ArMgX) in examples employing
pinacolboronate esters or triorganoboranes.13–15 Furthermore,
School of Chemistry, University of Manchester, M13 9PL, UK.
E-mail: Michael.ingleson@manchester.ac.uk
†Electronic supplementary information (ESI) available: Full experimental,
computational and X-ray crystallographic details and all relevant NMR spectra.
CCDC 1010524, 1010526 and 1058782. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c5dt03835a
Borate reagents incorporating tethered, trianionic ligands
are known,19 with the triolborates particularly noteworthy,20
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Fig. 1 Proposal to favour hydrocarbyl transmetallation over alkoxide
transfer to iron from boronate reagents.
however these are non-productive in Fe catalysed SM coup-
lings.12 Borates incorporating a trianionic 6,6-bicyclic chelate
are rare,21 but are attractive as they will afford a reduction in
ring strain in the conjugate 3-coordinate boronate ester
formed post hydrocarbyl transfer relative to 5,5-bicyclic and
6,6,6-tricyclic (e.g. triolboronate) systems. This would facilitate
the planarisation of the neutral 3-coordinate boronate ester by-
product and thus its stabilisation through increased π-orbital
overlap, an outcome enhanced further by the greater multiple
bond character present in B-NR2 relative to B-OR; both factors
were envisaged to enhance the nucleophilicity of the anionic
hydrocarbyl-borate.
Fig. 2 Synthesis of dipropanolamine chelated neutral boronate esters
1a–1q.
relative nucleophilicity of K[2a] and other common borate
Combining 1.1 equiv. of dipropanolamine (which can be anions. PhIAs were calculated from the isodesmic reactions
easily prepared on multi-gram scales from cheap, commer- between tetraphenylborate and the appropriate neutral boron
cially available reagents)22 with phenylboronic acid (1 equiv.) Lewis acid (Table 1). This methodology is analogous to pre-
in THF afforded the desired neutral boronate ester 1a within vious approaches for calculating hydride and chloride ion
10 minutes at ambient temperature. The poor solubility of 1a affinities (HIAs and CIAs) of boron Lewis acids, where calcu-
in THF allows for its isolation by filtration and subsequent lations of HIA correlated well with hydride transfer reactiv-
recrystallisation from hot acetone affords analytically pure 1a ity.26,27 Analysis of the calculated PhIA values revealed neutral
in 89% yield (9.74 g) (Fig. 2). The rigid nature of the 6,6-bi- borane 3 to have a substantially lower PhIA (hence K[2a] will
1
cyclic chelate is evident in the H NMR spectra of 1a through provide a better source of Ph−) than other common borate
the diastereotopic nature of the CH2 protons within the chelate reagents tested (Table 1). The alleviation of ring strain in 3
backbone, a feature also observed in related 5,5-bicyclic relative to the 5,5-bicyclic analogue derived from diethanol-
systems.23 It is noteworthy that 5,5-bicyclic analogues demon- amine (5,5-ONO, entry 4) and to a greater extent the 6,6,6-tri-
strate fluxional behaviour at temperatures above 40 °C, cyclic containing triolborane (entry 1) does indeed have a
whereas 1a is considerably more conformationally rigid, considerable effect on PhIA and thus the energetics of aryl
demonstrating no fluxional behaviour up to 110 °C (in d6- transfer.
DMSO), analogous to the extremely robust MIDA boronate
The trends observed through calculated PhIAs were probed
esters.24 This simple esterification protocol can be extended to experimentally by phenyl ion transfer reactions. Initial studies
a number of aryl- or heteroarylboronic acids, affording neutral commenced with K[2a]/3 and boron reagents possessing con-
boronate esters 1a–1q in good to excellent isolated yields siderably different calculated PhIAs. The addition of K[2a] to
(97–72%) (Fig. 2).
triolborane in THF resulted in slow dissolution of triolborane
Generation of the anionic borates is simple as exemplified (which is poorly soluble in THF), with initial 11B NMR spectra
by the formation of K[2a] which can be achieved in quantita- (after ca. 10 minutes at 20 °C) showing a mixture of K[2a]
tive yield via deprotonation of 1a with a variety of bases (KH, (δ11B = 2.5) and a new species (δ11B = 4.4) which is tentatively
tBuOK, KOMe) in anhydrous THF.25 The employment of KOH attributed to reversible Lewis adduct formation between K[2a]
leads only to the recovery of neutral boronate ester, 1a. This and triolborane, presumably by coordination of the N or O
observation correlates with the fact that K[2a] slowly hydrolyses nucleophilic sites in K[2a] to the Lewis acidic boron centre of
to 1a (and presumably KOH) with prolonged storage in the the triolborane. Nevertheless, heating this mixture at 60 °C for
solid or solution state under air. With methods for the gene- 18 h resulted in complete disappearance of both of these
ration of the anionic borates in hand, we next focused upon signals, full dissolution of the triolborane and formation of 3
calculating relative phenyl ion affinities (PhIAs) to assess the (δ11B = 20.0) and [Ph-triolborate]− (δ11B = 3.6) as the major
20578 | Dalton Trans., 2015, 44, 20577–20583
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Table 1 Relative phenyl ion affinities (PhIAs) of neutral boron Lewis
acids derived from common arylborate reagentsa
Entry
1
Lewis acid
Abbreviation
Triolborane
PhIAa (kcal mol−1
)
Scheme 1 Phenyl
trimethoxyborane.
ion
transfer
reaction
from
K[2a]
to
−22.8
2
3
BF3
BPh3
—
—
−16.3
[PhB(Pin)-(nBu)]− resulted in minimal formation of [2a]− after
16 h at 60 °C.
0.0
4
5,5-ONO
+17.0
Encouraged by the calculated PhIA value of 3 and the
stoichiometric phenyl ion transfer experiments, we moved on
to probe whether high hydrocarbyl nucleophilicity in [2a]−
would translate to facile boron-to-iron hydrocarbyl transmetal-
lation with common iron(II) (pre)catalysts. We began this
investigation with the (pre)catalyst, [Fe(dppe)Cl2]n 4 (dppe =
1,2-bis(diphenylphosphino)ethane) (4 was found by X-ray diffr-
action studies to exist as a 1D coordination polymer in the
solid state on recrystallisation from THF).28 In situ generation
of K[2a] with KH (1 equiv.) in anhydrous THF followed by the
addition of 4 (0.5 equiv.) and dppe (0.5 equiv.) then agitation
at ambient temperature led to no observable reaction (by
11B{1H} NMR spectroscopy, with K[2a] the only signal
observed) after 1 h. However, heating the reaction mixture for
10 minutes at 60 °C in THF (or in toluene) led to the pale
green reaction mixture becoming deep red in colour. Analysis
5
6
B(OMe)3
—
+22.7
+23.2
B(Pin)(tBu)
7
8
B(Pin)(nBu)
+23.9
3 (This work)
+36.3 (+31.3)b
a PhIAs calculated at the M06-2X/6-311G+(d,p) level of theory with
incorporation of
dichloromethane PCM solvent model. b In
parenthesis the PhIA is calculated using atomic coordinates from the
a
solid state structure of K[2a].
1
of the reaction mixture by H NMR spectroscopy was uninfor-
mative, however the 11B{1H} NMR spectra revealed the com-
plete disappearance of the 4-coordinate signal of K[2a] and the
presence of a single new boron containing species in the
three-coordinate region at δ11B 23.9 ppm attributed to 3.29 Iso-
species. The new boron species at δ11B = 20 was confirmed as
the expected neutral 3-coordinate boron by-product of hydro-
carbyl transmetallation via the independent synthesis and
characterization of 3 from dipropanolamine and Me2S·BH3.25
The reaction of K[2a] with Et2O-BF3 proceeded more rapidly
with no starting materials observed after <10 min at 20 °C (by
11B NMR spectroscopy), instead 3 and [PhBF3]− are the major
species observed, along with an unidentified intermediate
(δ11B = 0.1) observed at short times which is fully consumed on
heating to 60 °C.
It is more significant in the context of this investigation to
experimentally benchmark the PhIA of 3 against boron Lewis
acids whose Ph-borate anions transmetallate to Fe (with or
without additives). However, attempts to experimentally
confirm the relative PhIA of BPh3 and 3 were frustrated by
solubility issues (in THF and DCM) along with the formation
of an intermediate (δ11B = 0.8) that persisted even on heating
to 60 °C for 18 h, this species frustrated crystallisation
attempts and is tentatively assigned to a Lewis adduct between
K[2a] and BPh3. In contrast, combination of K[2a] and
B(OMe)3 resulted in rapid phenyl transfer at 20 °C (with a
short lived intermediate again observed, δ11B = 3.1), to ulti-
mately form 3 and [PhB(OMe)3]−, confirming the relative PhIA
of 3 and B(OMe)3 (Scheme 1). The combination of K[2a] and B
(Pin)(nBu) resulted in only a minor amount of 3 formed after
16 h at 60 °C. We attribute the limited phenyl transfer to a
kinetically slow process as opposed to an equilibrium favour-
ing B(Pin)-(nBu) and K[2a] as the reverse reaction of 3 with
t
lated samples of K[2a] generated from BuOK or KOMe pro-
duced analogous outcomes, provided all protic by-products
from boronate generation (tBuOH/MeOH) were removed prior
to use due to their detrimental effect on these specific iron
complexes. The requirement for raised temperatures for con-
version of K[2a] to 3 on addition of 4/dppe is in contrast to the
reaction of Li[(Ph)B(pin(tBu)] (5) with 4 (0.5 equiv. + 0.5 equiv of
dppe) which in our hands was found to react within minutes at
ambient temperature in the absence of MgBr2 to produce
tBuBPin (by 11B NMR spectroscopy). Thus whilst [2a]− is calcu-
lated to be thermodynamically a more nucleophilic source of
Ph− than 5 it is kinetically slower to react by phenyl transfer
with the Fe species present in solution under these conditions.
With the in situ 11B{1H} NMR data being consistent with
hydrocarbyl loss from [2a]− we sought evidence for the for-
mation of arylated iron species. Most notably the generation of
significant quantities of biphenyl (ca. 0.5 equiv.) post aryl
transfer was confirmed by analysis of the reaction mixture by
GC-MS against a calibrated internal standard,25 indicative of
Fe-Ph species.14b Consumption of all K[2a] on addition of 0.5
equiv. of 4 (by 11B{1H} NMR spectroscopy) suggests the trans-
fer of two aryl equivalents to iron, although the transfer of just
one aryl equivalent to Fe and formation of an adduct between
an Fe species and a second equivalent of [2a] (which may not
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be observable in the 11B NMR spectra) is also feasible. We dis- by Bedford and co-workers30 and with an independently pre-
favour a reaction stoichiometry other than 2 : 1 as increasing pared sample of 7 obtained from the reaction of 6 with
the ratio of K[2a] : Fe from 2 : 1 to 3 : 1 and to 4 : 1 resulted in p-TolMgBr (2 equiv.) (Fig. 3). Whilst the presence of EPR and
an increasing amount of K[2a] persisting in solution (by 11B NMR spectroscopy silent Fe complexes cannot be precluded
NMR spectroscopy). Analogous outcomes were observed on from these reactions the formation of significant quantities of
combining K[2c] (or K[2d]) and 4 in a 2 : 1 ratio, with signifi- the homocoupled biaryl compounds and the observation of 7
cant biaryl formed in each case (by GC-MS versus an internal by EPR spectroscopy combined do confirm significant boron
standard). Repeated efforts to isolate any Fe species from to iron hydrocarbyl transmetallation using [2x].
transmetallation to iron complexes with K[2a] were unsuccess-
With boron-to-iron hydrocarbyl transmetallation confirmed
ful in our hands.
we moved on to evaluating the efficiency of K[2a] in iron cata-
[Fe(dpbz)2Cl2], 6 (0.5 equiv.) (dpbz = 1,2-bis(diphenylphos- lysed Csp2–Csp3 SM cross-coupling. Employing K[2a] and
phino)benzene) reacted analogously with K[2a] or K[2b], with 3-methoxybenzyl bromide in the presence of 4 + dppe (both at
two equivalents of borate consumed per Fe centre (by 11B NMR 10 mol% loading) led to the complete consumption of
spectroscopy) and significant biphenyl or bi-tolyl (ca. 0.5 3-methoxybenzyl bromide and almost exclusive formation of
equiv. by GC-MS) formed again indicating significant transme- 1,2-bis-(3-methoxyphenyl)ethane with only
a very minor
tallation from boron to iron. The previously reported iron(I) amount of heterocoupled product (by GC-MS). This obser-
complex, [Fe(dpbz)2(p-Tol)] 730 (Fig. 3) was observed from this vation is in contrast to the outcomes reported employing the
reaction mixture by X-band continuous-wave EPR measure- tBuLi activated arylpinacol boronate esters, such as 5, or
ments at 120 K further confirming successful boron to Fe Grignard activated triorganoboranes, which under similar con-
transmetallation. The spectrum of 7 is in excellent agreement ditions, in the presence of magnesium or zinc additives led
with previous X-band CW-EPR measurements obtained for 7 predominantly to heterocoupling products.14,15 Analogous
homocoupling outcomes to that observed through the use of K
[2a] were found in the employment of the lithium salt Li[2a],
hence the difference in reactivity between M[2a] and 5 is not
cation derived. Furthermore, repeating the cross coupling reac-
tions using Li[2a] with MgBr2 additive, led to homocoupling
again being the dominant pathway.
Diarylzinc co-catalysts are used widely in Fe SM catalysis,
but it is important to note that species formulated as aryl2Zn
are actually often ionic zincates, and this can significantly
impact reactivity in cross-coupling.31 For example, the pre-
viously reported attempted synthesis of (p-tol)2Zn led instead
to the zincate [Mg(THF)4Br2(Zn(p-tol)2)2]n, 8 (on recrystallisa-
tion, Scheme 2, inset).32 Using 10 mol% of a stock solution of
8 (equating to a total of 0.08 mmol of tolyl nucleophile) and
5 mol% 4/dppe resulted in the heterocoupling product from K
[2a] and cyclo-heptylbromide being the major product
(Scheme 2). However, the product distribution from this reac-
tion is significantly less selective towards heterocoupling than
cross coupling cycloheptylbromide using [tBuB(Pin)Ar]− (or
[Ar(iPr)BBN]−) with magnesium or zinc additives.14,15 Repeat-
ing the coupling of cyclo-heptylbromide with K[2a] in the
absence of 8, with or without MgBr2 as an additive, led to no
Fig. 3 Top, reaction of K[2a] and 4 (0.5 equiv.). Middle, the formation
of the Fe(I) complex 7 from K[2b] and 6 (0.5 equiv.). Bottom, comparison
of the CW-EPR spectra recorded at 120 K of in situ generated 7 from
either K[2a] (black) or p-TolMgBr (grey).
Scheme 2 Zincate mediated Fe catalysed SM cross coupling using K
[2a].
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heterocoupling product formation under a range of con- MBraun UniLab glovebox, under an atmosphere of argon
ditions. The omission of 4 also resulted in no heterocoupling (<0.1 ppm O2/H2O). Unless otherwise indicated, solvents were
confirming the importance of Fe under these conditions.
distilled from appropriate drying agents: tetrahydrofuran (pot-
The disparity between magnesium and zinc additives led us assium); toluene (potassium); n-hexane (NaK); and dichloro-
to investigate the direct reaction between MX2 (M = Zn or Mg, methane (CaH2). Tetrahydrofuran and dichloromethane were
X = Br or Cl) and K[2a]. Combination of MgBr2 and K[2a] in stored over activated 3 Å molecular sieves while toluene and
THF resulted in no observable reaction at 20 °C and 60 °C n-hexane were stored over potassium mirrors. 3,3′-azanediyl-
(after 24 h by 11B NMR spectroscopy). In contrast, K[2a] reacts bis-(propan-1-ol),22 [Fe(dppe)Cl2],33 [Fe(dpbz)2Cl2],11 and [Fe-
rapidly with ZnCl2 with complete consumption of K[2a] and (dpbz)2(p-Tol)],30 were all prepared according to previously
formation of 3 as the only new observable boron containing reported literature procedures. All other compounds were pur-
product after 5 h in THF at 20 °C or after 30 min in toluene at chased from commercial sources and used as received. NMR
60 °C (by 11B NMR spectroscopy). This is suggestive of trans- spectra were recorded on Bruker AvanceIII-400, Bruker Avan-
metallation from boron in [2a]− to ZnCl2 but not to MgBr2, ceII-500 or Bruker Ascend-400 spectrometers. Chemical shifts
consistent with the disparate additive performance in Fe cata- are reported as dimensionless δ values and are frequency refer-
lysed cross coupling reactions using K[2a]. Related transmetal- enced relative to residual protio-impurities in the NMR sol-
lation from boron to ZnCl2 has been previously observed, e.g. vents for 1H and 13C{1H} respectively, while 11B{1H}, 19F{1H}
using [sBu3B(p-tolyl)][Mg(solv)Cl],15b which is a boron based and 7Li shifts are referenced relative to external BF3-etherate,
aryl nucleophile also effective in Fe catalysed SM reactions. It hexafluorobenzene and LiCl, respectively. Coupling constants
is notable that less nucleophilic borates such as [PhBF3]− and J are given in Hertz (Hz) as positive values regardless of their
[PhBtriolborane]− give no heterocoupling under Fe catalysed real individual signs. The multiplicity of the signals are indi-
SM conditions even in the presence of zinc additives.12 cated as “s”, “d”, “t” “pent”, “sept” or “m” for singlet, doublet,
In control reactions
2
equivalents of [PhBF3]− (or triplet, pentet, septet or multiplet, respectively. High resolution
[PhBtriolborane]−) were heated in THF at 60 °C for 16 h with mass spectra (HRMS) were recorded on a Waters QTOF mass
ZnCl2. Subsequent analysis of these reaction mixtures in d6- spectrometer. GC-MS analysis was performed on an Agilent
DMSO (in which all boron species, e.g., [PhBY3]− and the con- Technologies 7890A GC system equipped with an Agilent
jugate neutral borane, are soluble) revealed no evidence for Technologies 5975C inert XL EI/CI MSD with triple axis detec-
transmetallation to zinc with [PhBF3]− (or [PhBtriolborane]−) tor. The column employed was an Agilent J&W HP-5 ms ((5%-
being the only major boron product observed (by 11B NMR Phenyl)-methylpolysiloxane) of dimensions: length, 30 m;
spectroscopy).
internal diameter, 0.250 mm; film, 0.25 μm. Microanalysis was
performed by Mr Stephen Boyer at the London Metropolitan
University microanalytical service. X-band CW-EPR spectra were
recorded at 120 K on a Bruker EMXmicro spectrometer operat-
Conclusions
In summary, neutral aryl- and heteroarylboronate esters incor- ing at 9.35 GHz field modulation, 2 mW microwave power and
porating a 6,6-bicyclic chelate and their corresponding anionic equipped with a high sensitivity Bruker cavity (ER 4119HS).
borates can be readily synthesised from the parent boronic
General procedure for the synthesis of 1a–1q
acids. The novel borate species demonstrate enhanced hydro-
carbyl nucleophilicities in comparison to other common aryl- Under ambient conditions, with no additional precautions
borates, as assessed by PhIA calculations and stoichiometric taken to exclude air or moisture, a round bottom flask was
phenyl ion transfer experiments. The new borates undergo charged with 3,3′-azanediylbis(propan-1-ol) (1.1 equiv.) and
additive-free, direct boron-to-iron hydrocarbyl transmetalla- tetrahydrofuran. To this stirred solution was added the appro-
tions with simple bis-phosphine ligated iron(II) (pre)catalysts. priate boronic acid (1 equiv.) and the reaction mixture stirred
However, the application of the new boronates to iron cata- at ambient temperature. Typically, the desired products 1a–1q
lysed Csp2–Csp3 cross-coupling led predominantly to homo- began to deposit from solution after stirring for between 5 and
coupling in the absence of Zn additives, highlighting the 30 minutes, however in examples where this was not the case,
inherent intricacies associated with iron-catalysed Suzuki– stirring was continued at ambient temperature for 16 h. Iso-
Miyaura type Csp2–Csp3 cross-couplings. Presumably, the kine- lation of the insoluble material by filtration followed by
tics of aryl transfer to active Fe species (either direct or via aryl- washing with cold tetrahydrofuran typically afforded 1a–1q as
Zn species) has to be optimal to match other steps involved in colourless free-flowing solids of sufficient purity to be used
the heterocoupling cycle and without zinc additives the trans- without further purification. Alternatively, 1a–1q can be recrys-
fer of aryl from boron in [2a] to key Fe species is non-optimal.
tallised from hot acetone.
10-phenyloctahydro-[1,3,2]oxazaborinino[2,3-b][1,3,2]oxaza-
borinin-5-ium-10-uide21 (1a): Prepared according to the
general procedure. Phenylboronic acid (6.10 g, 50.0 mmol) and
3,3′-azanediylbis(propan-1-ol) (7.33 g, 55.0 mmol) in tetra-
Experimental
General experimental details
Unless otherwise stated, all manipulations were carried out hydrofuran (200 mL) afforded 1a as a free-flowing white solid
1
using standard Schlenk techniques under argon, or in an (9.74 g, 89%). H NMR (400 MHz, DMSO-d6, 298 K): 7.39 (2H,
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d, J = 6.7 Hz, o-CH); 7.19 (2H, t, J = 7.5 Hz, m-CH); 7.09 (1H, t, Chemistry Software (NSCCS), the EPSRC UK National Electron
J = 7.5 Hz, p-CH); 5.90 (1H, bs, NH); 3.76–3.70 (2H, m, CH2); Paramagnetic Resonance facility at the University of Man-
3.50–3.44 (2H, m, CH2); 3.09–3.02 (2H, m, CH2); 2.86–2.80 (2H, chester, Mr Daniel Sells for assistance with EPR measurements
m, CH2); 1.81–1.72 (2H, m, CH2); 1.51–1.42 ppm (2H, m, CH2). and Professor David Collison for helpful discussions.
13C{1H} NMR (100 MHz, DMSO-d6, 298 K): 131.8; 126.9; 125.5;
59.7; 46.0; 24.3 ppm. 11B{1H} NMR (128.4 MHz, DMSO-d6,
298 K): 3.2 ppm. HRMS (ESI) m/z: calculated for [M + H]+,
C12H19BNO2 , 220.1509, found: 220.1516. Crystals suitable for
References
+
X-ray diffraction were grown by the slow evapouration of a con-
centrated acetone solution of 1a at ambient temperature.
See ESI† for further details for 1b–1q synthesised via the
general procedure.
1 N. Miyaura and A. Suzuki, J. Chem. Soc., Chem. Commun.,
1979, 866–867.
2 N. Miyaura, K. Yamada and A. Suzuki, Tetrahedron Lett.,
1979, 20, 3437–3440.
3 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483.
4 For a review discussion the transmetallation in palladium
catalysed Suzuki–Miyaura see: A. J. J. Lennox and G. C. Lloyd-
Jones, Angew. Chem., Int. Ed., 2013, 52, 7362–7370.
5 E. Negishi, Hand-book of Organopalladium Chemistry for
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8 B. Plietker, Iron Catalysis in Organic Chemistry, Wiley-VCH,
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9 (a) M. J. Ingleson and R. A. Layfield, Chem. Commun., 2012,
48, 3579–3589; (b) B. D. Sherry and A. Fürstner, Acc. Chem.
Res., 2008, 41, 1500–1511.
10 For select recent articles in this area see: (a) S. L. Daifuku,
M. H. Al-Afyouni, B. E. R. Snyder, J. L. Kneebone and
M. L. Neidig, J. Am. Chem. Soc., 2014, 136, 9132–9143;
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General procedure for the synthesis of M[2a] (M = K or Li)
A flame dried Schlenk tube was charged with 1a (1 equiv.) and
dried under vacuum for ca. 1 h prior to the addition of anhy-
drous THF or toluene (X mL) under an argon atmosphere. To
this stirred suspension was added the appropriate base
(1 equiv.) and the reaction mixture stirred at ambient tempera-
ture until the solution became homogeneous. Removal of the
solvent generally afforded a pale residue which could be tritu-
rated with anhydrous n-hexane to afford M[2a] (M = K or Li) as
a free-flowing white solid after isolation by filtration and
drying under vacuum (see Table S1† for isolated yields).
K[2a]: Prepared according to the general procedure. 1a
(750.0 mg, 3.42 mmol) and KH (136.8 mg, 3.42 mmol) in tetra-
hydrofuran (10 mL) afforded K[2a] as a free-flowing white solid
(862 mg, 98%).1H NMR (400 MHz, THF-d8, 298 K): 7.54 (2H, d,
J = 6.9 Hz, o-CH); 7.09 (2H, t, J = 7.2 Hz, m-CH); 6.96 (1H, t, J =
7.5 Hz, p-CH); 3.61–3.56 (2H, m, CH2); 3.52–3.43 (2H, m, CH2);
3.34–3.25 (2H, m, CH2); 2.74–2.66 (2H, m, CH2); 1.88–1.75 (2H,
m, CH2); 1.36–1.26 (2H, m, CH2) ppm. 13C{1H} NMR (100 MHz,
THF-d8, 298 K): 135.1; 127.1; 125.0; 62.6; 51.8; 31.1 ppm. 11B
{1H} NMR (128.4 MHz, THF-d8, 298 K): 2.5 ppm. Anal Calcd
for C12H18BKNO2: C, 56.04; H, 6.66; N, 5.45. Found: C, 55.84;
H, 6.49; N, 5.39. Crystals suitable for X-ray diffraction were
grown by the slow evaporation of a concentrated THF solution
of K[2a] at ambient temperature.
Computational details
Calculations were performed using the Gaussian09 suite of
programmes.34 Structures were pre-optimised at the HF/3-21G
level followed by optimisation at the M06-2X/6-311G+(d,p)
level35 with inclusion of a PCM model for solvent correction
(DCM).36 Structures were confirmed as minima by frequency
analysis and the absence of imaginary frequencies.
11 R. B. Bedford, E. Carter, P. M. Cosgwell, J. N. Harvey,
N. J. Gower, M. F. Haddow, D. M. Murphy, E. C. Neeve and
J. Nunn, Angew. Chem., Int. Ed., 2013, 52, 1285–1288.
Acknowledgements
We gratefully acknowledge the Royal Society (for the award of a
University Research Fellowship for M. J. I.), the European 12 R. B. Bedford, M. A. Hall, G. R. Hodges, M. Huwe and
Research Council under Framework 7 (J. J. D. grant agreement M. C. Wilkinson, Chem. Commun., 2009, 6430–6432.
no. 305868), the Leverhulme Trust (E. R. C.) and the EPSRC 13 For see: T. Hashimoto,
[(alkenyl)B(pin)(tBu)]−
(grant number EP/K039547/1) for funding. We also acknowl-
edge the EPSRC UK National Service for Computational
T. Hatakeyama and M. Nakamura, J. Org. Chem., 2012, 77,
1168–1173.
20582 | Dalton Trans., 2015, 44, 20577–20583
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Paper
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This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 20577–20583 | 20583