1-Alkali-metal-2-alkyl-1,2-dihydropyridines: Soluble Hydride Surrogates for Catalytic Dehydrogenative Coupling and Hydroboration Applications

Article abstract of DOI:10.1002/chem.201703609

Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-tert-butyl-1,2-dihydropyridine (1tLi) functions as a precatalyst to convert Me₂NH?BH₃ to [NMe₂BH₂]₂ (89 % conversion) under competitive conditions (2.5 mol %, 60 h, 80 °C, toluene solvent) to that of previously reported LiN(SiMe₃)₂. Sodium and potassium dihydropyridine congeners produce similar high yields of [NMe₂BH₂]₂ but require longer times. Switching the solvent to pyridine induces a remarkable change in the dehydrocoupling product ratio, with (NMe₂)₂BH favoured over [NMe₂BH₂]₂ (e.g., 94 %:2 % for 1tLi). Demonstrating its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes, though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of Group 1 metal-catalysed processes are discussed.

Full text of DOI:10.1002/chem.201703609

A Journal of
Accepted Article
Title: 1-Alkali-metal-2-alkyl-1,2-dihydropyridines: soluble hydride
surrogates for catalytic dehydrogenative coupling and
hydroboration applications
Authors: Ross McLellan, Alan R Kennedy, Robert E. Mulvey,
Samantha A Orr, and Stuart D Robertson
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201703609
Link to VoR: http://dx.doi.org/10.1002/chem.201703609
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1
-Alkali-metal-2-alkyl-1,2-dihydropyridines: soluble hydride
surrogates for catalytic dehydrogenative coupling and
hydroboration applications
[
a]
[a]
[a]
[a]
Ross McLellan, Alan R. Kennedy, Robert E. Mulvey,* Samantha A. Orr and Stuart D.
[
a]
Robertson*
[
a]
Dr R. McLellan, Dr A. R. Kennedy, Prof. R. E. Mulvey, Dr S. A. Orr, Dr S. D. Robertson
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde
Glasgow G1 1XL (UK)
E-mail: r.e.mulvey@strath.ac.uk; stuart.d.robertson@strath.ac.uk
Supporting information for this article is given via a link at the end of the document.
Abstract: Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-t-butyl-1,2-dihydropyridine
1tLi) functions as a precatalyst to convert Me NH·BH to [NMe BH (89% conversion) under competitive conditions (2.5
mol%, 60h, 80°C, toluene solvent) to that of previously reported LiN(SiMe . Sodium and potassium dihydropyridine congeners
produce similar high yields of [NMe BH but require longer times. Switching the solvent to pyridine induces a remarkable
change in the dehydrocoupling product ratio, with (NMe BH favoured over [NMe BH (e.g., 94%:2% for 1tLi). Demonstrating
(
2
3
2
2 2
]
3 2
)
2
2 2
]
2
)
2
2
2 2
]
its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection
of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes,
though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of group 1
metal catalysed processes are discussed.
Introduction
The prevailing chemistry of dihydropyridines (DHPs) is dominated by their hydrogen transfer ability, a property resulting
from their propensity to (re)gain the classic 6 p electron aromaticity of the parent pyridine. The most important DHP is
NADH (nicotinamide adenine dinucleotide), through its role in biology as an electron transporter used for energy
1
2
creation. Two important examples of DHPs in synthetic chemistry are the Hantzsch esters, exploited, for example,
3
+
under the name Nifedipine as calcium antagonists in hypertension treatment; and Lansbury’s reagent Li [Al(1,4-
4
NC
5
H
6
)
4
]¯, a highly selective stoichiometric reducing agent. Hantzsch esters, and indeed most DHPs, exist as
with excess pyridine,
thermodynamically preferred 1,4-isomers. With Lansbury’s reagent, formed by reaction of LiAlH
4
the isomeric ratio (1,2-:1,4-), and hence the active species identity in any given reaction is less clear and depends on
reaction conditions, that is, the initially formed kinetic 1,2-isomer converts to the 1,4-isomer over time or with increased
5
0
temperature. An emerging advance in the chemistry of main group (or d ) DHPs is the realisation of their usefulness in
6
catalytic processes such as the hydroboration or hydrosilylation of pyridines and related heterocycles. Particularly
noteworthy are reports by Hill who utilised a DIPPnacnac-MgnBu (DIPPnacnac = [(2,6-iPr
2
C
6
H
3
)NC(Me)]
2
CH) (Figure
6
a
1
A) complex affording mixtures of 1,2- and 1,4-DHP products, (Scheme 1) and Harder who used (DIPPnacnac-
6
d
CaH∙THF)
2
(Figure 1B) to selectively give 1,2-DHP products.
Significantly, each group 2 catalysed reaction is
proposed to involve M-H intermediates.
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Figure 1. A) Depiction of DIPPnacnac-MgnBu precatalyst; B) Depiction of (DIPPnacnac-CaH∙THF)
2
precatalyst; C) 1-Li-2-tert-butyl-1,2-
3
4
dihydropyridine unit; D) Molecular structure of 1tLi∙Me AEE with all H atoms other than that bonded to the dihydropyridyl sp C atom omitted for
7
b
clarity.
Recently we began to systematically investigate the synthesis and reactivity of a series of kinetically stable 1-lithio-2-
alkyl-1,2-dihydropyridines (1, Figure 1C) (alkyl = n-, i-, s- or t-butyl) making the surprising finding that they can be
7
isolated as stable solids provided that a 1:1 stoichiometric alkyllithium:pyridine ratio is used in their preparation.
Significantly in the case of s-, and t-butyl isomers, 1sLi and 1tLi, the resulting cyclotrimeric aggregates were found to be
soluble in hexane at room temperature, thus offering a synthetically important advantage over the insoluble rock salt
8
lattice structure of LiH. 1tLi can also be isolated as a monomer by coordination with neutral Lewis bases such as bis-[2-
(
4 4
N,N-dimethylamino)ethyl]ether (Me AEE) in 1tLi∙Me AEE (Figure 1D). Promisingly, reactivity studies revealed that 1sLi
and 1tLi are effective LiH transfer agents to the unsaturated C=O bond in benzophenone. Metathetical reactions of 1tLi
with NaOtBu or KOtBu resulted in the production of isolable heavier alkali-metal congeners 1tNa or 1tK, both of which
9
exhibit similar reactivity to 1tLi in stoichiometric hydrometallation reactions. Moreover, we recently disclosed the first
example of a group 1 DHP complex (1tLi) functioning as an effective (pre)catalyst, in the dehydrogenative cyclisation of
1
0
diamine boranes, therein establishing the dual role of 2-t-butylpyridine as a LiH storage/release vessel, and moreover
delivering a rare example of a lithium based precatalyst.
6
b
Scheme 1. Depiction of catalytic hydroboration of pyridine.
With a series of soluble alkali-metal hydride surrogate congeners in hand we sought to examine their application in two
distinct catalytic processes, namely dehydrocoupling of amine boranes and hydroboration of aldehydes and ketones. In
each reaction metal hydride species have been found to either catalyse or have been identified as key intermediates in
the process. The controlled formation of boron-nitrogen bonds via dehydrocoupling of amine boranes, HNR
2
∙BH
3
(R =
1
1
H, alkyl) is a reaction that attracts widespread attention in the synthesis of novel polymers and ceramics, and in the
1
2
arena of hydrogen storage materials. Thus over the past two decades much activity has been directed at transition
metal catalysed dehydrocoupling of ammonia borane and amine boranes, and moreover much insight has been
1
3
garnered regarding mechanistic aspects of the various catalytic pathways. Recent insightful work from the groups of
1
4
15
16
17
0
Harder, Hill and Wright, among others, demonstrated that main group (d ) complexes are active in both
stoichiometric and catalytic dehydrocoupling of main group element-H bonds. Furthermore, Bertrand demonstrated that
1
8
cross-dehydrocoupling of secondary boranes with alcohols, thiols and amines can be accomplished without a catalyst.
It is also noteworthy that precatalysts discussed in these reports tended to be more economically viable and
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environmentally innocuous than their invariably expensive and toxic noble transition metal counterparts, albeit at this
point they do not (yet) match the best catalytic efficiencies. Among the most studied main group precatalysts are those
from group 2 and group 13 which typically contain bulky b-diketiminato or (silyl)amide ligands. Similarly these group 2
complexes and related species have been found to catalyse the hydroboration of a range of substrates, including
6
19
20
21
22
pyridines, aldehydes and ketones, nitriles, isonitriles, and esters. Impressively the hydrosilylation of alkenes
2
3
using a potassium hydride catalyst was reported by Harder. More recently Okuda has provided mechanistic evidence
2
4a
for potassium catalysed hydrosilylation of a range of alkenes using a K(18-Crown-6)(SiPh
3
) catalyst. Further, the
Okuda group has recently demonstrated that alkali-metal hydridotriphenylborates can catalyse the hydroboration of
2
4b
benzophenone. These transformations, for example converting an aldehyde into an alcohol, are of central importance
within organic chemistry and have historically been accomplished using stoichiometric metal hydride species, e.g.
2
5
4
LiAlH , which can suffer from poor functional group selectivity and low solubility in hydrocarbon solvents. Thus
utilisation of milder hydride sources (e.g. boranes) in tandem with a suitable catalyst remains a tantalising synthetic
strategy. Breakthroughs reported herein will extend the versatility of hydrocarbon soluble group 1 DHPs as metal
hydride surrogates in the catalytic dehydrocoupling of amine boranes and in hydroboration of aldehydes and ketones.
We also disclose the crucial importance of reaction solvent on catalytic efficiency.
Results and Discussion
Dehydrogenative coupling with a lithium dihydropyridyl precatalyst
A particularly well understood substrate is dimethylamine borane, HNMe
2
∙BH
(Figure 2A). Essentially the reaction follows four steps: A)
by metal amide; B) b-hydride elimination to afford a metal hydride and NMe -BH ; C) insertion
into another equivalent of the metallated amidoborane generated in step A (B-N bond forming step); D) b-
or d-hydride elimination to afford final products, (NMe BH (III) and (NMe BH (IV), and regenerate metal hydride
catalysts. Note step E is explained below. In certain cases intermediates containing a metal-bound [NMe
3
, and a general mechanism has been
1
5a
proposed to rationalise its dehydrocoupling process
metallation of HNMe BH
of NMe -BH
2
3
2
2
2
2
2
)
2
2
2 2
)
-
2
BH
] results from polar insertion of NMe
and is the immediate precursor of the final reaction product(s). Complementary theoretical studies
2
NMe
2
BH
3
]
1
5a,b
-
anion (II) were isolated and structurally characterised.
into M-NMe BH
[NMe
2
BH
2
NMe
2
BH
3
2
-BH
2
2
3
2
6
support this general mechanistic picture, though the b-hydride elimination pathway (an apparent two-step process) is
reportedly energetically disfavoured.
Hill recently noted the first example of group 1 silylamide precatalysts [MN(SiMe
3
)
2
, M = Li, Na, K] for
in toluene gave the best conversion, determined via
BH after heating at 80 °C for 124 h. In this study an intermediate
3
] complex was isolated, indicating that the catalysis likely follows that suggested for
2
7
3 2
dehydrocoupling of dimethylamine borane. 5 mol% of LiN(SiMe )
1
1
B NMR, to 72% [NMe
2
BH
2
]
2
and 5% (NMe
2
)
2
-
potassium [NMe BH NMe
2
2
2
BH
Group 2 and 13 precatalysts (Figure 2A). These important results are more impressive given that the catalytically active
metal-hydride species are reported to form insoluble aggregates during the experiments, slowing down the process,
particularly for the heavier alkali metal silylamides NaN(SiMe
encountered by Wright on employing LiAlH as a catalyst in a similar reaction with HNMe
LiN(SiMe catalysed cross-dehydrocoupling of HBpin or 9-BBN with a range of amines, another rare example of group
catalysis. It is therefore apparent that effective solubility of key metal-hydrides is critical for high catalytic efficiency.
3
)
2
and KN(SiMe
3
)
2
. Solubility problems have also been
4
2
∙BH , and by Panda in the
3
3 2
)
2
8
1
Given that 1tLi represents a soluble source of lithium hydride in hexane, we reasoned that the in situ generated metal-
hydride would exist as a soluble dihydropyridine species, thus enhancing the catalytic process.
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0
11
Figure 2. A) Proposed mechanism for d based dehydrogenative coupling of HNMe
mol%) and HNMe ∙BH in d -toluene.
2 3
∙BH ; B) B NMR spectra of the reaction between 1tLi (2.5
2
3
8
1
1
Reaction between 2.5 mol% 1tLi and HNMe
integrals) to 89% of [NMe BH ] and 4% of (NMe
faster than that of 5 mol% [Mg{CH(SiMe (THF)
The in situ 1tLi induced reaction was monitored by B NMR spectroscopy
Figure 2B) revealing the presence of several species (identified by comparison with literature data where appropriate).
Initial mixing of the reagents in a J. Young’s NMR tube resulted in immediate H gas evolution. This observation may be
tentatively ascribed to the initial reaction between 1tLi and HNMe ∙BH forming Li[NMe BH ] (I), 2-t-butylpyridine and H
Scheme 2A). To be consistent with our hypothesis we expect that 2-t-butylpyridine will act as a LiH storage/release
vessel during the process, by forming dihydropyridines as a result of interaction with Li[amidoborane] species (Scheme
2
∙BH
3
in d
BH after 60 h (Table 1 entry 2). Significantly this reaction proceeded
] with dimethylamine borane in d -benzene (72 h at 60 °C), indicating
8
-toluene at 80 °C results in conversion (determined via B NMR
2
2
2 2
)
3
5a
)
2
}
2
2
6
1
11
that 1tLi is a competitive precatalyst.
(
2
2
3
2
3
2
(
1
1
1
2
B). At the initial time point the B NMR spectrum displays two resonances: a triplet at d 3.4 ppm ( JBH 100.1 Hz)
2
7
16c,27
corresponding to Li[NMe
2
BH
2
NMe
2
BH
3
] (II) and a quartet composed of the mutually coincident signals
of
] (II) centred at d -13.6 ppm ( JBH 96.2 Hz). The last named is
into Li[NMe BH ], in line with the literature mechanism. Analysis
of the B NMR spectrum after heating the solution at 80 °C for 24 hours reveals the presence of several new species: a
1
HNMe
2
BH
3
, Li[NMe
2
BH
3
] (I) and Li[NMe
2
BH
2
NMe
2 3
BH
formed by polar insertion of highly reactive NMe
2
-BH
2
2
3
1
1
1
29
1
2 2
doublet at d 28.9 ppm ( JBH 129.9 Hz) confirmed as (NMe ) BH (III); a triplet at d 5.4 ppm ( JBH 113.1 Hz) assigned to
3
0
1
2 2 2
cyclic dimer [NMe BH ] (IV); a partially obscured quartet centred around d -11.0 ppm ( JBH 91.1 Hz) assigned to
1
6c
1
2 3 2 4
Li[NMe (BH ) ] (V); and a quintet at d -40.9 ppm ( JBH 81.0 Hz) corresponding to the borohydride Li[BH ].
Scheme 2. A) Proposed initial consumption of 1tLi; B) Suggested formation of intermediate dihydropyridine (LHS - depicted here as 1,2-DHP; but
1
,4-DHP or 1,6-DHP isomers are also possible) from amidoborane intermediate and 2-tert-butylpyridine.
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The emergence of Li[BH
4
] and Li[NMe
2
(BH
3
)
2
] (V) can be readily explained (Step E Figure 2A). Borohydride Li[BH
BH NMe BH ] is followed. Li[NMe (BH
] and has been noted before by Wright, who rationally synthesized
4
] is
the coproduct formed when the b-hydride elimination pathway from Li[NMe
the result of deprotonation of HNMe ∙BH by Li[BH
and structurally characterised the compound. As the reaction progresses it is apparent from B NMR data that the
metallated amidoboranes are consumed. In the case of Li[NMe BH NMe BH ] it is clear that the major process is d-
hydride elimination to produce [NMe BH (IV). We propose that Li[NMe (BH ] is consumed via one (or both) of two
similar routes. The first scenario involves a hydride transfer which would reform Li[BH ] and also generate BH -NMe
Scheme 3A). Both compounds could then re-enter the catalytic cycle, or in the latter case an off-metal dimerization
pathway is conceivable. Alternatively, a molecule of NMe -BH could insert into Li[NMe (BH ] giving [NMe BH and
Li[BH ] directly (Scheme 3B). Although a definitive pathway has not been discovered it is clear that Li[NMe (BH ] is an
important product-forming intermediate in main group catalysed dehydrocoupling processes. A further important
2
2
2
3
2
3 2
)
] is
2
3
4
1
6c
11
2
2
2
3
2
2
]
2
2
3 2
)
4
2
2
(
2
2
2
3
)
2
2
2 2
]
4
2
3 2
)
1
1
observation from B NMR data is that at high conversions to products, that is, low concentrations of HNMe
2
BH
. The
presence of this intermediate is somewhat surprising since it reacts/inserts very rapidly at early stages in the reaction.
The inference is that the off-metal dimerization step is likely to be very slow and thus insertion is preferred for NMe -BH
3
/
1
2 3 2 2
Li[NMe BH ] a triplet of very low intensity is observed at d 38.1 ppm ( JBH 132.8 Hz) corresponding to NMe -BH
2
2
giving credence to the amidoborane insertion path proposed in scheme 3B. Altogether, the higher conversion, lower
catalyst loading and shorter timescale found with 1tLi, compared to the current state of the art, suggests that the
presence of DHP species is important in the enhancement observed in these reactions.
2 3 2 2 2
Scheme 3. Proposed consumption of intermediate Li[NMe (BH ) ] (V) by A) a hydride transfer, and/or B) polar insertion of NMe -BH .
Dehydrogenativeꢀcouplingꢀwithꢀtheꢀsodiumꢀandꢀpotassiumꢀdihydropyridylꢀprecatalystsꢀ
Next we assessed the role of alkali metal on the reaction. Sodium (1tNa) and potassium (1tK) variants were prepared
9
via a simple and high yielding metathetical approach. Employing 1tNa or 1tK in catalytic reactions (Table 1 entries 3 -
4
) under analogous conditions used for 1tLi resulted in similar conversions in both cases. All three reactions appear to
1
1
proceed via similar routes since the analogous intermediates are observed in each case in the B NMR spectra (See
Supporting Information). Notably these results compare very favourably with literature values (conversions to > 85%
2
7
[
NMe
timescales were comparatively long (72 h for 1tNa and 144 h for 1tK) with respect to 1tLi (60 h), albeit considerably
shorter than the reported values for NaN(SiMe and KN(SiMe (both 172 h). Thus, it seems clear that the issues with
2 2 2 3 2 3 2
BH ] with 1tNa or 1tK compared with approximately 43% with NaN(SiMe ) or KN(SiMe ) ). Reaction
3
)
2
3 2
)
modest conversion in previous Na and K based catalysis, which was attributed to poorly soluble M-H species, has been
somewhat resolved via use of “M-H solubilising” alkali metal alkyldihydropyridine precatalysts. That 1tLi outperforms the
Na and K precatalysts agrees with both the enhanced solubility and the trend observed previously in main group
2
7
dehydrocoupling systems, where slower activity may be attributed to: increasing cation radius which promotes a
0
longer, looser M∙∙∙H-B contact and slows down hydride elimination; or the more dispersed charge density at the d metal
which affects steps involving polar insertion of unsaturated fragments or s-bond metathesis leading to product
formation.
The influence of reaction solvent was also investigated using 1tLi as a representative precatalyst. (Table 1 entries 5 - 8).
Conducting the reaction in d12-cyclohexane results in high conversion (94%) to [NMe
5 °C for 168 h. This comparatively long timescale is attributed to poor solubility of the dimethylamine borane starting
material in cyclohexane slowing down the reaction. By moving to a more polar reaction medium, d -tetrahydrofuran, the
reaction slowed considerably more, only reaching a conversion of 88% [NMe BH after 360 h. Presumably efficient
stabilising Lewis base solvation of lithiated amidoboranes inhibits the polar insertion of NMe -BH into Li[NMe BH
and/or the hydride elimination steps. Moreover, it suggests that in this case fast catalytic turnover is reliant on the level
2 2 2
BH ] , albeit only after heating at
7
8
2
2 2
]
2
2
2
3
]
1
6c
of alkali-metal solvation. The solvent effect here is in contrast to that reported by Wright, where both toluene and THF
gave similar results with LiAlH as catalyst, albeit the poor solubility of LiAlH in hydrocarbon solvents may be a factor in
this report. To assess the donor effect more thoroughly, the reaction was repeated with a donor solvated complex of
tLi in d -toluene, thereby differentiating any effect from bulk donor solvent (Table 1 entry 7). We selected previously
reported chelate complex 1tLi∙Me
coordination sites. Reaction using 1tLi∙Me
4
4
1
8
7
b
4
AEE, where two N and one O donor sites of the tridentate ligand fill three Li
AEE in toluene is faster than 1tLi in bulk THF (120 h versus 360 h) although
4
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Chemistry - A European Journal
10.1002/chem.201703609
it is still much slower than unsolvated 1tLi in toluene. Therefore it is clear that the level of solvation of the alkali metal is
pivotal in this process.
Table 1. Catalytic conversion of HNMe
NMe BH (III) using DHP precatalysts.
2
∙BH
3
to [NMe
2
BH
2
]
2
(IV) and
(
2 2
)
Precatalyst
mol %)
Deuteratedꢀ
solventꢀ
t(h)ꢀ
T(°C)ꢀ (IV)(%)ꢀ (III)(%)ꢀ
(
[
a]
1
2
3
4
5
6
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1tLiꢀ(5%)ꢀ
1tLiꢀ(2.5%)ꢀ
1tNaꢀ(2.5%)ꢀ
1tKꢀ(2.5%)ꢀ
1tLiꢀ(2.5%)ꢀ
1tLiꢀ(2.5%)ꢀ
toluene
ꢀ
72ꢀ
60ꢀ
80 ꢀ
80ꢀ
80ꢀ
80ꢀ
75ꢀ
65ꢀ
86ꢀ
89ꢀ
89ꢀ
86ꢀ
94ꢀ
88ꢀ
7ꢀ
4ꢀ
4ꢀ
8ꢀ
2ꢀ
4ꢀ
tolueneꢀ
tolueneꢀ
tolueneꢀ
72ꢀ
144ꢀ
168ꢀ
360ꢀ
6 12ꢀ
C D
THFꢀ
1
tLi·Me
4
AEEꢀ
7
ꢀ
tolueneꢀ
120ꢀ
80ꢀ
81ꢀ
8ꢀ
(
2.5%)ꢀ
[
b]
8
ꢀ
1tLiꢀ(2.5%)ꢀ
2ꢀ(1.25%)ꢀ
2ꢀ(1.25%)ꢀ
LiAlH4ꢀ
pyridineꢀ
pyridineꢀ
tolueneꢀ
5ꢀ
5ꢀ
80ꢀ
80ꢀ
80ꢀ
2ꢀ
4ꢀ
94ꢀ
94ꢀ
9ꢀ
ꢀ
[b]
9
ꢀ
1
0ꢀ
146ꢀ
78ꢀ
[
b]
1
1 ꢀ
pyridineꢀ
9ꢀ
80ꢀ
2ꢀ
88ꢀ
(
2.5%)ꢀ
1
2ꢀ
3ꢀ
1tNaꢀ(2.5%)ꢀ
1tKꢀ(2.5%)ꢀ
pyridineꢀ
pyridineꢀ
8ꢀ
7ꢀ
80ꢀ
80ꢀ
2ꢀ
91ꢀ
98ꢀ
1
<1ꢀ
[
a] Initial 24h at 22 °C. [b] Resonance corresponding to III obscures a
second reaction product, that increases wrt III when heating is prolonged
after consumption of HNMe ·BH
2
3
.
Surprisingly, moving to bulk pyridine (Table 1 entry 8) results in a remarkable acceleration of the reaction. Even more
unexpected is the ratio of products dramatically switches such that near quantitative conversions of the diamine borane
(
94% in 5 h) to (NMe
stoichiometric quantity of boron remains unaccounted for by analysing the products observed in the B NMR spectrum.
The identity of the “missing” boron has not been proven, however it is unlikely to be lost as B , since diborane was not
identified in NMR reaction monitoring). At this point it is unclear why the presence of bulk pyridine results in such a
2 2 2 2 2 2 2
) BH rather than [NMe BH ] are obtained (note that since (NMe ) BH is the major product, a
1
1
2
H
6
1
1
pronounced switch in reactivity. Analysis of B NMR data reveals the presence of Li[NMe
Li[NMe (BH ] (V), the same intermediates observed in the catalysis conducted in d -toluene, alongside an additional
overlapping quartet resonance. Therefore, the main catalytic process may be considered to proceed via a similar route
as in toluene, except that the product formation step is b-H elimination from Li[NMe BH NMe BH ] (vide supra), which
2 2 2 3
BH NMe BH ] (II) and
2
3
)
2
8
2
2
2
3
can be tentatively explained by some pyridine “induced” change in charge polarisation over the intermediate, that is,
coordination of pyridine to a boron atom in the intermediate would lead to a change in the charge distribution across the
molecule. Sicilia previously disclosed that the in silico energetics of the (NMe
2
)
2
BH product forming steps are very high
in energy for a related Mg system. Clearly the solvation effect of excess pyridine in some way promotes the hydride
transfer from Li[NMe BH NMe BH ] giving (NMe BH. An alternative explanation for preferential (NMe BH formation
is that in a secondary competing process, a BH group is transferred to pyridine at some stage in the process forming
the Py∙BH adduct, which is in line with the additional low intensity quartet present in the B NMR spectrum. A control
reaction of HNMe ∙BH in d -pyridine at 80 °C for 20 h. confirms that BH transfer from HNMe ∙BH to pyridine does not
II
26
2
2
2
3
2
)
2
2 2
)
3
1
1
3
2
3
5
3
2
3
occur to any significant extent (ca. 15 % is present at d -11.2 ppm after prolonged heating). An alternative proposed
reaction sequence; accounting for the unexpected reactivity in pyridine is given in scheme 4.
Scheme 4. Alternative proposed reaction sequence accounting for the role of pyridine.
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Chemistry - A European Journal
10.1002/chem.201703609
2 2 2 3
The initial deprotonation and insertion steps remain the same. However, the intermediate Li[NMe BH NMe BH ] has
been depicted in an alternative conformation, ideally suited to transfer BH
here, elimination of LiH (possibly as a dihydropyridine species), and reaction with the pyridine borane adduct would
account for the formation of LiBH (Scheme 4B). It is also important to state that the identity of the precatalyst in
pyridine solution is likely to be different from 1tLi. Reaction of the n-butyl isomer of 1tLi with excess pyridine results in a
,4-dihydropyridyl bridged lithium dimer, [py ·Li(-µ-1,4-DHP)] (2), with each Li atom solvated by two pyridine molecules
Scheme 5). Therefore it is likely that the active catalytic species more closely resembles 2 than 1tLi. 2 was
synthesised and tested as a precatalyst (1.25 mol%) in d -pyridine and in d -toluene (entries 9 - 10). In d -pyridine the
3
to a molecule of pyridine (Scheme 4A). From
4
1
2
2
3
1
(
5
8
5
reaction is complete in 5 hours, essentially replicating the reactivity observed using 1tLi, reinforcing the idea that in
pyridine 1tLi converts to a species resembling 2.
Scheme 5. Synthesis of 2.
8
In d -toluene the catalysis is much slower. Initially the product ratio is only approximately 3:1 in favour of IV over III,
highlighting the influence of pyridine in product determination (here there are two equivalents of pyridine for each
LiDHP). However, as the reaction proceeds the ratio changes to approximately 9:1 after 146 hours. Exploring the
concept of solvent control further we elected to employ LiAlH
usually stoichiometrically employed Lansbury’s reagent). Further, Wright demonstrated that LiAlH
catalyst in dehydrocoupling of dimethylamine borane in THF and toluene. Once more, the use of pyridine as reaction
solvent results in high consumption of HNMe ·BH , after 9 hours at 80 degrees, forming III as the major product (entry
1). Together these findings outline the importance of reaction solvent and suggest that a control of various
4
as a catalyst in d
5
-pyridine (i.e., a catalytic amount of the
4
is an effective
2
3
1
dehydrocoupling reactions can be achieved with careful selection of precatalyst/solvent combinations. Interestingly, in
each case where pyridine was used as a reaction solvent, prolonged heating of the reaction, after consumption of
starting material results in the appearance of a partially obscured singlet resonance at ca. d 26 ppm, alongside that
1
1
corresponding to (NMe
reagents pinacol or catechol borane, prompted us to consider whether, once formed, could then III hydroborate pyridine
in the presence of a lithium DHP catalyst. A stoichiometric reaction between LiAlH and HNMe ·BH at 80 °C in bulk
2 2 2 2
) BH in the B NMR spectra. The similarity of (NMe ) BH to the commonly used hydroboration
4
2
3
pyridine was conducted to test this hypothesis (Scheme 6). After removal of solvent, the crude solid, identified as
primarily Lansbury’s reagent, was washed with hexane and the hexane washings were subsequently analysed via NMR
1
1
1
spectroscopy. Crucially the B NMR spectrum revealed the expected singlet at d 26.4 ppm. The H NMR spectrum
displayed three equal intensity multiplets at 5.96, 4.53 and 2.95 ppm, characteristic of a 1,4 dihydropyridine species. A
singlet at 2.31 ppm can be assigned as the methyl hydrogens of an NMe
2
group. The ratio of the peaks are in
agreement with those of (DHP) B(NMe ) (VI), indicating HNMe has been lost from III during the reaction.
2
2
2
Scheme 6. Synthesis of VI, formed by hydroboration with III.
Importantly this result indicates that a DHP based catalyst is still active in pyridine after expected product formation, and
further, proving the hypothesis provided us with an impetus to test 1tLi as a hydroboration precatalyst under more
5
controlled conditions. Finalising our investigations in d -pyridine the reaction was repeated using precatalysts 1tNa and
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Chemistry - A European Journal
10.1002/chem.201703609
1
2 3 2 2
tK under analogous conditions (Table 1, entries 12 – 13). In both cases conversion of HNMe ·BH to (NMe ) BH was
rapid (ca > 90% in 8 h), albeit again slower than for 1tLi, and interestingly the product resonances were clean with no
presence of the hydroboration product.
Hydroboration of aldehydes and ketones
Seeking to achieve our aim of extending the versatility of 1tLi (the best performing precatalyst from the preceding
section) in a catalytic regime we next attempted a series of hydroboration reactions with a selection of aldehydes and
ketones using pinacol borane (HBpin). Traditionally HBpin is employed in hydroboration due to its hydridic hydrogen
and electrophilic boron, however a recent break-through has demonstrated it can also be employed as an easily
3
2
accessed source of nucleophilic boron. These hydroboration products are important intermediates in the synthesis of
alcohols from aldehydes and ketones, and remove the necessity to use a stoichiometric amount of metal reducing
agent. Hill reported that DIPPnacnac-MgnBu is an excellent precatalyst for this reaction, which proceeds with low
1
9
catalyst loadings, high conversions and mild conditions. Moreover a Mg-H species was pinpointed as the active
catalyst, involved in the first step of a two-step process. The first step is insertion of the unsaturated carbonyl compound
into the Mg-H bond. The second step, a metathesis with HBpin, affords hydroborated product and regenerates the
7
,9
active catalyst. We have already disclosed that alkali-metal DHPs can efficiently transfer Li-H to benzophenone,
a
reaction that mirrors the first step in the catalytic process since Li-H from 1tLi adds across the C=O bond. Provided that
the subsequent metathetical reaction with HBpin, in the presence of 2-t-butylpyridine, regenerates an active 1-lithio-
DHP then catalysis should proceed as described. Testing the hypothesis, benzaldehyde and HBpin were placed in a J.
1
11
Young’s NMR tube in d
tLi. After 15 minutes at room temperature the H and B NMR spectra indicate essentially clean quantitative
conversion to the hydroborated product, (Table 2 entry 1).
6
-benzene and the H and B NMR spectra were monitored over time after addition of 5 mol%
1
11
1
Table 2. Catalytic hydroboration of aldehydes and ketones using 1tLi
6 6
precatalyst in C D .
1
Aldehyde /
Ketone
Yield by H NMR
t (h)
0.25
[a]
(%)
O
[
c]
1
2
3
H
>99 [>95%]
O
H
0.25
0.25
93
98
OMe
O
H
H
4
5
Fe
0.25
0.25
>99
>99
O
O
H
Br
O
[
b]
6
H
24
>99
97
O
7
8
9
0.5
O
0.25
>99
I
O
CF
3
0.25
0.25
>99
>99
O
1
0
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10.1002/chem.201703609
1
1
1
2
Fe
0.25
0.25
97
O
O
>98
N
O
[
b]
1
3
24
89
O
1
4
0.25
24
96
69
O
[
b]
1
5
[
a] yield determined by formation of RR’CHOBpin relative to internal standard
hexamethylcyclotrisiloxane. [b] heated at 70 °C. [c] 1% catalyst loading.
Importantly the result demonstrates the versatility of group 1 DHP based precatalysts since they can effectively catalyse
both dehydrocoupling and hydroboration reactions. Next we turned our attention to extending the scope of aldehydes
and ketones employed in hydroboration reactions using the same conditions. 2-methoxybenzaldehyde, 2-
naphthaldehyde and ferrocene carboxaldehyde (entries 2-4) are all cleanly converted into the corresponding protected
alcohols after only 15 minutes at room temperature in high NMR yields (ca. 95%) versus an internal standard. Notably
1
9
the analogous reaction of 2-methoxybenzaldehyde using DIPPnacnac-MgnBu (0.5 mol%) is complete in one hour.
Further, the hydroboration of 2-naphthaldehyde is faster than that catalysed by the ruthenium complex [Ru(p-
3
3
2 2
cymene)Cl ] (0.1 mol%, 4 h), albeit lower catalyst loadings were used in each case. Hydroboration of 4-
bromobenzaldehyde (entry 5) is also complete within 15 minutes, indicating a tolerance to Li/halogen exchange under
the reaction conditions, thereby increasing the range of useful substrates able to participate in these reactions.
3
4
Furthermore this reaction occurs quicker than those using either 0.05 mol% Ar*N(Si(i-Pr)
3
)SnOtBu, in 4.5 hours (Ar* =
6 2 2 2 2 2
(C H {C(H)Ph } i-Pr-2,6,4), (IPr)CuOtBu, (0.1 mol%, 1 h) or [Ru(p-cymene)Cl ] (0.1 mol%, 3 h), although again 1tLi
3
5
3
3
has a higher loading (5 mol%). Interestingly, hydroboration of mesitaldehyde (entry 6) takes longer for complete
conversion (24 h at 70 °C). We attribute this to the steric hindrance of two ortho mesityl methyl groups, which slows
down the process, presumably by either inhibiting the hydrometallation step and/or by preventing efficient reformation of
the putative active DHP catalyst. Moving to ketones, the hydroboration potential of 1tLi was examined with
benzophenone as substrate (entry 7). Under the same conditions outlined above, clean conversion was achieved albeit
after 30 minutes at room temperature. 4-Iodoacetophenone and trifluoroacetophenone (entries 8-9) both react in high
yields and with short reaction times (ca. >95% in 15 minutes). In the latter case, Jones reports Ar*N(Si(i-Pr)
3
)GeOtBu
(
2.5 mol%, 15 minutes) and Ar*N(Si(i-Pr) )SnOtBu (0.5 mol%, <15 mins) precatalysts that perform the reaction with
3
3
4
lower loadings or are slightly faster in the Sn case. Hydroboration of 2-phenylacetophenone, 2-acetylferrocene and 2-
benzoylpyridine (entry 10-12) are also complete in 15 minutes at room temperature, with in the third case efficient
hydroboration occurring only at the carbonyl functionality. Once more the increased sterics of a mesityl substituted
carbonyl (entry 13) necessitates a longer reaction (24 hours) and increased temperature (70 °C) to achieve full
conversion. Dialkylketones are smoothly hydroborated, with 2-butanone taking 15 minutes at room temperature (entry
1
4). Like the aryl systems, increased steric bulk necessitates longer times and higher temperatures, with di-t-butyl
o
ketone requiring 24h at 70 C to give almost 70% conversion (entry 15). To assess whether the reaction may proceed
1
9
via an alternative reaction pathway to that postulated for other main group systems (vide supra) a series of control
reactions were performed. As dihydropyridines and their parent aromatic counterparts would be present in the reaction
mixture, the reactivity between HBpin and 1tLi and with pyridine (as a model variant of 2-tert-butylpyridine) were probed.
The stoichiometric reaction between HBpin and 1tLi in toluene at room temperature (Scheme 7A) results in complete
1
trans-elementation giving in situ generated 1tBpin as evidenced via H NMR studies (Figure 3). Here the five proton
resonances from the dihydropyridyl ring 1tLi are replaced by five new dihydropyridyl resonances, consistent with
1
1
replacement of lithium with a Bpin unit and presumably generating LiH as a coproduct. Furthermore the B NMR
displays a singlet resonance at d 24.5 ppm corresponding to the newly installed B-N bond.
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1
Figure 3. H NMR spectra (dihydropyridyl region) of the reaction between 1tLi and HBpin in d
6
benzene showing formation of 1tBpin. * = toluene.
Potentially 1tBpin could act as an active catalytic entity in the hydroboration process, therefore benzophenone was
added to a reaction mixture containing 1tBpin to investigate whether it would convert to hydroborated product, and the
1
1
reaction was monitored by B NMR spectroscopy. The emergence of a singlet at d 23 ppm corresponds to the
hydroborated product. For 1tBpin to act as a viable catalytic intermediate, conversion of the parent pyridine into a
dihydropyridine species must occur by some mechanism. It is long established that commercial LiH, owing to its
insolubility in organic media (originating from its considerable lattice energy), on its own does not add across pyridine,
3
1
indicating this pathway is unlikely, albeit in situ generated LiH may exhibit higher reactivity in this regard. A second
possibility is the direct addition of HBpin across the parent pyridine.
Scheme 7. Control reaction of A) 1tLi with HBpin giving 1tBpin and B) Pyridine with HBpin giving 3.
Direct reaction between HBpin and pyridine (Scheme 7B) suggests that hydroboration and concomitant dearomatisation
of the pyridine does not readily occur. This was duly confirmed with an X-ray crystallographic study, revealing the major
product as the simple donor-acceptor adduct HBpin·py (3) in a 58% yield. This structure represents the ‘pyridine-
activated HBpin’ intermediate postulated by Wright and co-workers in their very recently reported boronium cation
3
6
initiated hydroboration of pyridine. In 3 B1 is in a distorted tetrahedral geometry [range of angles 103.4(9) – 116.7(9)°]
with respect to N1, O1, O2 and H1 (which was located and refined crystallographically, Figure 4). A search of the
Cambridge Structural Database (CSD) surprisingly resulted in zero hits for HB(O)
2
units bonded to pyridine. Crystals of
1
1
3
appear to decompose into a colourless oil after storage in an inert atmosphere glovebox. B NMR studies of the
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decomposition product reveal that as expected the major resonance is that of 3, a doublet at d 28.3 ppm accounting for
ca. 80% of the material via integration of the boron NMR spectrum. The remainder of the material is represented by a
1
singlet at d 23.9 ppm indicating a minor amount of hydroborated pyridine. In agreement the H NMR displays
resonances potentially attributable to a DHP species, alongside the expected HBpin and pyridine resonances.
Figure 4. Molecular structure of 3. Hydrogen atoms other than that attached to boron are omitted for clarity. Thermal ellipsoids are drawn at 30%
probability. Selected bond lengths (Å) and angles (°): B1-N1 1.651(2); B1-O1 1.442(2); B1-O2 1.452(2); B1-H1 1.164(18); N1-B1-O1 107.76(13);
N1-B1-O2 108.41(13); N1-B1-H1 103.4(9); O1-B1-O2 107.45(15); O1-B1-H1 116.7(9); O2-B1-H1 112.6(9).
Scheme 8 displays two potential routes for catalysis to proceed. Pathway A follows one commonly accepted mechanism
1
9
of main group hydroboration catalysis (insertion/metathesis), albeit in this case pyridine/dihydropyridine plays an
active role as a metal hydride storage/release vehicle. Alternatively pathway B describes a concerted process between
1
tBpin, the carbonyl substrate, and the in situ generated LiH, explaining both hydroboration and catalyst reformation. It
may be significant that in pathway B, LiH is generated in a step prior to aromatic pyridine formation. Due to the poor
hydrocarbon solubility of LiH, polymeric LiH aggregates are likely to precipitate. Therefore one may expect pathway A to
be the favoured catalytic manifold since LiH is generated in the presence of the aromatic pyridine and can therefore add
across it in this regime. A second consideration in pathway B is that the incipient LiH may simply associate with excess
HBpin giving a substituted borohydride species of the form Li[H
see no spectroscopic evidence to support such a scenario.
2
Bpin], and thereby remaining solubilized. However we
Scheme 8. Proposed catalytic pathways A and B with hypothetical transition states for hydroboration of aldehydes and ketones with 1tLi as
precatalyst. DHP species are depicted as 1,2-isomers; other isomers (1,4- or 1,6-) are also possible.
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Conclusions
In conclusion, this study showcases the benefits of making molecular modifications of the classical salt lattice structures
of the alkali metal hydrides. Dispensing metal hydrides in the form of molecular alkyl-dihydropyridines has a profound
positive impact on the dehydrogenative coupling of dimethylamine borane. Excellent hydrocarbon solubility of these
alkali metal dihydropyridines and presumably of the metal hydride intermediates involved in the catalysis, are almost
certainly key factors in the successful dehydrocoupling reactions. The usefulness of the lithium-t-butyl-dihydropyridine
as a precatalyst was extended to pinacolborane sourced hydroboration reactions with a range of aldehydes and
ketones. These catalytic applications demonstrate rare examples of group one based pre-catalysts that advance the
growing body of recent literature demonstrating that main group metal systems can in certain cases be successful in
catalytic reactions previously thought to be the exclusive domain of transition metal systems. Future work will focus on
just how far this analogy can be extended for these remarkable soluble hydride surrogates.
Experimental Section
Full details of experimental procedures are provided in the electronic supplementary information.
Acknowledgements
The authors thank the following sponsors for their generous support of this research: George Fraser (scholarship
awarded to S.A.O.), the EPSRC (grant award number EP/L027313/1 and a DTP award to S.A.O), the Royal Society of
Edinburgh (BP Trust Fellowship to S.D.R.), and the University of Strathclyde (Chancellors Fellowship to S.D.R.). The
data set underlying this research can be located at http://dx.doi.org/10.15129/f26d3563-55d1-47a6-860c-a2fa8e643c26
Keywords: Dehydrocoupling • Hydroboration • Catalysis • Lithium • Main group
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