Borohydrides from organic hydrides: Reactions of Hantzsch's esters with B (C6F5)3

Article abstract of DOI:10.1002/chem.200903005

We report herein that the reaction between a series of Hantzsch's ester analogues 1a-d with the Lewis acidic species B(C₆F₅) ₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridini

Full text of DOI:10.1002/chem.200903005

FULL PAPER
DOI: 10.1002/chem.200903005
Borohydrides from Organic Hydrides:
Reactions of Hantzschꢀs Esters with B(C6F5)3
AHCTUNGTRENNUNG
Jonathan D. Webb,[a] Vꢁronique S. Laberge,[a] Stephen J. Geier,[b]
Douglas W. Stephan,*[b] and Cathleen M. Crudden*[a]
Abstract: We report herein that the re-
action between a series of Hantzschꢀs
ester analogues 1a–d with the Lewis
acidic species BACHTUNGTRENNUNG(C6F5)3 results in facile
borohydride salt, with the balance of
the material sequestered as the ester-
bound Lewis acid–base adduct 3a. For-
mation of the Lewis acid–base adduct
could be minimized by increasing the
steric bulk about the ester groups as in
1d. The connectivity of the carbonyl-
bound adduct was confirmed by an X-
ray crystallographic analysis of 3e the
product of the reaction of methyl
plored the generation of these pyridini-
um salts by employing frustrated Lewis
pair methodology. However, the reac-
tion of mixtures of the corresponding
transfer of hydride to boron. The main
products of this reaction are pyridini-
um borohydride salts 2a–d, which are
obtained in high to moderate yields.
The N-substituted substrates (N-Me,
N-Ph) reacted in high yield 90–98%
and the connectivity of the products
were confirmed by an X-ray crystallo-
graphic analysis of the N-Me borohy-
dride salt 2a. Unsubstituted Hanztschꢀs
ester 1a reacted less effectively gener-
ating only 60% of the corresponding
pyridine and BACTHNGUTERN(NUG C6F5)3 with hydrogen
gas only resulted in formation of trace
amounts of the pyridinium borohy-
dride, along with the Lewis acid–base
adduct of the starting material and B-
A
The
1,2-dihydropyridine
ketone 1e with B
N
adduct was the final product of this re-
action. This was ascribed to the low ba-
sicity of the pyridine nitrogen and the
complicating formation of an ester
bound Lewis acid–base adduct.
À
Keywords: boron · C H activation ·
Frustrated Lewis pairs · Hantzschꢀs
ester · hydride transfer
Introduction
of borohydride synthesis involve the use of inorganic hy-
drides such as sodium hydride.[4,7–14] While these methods
are effective, the preparation of inorganic hydrides is energy
intensive. This is particularly problematic for borohydrides
destined for use in energy storage applications where cost
effective reconstitution of the boron hydride is essential.[4,6]
Other methods for hydride transfer to boron have included
less reactive species such as R3SiH,[9] Bu3SnH,[10] and Rh hy-
drides,[14] although the use of these reagents on stoichiomet-
ric scale is prohibitive.
À
Reactions that utilize the reactivity of boron hydrogen
bonds are of considerable importance. In organic chemistry,
hydroboration and borohydride reagents that perform selec-
À
tive reductions as well as C H functionalizations are criti-
cally important.[1–3] On the other hand, boron hydride spe-
À
cies such as ammonia borane[4–7] are of considerable interest
as molecular hydrogen storage materials. Standard methods
À
An alternative strategy for the formation of B H species
[a] J. D. Webb, V. S. Laberge, Prof. Dr. C. M. Crudden
Department of Chemistry
that would involve the use of electron-rich “organic hy-
drides” has received surprisingly little attention.[15–20] In scat-
tered examples, bulky amines have been shown to act as hy-
À
Queenꢀs University
90 Bader Lane
Kingston, Ontario, K7L3N6
E-mail: cruddenc@chem.queensu.ca
Homepage: http://www.chem.queensu.ca/people/faculty/crudden
dride donors[21] in reactions with B
ACHTNUTRGNE(UNG C6F5)3 via a C H activa-
tion alpha to nitrogen [Eqs. (1–3)].[15–18]
[b] S. J. Geier, Prof. Dr. D. W. Stephan
Department of Chemistry
University of Toronto
80 St. George St. Toronto, Ontario, M5S3H6
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/dstephan
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903005.
Chem. Eur. J. 2010, 16, 4895 – 4902
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4895

to the product that would be prepared by the direct C-H ac-
tivation using Hantzschꢀs ester as the hydride donor and
BACHTUNRTGNEUNG(C6F5)3 as the acceptor [Eq. (7)]. Herein, we report the use
À
of both of these methods, C H activation from 1,4-dihydro-
pyridines and H2 activation via the corresponding pyridine,
The quintessential organic hydride donor is NADH,[22] its
to generate B H bonds with Lewis acidic boranes such as
À
stable analogues Hantzschꢀs ester[23] and derivatives 1a–e,
B
G
N
(Figure 1) have a reducing power comparable to that of
reagents has been shown to occur via a stepwise process in-
volving an initial electron transfer followed by hydrogen-
atom transfer,[21] based on the oxidation potential of
Hantzschꢀs ester and the estimated reduction potential of B-
[24]
À
NEt3BH3 and unusually low C4 H heterolytic bond disso-
ciation energies of ~70 kcalmolÀ1.[25,26] 1,4-Dihydropyri-
dines[24–27] such as Hantzschꢀs ester have found widespread
application as reducing agents in organic and bioorganic
chemistry, however, few reports have probed their applica-
tion in hydride transfer reactions to Lewis acidic organoele-
AHCTNUGTER(GNNNU C6F5)3, we anticipate that the hydride transfers described
herein occur via a concerted mechanism.[40] However, defini-
tive mechanistic studies have not been carried out on these
particular systems to rule out stepwise processes.
ment species such as BACHTNUTRGNEUNG(C6F5)3.
Results and Discussion
Initially focusing on C-H activation chemistry, we examined
the reaction of N-alkylated 1,4-dihydropyridines 1a (N-Me)
and 1b (N-Ph) with BACHTUNRGTNE(UNG C6F5)3 (Table 1). These reactions
Figure 1. Hantzschꢀs ester and its derivatives
cleanly transfer hydride to boron generating the salts 2a and
2b [Eq. (8)] in good yields, 98 and 90%, respectively
(Table 1, entries 1–3).
À
An alternative strategy for the formation of B H bonds
would employ the recently developed concept of “frustrated
Lewis pairs”.[28–34] In these cases, sterically hindered phos-
phines,[29] carbenes,[33,35,36] amines[17] or pyridines[37,38] act in
concert with Lewis acids to affect the heterolytic activation
of H2 [Eqs. (4–6)].
Formation of the borohydride anion was confirmed by its
characteristic doublet at À25 ppm in the 11B NMR spectra.
The reactions initiate rapidly at temperatures below À408C,
with no further conversion being observed after 30 min at
À208C.[40]
Table 1. Product distribution for reactions of 1a–e with B(C6F5)3.
Entry
Substrate
R, R’
1/B(C6F5)3
Yield 2 [%][a]
1
2
3
4
5
6
7
1a
1a
1b
1c
1c
1c
1d
Me, OEt
Me, OEt
Ph, OEt
H, OEt
H, OEt
H, OEt
H, OtBu
1:1
1:1
1:1
1:1
2:1
1:2
1:1
95
98[b]
90[b]
60
Judicious tuning of the steric and electronic features of
the reagents allows some of these reactions to be reversi-
ble.[28,34,39] Most notably, Equation (6) describes the use of
>90
60
90[c]
substituted pyridines which are capable of promoting the ac-
1
[a] Reactions carried out in 0.03m 1 in CD2Cl2 at À208C, H NMR yields
based on limiting reagent with bibenzyl as an internal standard. [b] Re-
action carried out at 258C. [c] Reaction carried out at À308C.
À
tivation of hydrogen to yield HB
G
sponding pyridinium cation.[37,38] This salt is closely related
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Borohydrides from Organic Hydrides
FULL PAPER
Salts 2a and 2b can also be formed at room temperature
in less than 5 min, although decomposition is observed after
extended times in solution (see below). An X-ray crystal
structure of the salt 2a confirmed the expected connectivity
(Figure 2). The metric parameters are analogous to those re-
cently reported for pyridinium hydridoborate [C5H3Me2NH]
À
Upon warming from À208C to room temperature, boro-
hydride salt 2c converts into the corresponding 1,2-dihydro-
pyridine-BACHTNUTRGNEG(UN C6F5)3 adduct 4c. A significant amount (55%
yield) of the 1,2-dihydropyridine adduct is observed after
30 min at room temperature with a maximum yield of 70%
after 24 h in CD2Cl2 [Eq. (10)].
[HBACHTUNGTRENNUNG
(C6F5)3].[37] The closest approach of the BH and N Me
fragments in the solid state is 3.59 ꢂ.
This stands in contrast to the corresponding 1,2-dihydro-
pyridine adducts 4a and 4b, derived from 1a and 1b, which
are formed in solutions of 2a and 2b, respectively, upon
standing at room temperature for several hours. The 1,2-di-
hydropyridine adduct 4c is readily identifiable by its
1H NMR spectrum,[42,43] which is characterized by a pentet
3
at d 4.60 ppm (p, 1H, JH–H =7 Hz) in CD2Cl2 representing
the methine proton and a doublet at d 1.10 ppm (d, 3H,
3JH–H =7 Hz) from the adjacent methyl group.[44] The 19F and
Figure 2. POV-ray depiction of the X-ray crystal structure of 2a. All hy-
drogen atoms except for the BH have been omitted for clarity
11B NMR spectra were consistent with B
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
In a similar fashion, the reaction of Hantzschꢀs ester itself,
1c, generates the borohydride salt 2c in 60% yield at
À208C. At this temperature, the remaining 1,4-dihydropyri-
dine is sequestered as the Lewis acid–base adduct 3c
(entry 4, [Eq. (9)]).
out in a mixture of CH2Cl2 and hexanes at room tempera-
ture for 1 h and cooled to induce crystallization.
The structure of 4c, in which the BACHTNUTRGNEUNG(C6F5)3 fragment is
bound to the ester trans to the center of hydride addition,
was confirmed by X-ray crystallographic analysis (Figure 3).
However, solution data indicate that the BACHTUNRGTNEUNG(C6F5)3 moiety is
likely exchanging between both ethyl esters in solution. The
geometry about the B centre of 4c is pseudo-tetrahedral
with an O–B distance of 1.547(4) ꢂ, similar in length to
analogous ester adducts.[45,46] Formation of the correspond-
ing 1,2-dihydropyridine adduct is negligible for all substrates
at À208C even after several hours.
Although the carbonyl oxygen atoms are expected to be
less basic than the amine nitrogen, the steric congestion
about nitrogen and its vinylogous placement presumably ac-
counts for the formation of these adducts. When a solution
that contained 3c was cooled below À408C, the peaks repre-
senting the distinct ethyl ester moiety were resolved, sug-
À
gesting a dissymmetric complex resulting from a carbonyl
borane adduct. A NOESY experiment revealed a correla-
tion between the protic NH (d 12.5 ppm) and the BH hy-
dride (d 3.5 ppm) for compound 2c in solution, consistent
with ion pairing. Similar interactions have been observed in
related compounds.[16,17,29,37]
The product distribution was altered by varying the stoi-
chiometry. Formation of the adduct 3c was minimized in the
presence of excess 1c relative to BACHTNUTRGNE(UNG C6F5)3; alternatively for-
Figure 3. POV-ray depiction of one of the molecules of 4c in the asym-
metric unit of the X-ray crystallographic structure. All hydrogen atoms
were omitted for clarity.
mation of 2c was driven by increasing the amount of the
1,4-dihydropyridine (entry 5).[41]
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D. W. Stephan, C. M. Crudden et al.
Clearly for the reactions of 1a–c with BACTHUNRGTNEUNG(C6F5)3, the forma-
tion of 1,2-dihydropyridine adducts and Lewis acid–base ad-
ducts has an impact on the effectiveness of hydride transfer.
In an effort to disfavor the formation of these adducts, the
reaction of the tert-butyl-ester derivative 1d with BACHTUNRGTNEUNG(C6F5)3
was examined. This approach proved successful, as a 1:1
molar ratio of these species afforded the pyridinium hydri-
doborate salt 2d in good yield (90%) [Eq. (11)].
Figure 4. POV-ray depiction of one of the molecules of 3e in the asym-
metric unit of the X-ray crystallographic structure. All hydrogen atoms
were omitted for clarity.
However, isolation of borohydride salt 2d was hampered
by its low solubility and instability above À208C. Thus the
reaction was carried out at À308C and characterization was
performed at low temperature. Nonetheless, the formation
of 2d in good yield without the need for excess hydride
donor is consistent with steric congestion at the carbonyl
oxygen precluding Lewis acid–base adducts formation. Fur-
thermore, the low temperature at which hydride transfer
occurs illustrates the very high reactivity of 1,4-dihydropyri-
extreme in these reactions of 1,4-dihydropyridines with B-
AHCTUNGTRENNUNG
borohydride salts employs the recently developed frustrated
Lewis pair (FLP) methodology.[30,37,38] As was previously
shown [Eq. (6)], sterically hindered pyridines and BACHTUNGTRENNUNG(C6F5)3
could be employed to affect the heterolytic cleavage of H2,
the resulting pyridinium–borohydride salts would be similar
À
dines with B
In contrast, the dimethyl ketone analogue 1e reacts with
(C6F5)3 affording exclusively the Lewis acid–base adduct
N
to those obtained from C H activation from a 1,4-dihydro-
pyridine. Thus we explored this methodology as a comple-
B
N
mentary synthetic approach for the synthesis of 2c.
3e with no evidence of the hydride transfer product
[Eq. (12)].
The pyridine analogue of 1c, (C5HMe
combined with B(C6F5)3 in a 1:1 ratio. This produced 6c, at-
tributable to the Lewis acid–base adduct formed via binding
of B(C6F5)3 to the carbonyl group [Eq. (13)]. Compound 6c
2ACHTUNGTREN(UNNG CO2Et)2N) 5c was
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
was isolated and crystallographically characterized confirm-
ing the proposed binding of B to one of the carbonyl
À
oxygen atoms (Figure 5). In this case, the O B bond length
was determined to be 1.589(2) ꢂ which was slightly longer
than that observed for 4c.[45] In solution, 6c gives rise to
three resonances in the 19F NMR spectrum, even at À608C,
consistent with binding at oxygen. It should be noted that
The preferential formation of the adduct 3e is consistent
with the decreased steric congestion around the carbonyl,
and increased Lewis basicity of the carbonyl oxygen,[45,47] 1e
is a vinylogous amide. Again, exchange between the two car-
bonyl groups is facile at room temperature, as desymmetri-
zation of the Me resonances is observed in CD2Cl2 only on
cooling to 08C. Binding to the carbonyl group is also sup-
ported by crystallography data for 3e (Figure 4). Similar to
2,6-disubstituted pyridine adducts of BACTHUNRGTNEUNG(C6F5)3 show inequi-
1
valent C6F5 rings.[37,38] Low-temperature H NMR spectros-
copy did not resolve the methyl and ethyl resonances, sug-
gesting the borane is rapidly exchanging between the two
carbonyl groups.
The equilibrium between 5c and 6c, allows the combina-
tion of Lewis acid and base to react as a FLP with H2 in
[D8]-toluene, albeit slowly [Eq. (14)]. At room temperature,
over two days this reaction affords a product mixture includ-
ing the oxygen-bound pyridine adduct 6c, oxygen-bound
1,2-dihydropyridine adduct 4c, and ion pair 2c in a 40:51:9
ratio. At low temperature no H2 activation is observed. At
À158C, only trace amounts of 2c and 4c appear slowly on
standing (weeks), whereas no reaction was observed at
À308C. In contrast, reaction of 5c with two equivalents of
4c, the structure of 3e shows the BACHTNUTRGNE(UNG C6F5)3 fragment bound
with a pseudo-tetrahedral geometry about B and an O–B
distance of 1.538(5) ꢂ.[45] The adduct 3e is thermally sensi-
tive, degrading slowly at room temperature in solution to an
inseparable mixture of the corresponding pyridine and 1,2-
dihydropyridine adducts.
The formation of 2d and 3e demonstrate that electronic
and steric factors impact the stability of the competing prod-
ucts derived from hydride transfer and Lewis acid–base
adduct formation. Indeed these two cases represent either
BACHTUNGTRENNUNG
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Borohydrides from Organic Hydrides
FULL PAPER
er, the products of this reaction display limited stability. For
the most stable products, resulting from hydride transfer
from the N-Me and N-Ph analogues of Hantzschꢀs ester, a
slow reaction between the borohydride and the pyridinium
ions is observed which results in the gradual accumulation
of 1,2-dihydropyridine adduct in solution at room tempera-
ture. FLP-type activation of H2 by the fully aromatic pyri-
dine analogue of Hantzschꢀs ester was also shown to yield
the borohydride/pyridinium salt, although this reaction was
considerably slowed by comparison with the hydride trans-
fer from the 1,4-dihydropyridine analogues. The increased
time for reaction provided the opportunity for the transfer
of hydride back to the pyridinium salt such that the major
Figure 5. POV-ray depiction of the X-ray crystal structure of 6c. All hy-
drogens were omitted for clarity.
products observed were the pyridine–BACTHUNRGTNEUNG(C6F5)3 adduct, and
the 1,2-dihydropyridine. This decreased reactivity is attribut-
ed to the lower basicity of the pyridine nitrogen and the
presence of a second complexation site for boron at the car-
bonyl oxygen. Work is in progress to identify hydride
donors that are as effective as Hantzschꢀs ester without pro-
viding alternative coordination sites for boron.
Experimental Section
General: Manipulations were performed either in an mBraun glovebox
or on a double manifold Ar/vacuum line using standard Schlenk tech-
nique unless otherwise noted. All solvents were of Certified A.C.S.
grade, purchased from Aldrich Chemical Co. unless otherwise note and
distilled prior to use from an appropriate drying agent (pentane: P2O5;
hexanes: Na; THF: Na/benzophenone; Et2O, CH2Cl2, CD2Cl2 and 1,2-di-
chloroethane: CaH2; CDCl3: Na2SO4) degassed by three freeze–pump–
thaw cycles and stored in a glovebox over 4 ꢂ molecular sieves and basic
Al2O3. CD2Cl2 was purchased from Cambridge Isotopes, ethyl acetoace-
tate from Acros Organics and ammonium acetate from BDH Chemicals,
These results are consistent with increased hydrogen acti-
vation with increased availability of free BACHTUNRGTNEUNG(C6F5)3. More-
BACHTUNGTRENNUNG
over, it suggests the barrier to activation of H2 by the pyri-
dine–borane pair is significantly higher than the barrier for
hydride abstraction from 1c which occurs rapidly at low
temperature. This barrier to activation of H2 and the facile
conversion of 2c to 4c thus precludes the isolation of the de-
sired salt 2c. These observations also demonstrate that 5c/
borane activates hydrogen to a much lesser extent than the
A
CHTUNGTRENNUNG
CTHUNGTRENNUNG
CTHUNGTRENNUNG
A
CHTUNGTRENNUNG
by literature methods. 1H, 13C, 11B and 19F NMR spectra were obtained
on Bruker Advance 400, 500 and 600 MHz NMR spectrometers as indi-
cated and referenced to residual solvent (1H, 13C) or externally (11B:
BF3OEt2, 19F: CFCl3).
mixture of 2,6-lutidine/BACHTNUGRTNEUNG
(C6F5)3,[37,38] consistent with the re-
ACHUTNGRENUN(G C5H2Me2AHCUTNGTRENN(GUN CO2Et)2NMe) (1a): NaH (90 mg, 3.79 mmol) was weighed in a
duced basicity at the pyridine nitrogen in 5c, and the effect
of the formation of the adduct, 6c on the reaction with H2.
glovebox, suspended in dry THF (5 mL) and transferred via cannula to
1c (400 mg, 1.58 mmol) suspended in THF (5 mL) in a 25 mL round
bottom flask at 08C. The solution developed a bright orange colour over
10 min and was warmed to room temperature. Methyl p-toluene sulfo-
nate (0.60 mL, 5.91 mmol) was added drop wise over 30 min. The suspen-
sion was stirred for a further 2 h after which the solvent was removed
under vacuum. The residue was cooled in an ice bath, the excess NaH
was quenched by slow addition of a dilute aqueous solution of p-toluene
sulfonic acid (0.05m, 25 mL) and the crude product was recovered by ex-
traction with CH2Cl2. The organic layer was washed with brine and dried
Conclusion
In summary, we have shown that Hantzschꢀs ester and par-
ticularly its N-alkylated or arylated analogues are highly ef-
fective hydride donors for the organic Lewis acid BACHTUNRGTNEUNG(C6F5)3,
with MgSO4, followed by the addition of 1,4-diazabicycloACTHNUTRGENUGN[2.2.2]octane
such that hydride transfer is observed at temperatures as
low as À508C. The analogue in which the ethyl esters are re-
placed by methyl ketones reacts only by formation of a
Lewis acid–base adduct at the carbonyl carbon, and no hy-
dride transfer is observed. Increasing steric bulk at this loca-
tion by the use of a tBu ester results in significantly less
adduct formation and a very facile hydride transfer; howev-
(500 mg, 4.44 mmol) to quench excess methyl p-toluene sulfonate. The
solvent was removed under vacuum and the residue was purified by chro-
matography (hexanes/ethyl acetate 3:1). The recovered white solid was
dried over P2O5 under vacuum (210 mg, 50%). 1H NMR (400 MHz,
CD2Cl2, 298 K): d = 1.31 (t, 6H, 3JH–H =6 Hz, OCH2CH3), 2.39 (s, 6H,
3
CH3), 3.14 (s, 2H, CH2), 3.18 (s, 3H, N-CH3), 4.17 ppm (q, 4H, JH–H
=
6 Hz, OCH2CH3); 13C NMR (100 MHz, CDCl3, 298 K): d = 14.4, 15.9,
23.9, 33.8, 59.7, 101.5, 150.6, 167.8 ppm.
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D. W. Stephan, C. M. Crudden et al.
A
E
N
U
alytically pure sample. X-ray quality crystals were grown from CH2Cl2
1
was weighed in a glovebox, dissolved in CH2Cl2 (1.5 mL) and was added
drop wise, in the dark, to a solution of 1b (50 mg, 0.19 mmol) in CH2Cl2
(1.5 mL) in a 4 dram glass vial at room temperature. A bright yellow so-
lution formed on addition and after 5 min Et2O (15 mL) was added. The
solvent was removed under vacuum to yield a white powder. X-ray quali-
layered with pentane at À258C. H NMR (400 MHz, CDCl3, 298 K): d =
1.10 (ov d, 3H, 3JH–H =7 Hz, CHCH3), 1.13 (ov br m, 3H, OCH2CH3),
1.44 (br, 3H, OCH2CH3), 2.39 (s, 3H, CH3), 4.13 (brm, 2H, OCH2CH3),
4.22 (brm, 2H, OCH2CH3), 4.56 (p, 1H, 3JH–H =7 Hz, CHCH3), 6.06
(brs), 7.41 ppm (brs); partial 13C (125 MHz, CD2Cl2, 298 K, HSQC): d =
13.5, 13.7, 21.7, 23.7, 48.6, 62.1, 65.4, 138.6 ppm; 19F NMR (376 MHz
CD2Cl2, 298 K): d = À134.2 (brm, 6F, ortho-C6F5), À157.3 (brm, 3F,
para-C6F5), À164.3 ppm (brm, 6F, meta-C6F5); 11B NMR (160 MHz,
C6D6, 298 K): d = 2.9 ppm (brs, B-O).
ty crystals were grown from Et2O at À258C. H NMR (500 MHz, CD2Cl2,
298 K): d = 1.45 (t, 6H, 3JH–H =7 Hz, OCH2CH3), 3.16 (ov s, 6H, CH3),
3.42 (br ov q, 1H, 1JB–H =88 Hz, B-H), 4.25 (s, 3H, N-CH3), 4.52 (q, 4H,
3JH–H =7 Hz, OCH2CH3), 9.14 ppm (s, 1H, para-H); 13C NMR (125 MHz,
CD2Cl2 273 K): d = 14.1, 20.3, 42.5, 64.6, 125.4 (brm, C-B), 130.4, 136.8
(dm, 1JC–F =257 Hz, C-F), 138.1 (dm, 1JC–F =244 Hz, C-F), 146.7, 148.28
(dm, 1JC–F =244 Hz, C-F), 160.7, 162.6 ppm; 19F NMR (376 MHz, CD2Cl2,
298 K): d = À134.6 (brm, 6F, ortho-C6F5), À164.6 (brm, 3F, para-C6F5),
À167.6 ppm (brm, 6F, meta-C6F5); 11B NMR (160 MHz, CD2Cl2, 298 K):
ACHUTNGTERG[NNUN C5HMe2CAHTUNGTREN(NUG CO2tBu)2NH][HBAHCTUNGTRENNG(UN C6F5)3] (2d): BCATHNUGTREN(NUGN C6F5)3 (9.7 mg, 0.02 mmol)
was weighed in a glovebox, dissolved in CD2Cl2 (0.25 mL) and was added
in the dark, to a suspension of 1c (5 mg, 0.02 mmol) frozen in CD2Cl2
(0.25 mL) in an NMR tube. On warming to À508C, a bright yellow heter-
ogeneous solution formed. The yellow precipitate dissolved on shaking
and the solution was characterized by VT multi-nuclear NMR. 1H
d = À25.8 ppm (d, JB–H =88 Hz, B-H); elemental analysis calcd (%) for
C32H21BF15NO4·0.5CH2Cl2: C 47.50, H 2.70, N 1.70; found: C 47.51, H
2.50, N 1.71.
(600 MHz, CD2Cl2, 233 K): d = 1.58 (s, 18H, OCACTHUNGTRNEUNG(CH3)3), 2.90 (s, 6H,
CH3), 3.50 (br m, B-H), 9.28 (s, 1H, para-H), 12.81 ppm (brs, 1H, N-H);
partial 13C (150 MHz, CD2Cl2, 223 K, HSQC and HMBC): d = 25.1 (s,
ACHTUNGTRENNUNG[C5HMe2ACHTUNGTRENNUNG(CO2Et)2NPh][HBAHCTUNGTRENNG(UN C6F5)3] (2b): BCAHTNUGTREN(NUGN C6F5)3 (8.6 mg, 0.015 mmol)
CCH3), 27.5 (s, OCACHTNUGTRENN(GU CH3)3), 85.7 (s, OCACHUTGNTREN(NNGU CH3)3), 127.9 (s, pyC), 149.2 (s,
was weighed in a glovebox and dissolved in CD2Cl2 (0.25 mL) and was
added in the dark to a suspension of 1b (5 mg, 0.015 mmol) frozen in
CD2Cl2 (0.25 mL) in an NMR tube. On warming, a bright yellow solution
formed. The solution was characterized by VT multi-nuclear NMR. 1H
(400 MHz, CD2Cl2, 243 K): d = 1.43 (t, 6H, 3JH–H =5 Hz, OCH2CH3),
2.76 (s, 6H, CH3), 3.45 (brm, 1H, B-H), 4.51 (q, 4H, 3JH–H =5 Hz,
OCH2CH3), 7.24 (d, 2H, 3JH–H =4 Hz, ortho-ArH), 7.79 (ov m, 3H, meta
& para-ArH), 9.32 ppm (s, 1H, para-H); partial 13C NMR (150 MHz,
CD2Cl2, 258 K): d = 13.8, 21.5, 64.2, 124.6, 129.4, 129.5, 129.9, 132.2,
132.5, 136.3 (dm, 1JC–F =244 Hz, C-F), 136.9 (dm, 1JC–F =238 Hz, C-F),
138.2, 147.6 147.8 (dm, 1JC–F =244 Hz, C-F), 161.5, 162.0 ppm; 19F NMR
(376 MHz, CD2Cl2, 253 K): d = À135.3 (brm, 6F, ortho-C6F5), À164.3
(brm, 3F, para-C6F5), À167.3 ppm (brm, 6F, meta-C6F5); 11B (168 MHz,
pyCH), 158.4 (s, pyC), 160.4 ppm (s, COOtBu); 19F NMR (376 MHz,
CD2Cl2, 233 K): d = À134.7 (d, 6F, 3JF–F =22 Hz, ortho-C6F5), À162.5 (t,
3F, 3JF–F =20 Hz, para-C6F5), À166.2 ppm (t, 6F, 3JF–F =20 Hz, meta-
C6F5); 11B (168 MHz, CD2Cl2, 240 K): d = À24.6 ppm (d, 1JB–H =74 Hz,
B-H).
A
E
N
(C6F5)3)NH)
(3e):
BN
(25 mg,
0.05 mmol) was weighed in a glovebox, dissolved in CD2Cl2 (0.25 mL)
and was added in the dark, to a suspension of 1e (9 mg, 0.05 mmol) in
CD2Cl2 (0.25 mL) in an NMR tube. The solution was characterized by
VT multi-nuclear NMR. X-ray quality crystals were grown from 1,2-
1
DCE layered with pentane at À258C. H (600 MHz, CD2Cl2, 273 K): d =
2.10 (s, 3H, CH3), 2.21 (s, 3H, CH3), 2.28 (s, 3H, CH3), 2.45 (s, 3H, CH3),
3.34 (s, 2H, CH2), 7.10 ppm (s, 1H, NH); 13C NMR (125 MHz, CD2Cl2,
273 K): d = 19.1, 23.9, 25.3, 27.0, 30.4, 105.9, 116.6, 119.6 (brm, C-B),
137.3 (dm, 1JC–F =246 Hz, C-F), 139.3, 140.2 (dm, 1JC–F =253 Hz, C-F),
148.0 (dm, 1JC–F =240 Hz, C-F), 164.7, 197.9, 198.5 ppm; 19F NMR
1
CD2Cl2, 298 K): d = À23.3 ppm (d, JB–H =93 Hz, B-H); elemental analy-
sis calcd (%) for C37H23BF15NO4: C 52.82, H 2.76, N 1.67; found: C 53.25,
H 2.68, N 1.67.
A
E
G
(2c)
and
(C5H2Me2ACTHUNGTRENNU(G CO2Et)
(376 MHz, CD2Cl2, 250 K): d
=
À134.0 (brm, 6F, ortho-C6F5), À155
E
(br m, 3F, para-C6F5), À163.3 ppm (brm, 6F, meta-C6F5); 11B (168 MHz,
CD2Cl2, 273 K): d = À2.0 ppm (brs, B-O); elemental analysis calcd (%)
for C32H21BF15NO4·0.5CH2Cl2: C 49.39, H 2.14, N 1.99; found: C 49.22, H
2.03, N 1.96.
a glovebox, dissolved in CD2Cl2 (0.25 mL) and was added in the dark to
a suspension of 1c (5 mg, 0.02 mmol) frozen in CD2Cl2 (0.25 mL) in an
NMR tube. On warming, a bright yellow solution formed. The solution
was characterized by VT multi-nuclear NMR. 2c: 1H (400 MHz, CD2Cl2,
3
A
E
N
(C6F5)3)N)
(6c):
B
N
(100 mg,
253 K): d = 1.46 (t, 6H, JH–H =7 Hz, OCH2CH3), 3.06 (s, 6H, CH3), 3.53
(br ov q, 1H, 1JB–H =80 Hz, B-H), 4.50 (q, 4H, 3JH–H =7 Hz, OCH2CH3),
9.42 (s, 1H, para-H), 13.05 ppm (brs, 1H, N-H); 13C NMR (125 MHz,
CD2Cl2 253 K, HSQC and HMBC): d = 14.3 (s, OCH2CH3), 21.2 (s,
0.20 mmol) was added to 5c (48 mg, 0.20 mmol) in toluene (2 mL). The
solution was allowed to stir for 4 h and then pumped to dryness. The
solid was washed with pentane (2ꢃ2 mL) and again pumped to dryness
(110 mg, 74%). X-ray quality crystals were grown from pentane at
À358C. Cooling to À608C resulted in only broadening of the peaks, not
in resolution. 1H NMR (CD2Cl2): d = 1.40 (t, 3JH–H =8 Hz, CH2-CH3),
2.75 (s, C-CH3), 4.47 (q, 3JH–H =8 Hz, CH2-CH3), 8.48 ppm (s, CH);
CH3), 64.3 (s, OCH2CH3), 124.0 (brm, C-B), 128.0 (s, pyC), 136.8 (dm,
1
1JC–F =244 Hz, C-F), 138.4 (dm, 1JC–F =235 Hz, C-F), 148.2 (dm, JC–F
=
231 Hz, C-F), 149.5 (s, pyCH), 159.3 (s, pyC), 161.7 ppm (s, COOEt);
19F NMR (376 MHz, CD2Cl2, 253 K): d = À135.3 (d, 6F, 3JF–F =22 Hz,
ortho-C6F5), À164.1 (t, 3F, 3JF–F =20 Hz, para-C6F5), À167.7 ppm (t, 6F,
3JF–F =20 Hz, meta-C6F5); 11B (168 MHz, CD2Cl2, 263 K): d = À24.4 ppm
(d, 1JB–H =80 Hz, B-H). 3c 1H (400 MHz, CD2Cl2, 253 K): d = 1.23 (br,
1
13C NMR (CD2Cl2) (partial): d = 13.9, 24.4, 64.0, 122.5, 137.7 (dm, JC–
F =252 Hz, CF), 140.3, 148.1 (dm, 1JC–F =252 Hz, CF), 162.1, 168.3 (m);
19F NMR (CD2Cl2): d
=
À131.5 (brd, 3JF–F =17 Hz, 6F, ortho-C6F5),
À150.5 (brs, 3F, para-C6F5), À162.9 ppm (brs, 6F, meta-C6F5); 11B NMR
3
6H, OCH2CH3), 2.11 (s, 6H, CH3), 3.22 (s, 2H, CH2), 4.20 (q, 4H, JH–
H =7 Hz, OCH2CH3), 5.94 ppm (brs, 1H, N-H); partial 13C (125 MHz,
CD2Cl2, 263 K, HSQC and HMBC): d = 13.1 (s, OCH2CH3), 18.9 (s,
CH3), 23.3 (s, CH2), 66.1 (s, OCH2CH3), 97.1 (s, C=C), 149.8 (s, C=C),
174.9 ppm (s, COOEt); 19F NMR (376 MHz, CD2Cl2, 253 K): d = À135.6
(brm, 6F, ortho-C6F5), À159.1 (brm, 3F, para-C6F5), À165.9 ppm (brm,
6F, meta-C6F5); 11B (168 MHz, CD2Cl2, 263 K): d = 2.4 ppm (brs, B-O).
(CD2Cl2): d
= 42.2 ppm (brs, BO); elemental analysis calcd (%) for
C31H17BF15NO4: C 48.78, H 2.24, N 1.84; found: C 48.59, H 2.17, N 1.85.
X-ray data collection and reduction: Crystals were coated in Paratone-N
oil in the glovebox, mounted on a MiTegen Micromount and placed
under an N2 stream, thus maintaining a dry, O2-free environment for
each crystal. The data were collected on a Bruker Apex II diffractometer.
The data were collected at 150(Æ2) K for all crystals. The frames were in-
tegrated with the Bruker SAINT software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the empirical
multi-scan method (SADABS).
A
E
N
(C6F5)3)NH]
(4c):
BN
(100 mg,
0.19 mmol) was weighed in a glovebox, dissolved in CH2Cl2 (1.5 mL) and
was added drop wise, in the dark, to a solution of 1c (50 mg, 0.19 mmol)
in CH2Cl2 (1.5 mL) in a 4 dram glass vial at room temperature. A bright
yellow solution formed on addition, after 5 min hexanes (10 mL) was
added and the solution was allowed to stand at À258C. The resulting pre-
cipitate was filtered and recrystallized from CH2Cl2 layered from pen-
tane, filtered and dried under vacuum (60 mg, 40%). 3c slowly decom-
posed in solution to 2c and as a result it was not possible to obtain an an-
Structure solution and refinement: Non-hydrogen atomic scattering fac-
tors were taken from the literature tabulations.[24] The heavy atom posi-
tions were determined using direct methods employing the SHELXTL
direct methods routine. The remaining non-hydrogen atoms were located
from successive difference Fourier map calculations. The refinements
4900
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4895 – 4902

Borohydrides from Organic Hydrides
FULL PAPER
were carried out by using full-matrix least squares techniques on F, mini-
mizing the function wACHTUNGTRENNUNG
newable Resources, 2009 (http://www1.eere.energy.gov/hydroge-
nandfuelcells/storage/pdfs/targets_onboard_hydro_storage.pdf).
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2s(Fo2) and Fo and Fc are the observed and calculated structure factor
amplitudes, respectively. In the final cycles of each refinement, all non-
hydrogen atoms were assigned anisotropic temperature factors in the ab-
sence of disorder or insufficient data. In the latter cases atoms were treat-
À
ed isotropically. C H atom positions were calculated and allowed to ride
À
on the carbon to which they are bonded assuming a C H bond length of
0.95 ꢂ. H-atom temperature factors were fixed at 1.10 times the isotropic
temperature factor of the C atom to which they are bonded. The H-atom
contributions were calculated, but not refined. The locations of the larg-
est peaks in the final difference Fourier map calculation as well as the
magnitude of the residual electron densities in each case were of no
chemical significance. Additional details are provided in Table 2 and the
Supporting Information.
Table 2. Crystallographic data.
Crystal
2a
4c
3e
6c
formula
formula
weight
crystal
system
space
group
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
C32H21BF15NO4 C31H19BF15NO4 C29H15BF15NO2 C31H17BF15NO4
780.35
765.28
705.23
763.27
triclinic
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/c
¯
P1
9.7091(9)
12.2814(12)
14.0010(14)
78.055
13.7038(3)
27.7497(5)
18.4929(3)
90.00
15.7452(13)
24.0777(18)
20.9825(18)
90.00
11.4131(2)
18.7242(3)
15.061(2)
90.00
b [8]
g [8]
82.764(1)
86.857(1)
1619.6(3)
2
97.925(1)
90.00
6965.2(2)
8
109.924(2)
90.00
7478.5(11)
8
108.4620(10)
90.00
3052.72(8)
4
[17] V. Sumerin, F. Schulz, M. Nieger, M. Leskelꢄ, T. Repo, B. Rieger,
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2002, 3328–3335.
V [ꢂ3]
Z
1calcd
1.598
1.460
1.253
1.661
[gcmÀ1
]
]
m [cmÀ1
0.162
0.149
13625
0.129
14587
0.170
5983
[19] G. Erker, Dalton Trans. 2005, 1883–1890.
data: total 6337
(indep)
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unclear whether the hydride is transferred by a concerted or step-
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76.
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data
5062
7111
6244
4479
Fo2 >3s(Fo2)
variables
R[a]
494
0.0374
0.1013
904
871
473
0.0576
0.1652
0.933
0.0616
0.1747
0.819
0.0357
0.0856
1.006
[b]
Rw
goodness of 1.036
fit
[a] R=S(FoÀFc)/SFo. [b] Rw =(S[w(Fo2ÀFc2)2]/S[w(Fo)2])1/2
.
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15298–15299.
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2003, 9, 871–880.
Acknowledgements
C.M.C. and D.W.S. gratefully acknowledge the financial support of
NSERC of Canada. D.W.S. is also grateful for the award of a Killam Fel-
lowship. J.D.W. and S.J.G. are grateful for the award of NSERC of
Canada scholarships.
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[26]
Fc+/0
.
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Received: October 30, 2009
Published online: March 22, 2010
4902
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4895 – 4902