Tuning the lewis acidity of boranes in frustrated lewis pair chemistry: Implications for the hydrogenation of electron-poor alkenes

Article abstract of DOI:10.1002/chem.201301158

An analysis of the metal-free reduction of electron deficient olefins by frustrated Lewis pairs indicates that the rate-determining step might be either the heterolytic cleavage of H₂ to form an -onium borohydride salt, or the subsequent transfer of the hydride moiety to the substrate following a Michael-type addition reaction. While the use of strong Lewis acids such as B(C₆F₅)₃ facilitates the first of these processes, hydride transfer to the olefin should be contrarily favoured by the use of weak Lewis acids which, for this very same reason, might be unable to promote the prior H₂ split. After systematic testing of several boranes of different Lewis acidity (assessed by using the Childs' method) and steric demand, an optimal situation that employs tris(2,4,6-trifluorophenyl) borane was reached. Mixtures of this borane with 1,4-diazabicyclo[2.2.2]octane (DABCO) exhibited excellent catalytic activity for the hydrogenation of alkylidene malonates. In fact, this transformation could be achieved under milder conditions than those we reported previously. Moreover, the reaction scope could be expanded to other electron deficient olefins containing esters, sulfones or nitro functionalities as electron-withdrawing substituents.

Full text of DOI:10.1002/chem.201301158

DOI: 10.1002/chem.201301158
Tuning the Lewis Acidity of Boranes in Frustrated Lewis Pair Chemistry:
Implications for the Hydrogenation of Electron-Poor Alkenes
Juan A. Nicasio, Sebastian Steinberg, Blanca Inꢀs, and Manuel Alcarazo*[a]
Abstract: An analysis of the metal-free
reduction of electron deficient olefins
by frustrated Lewis pairs indicates that
the rate-determining step might be
either the heterolytic cleavage of H2 to
form an -onium borohydride salt, or
the subsequent transfer of the hydride
moiety to the substrate following a Mi-
chael-type addition reaction. While the
use of strong Lewis acids such as
use of weak Lewis acids which, for this
very same reason, might be unable to
promote the prior H2 split. After sys-
tematic testing of several boranes of
different Lewis acidity (assessed by
using the Childsꢀ method) and steric
demand, an optimal situation that em-
ploys tris(2,4,6-trifluorophenyl)borane
was reached. Mixtures of this borane
with 1,4-diazabicyclo[2.2.2]octane
AHCTUNGTRENNUNG
(DABCO) exhibited excellent catalytic
activity for the hydrogenation of alkyli-
dene malonates. In fact, this transfor-
mation could be achieved under milder
conditions than those we reported pre-
viously. Moreover, the reaction scope
could be expanded to other electron
deficient olefins containing esters, sul-
fones or nitro functionalities as elec-
tron-withdrawing substituents.
Keywords: alkenes
·
boranes
·
BACHTUNGTRENNUNG(C6F5)3 facilitates the first of these
processes, hydride transfer to the olefin
should be contrarily favoured by the
Childsꢀ method · fluorine · hydroge-
nation · Lewis acids
Introduction
very reasonable due to the electron-poor nature of the al-
lenes and, therefore, an alternative reaction pathway might
take place. In an attempt to gain some evidence about the
operating mechanism, a series of experiments was carried
out. Interestingly, the equimolar reaction of the alkylidene
The activation of small molecules by the cooperative action
of Lewis acid and base centres, so-called frustrated Lewis
pairs, has been explored intensively over the past few
years.[1] These unquenched combinations of Lewis acids and
bases show remarkable reactivity towards the fixation and
activation of CO2,[2] NO,[3] alkenes and alkynes,[4] disufides[5]
or silanes.[6] However, probably the most important applica-
malonate 1 with [HDABCO][DBACHTNUTRGNE(UNG C6F5)3] afforded the hy-
drogenated product [D1]-2, which has a deuterium label ex-
clusively in the b-position. This clearly indicates that the hy-
dride from the borohydride anion is transferred at the elec-
trophilic position of the substrate (Scheme 1). No reaction
[7]
tion to date is the heterolytic spliting of H2 and the subse-
quent development of metal-free catalytic hydrogenations of
the polarized double bonds that are present in imines,[8] en-
amines,[9] nitrogenated heterocycles[10] or silyl enol ethers.[11]
Moreover, this reactivity has been very recently extended to
polyarenes[12] and olefins that are able to stabilise carboca-
tions[13] by the employment of electron-poor phosphines as
bases. Experimental and theoretical data support that these
hydrogenation processes take place first through a protona-
tion of the substrate, followed by hydride delivery.
In our former communication on this topic, we described
the catalytic reduction of alkylidene malonates and elec-
Scheme 1. Plausible mechanism for the FLP-promoted hydrogenation of
alkylidene malonates. Reagents and conditions: a) ClC6H5, 808C, 4 h,
29%.
tron-poor allenes by using 1,4-diazabicycloACTHUNGTRNEN[UNG 2.2.2]octane
(DABCO)/B
ACHTUNGTRENNUNG
seemed that the direct protonation of the substrates was not
was detected when the reduction was attempted with K[HB-
AHCTNUGERTG(NNUN C6F5)3] followed by quenching with DABCO·HCl, demon-
strating that the [HDABCO]+ cation plays an active role
during the hydrogenation process. Moreover, screening of
different phosphorus bases revealed that these are less effi-
cient than their nitrogen congeners. Thus, we rationalised
that in this transformation the [HDABCO]+ moiety should
activate the substrate, presumably through the formation of
[a] J. A. Nicasio, S. Steinberg, Dr. B. Inꢁs, Dr. M. Alcarazo
Max-Planck-Institut fꢂr Kohlenforschung
45470 Mꢂlheim an der Ruhr (Germany)
Fax : (+49)208-306-2994
E-mail: alcarazo@mpi-muelheim.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301158.
11016
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 11016 – 11020

FULL PAPER
a hydrogen bond, followed by nucleophilic attack of the hy-
dride (Scheme 1).
The fact that activation of the alkylidene malonate is nec-
essary to carry out the desired reduction suggests that, for
these substrates, hydride transfer from the borohydride
moiety is the rate-determining step. Hence, the use of bor-
boranes 5 and 7 are slightly more acidic than their respec-
tive di-ortho-substituted analogues 4 and 6. These higher Dd
values might be attributed to steric factors: the smaller size
of a hydrogen atom compared with a fluorine atom allows
the easier coordination of crotonaldehyde to those boranes
with one unsubstituted ortho position. Moreover, the steric
penalty of pyramidalisation at the boron atom in these com-
pounds is reduced.[21,17]
anes with weaker Lewis acidity than BACHTNURGTNEUNG(C6F5)3 (3) should fa-
cilitate this elemental process. However, weak Lewis acids
might not be able to promote the H2 cleavage that is neces-
sary for the reduction to take place and, therefore, a com-
promise regarding the Lewis acidity of the borane partner
must be found to optimise this transformation in terms of
reaction conditions and substrate scope.[15,16]
To compare the catalytic performance of 3–7, our previ-
ously described hydrogenation of alkylidene malonate 1 was
chosen as a model reaction. Table 1 shows the experimental
Table 1. Borane screening for the catalytic hydrogenation of alkylidene
malonates.
Results and Discussion
To put this concept into practice, the formal elimination of
À
F substituents from selected positions on the parent
Entry
Borane
T [8C]
Pressure [bar]
Conversion [%]
borane 3 was employed as an effective strategy to deliver
new boranes with reduced Lewis acidity but similar steric
demands.[17] Scheme 2 depicts the boranes 3–7 that we pre-
pared for this study. Compounds 4[18] and 5 are both deco-
1
2
3
4
5
3
4
5
6
7
80
50
80
50
80
30
5
30
10
30
>98
>98
n.r.
>98
n.r.
[a] All reactions were carried out with a catalytic loading (DABCO/
borane) of 5 mol% for 24 h. Pressure and temperature were increased
until >98% conversion was observed by GC analysis.
conditions required to obtain full conversion (>98%, GC)
of the substrate into the reduced product 2 after reaction
times of one day with a catalyst loading of 5 mol%. In our
hands, boranes 5 and 7, both of which contain mono-ortho-
substituted phenyl rings, did not catalyse the hydrogenation
reaction (Table 1, entries 3 and 5). Formation of a strong
Lewis adduct between DABCO and these two slightly less
sterically hindered boranes is probably the reason for their
lack of reactivity.[22] Contrarily, compounds 4 and 6 (Table 1,
entries 2 and 4) behaved better than the parent borane 3
(Table 1, entry 1), indicating that the combination of
DABCO and 4 is optimal in terms of reactivity. With this
catalytic mixture, the hydrogenation reaction took place
under very mild conditions: 508C and a H2 pressure of only
5 bar. Finally, the equimolar reaction of alkylidene malonate
Scheme 2. Boranes employed in this study and their relative Lewis acidi-
ties, as determined by the Childsꢀ method, in parentheses.
rated with three trifluorophenyl substituents and only differ
À
in the relative positions of the F groups on the phenyl
1 with [HDABCO]ACHTGUNTERNNU[G D·4] also gave exclusively the hydrogen-
rings. The same can be said about boranes 6[19] and 7, al-
though in this case the phenyl rings are difluorinated.
ated product [D1]-2 with the deuterium label in the b-posi-
tion (see the Supporting Information). This result indicates
that a similar mechanism is probably operative, although it
can not yet be asserted whether or not the order of addition
of H+ versus HÀ changes when employing borane 4 instead
of 3.
Once the best catalyst was established, we surveyed the
scope of the method by subjecting a representative set of
differently substituted alkylidene malonates 8–12 to the
same reaction conditions. Interestingly, regardless of the
nature of the substituents, they could all be reduced in very
good to excellent yields (Table 2).
The relative Lewis acid strength of these boranes was
quantified by using the Childsꢀ method, with crotonaldehyde
as the reference base and normalized against 3, which was
given a value of 100.[20] The values obtained are shown in
Scheme 2 (see also the Experimental Section and Support-
ing Information). As expected, ranking the Dd values places
the acidity of nonafluoro-substituted boranes 4 and 5 (Dd=
0.66 and 0.69 ppm respectively) as higher than that of hexa-
fluoro-substituted boranes (Dd=0.53 ppm for
0.63 ppm for 7). Interestingly, both mono-ortho-substituted
6
and
Chem. Eur. J. 2013, 19, 11016 – 11020
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11017

M. Alcarazo et al.
Table 2. Substrate scope for the catalytic hydrogenation of alkylidene
malonates.
Entry
Alkene
Product
Yield [%][b]
1
2
3
4
5
8; R=p-
9; R=Cy
10; R=2-naphthyl
11; R=p-(F)Ph
12; R=p-(OMe)Ph
(CF3)Ph
13
14
15
16
17
96
91
95
96
92
Figure 1. Molecular structure of 29 in the solid state (excluding the newly
[a] All reactions were carried out with a catalytic loading (DABCO/4) of
5 mol% for 24 h, under 5 bar of H2 and at 508C. [b] Yield of isolated
product.
À
formed C H bonds, hydrogen atoms are omitted for clarity; ellipsoids
are set at 50% probability).[23]
In view of the enhanced reactivity of this catalytic mix-
ture, we decided to explore whether the activation provided
by the two ester substituents was necessary to accomplish
the desired reduction, or if other structurally related elec-
tron deficient alkenes could eventually also undergo the
same transformation. Thus, alkenes 18—23, which contain
sulphones and esters functionalities, were subjected to the
newly developed reduction conditions. Although an increase
of the H2 pressure from 5 to 10 bar was necessary, yields
were excellent for sulphonoesters 18 and 19 with an elec-
tron-neutral or electron-poor substituent (Table 3, entries 1
The above results clearly indicate that for electron-poor
substrates the hydrogenation reaction only takes place if the
C=C double bond is strongly polarised, a requirement that
has been fulfilled by introducing two electron-withdrawing
substituents. However, we recognised that stronger electron-
withdrawing groups than esters or sulfones could, in princi-
ple, activate the olefin moiety to a level that permits the de-
sired reduction to be accomplished, even in the absence of a
second activating group. To this end, a representative set of
nitroalkenes was submitted to hydrogenation following our
newly developed protocol. Table 4 summarises the obtained
Table 4. Catalytic hydrogenation of nitroalkenes.
Table 3. Catalytic hydrogenation of electron poor alkenes.
Entry
Alkene
Product
Yield [%][b]
1
2
3
4
34; R=Ph
38
49
40
41
95
97
96
96
Entry Alkene
Product Yield
[%][b]
35; R=2-furyl
36; R=p-(F)Ph
37; R=o-(Br)Ph
1
2
18; R1 =CO2Et; R2 =SO2Ph; R3 =Ph
26
27
96
97
19; R1 =CO2Et; R2 =SO2Ph; R3 =p-
(CF3)Ph
[a] All reactions were carried out with a catalytic loading (DABCO/4) of
5 mol% for 24 h, under 5 bar of H2 and at 508C. [b] Yield of isolated
product.
3
20; R1 =CO2Et; R2 =SO2Ph; R3 =p-
(OMe)Ph
28
65
4
5
6
7
8
21; R1 =R2 =SO2Ph; R3 =Ph
22; R1 =R2 =SO2Ph; R3 =p-(CF3)Ph
23; R1 =R2 =SO2Ph; R3 =p-(OMe)Ph
24; R1 =H; R2 =CO2Et; R3 =Ph
25; R1 =H; R2 =SO2Ph; R3 =Ph
29
30
31
32
33
97 [c]
95 [c]
96
n.r.
n.r.
[c]
results. As can be seen, our hypothesis was confirmed and in
all of the examples studied the DABCO/4 mixture displayed
an excellent ability to perform the desired hydrogenations
under the same mild conditions developed for alkylidene
malonates.
[a] All reactions were carried out with a catalytic charge (DABCO/4) of
5 mol% for 24 h, under 10 bar of H2 and at 508C; [b] Yield of isolated
product; [c] Reactions carried out at 20 bar of H2.
and 2). The introduction of electron-donating groups, such
as p-(OMe)Ph, to 20 decreased the yield as expected from
the reaction mechanism (Table 3, entry 3). A further in-
crease in the H2 pressure to 20 bar also allowed the reduc-
tion of disulphones 21–23 in very good yields, regardless of
the electronic nature of the third substituent on the olefin
(Table 3, entries 4–6 and Figure 1). Unfortunately, even
under forcing conditions (808C, 60 bar) we could not hydro-
genate olefins containing only one activating group, such as
24 and 25 (Table 3, entries 7 and 8).
Conclusion
After fine-tuning the Lewis acidity of the borane partner,
the hydrogenation of alkylidene malonates by employing
frustrated Lewis pair (FLP)-based catalysis could be ach-
ieved under milder conditions than those previously descri-
bed by our group. In addition, the employment of DABCO/
4 as catalytic mixture allowed the extension of this transfor-
mation to other electron-poor olefins, such as alkylidene sul-
fonoesters, alkylidene disulfones and, importantly, also
11018
www.chemeurj.org
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 11016 – 11020

Lewis Acidity of Boranes in Frustrated Lewis Pairs
FULL PAPER
CHCHO·BACHTUNRGTEN(UNG C6F5)3 adduct d=7.81 ppm (m, 1H). Reference shift Dd=
0.94 ppm.
nitro-olefins. These results also evidence the compatibility
of FLP chemistry with substrates containing densely oxygen-
ated functional groups.
General procedure for the reduction of alkenes: A toluene solution
(4 mL) containing DABCO (1.1 mg, 0.010 mmol), tris(2,4,6-trifluorophe-
nyl)borane (4 mg, 0.010 mmol) and the desired alkene (0.2 mmol) was
transferred under Ar to an autoclave, pressurised with H2 (5 bar for 8–12,
10 bar for 18–20 or 20 bar for 21–23) and heated to 508C for 24 h.
Experimental Section
Determination of the Lewis acidity of boranes 3–7 by the Childsꢀ
method, complete characterisation of the newly prepared olefins and hy-
drogenated products and NMR spectra can be found in the Supplementa-
ry Information and Reference [14].
Synthesis of 4: iPrMgCl (2.0m in Et2O, 2.4 mL, 4.8 mmol) was added
dropwise to
a solution of 1-bromo-2,4,6-trifluorobenzene (0.57 mL,
4.8 mmol) in THF (100 mL) at À208C and the reaction mixture was
slowly warmed to 08C. After 1 h, the reaction mixture was cooled to
À508C and BF3·Et2O (0.2 mL, 1.6 mmol) was added. After stirring for
1 h the cooling bath was removed and the mixture was allowed to reach
room temperature over another hour. Removal of all volatiles and subli-
mation of the crude product (1208C, 1ꢄ10À3 mbar) gave 4 as a white
solid (550 mg, 85%). 1H NMR (300 MHz, CD2Cl2): d=6.68 ppm (dd, J=
8.9, 8.0 Hz, 6H); 13C NMR (75 MHz, CD2Cl2): d=100.8 (ddd, J=30.2,
25.0, 3.2 Hz), 166.5 (dt, J=252.2, 14.9 Hz), 167.1 ppm (dt, J=255.1,
16.8 Hz); 11B NMR (96 MHz, CD2Cl2): d=59.4 ppm; 19F NMR
(282 MHz, CD2Cl2): d=À100.9 (t, J=10.1 Hz, 3F), À96.4 (d, J=9.9 Hz,
Acknowledgements
Generous financial support from the Fonds der Chemischen Industrie
(Dozentenstipendium) and the European Research Council (ERC Start-
ing Grant agreement no. 277963) is gratefully acknowledged. We also
thank Prof. Alois Fꢂrstner for constant support and the X-ray crystallog-
raphy department of our institute for solving the molecular structure of
compound 29.
˜
6F) ppm; IR (neat): n=720, 838, 875, 891, 998, 1022, 1113, 1164, 1224,
1291, 1326, 1412, 1424, 1479, 1518, 1579, 1602, 1629, 3112 cmÀ1; HRMS
(EI): m/z calcd for C18H6BF9: 404.041883 [M]+; found: 404.041906.
Synthesis of 5: Prepared following the procedure described for the syn-
thesis of 4, employing 1-bromo-2,4,5-trifluorobenzene (0.50 mL,
4.2 mmol), iPrMgCl (2.0m in Et2O, 2.1 mL, 4.2 mmol) and BF3·Et2O
(0.18 mL, 1.4 mmol). White solid (425 mg, 75%). 1H NMR (300 MHz,
CD2Cl2): d=6.97–7.05 (m, 3H), 7.16 ppm (dt, J=9.8, 5.5 Hz, 3H);
13C NMR (75 MHz, CD2Cl2): d=106.3 (dd, J=31.6, 20.3 Hz), 125.5 (ddd,
J=17.6, 9.0, 2.1 Hz), 147.4 (ddd, J=246.0, 12.1, 3.4 Hz), 154.1 (dt, J=
257.9, 14.1 Hz), 162.6 ppm (ddd, J=250.7, 9.7, 2.1 Hz); 11B NMR
(96 MHz, CD2Cl2): d=61.8 ppm; 19F NMR (282 MHz, CD2Cl2): d=
À143.9 (dd, J=22.1, 15.7 Hz, 3F), À126.0 (dd, J=22.1, 8.8 Hz, 3F),
[1] For some reviews on the chemistry of frustrated Lewis pairs, see:
a) D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50; Angew.
Chem. Int. Ed. 2010, 49, 46; b) D. W. Stephan, Org. Biomol. Chem.
2008, 6, 1535.
[2] a) C. Rosorius, G. Kehr, R. Frçhlich, S. Grimme, G. Erker, Organo-
metallics 2011, 30, 4211; b) E. Theuergarten, J. Schlçsser, D. Schluns,
M. Freytag, C. G. Daniliuc, P. G. Jones, M. Tamm, Dalton Trans.
2012, 41, 9101.
[3] A. J. P. Cꢅrdenas, B. J. Culotta, T. H. Warren, S. Grimme, A. Stute,
R. Frçhlich, G. Kehr, G. Erker, Angew. Chem. 2011, 123, 7709;
Angew. Chem. Int. Ed. 2011, 50, 7567.
[4] a) J. S. J. McCahill, G. C. Welch, D. W. Stephan, Angew. Chem. 2007,
119, 5056; Angew. Chem. Int. Ed. 2007, 46, 4968; b) M. Ullrich,
K. S. H. Seto, A. J. Lough, D. W. Stephan, Chem. Commun. 2009,
2335; c) C. M. Mçmming, G. Kehr, B. Wibbeling, R. Frçhlich, B.
Schirmer, S. Grimme, G. Erker, Angew. Chem. 2010, 122, 2464;
Angew. Chem. Int. Ed. 2010, 49, 2414.
[5] a) M. A. Dureen, G. C. Welch, T. M. Gilbert, D. W. Stephan, Inorg.
Chem. 2009, 48, 9910; b) B. Inꢁs, S. Holle, R. Goddard, M. Alcarazo,
Angew. Chem. 2010, 122, 8567; Angew. Chem. Int. Ed. 2010, 49,
8389; c) M. Alcarazo, Dalton Trans. 2011, 40, 1839; d) D. Palomas, S.
Holle, B. Inꢁs, H. Bruns, R. Goddard, M. Alcarazo, Dalton Trans.
2012, 41, 9073.
[6] a) D. J. Parks, W. E. Piers, J. Am. Chem. Soc. 1996, 118, 9440;
b) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000, 65,
3090; c) M. Alcarazo, C. Gomez, S. Holle, R. Goddard, Angew.
Chem. 2010, 122, 5924; Angew. Chem. Int. Ed. 2010, 49, 5788; d) D.
Chen, V. Leich, F. Pang, J. Klankermayer, Chem. Eur. J. 2012, 18,
5184.
˜
À100.4 ppm (dd, J=15.8, 8.6 Hz, 3F). IR (neat): n=681, 719, 746, 778,
815, 893, 1040, 1085, 1128, 1140, 1183, 1193, 1276, 1298, 1393, 1419, 1491,
1618, 3080, 3476, 3547 cmÀ1
; HRMS (EI): m/z calcd for C18H6BF9:
404.041883 [M]+; found: 404.041715.
Synthesis of 6: Prepared following the procedure described for the syn-
thesis of 4, employing 1-bromo-2,6-difluorobenzene (1.0 mL, 8.8 mmol),
iPrMgCl (2.0m in Et2O, 4.4 mL, 8.8 mmol) and BF3·Et2O (0.37 mL,
1
2.9 mmol). White solid (609 mg, 59%). H NMR (300 MHz, CD2Cl2): d=
6.91 (t, J=8.2 Hz, 6H), 7.46–7.56 ppm (m, 3H); 13C NMR (75 MHz,
CD2Cl2): d=111.6 (dd, J=26.5, 1.7 Hz), 135.5 (t, J=11.6 Hz), 165.5 ppm
(dd, J=250.5, 11.9 Hz); 11B NMR (96 MHz, CD2Cl2): d=63.6 ppm;
19
˜
F NMR (282 MHz, CD2Cl2): d=À99.8 ppm.; IR (neat): n=699, 732,
780, 871, 966, 1120, 1169, 1211, 1239, 1283, 1316, 1439, 1558, 1614,
3503 cmÀ1; HRMS (EI): calcd for C18H9BF6: 350.070153 [M]+; found:
350.070229.
Synthesis of 7: Prepared following the procedure described for the syn-
thesis of 4, employing 1-bromo-2,4-difluorobenzene (1.0 mL, 8.8 mmol),
iPrMgCl (2.0m in Et2O, 4.4 mL, 8.8 mmol) and BF3·Et2O (0.37 mL,
1
2.9 mmol). White solid (408 mg, 41%). H NMR (300 MHz, CD2Cl2): d=
6.86 (dt, J=9.5, 2.3 Hz, 3H), 6.97 (dt, J=8.3, 2.4 Hz, 3H), 7.36 ppm (q,
J=7.8 Hz, 3H); 13C NMR (75 MHz, CD2Cl2): d=104.1 (dd, J=29.2,
24.5 Hz), 111.7 (dd, J=20.4, 3.3 Hz), 140.4 (t, J=9.8 Hz), 165.6 (dd, J=
74.2, 12.7 Hz), 169.0 ppm (dd, J=72.5, 12.6 Hz); 11B NMR (96 MHz,
CD2Cl2): d=63.0 ppm; 19F NMR (282 MHz, CD2Cl2): d=À104.1 (d, J=
[7] For theoretical mechanistic studies, see: a) T. A. Rokob, A. Hamza,
A. Stirling, T. Soꢆs, I. Pꢅpai, Angew. Chem. 2008, 120, 2469; Angew.
Chem. Int. Ed. 2008, 47, 2435; b) T. A. Rokob, A. Hamza, I. Pꢅpai,
J. Am. Chem. Soc. 2009, 131, 10701; c) S. Grimme, H. Kruse, L.
Goerigk, G. Erker, Angew. Chem. 2010, 122, 1444; Angew. Chem.
Int. Ed. 2010, 49, 1402; d) for the splitting of dihydrogen using a
single carbon centre, see: G. D. Frey, V. Lavallo, B. Donnadieu,
W. W. Schoeller, G. Bertrand, Science 2007, 316, 439.
[8] a) P. A. Chase, G. C. Welch, T. Jurca, D. W. Stephan, Angew. Chem.
2007, 119, 8196; Angew. Chem. Int. Ed. 2007, 46, 8050; b) P. A.
Chase, T. Jurca, D. W. Stephan, Chem. Commun. 2008, 1701; c) C. F.
Jiang, O. Blacque, H. Berke, Chem. Commun. 2009, 5518; d) G.
Erçs, H. Mehdi, I. Pꢅpai, T. A. Rokob, P. Kirꢅly, G. Tꢅrkꢅnyi, T.
Soꢆs, Angew. Chem. 2010, 122, 6709; Angew. Chem. Int. Ed. 2010,
49, 6559.
˜
11 Hz, 3F), À95.7 ppm (d, J=11 Hz, 3F); IR (neat): n=674, 712, 732,
742, 815, 849, 968, 1081, 1090, 1113, 1136, 1227, 1248, 1289, 1402, 1495,
1575, 1596, 3075 cmÀ1; HRMS (EI): calcd for C18H9BF6: 350.070154 [M]+;
found: 350.070459.
Childsꢁ Lewis acidity tests: A solution of crotonaldehyde in CD2Cl2
(0.1m) was prepared and its 1H NMR spectrum was recorded at 298 K.
One equivalent of BACHTUNGTRENNUNG(C6F5)3 was then added, and the spectrum was again
recorded at the same temperature. An excess amount of borane was then
added to ensure that all crotonaldehyde was coordinated. 1H NMR
(298 K): H3CCH=CHCHO reference d=6.87 ppm (m, 1H); H3CCH=
Chem. Eur. J. 2013, 19, 11016 – 11020
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11019

M. Alcarazo et al.
[9] P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Frçhlich, G.
Erker, Angew. Chem. 2008, 120, 7654; Angew. Chem. Int. Ed. 2008,
47, 7543.
[10] a) S. J. Geier, P. A. Chase, D. W. Stephan, Chem. Commun. 2010, 46,
4884; b) G. Erꢆs, K. Nagy, H. Mehdi, T. Soꢆs, I. Pꢅpai, P. Nagy, P.
Kirꢅly, Dr. G. Tꢅrkꢅnyi, Chem. Eur. J. 2012, 18, 574.
7871; c) T. J. Herrington, A. J. W. Thom, A. J. P. White, A. E.
Ashley, Dalton Trans. 2012, 41, 9019.
[18] D. Chakraborty, A. Rodrꢇguez, E. Y. X. Chen, Macromolecules 2003,
36, 5470.
[19] D. Naumann, H. Butler, R. Gnann, Z. Anorg. Allg. Chem. 1992,
618, 74.
[11] H. D. Wang, R. Frçhlich, G. Kehr, G. Erker, Chem. Commun. 2008,
5966.
[20] R. F. Childs, D. L. Mulholland, A. Nixon, Can. J. Chem. 1982, 60,
801.
[12] Y. Segawa, D. W. Stephan, Chem. Commun. 2012, 48, 11963.
[13] L. Greb, P. OÇa-Burgos, B. Schirmer, S. Grimme, D. W. Stephan, J.
Paradies, Angew. Chem. Int. Ed. 2012, 51, 10164.
[14] B. Inꢁs, D. Palomas, S. Holle, S. Steinberg, J. A. Nicasio, M. Alcara-
zo, Angew. Chem. Int. Ed. 2012, 51, 12367.
[21] M. M. Morgan, A. J. V. Marwitz, W. E. Piers, M. Parvez, Organome-
tallics 2013, 32, 317.
[22] The addition of DABCO (1 equiv) to a solution of 5 or 7 in CD2Cl2
caused the formation of white precipitates showing 11B NMR reso-
nances at d=5.07 and 6.03 ppm, respectively, which can be attribut-
ed to the borane–DABCO adduct. The free boranes 5 and 7 show
11B NMR resonances at d=61.8 and 63.0 ppm, respectively (see the
Supporting Information).
[23] CCDC-929706 (29) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
[15] For previous reports about the effect of modified boranes on the re-
˝
duction of electron-poor alkenes, see: a) G. Eros, H. Mehdi, I.
Pꢅpai, T. A. Rokob, P. Kirꢅly, G. Tꢅrkꢅnyi, T. Soꢆs, Angew. Chem.
2010, 122, 6709; Angew. Chem. Int. Ed. 2010, 49, 6559; b) J. S.
Reddy, B. H. Xu, T. Mahdy, R. Frçhlich, G. Kehr, D. W. Stephan, G.
Erker, Organometallics 2012, 31, 5638.
[16] J. Paradies and co-workers, following a similar line of reasoning, in-
dependently arrived at the same conclusions, see: L. Greb, C. G.
Daniliuc, K. Bergander, J. Paradies, Angew. Chem. Int. Ed. 2013,
125, 5989–5992; Angew. Chem. Int. Ed. 2013, 52, 5876–5879.
[17] a) K. Vanka, M. S. W. Chan, C. C. Pye, T. Ziegler, Organometallics
2000, 19, 1841; b) B. L. Durfey, T. M. Gilbert, Inorg. Chem. 2011, 50,
Received: March 26, 2013
Revised: May 25, 2013
Published online: June 28, 2013
11020
www.chemeurj.org
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 11016 – 11020