Thermal Dehydrogenation of Base-Stabilized B2H5+ Complexes and Its Role in C-H Borylation

Article abstract of DOI:10.1002/anie.201507647

Thermally induced dehydrogenation of the H-bridged cation L₂B₂H₅⁺ (L=Lewis base) is proposed to be the key step in the intramolecular C-H borylation of tertiary amine boranes activated with catalytic amounts of

Full text of DOI:10.1002/anie.201507647

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507647
German Edition: DOI: 10.1002/ange.201507647
À
C H Activation
+
Thermal Dehydrogenation of Base-Stabilized B2H5 Complexes and Its
À
Role in C H Borylation
Aleksandrs Prokofjevs*
Dedicated to Professor Edwin Vedejs
Abstract: Thermally induced dehydrogenation of the H-
+
bridged cation L2B2H5 (L = Lewis base) is proposed to be
À
the key step in the intramolecular C H borylation of tertiary
amine boranes activated with catalytic amounts of strong
+
“hydridophiles”. Loss of H2 from L2B2H5 generates the
+
highly reactive cation L2B2H3 , which in its sp2-sp3 diborane(4)
À
form then undergoes either an intramolecular C H insertion
+
À
with B B bond cleavage, or captures BH3 to produce L2B3H6 .
The effect of the counterion stability on the outcome of the
Scheme 1. Lewis base borane complex activation by electrophiles.
L=tertiary amine or phosphine, E=“hydridophile”, X=counterion,
Solv=Lewis-basic solvent.
reaction is illustrated by formation of LBH2C6F5 Àcomplexes
+
through disproportionation of L2B2H5 HB(C6F5)3
.
À
E
lectrophilic activation of B H bonds is the essential step in
metal-free dehydrogenation of amine borane complexes,[1] as
Tertiary amine boranes possessing suitably placed C H
bonds have been shown previously to undergo intramolecular
borylation upon stoichiometric activation with strong electro-
philes (Scheme 2).[2a–c] In the reaction sequence involving
À
[2,3]
À
well as in some electrophilic C H borylations.
majority of studies to date have focused on dehydrogenation
of amine borane complexes possessing at least one hydrogen
atom at the amine nitrogen atom (i.e. B N dehydrocoupling),
it appears that other dehydrogenative events can also occur in
activated Lewis base borane complexes. Highlighted herein is
the ability of activated amine and phosphine boranes to
undergo thermal B B dehydrocoupling, and the signifi-
cance of this event in certain electrophilic C H borylations.
While the
1 equivalent of Ph3C+ B(C6F5)4 in either PhBr or PhF,
hydride abstraction from N,N-dimethylneopentylamine
borane (6) leads to formation of the unstabilized borenium
À
À
+
8 through the 3c2e bonded L2B2H5 intermediate 7. Avail-
[4]
À
ability of both an empty orbital and a partially hydridic H at
the borenium center of 8 determines its high reactivity
towards adjacent s-bonds, thus resulting in a facile dehydro-
genative borenium C H insertion and leading to the cyclic
cation 9. Quenching 9 with a suitable hydride source afforded
the isolable cyclic amine borane 10.
It is also possible to perform intramolecular borylations
by using only a catalytic amount of a strong electrophile.[2b,c]
In this case only a small amount of 7 was produced by
activation of the starting amine borane, and heating above
1208C (typically in PhMe) was required to drive the process
towards formation of 10 and H2. The initial mechanistic
rationale assumed that the process involves intramolecular
À
À
The partial hydridic character of B H bonds in a tetra-
À
coordinate boron environment dictates their high reactivity
towards electrophilic agents, and is central to the reducing
properties of borane complexes.[5] The outcome of the
electrophilic activation of tertiary amine and phosphine
borane complexes (1) depends on availability of Lewis
bases in the reaction medium (Scheme 1).[6] Thus, while the
neutral covalent adducts 2 and boronium ions 3 are prefer-
entially formed under more basic conditions, the tricoordi-
nate borenium ions 5[2,7] (or even their respective dicationic
dimers)[8] are accessible when the coordinating ability of the
reaction medium is limited. Depending on the ratio of
electrophile to 1, 5 can also form a three-center-two-electron
(3c2e) bonded adduct with 1, thus giving rise to the H-bridged
L2B2H5+ complex 4,[1a,4a,9] which is the cationic analogue of the
À
well-known B2H7 anion.
[*] Dr. A. Prokofjevs[+]
Department of Chemistry, University of Michigan
930 N. University Ave, Ann Arbor, MI 48109 (USA)
E-mail: aprokofj@umich.edu
[+] Current address: Department of Chemistry, Northwestern University
2145 Sheridan Rd, Evanston, IL 60208 (USA)
À
Scheme 2. The mechanism of dehydrogenative C H borylation of N,N-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507647.
dimethylneopentylamine borane (6) using stoichiometric Ph3C+ activa-
tion.[2a–c]
Angew. Chem. Int. Ed. 2015, 54, 13401 –13405
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13401

.
Angewandte
Communications
À
À
C H insertion of 8, which is generated by the thermal
the B B bonded L2B2H3+ cation 14 and H2 (Figure 1, Path A).
In the most stable conformation of 14, the tricoordinate boron
dissociative loss of 6 from 7. However, multiple discrepancies
between this simple mechanism and experimental observa-
tions were soon identified and explicitly summarized in
a previous communication,[2c] thus outlining a clear need for
a new mechanistic picture.
À
atom enjoys some stabilization from a C H bond in the
proximal neopentyl group, much like the borenium center in
the lowest energy conformation of 8. Because of this
stabilizing interaction, 14 preferentially exists in the open
borenium form, although the isomeric C2-symmetric H-
bridged form is higher in energy by only 1.4 kcalmolÀ1 (gas
phase; see the Supporting Information).
Natural bond orbital (NBO) analysis of the lowest-energy
structure of 14 reveals that natural charges of the two
interconnected boron atoms are distinctly different, +0.80
and À0.42 for the tri- and tetracoordinate B centers,
respectively. Additionally, the LUMO of 14 has a substantial
contribution from the unoccupied p orbital of the tricoordi-
In proposing an alternative mechanism for the catalytic
borylation, the following experimental observations must be
accounted for:[2c] 1) The role of H-bridged cations such as 7 is
clearly significant, since the catalytic reaction was only shown
to proceed efficiently with those electrophiles that led to
generation of the L2B2H5 species. 2) The catalytic and
stoichiometric processes display different regiochemical
trends. 3) The rate-limiting step of the catalytic process
+
À
appears to be a C H insertion, and is primarily dominated
by steric factors.
À
nate B center, while the B B s-bonding orbital is contributing
À
For C H borylation to occur in a concerted insertion step,
to the HOMO of the cation.[11] Therefore, since the tricoor-
the key intermediate must be a coordinatively unsaturated
boron species possessing both a sufficiently empty p orbital,
and a perpendicular filled orbital, such as an unshared
dinate B atom of 14 possesses an empty p orbital (acid)
orthogonal to the strongly polarized B B s bond (base), it
satisfies the requirements for a species capable of inserting
into adjacent C H s bonds. Indeed, the ability of diborane(4)
À
electron pair in borylenes,[10] or a hydridic B H in boranes
À
À
À
or borenium ions. The search for an alternative mechanism
for the catalytic borylation began by noting that the L2B2H5
species are unstable with respect to loss of H2 at 908C in
[D5]PhBr solutions, even when no intramolecular borylation
is possible. This observation suggested that the key borylating
intermediate may be generated by the thermally induced loss
14 to react with proximal C H bonds was confirmed by
identifying the appropriate TS 15 (G° = 29.4 kcalmolÀ1), thus
leading directly to a new H-bridged product (16) containing
the cyclized amine borane subunit. Hydride exchange
between 16 and 6 delivers the final product 10, and
regenerates the crucial intermediate 7, thus completing the
catalytic cycle.[12] The new mechanism is in good agreement
with the requirements defined by the previous experimental
+
+
of H2 from L2B2H5 , and computational studies (M06-2X/
6-311++G(3df,2p)//MP2/cc-pVDZ, SMD solvation) were
performed to outline a plausible mechanism for the cycliza-
tion of 6 in PhMe (Figure 1). Since 7 can be viewed as the
s complex of 8 and 6, it follows that the 3c2e-bonded cation 7
preorganizes the components for a dehydrogenative insertion
of 8 into the B H bond of 6, in a fashion similar to C H
insertion in the stoichiometric borylation process. Indeed,
a sufficiently low-lying transition state (TS 13; G° = 27.8 kcal
molÀ1) was identified for the process, thus leading from 7 to
À
work (see below), as H2 loss from 7 indeed precedes the C B
bond formation, and the steric demands in 15 are more
prominent than in 11, which is consistent with the higher
selectivity of the catalytic process for less hindered borylation
2
products.[2c] The open form of 14 can also be viewed as a sp
À
À
À
sp3 diborane(4), and the synthetic potential of this structural
motif has recently been recognized in an anionic and neutral,
but not cationic, setting.[13] It should also be noted that this
Figure 1. The major features of the reaction profile calculated at the M06-2X/6-311+ +G(3df,2p)//MP2/cc-pVDZ level of theory in PhMe (SMD
À
solvation). Path A represents the B B dehydrocoupling mechanism proposed in this article, while Path B shows the previously considered
mechanism inspired by the stoichiometric reaction. See the Supporting Information for details.
13402 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 13401 –13405

Angewandte
Chemie
2
3
À
appears to be the first example of a cationic sp sp
a triplet with JP-B = 90 Hz, and was thus identified as
diborane(4) being formed in situ from L-BH3 complexes,
and functioning as the key species in the catalytic cycle.
In contrast, the borylation pathway proceeding via the
same primary borenium 8, as in the stoichiometric trans-
formation, is characterized by a higher activation barrier
originating from the unremarkable boronium cation 19a.
The H-coupled broad signal at d = 11B À9.8 ppm did not show
any detectable coupling to 31P, while the complex multiplet at
d = 11B À38.9 ppm was transformed into a doublet with JP-B
=
110 Hz upon proton decoupling. Both signals were shown to
originate from non-equivalent B atoms in the same boron
cation, which was ultimately identified as the base-stabilized
(Figure 1, Path B). Dissociation of 7 to the starting amine
borane 6 and 8 is endergonic by 16.4 kcalmol , and the C H
À1
À
+
insertion transition state 11 is located even higher on the free-
energy profile (G° = 35.9 kcalmolÀ1).[14] The chain of events
leading from 11 to the cyclized product via the thermody-
namically unstable borenium H2 complex 12 follows that of
the stoichiometric borylation.[2a]
B3H6 species 20a.[16] Several triboron cations of this type
have been prepared previously by either basic cleavage of
tetraborane(10),[17] or by reacting diborane(4) derivatives
with boron Lewis acids,[18,19] but formation of L2B3H6+ cations
+
from L2B2H5 has not been reported previously. The cation
+
Seeking experimental support for the reaction mechanism
outlined in the computational studies, a controlled thermal
20a can be viewed as an adduct of L2B2H3 (fragment
highlighted in Figure 2) and “BH3”, thus explaining formation
of 19a.[20] Electrospray ionization mass spectra (ESI MS+) of
the reaction mixture showed signals with m/z 165, 177 and
191, and the isotope peaks of the latter two signals were
consistent with the presence of two and three B atoms,
respectively. While the signals with m/z 165 and 191 are due to
19a and 20a, respectively, the intense m/z 177 signal is due to
the L2B2H3+ cation, apparently arising from fragmentation of
20a and thus emphasizing the close relationship of the two
species.
decomposition of L2B2H5 salts was performed.[15] Hydride
+
abstraction from Me3P BH3 by 0.5 equivalents of Ph3C+
À
À
B(C6F5)4 in [D5]-PhBr resulted in formation of the H-
bridged cation 18a (d = 11B À25.6 ppm,
JP-B = 90 Hz;
Figure 2). Heating to 908C led to the disappearance of 18a,
1
and formation of H2 was detected by H NMR spectroscopy
(d = 1H 4.51 ppm). Three new signals (d = 11B À9.8, À33.4,
and À38.9 ppm in about a 1:1:2 ratio), attributable to cationic
boron species, were observed by 11B NMR spectroscopy at
this point, while stability of the counterion and Ph3CH
Similar observations were made when the activated
trimethylamine borane derivative 18b [L = Me3N, X = B-
byproduct under the reaction conditions was confirmed by 19
F
and 1H NMR spectroscopy, respectively. The multiplet at d =
11B À33.4 ppm upon proton decoupling transformed into
(C6F5)4 ] was heated at 908C in [D5]-PhBr. In this case
À
disappearance of the L2B2H5+ signal at d = 11B À0.3 ppm was
accompanied by appearance of new H-coupled peaks at d =
11B À10.2 and À15.8 ppm, although the latter partially
À
overlapped with the B(C6F5)4 peak at d = À16.1 ppm. The
signals were identified as the BH2 (d = À10.2 ppm) and LBH
(d = À15.8 ppm) subunits of 20b,[18a] and formation of 19b
(d = 11B 3.6 ppm) was also observed. In this case the ESI MS+
+
pattern [m/z 131 (19b), 143 (L2B2H3 ), 157 (20b)] paralleled
that observed for Me3P derivatives, thus supporting the
general similarity of events for activated amine and phos-
phine boranes.[21]
Conversion of L2B2H5+ (18) into L2B3H6+ (20) is reminis-
cent of the processes observed in isostructural boron anions,
À
such as formation of the stable s-aromatic B3H8 anion by
À
thermolysis of B2H7
poorly known B2H5
,
[22] which is believed to proceed via the
À
.
[23,24] Thermal dehydrogenation of 7 and
À
18 is thus expected to generate cationic analogues of B2H5
(i.e., 14), which subsequently stabilize either by intramolec-
À
ular C H insertion (formation of 16) or BH3 incorporation
leading to L2B3H6 cations such as 20. Since 20 was shown
+
previously to produce diborane(4) derivatives upon treatment
with Lewis bases,[17,18] thermolysis of 18, followed by basic
cleavage of the resulting triboron cation 20 can be viewed as
À
a method for building electron-precise B B bonds from
[4c,25]
À
mononuclear L BH3 complexes.
While instability of the
corresponding neutral L2B2H4 complex prevented independ-
ent generation of 14 by hydride abstraction,[26] it should be
noted that the ability of L2B2H3+ to insert into s bonds is not
without precedent.[27]
Figure 2. Formation of the triboron cations 20 (L2B2H3+ fragment
highlighted) in the thermal decomposition of the H-bridged complexes
18. The 11B NMR spectra [L=Me3P, X=B(C6F5)4] are shown: a) imme-
diately following generation oÀf 18a in [D5]-PhBr, and b) after 18 h at
It was also of interest to explore the thermal behavior of
H-bridged cations paired with anions which are less stable to
electrophilic attack, and the reactivity of amine and phos-
À
908C. Other signals: B(C6F5)4 : d=À16.2 ppm; Me3P BH3:
d=À36.4 ppm.
Angew. Chem. Int. Ed. 2015, 54, 13401 –13405
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13403

.
Angewandte
Communications
Acknowledgements
This work was supported in part by the National Institute of
General Medical Sciences of the NIH (GM067146). This work
would not have been possible without the generous support
and encouragement from Prof. Edwin Vedejs.
Scheme 3. Formation of 22 by disproportionation of 21.
phine boranes with B(C6F5)3 was thus explored. Addition of
À
Keywords: boron · C H activation ·
density-functional calculations · homogeneous catalysis ·
Lewis bases
À
Et3N BH3 to 0.5 equivalents of B(C6F5)3 in CD2Cl2 at room
temperature resulted in a rapid hydride abstraction and
1
formation of the H-bridged cation 21 (B-H-B, H d À2.0–
3.3 ppm; 11B d À3.0 ppm, unres. t.; Scheme 3). Unlike in the
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13401–13405
Angew. Chem. 2015, 127, 13599–13603
trityl activation experiments described above, the other
À
product of the hydride abstraction was the HB(C6F5)3
[1] a) F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant, D. A.
Dixon, Angew. Chem. Int. Ed. 2007, 46, 746 – 749; Angew. Chem.
2007, 119, 760 – 763; b) A. J. M. Miller, J. E. Bercaw, Chem.
Commun. 2010, 46, 1709 – 1711; c) O. J. Metters, A. M. Chap-
man, A. P. M. Robertson, C. H. Woodall, P. J. Gates, D. F. Wass,
I. Manners, Chem. Commun. 2014, 50, 12146 – 12149; d) For
a recent review of amine borane dehydrogenations, see: N. E.
Stubbs, A. P. M. Robertson, E. M. Leitao, I. Manners, J. Organo-
met. Chem. 2013, 730, 84 – 89.
counterion (d = 11B À25.4 ppm, d, JB-H = 80 Hz), and the
difference in the anion structure and stability was found to
have a prominent effect on the subsequent events. Thus, even
at room temperature degradation of 21 was evident, and
heating the solution to 408C resulted in formation of B2H6,
and another compound, identified as 22 (d = 11B À14.2 ppm),
upon isolation. Formation of 22 is the result of disproportio-
nation involving some HB(C6F5)3À derivative, and despite the
À
[2] Electrophilic C H borylations initiated by hydride abstraction
À
1:1.5 R3N BH3/C6F5 reaction stoichiometry, only C6F5BH2
from L-BH3: a) T. S. De Vries, A. Prokofjevs, J. N. Harvey, E.
Vedejs, J. Am. Chem. Soc. 2009, 131, 14679 – 14687; b) A.
Prokofjevs, E. Vedejs, J. Am. Chem. Soc. 2011, 133, 20056 –
20059; c) A. Prokofjevs, J. Jermaks, A. Borovika, J. W. Kampf,
E. Vedejs, Organometallics 2013, 32, 6701 – 6711; d) C. Cazorla,
T. S. De Vries, E. Vedejs, Org. Lett. 2013, 15, 984 – 987; e) T.
Stahl, K. Müther, Y. Ohki, K. Tatsumi, M. Oestreich, J. Am.
Chem. Soc. 2013, 135, 10978 – 10981; f) J. M. Farrell, D. W.
Stephan, Angew. Chem. Int. Ed. 2015, 54, 5214 – 5217; Angew.
Chem. 2015, 127, 5303 – 5306.
complexes were formed.[28]
The reaction was further developed into a preparative
protocol, optimized with respect to decreasing the amount of
B(C6F5)3 used (0.36 equiv versus theoretical 0.33 equiv), and
simplified product isolation, which in most cases was accom-
plished by filtering the reaction mixture through a plug of
silica gel, followed by removal of the solvent. The results
listed in Table 1 suggest that this method can be conveniently
used to access C6F5BH2 complexes of simple tertiary amines
and phosphines.[29]
À
[3] Other representative electrophilic C H borylations: a) A.
Del Grosso, J. Ayuso Carrillo, M. J. Ingleson, Chem. Commun.
2015, 51, 2878 – 2881; b) V. Bagutski, A. Del Grosso, J. A.
Carrillo, I. A. Cade, M. D. Helm, J. R. Lawson, P. J. Singleton,
S. A. Solomon, T. Marcelli, M. J. Ingleson, J. Am. Chem. Soc.
2013, 135, 474 – 487; c) S. A. Solomon, A. Del Grosso, E. R.
Clark, V. Bagutski, J. J. W. McDouall, M. J. Ingleson, Organo-
metallics 2012, 31, 1908 – 1916; d) A. Prokofjevs, J. W. Kampf, E.
Vedejs, Angew. Chem. Int. Ed. 2011, 50, 2098 – 2101; Angew.
Chem. 2011, 123, 2146 – 2149; e) A. Del Grosso, M. D. Helm,
S. A. Solomon, D. Caras-Quintero, M. J. Ingleson, Chem.
Commun. 2011, 47, 12459 – 12461; f) A. Del Grosso, P. J. Single-
ton, C. A. Muryn, M. J. Ingleson, Angew. Chem. Int. Ed. 2011, 50,
2102 – 2106; Angew. Chem. 2011, 123, 2150 – 2154; g) N. Ishida,
T. Moriya, T. Goya, M. Murakami, J. Org. Chem. 2010, 75, 8709 –
8712; h) A. Del Grosso, R. G. Pritchard, C. A. Muryn, M. J.
Ingleson, Organometallics 2010, 29, 241 – 249. Also see Ref. [6a]
and references therein.
To summarize, involvement of highly electrophilic
+
L2B2H3 complexes in the high-temperature intramolecular
À
C H borylation of amine boranes activated with catalytic
amounts of strong “hydridophiles” is postulated based on
2
3
À
experimental and theoretical studies. Such sp sp diborane-
(4) cations are formally isoelectronic to B2H5À, and appear to
arise from the thermally induced dehydrogenative borenium
+
À
B H insertion within the isolable H-bridged cations L2B2H5 .
High reactivity of the cation L2B2H3+ manifests itself either in
À
high-yielding intramolecular C H insertions proceeding with
À
the cleavage of the B B bond, or in BH3 incorporation
resulting in formation of the more stabilized L2B3H6+ cation.
In view of these findings, it appears plausible that new
borylating reagents can be identified among electrophilically
activated diborane(4) derivatives, particularly those of the
À
[4] Transition metal catalyzed B B dehydrocoupling: a) O. Cio-
banu, E. Kaifer, M. Enders, H.-J. Himmel, Angew. Chem. Int.
Ed. 2009, 48, 5538 – 5541; Angew. Chem. 2009, 121, 5646 – 5649;
b) H. C. Johnson, C. L. McMullin, S. D. Pike, S. A. Macgregor,
A. S. Weller, Angew. Chem. Int. Ed. 2013, 52, 9776 – 9780;
Angew. Chem. 2013, 125, 9958 – 9962; c) A. Wagner, S. Litters, J.
Elias, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2014, 20, 12514 –
12527; d) S. Muhammad, S. Moncho, E. N. Brothers, A. A.
Bengali, Chem. Commun. 2014, 50, 5874 – 5877.
2
3
À
sp sp type.
Table 1: Synthesis of C6F5BH2 complexes.[a]
Entry Substrate
Solvent Product
Yield [%]
1[b]
2
Et3N BH3
PhF
CH2Cl2
Et3N BH2C6F5 (22)
>99
97
>99
71
À
À
À
À
Me3N BH3
Me3N BH2C6F5 (23)
[5] A. Staubitz, A. P. M. Robertson, I. Manners, Chem. Rev. 2010,
110, 4079 – 4124.
[6] a) M. J. Ingleson, Top. Organomet. Chem. 2015, 49, 39 – 71;
b) T. S. De Vries, A. Prokofjevs, E. Vedejs, Chem. Rev. 2012, 112,
4246 – 4282; c) W. E. Piers, S. C. Bourke, K. D. Conroy, Angew.
À
À
3
BnMe2N BH3 PhF
BnMe2N BH2C6F5 (24)
4[c]
Ph3P BH3
CH2Cl2
Ph3P BH2C6F5 (25)
À
À
À
[a] 0.36:1 B(C6F5)3/L BH3, 508C, 1 h, the reaction performed in sealed
vials. [b] 3 h. [c] 408C.
13404 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 13401 –13405

Angewandte
Chemie
Chem. Int. Ed. 2005, 44, 5016 – 5036; Angew. Chem. 2005, 117,
5142 – 5163; d) P. Kçlle, H. Nçth, Chem. Rev. 1985, 85, 399 – 418.
[7] a) M. A. Dureen, A. Lough, T. M. Gilbert, D. W. Stephan,
Chem. Commun. 2008, 4303 – 4305; b) D. McArthur, C. P. Butts,
D. M. Lindsay, Chem. Commun. 2011, 47, 6650 – 6652; c) J. M.
Farrell, J. A. Hatnean, D. W. Stephan, J. Am. Chem. Soc. 2012,
134, 15728 – 15731; d) A. Solovyev, S. J. Geib, E. Lacôte, D. P.
Curran, Organometallics 2012, 31, 54 – 56; e) P. Eisenberger,
B. P. Bestvater, E. C. Keske, C. M. Crudden, Angew. Chem. Int.
Ed. 2015, 54, 2467 – 2471; Angew. Chem. 2015, 127, 2497 – 2501.
[8] A. Prokofjevs, J. W. Kampf, A. Solovyev, D. P. Curran, E. Vedejs,
J. Am. Chem. Soc. 2013, 135, 15686 – 15689.
[9] a) M. Kameda, G. Kodama, Inorg. Chem. 1997, 36, 4369 – 4371;
b) T. S. De Vries, E. Vedejs, Organometallics 2007, 26, 3079 –
3081; c) B. InØs, M. Patil, J. Carreras, R. Goddard, W. Thiel, M.
Alcarazo, Angew. Chem. Int. Ed. 2011, 50, 8400 – 8403; Angew.
Chem. 2011, 123, 8550 – 8553; d) A. Prokofjevs, A. Bousson-
ni›re, L. Li, H. Bonin, E. Lacôte, D. P. Curran, E. Vedejs, J. Am.
Chem. Soc. 2012, 134, 12281 – 12288.
À9.3 and À40.9 ppm are within 2 ppm of the experimental
values.
[17] M. Kameda, G. Kodama, J. Am. Chem. Soc. 1980, 102, 3647 –
3649.
[18] a) R. DePoy, G. Kodama, Inorg. Chem. 1985, 24, 2871 – 2872;
b) R. E. DePoy, G. Kodama, Inorg. Chem. 1988, 27, 1116 – 1118;
c) R. E. DePoy, G. Kodama, Inorg. Chem. 1988, 27, 1836 – 1839.
[19] N. Schulenberg, H. Wadepohl, H.-J. Himmel, Angew. Chem. Int.
Ed. 2011, 50, 10444 – 10447; Angew. Chem. 2011, 123, 10628 –
10631.
[20] The exact path for the formation of 19 was not investigated,
although a role for trace L-BH3 appears plausible.
[21] L2B2H5+ (d = 11B À20.9 ppm) prepared from 1,3,4,5-tetramethy-
À
limidazolidene borane and 0.5 equiv Ph3C+ B(C6F5)4 in [D5]-
PhBr upon heating to 908C yielded qualitatively similar results
(d = 11B À7.2 (H-coupled, broad), À32.3 (t,
À34.2 ppm (H-coupled, broad)).
JB-H = 80 Hz),
[22] A. Y. Bykov, K. Y. Zhizhin, N. T. Kuznetsov, Russ. J. Inorg.
Chem. 2014, 59, 1539 – 1555.
[10] a) P. Bissinger, H. Braunschweig, A. Damme, R. D. Dewhurst, T.
Kupfer, K. Radacki, K. Wagner, J. Am. Chem. Soc. 2011, 133,
19044 – 19047; b) D. P. Curran, A. Boussonni›re, S. J. Geib, E.
Lacôte, Angew. Chem. Int. Ed. 2012, 51, 1602 – 1605; Angew.
Chem. 2012, 124, 1634 – 1637; c) R. S. Ghadwal, C. J. Schürmann,
F. Engelhardt, C. Steinmetzger, Eur. J. Inorg. Chem. 2014, 4921 –
4926.
[23] a) H. Beall, D. F. Gaines, Inorg. Chim. Acta 1999, 289, 1 – 10;
ˇ
ˇ
b) S. Hermµnek, J. Plesek, Collect. Czech. Chem. Commun. 1966,
31, 177 – 189.
[24] a) M. Krempp, R. Damrauer, C. H. DePuy, Y. Keheyan, J. Am.
Chem. Soc. 1994, 116, 3629 – 3630; b) R. C. Dunbar, J. Am.
Chem. Soc. 1968, 90, 5676 – 5682.
[25] For a recent review of approaches to building electron-precise
À
À
[11] Substantial B B bond polarization is consistent with previous
B B bonds, see: H. Braunschweig, R. D. Dewhurst, S. Mozo,
2
3
À
theoretical investigations on sp sp diboranes(4) used as
nucleophilic boron sources: a) N. Miralles, J. Cid, A. B.
Cuenca, J. J. Carbó, E. Fernµndez, Chem. Commun. 2015, 51,
1693 – 1696; b) A. Bonet, C. Pubill-Ulldemolins, C. Bo, H.
Gulyµs, E. Fernµndez, Angew. Chem. Int. Ed. 2011, 50, 7158 –
7161; Angew. Chem. 2011, 123, 7296 – 7299; c) C. Pubill-Ullde-
molins, A. Bonet, C. Bo, H. Gulyµs, E. Fernµndez, Chem. Eur. J.
2012, 18, 1121 – 1126.
ChemCatChem 2015, 7, 1630 – 1638.
[26] Attempts were made to prepare L2B2H4 complexes by reacting
THF·B3H7 with tBuCH2NMe2, BnNMe2, tBuCH2PPh2, (o-Tol)3P,
nBu3P, BnMeImd, and in all cases the desired neutral complex
either did not form at all, or persisted in the reaction mixture
only for a short time, before it could be isolated. This appears to
be due to 1) Fast reaction between LB3H7 and L2B2H4 to produce
LBH3 and higher boranes (M. Ishii, G. Kodama, Inorg. Chem.
1990, 29, 817 – 820) 2) Inherent instability of L2B2H4 complexes:
(Me3P)2B2H4 decomposes at room temperature under N2 (R. K.
Hertz, M. L. Denniston, S. G. Shore, Inorg. Chem. 1978, 17,
2673 – 2674). Even (Ph3P)2B2H4, which is the most robust L2B2H4
complex because of its high crystallinity, decomposes at only
908C (M. D. Levicheva, L. V. Titov, L. V. Zhemchugova, Izv.
Akad. Nauk SSSR 1987, 2120 – 2122). In full agreement with
literature reports, complexes with L = Me3N and Ph3P were
prepared successfully, and therefore the process appears to be
highly dependent on the nature of L. Reacting (Ph3P)2B2H4 with
the N-heterocyclic carbene BnMeImd produced complex mix-
tures containing some of the desired L2B2H4 (d = 11B À34 ppm,
br t) which could not be isolated, in agreement with the previous
report by Curran et al. (Ref. [10b]).
[12] Facile hydride exchange in L2B2H5+ was previously confirmed by
NMR studies. See Refs. [2a,b] and [9b].
[13] a) R. D. Dewhurst, E. C. Neeve, H. Braunschweig, T. B. Marder,
Chem. Commun. 2015, 51, 9594 – 9607, and references therein;
b) H. Asakawa, K.-H. Lee, Z. Lin, M. Yamashita, Nat. Commun.
2014, 5, 4245; c) S. Pietsch, E. C. Neeve, D. C. Apperley, R.
Bertermann, F. Mo, D. Qiu, M. S. Cheung, L. Dang, J. Wang, U.
Radius, Z. Lin, C. Kleeberg, T. B. Marder, Chem. Eur. J. 2015, 21,
7082 – 7098; d) sp2-sp2 diborane(4) cations: H. Braunschweig, A.
Damme, R. D. Dewhurst, T. Kramer, T. Kupfer, K. Radacki, E.
Siedler, A. Trumpp, K. Wagner, C. Werner, J. Am. Chem. Soc.
2013, 135, 8702 – 8707; e) Diborane(4) cations may have been
involved in other C-H insertion events: H. Braunschweig, A.
Damme, T. Kupfer, Chem. Commun. 2013, 49, 2774 – 2776.
[14] Gas-phase calculations indicate even stronger preference for the
diborane(4) pathway. See the Supporting Information for details.
As requested by a reviewer, calculations were also performed in
PhBr solvent. In this case the diborane(4) pathway is still
favored, although the difference between the two mechanisms is
less prominent, and both may contribute in some cases. The
majority of experimental work on high-temperature catalytic
borylations, however, was performed in PhMe (see Refs. [2b,c]).
Loss of the H2 byproduct from the reaction vessel is expected to
further increase preference for the diborane(4) pathway.
[15] Several Me3P-based complex boron cations were reported
previously (Refs. [9a, 17, 18]), thus justifying the choice of L
for the initial studies.
[27] In view of the mechanism outlined in Figure 1, previously
+
reported formation of L3B3H4 cations in the electrophilic
activation of L2B2H4 (Ref. [9a]) can now be interpreted as
+
À
L2B2H3 insertion into B B bond.
[28] No H2 was produced, thus ruling out the dehydrogenative
pathway proposed above for salts with more stable anions.
Involvement of L-BH2-H-B(C6F5)3 appears plausible based on
limited NMR evidence.
À
[29] a) Disproportionation between Me2S BH3 and B(C6F5)3: A.-M.
Fuller, D. L. Hughes, S. J. Lancaster, C. M. White, Organo-
metallics 2010, 29, 2194 – 2197; b) C6F5-transfer in activated
À
Me3N BH3: G. MØnard, D. W. Stephan, Dalton Trans. 2013, 42,
5447 – 5453.
[16] According to computational modeling, the cation 20a is
structurally similar to other species related to B3H8À. See the
Supporting Information for details. The calculated 11B NMR
chemical shifts of 20a in PhBr (GIAO, SMD solvation) of d =
Received: August 15, 2015
Published online: September 17, 2015
Angew. Chem. Int. Ed. 2015, 54, 13401 –13405
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13405