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