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Similarly, catalytic hydrosilylation of pyridine with 2.5 mol%
of 5 is possible, but is slow and does not go to completion.
Upon addition of 5 to a mixture of pyridine and PhSiH3 at
room temperature, [LMg(py)2(1,2-dhp)] 11 is formed immedi-
ately, which is also the kinetic product in stoichiometric reac-
tions of 5 with excess pyridine (vide supra). More forcing con-
ditions are needed to achieve any catalytic conversion. After
48 h at 808C, only around 52% of pyridine has been trans-
formed into a dihydropyridide species with [(1,4-dhp)2PhSiH]
being the main product, though the exact product distribution
is unknown, as [(1,4-dhp)2PhSiH], [(1,2-dhp)2PhSiH], and [(1,4-
dhp)(1,2-dhp)PhSiH] are all formed alongside each other show-
(see the previous section and Figures S21 and S22 in the Sup-
porting Information). This is also illustrated by ligand rear-
rangements in the molecular structure of 8 compared with
compound 5. Given the highly symmetrical solution NMR spec-
tra found for 1 that suggest flexible solution dynamics (vide
supra) we also reacted this compound with four equivalents of
HBPin, and found that it reacted rapidly at room temperature
to form 15 (Figures S49–S51 in the Supporting Information). By
implication, freshly generated “MgH2” may be the byproduct,
though this could not be spectroscopically verified. A broad
1
resonance at approximately 1.5 ppm in the H NMR spectrum
(Figure S49) could be attributed to a new reactive species. The
fast ligand-loss from 1 when treated with HBPin, especially
compared to the very slow reaction of 5 with HBPin, and the
fact that the phosphinoamide ligands in our catalytic studies
of 5 and 8 eventually end up in 7 or 15, led us to test 1 as
a catalyst, which yielded the equal-best results.
1
ing overlapping H NMR resonances. The phosphinoamide-con-
taining component at the end of the reaction is yet again 7.
Mechanistic considerations
The proposed mechanism for the catalytic hydroboration and
hydrosilylation of ketones for several previously reported sys-
tems often follows a general insertion/s-bond metathesis
cycle,[10,23] although other possibilities remain. To shed light on
the mechanism using the phosphinoamide-magnesium com-
plexes reported here (1, 5 and 8), and to identify the possible
active species, we performed some stoichiometric reactions to
assess the feasibility of individual steps for this mechanism.
Complex 5 reacts rapidly with 2-adamantanone to give the in-
sertion product 14 as previously stated (Figure 5). Similarly, 5
readily reacts with benzophenone in a hydromagnesiation re-
action, as judged by in situ 1H NMR spectroscopic data. No
well-defined products could be isolated from the latter reac-
tion mixtures, and the formed complexes likely depend on the
Mg:Ph2CO ratio including coordination of unreacted benzo-
phenone. These findings support that C=O insertion into the
MgH moiety could be the first step in the catalytic cycle.
Next, we treated complex 14 with 2.5 equivalents of HBPin
or PhSiH3 to investigate whether our ketone insertion product
can easily undergo s-bond metathesis with each hydride
source. Compound 14 and HBPin react instantaneously to give
the borate ester 2-Ad(H)OBPin and 15, and the resonances of
the starting materials have completely vanished from multinu-
clear NMR spectra of the reaction mixtures. The fate of the Mg
moiety is unknown, and NMR spectroscopic experiments sug-
gest that the magnesium hydride complex 5 is not reformed
in detectable quantities after release of the hydroborated
product (vide supra). A reaction of complex 14 with PhSiH3 at
room temperature was not observed. However, within several
minutes at 708C, complex 14 is consumed, alkoxysilanes form,
and the phosphinoamide groups are present as 7. The rapid
formation of Ad(H)OBPin from 14 at room temperature when
treated with pinacolborane, and the comparable reaction of 14
with phenylsilane at an elevated temperature gives credibility
to a general insertion/s-bond metathesis mechanism. Impor-
tantly, the conditions and reaction rates of the s-bond meta-
thesis of the alcoholate complex 14 with the hydride source
(HBpin or PhSiH3) correlate with the best observed catalysis
conditions of the respective reaction, and thus, may corre-
spond to the rate-determining step in the mechanism.
The reaction of 5 with HBpin alone in deuterated benzene is
very slow, and the starting materials remain largely unreacted
for a few days, although some 15 is produced eventually, and
later other compounds including LH. Although a large excess
of HBpin is typically used in catalytic studies, reactions of 5
with unsaturated substrates such as ketones were found to be
very rapid in comparison. The treatment of complex 5 with
PhSiH3 in deuterated benzene at elevated temperature in the
absence of donor molecules or substrates showed no reaction.
Complex 5 alone is furthermore stable towards heat in the
solid state, in non-coordinating solvents and towards excess
phosphinoamine proligand LH; that is, none of complex 1 is
formed, even at elevated temperatures, to our surprise. Fur-
thermore, it does not appear to react at room temperature
with unsaturated organic molecules lacking good donor
atoms. Consequently, the ketone addition must play a role in
removing L from magnesium, and in the formation of 15 and
7. Hydromagnesiation of LMgH units with ketones forms hard
alcoholate ligands that bind strongly to hard Mg2+ ions, likely
labilising the Mg-phosphinoamide bonds, and inducing ligand-
exchange processes. Additionally, the excess unreacted ketone
can act as additional donor molecules, and presumably induce
equilibria similar to those found when 5 is dissolved in THF
The stoichiometric chemistry reported in the previous sec-
tions already shows several possibilities, such as donor-induced
(THF, PMDETA) rearrangements and redistributions. Catalytic re-
actions with an excess of unsaturated organic substrates (e.g.,
ketones, pyridines) possessing good donor atoms (O, N), likely
induce a series of equilibria to generate compounds of the
type [{LMg(Do)nH}m] A [Scheme 4A, Eq. (1)], that could be
active catalytic species. The donor species (Do) can be neutral
donor groups such as R3P (from LÀ), the large excess of sub-
strates such as ketones (or pyridines) under catalytic regimes,
and even the generated reaction products (e.g., PhSiH(OR)2).
Depending on the donor properties and concentrations, sever-
al aggregates can be present in solution. As previously ob-
served, for example by dissolving 5 in THF, Schlenk-like equili-
bria with numerous redistribution possibilities [formulated in
a simplified form as Eq. (2)], could rapidly form donor-stabi-
lised and soluble MgH2 complexes such as B. That these equili-
Chem. Eur. J. 2016, 22, 1 – 13
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Fohlmeister, Lea
