Angewandte
Communications
Chemie
Table 1: 2-catalyzed hydrosilylation of acrylates and methacrylates.
differentiation of 2 from B(C F ) required study of the
6 5 3
[
a]
latterꢁs catalytic chemistry with secondary silanes. In contrast
Silane
Acrylate
Product
Yield [%]
isolated)
(
to the reactions of tertiary silanes, PhMeSiH and methyl
2
acrylate react in the presence of 1 mol% B(C F ) to give
6
5 3
a complex mixture of unidentified species. Additional evi-
PhMeSiH2
84 (82)
dence distinguishing the chemistry of 2 versus B(C F )
6
5 3
involves the latterꢁs proposed two-step pathway for formation
of a-silyl esters by 1,4-addition to give silyl ketene acetals and
subsequent retro-Brook rearrangement. This pathway was
tested for 2-catalyzed hydrosilylation by treatment of the
8
7
1 (80);
Ph SiH2
2
[b]
[c]
2
(72)
Et SiH2
2
82 (81)
88 (85)
77 (74)
82 (79)
83 (81)
78 (77)
89 (88)
82 (81)
proposed silyl ketene acetal intermediate Me C=C-
2
M
(
OSiPhMeH)OCH Ph, synthesized by To MgHB(C F ) -cat-
2
6
5 3
alyzed hydrosilylation of benzyl methacrylate with phenyl-
PhMeSiH2
[
11]
methylsilane, with either 2 or B(C F ) . However, the a-
6
5 3
silyl ester product Me (PhMeHSi)CCO CH Ph was not
2
2
2
Ph SiH2
2
detected in either reaction mixture after even 1 day.
Moreover, Ph SiH and either methyl methacrylate or
2
2
trans-4-ethyl bromocinnamate react in the presence of
B(C F ) to give mixtures involving CÀO cleavage. In
Et SiH2
2
6
3 3
contrast, the apparent hydrogenation products 3,4-dihydro-
trans-4-ethyl bromocinnamate and dihydrocoumarin are
obtained upon addition of either Ph SiH or Et SiH with
PhMeSiH2
2
2
2
2
the corresponding cinnamate ester or lactone in the presence
of a catalytic amount of 2 (after chromatographic workup of
the silyl ketene acetal). These results indicate that the
catalytic hydrosilylation chemistry of 2 with secondary silanes
is distinct from that of B(C F ) in terms of the identity of the
2
Et SiH2
[
d]
PhMeSiH
2
6
5 3
active species and its mode of action, as well as the nature of
accessible silane and a,b-unsaturated esters reaction partners.
Although the active catalytic species is not observed
under the reaction conditions, alkane byproducts such as
HC(SiHMe ) are not detected during the rapid conversions.
PhMeSiH2
Ph SiH2
2
85 (82)
84 (81)
2 3
From this outcome, we infer that tris(dimethylsilyl) groups
remain bonded to the Ce center during and after catalysis.
The relatively inert nature of the Ce-C(SiHMe ) coordina-
Et SiH2
2
2
3
tion may be related to the two Ce(H-Si interactions per
ligand found in the precatalyst and its new homoleptic
tris(alkyl) cerium precursor. As noted above, these interac-
tions are characterized by low-energy SiH in IR spectra,
fluxional processes frozen at low temperature in NMR
studies, and distorted structures revealed by X-ray crystallog-
raphy. Remarkably, the observed SiÀC bond formation is not
[
[
a] Reaction conditions: 1 mol% 2, CHCl , RT, 10 min. [b] [D ]benzene.
3 6
c] Toluene. [d] 08C, 1 h.
observed between the carbonyl resonance at d = 175.93 ppm
and the H NMR signal of the a-methyl. Finally, a band in the
infrared spectrum at 1725 cm was consistent with the nCO of
an ester.
Although the transformation occurs efficiently in chloro-
form, reactions performed in other solvents including ben-
zene, toluene, and methylene chloride also afford a-silyl
esters. In addition, the catalyst is extremely active, thus giving
full conversion of methyl acrylate and PhMeSiH2 with
1
À1
impeded by the oxophilic Ce and Si centers in the catalyst and
reductant. Furthermore, the apparent ancillary nature of the
alkyl groups suggest that other ancillary ligands may be
accessible for controlling enantioselectivity in these addition
reactions.
0
.05 mol% 2 within 5 minutes. During these reactions, the
pale yellow reaction mixture turns colorless upon catalyst Acknowledgments
deactivation. The timing of this color change, the yield
determined by NMR spectroscopy (from reactions that
deactivate before complete conversion), and the catalyst/
substrate ratio provides turnover number of 2200 and an
We gratefully thank the National Science Foundation (CHE-
1464774) for financial support. Compound 1 was initially
prepared with support from Critical Materials Institute, an
Energy Innovation Hub funded by the U.S. DOE Advanced
Manufacturing Office. W.C.E. and T.L.W. gratefully acknowl-
edge support by the Air Force Office of Scientific Research
under the AFOSR award FA9550-12-1-0476.
À1
À1
estimate of the turnover frequency of 37 s (133200 h ).
Because B(C F ) catalyzes the addition of tertiary silanes
6
5 3
[17]
to esters and a,b-unsaturated esters to give 1,2-addition and
a-silyl esters, respectively, and small amounts of B(C F )
5 3
[
10]
6
could be present in the reaction from dissociation of 2,
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
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