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tronic effects at the para position of the aromatic ring of the
phenylacetate derivatives were limited, and the correspond-
ing aldehydes were obtained selectively in good yields
regardless of the para substituent (71–85%; Table 2, entries 3,
6, and 7). More strikingly, aliphatic esters, such as ethyl
acetate or methyl decanoate, could be reduced to the
corresponding aliphatic aldehydes in good yields (up to
90% yield; Table 2, entries 12 and 13). Moreover, methyl 10-
undecenoate was transformed into 10-undecenal in 70%
=
yield; products resulting from the isomerization of the C C
bond were obtained in only 10% yield (Table 2, entry 14).
Importantly, the reduction of dimethyl tridecanedioate led
selectively to the corresponding dialdehyde, which was
isolated in very good yield (90%; Table 2, entry 15).
Interestingly, this reaction was also successful with
aromatic esters. When the reaction was performed with the
catalyst 4a (2.5 mol%) in the presence of Et2SiH2 (1.1 equiv)
at room temperature under UV irradiation (350 nm) for 4 h,
58% conversion of methyl p-bromobenzoate into the corre-
sponding aldehyde occurred. Indeed, the use of diphenylsi-
lane (1.5 equiv) led to full conversion, and p-bromobenzalde-
hyde was isolated in 86% yield (Table 2, entry 16). Under
similar conditions, p-methylbenzaldehyde was also formed in
good yield (80%), whereas the reduction of methyl p-
trifluoromethylbenzoate occurred with only 40% conversion,
probably because of the strongly electron withdrawing
substituent (Table 2, entries 17 and 18). Heteroaromatic
esters can be reduced under similar conditions: methyl 5-
bromofur-2-yl-methanoate was converted into the corre-
sponding aldehyde, which was isolated as its hydrazone
derivative in 55% yield (Table 2, entry 20).
Scheme 2. [(IMes)Fe(CO)4]-catalyzed hydrosilylation of lactones. Reac-
tion conditions: 1) lactone (0.5 mmol), Et2SiH2 (1.1 equiv), complex
4a (1 mol%), C6D6 (solvent), room temperature, UV irradiation
(350 nm), 3 h; 2) hydrolysis with 1m aqueous HCl (2 mL) and THF
(2 mL), room temperature, 2 h. [a] Yield determined by 1H NMR
spectroscopy for the silylacetal intermediate.
proton resonates at d = À9.30 ppm, and the silicon resonance
is at d =+ 28.4 ppm.
The molecular structures of 8a and 9 are depicted in
Figure 2. The coordination geometry at the iron center is
distorted trigonal bipyramidal; the N-heterocyclic carbene
We next focused our attention on the reduction of
lactones to lactols (Scheme 2).[9a,20] Such reductions are
usually performed with DIBAL-H at low temperature
(À788C). Under the optimized conditions (complex 4a
(1 mol%), Et2SiH2 (1.1 equiv), room temperature, UV irra-
diation (350 nm), 3 h), the g-lactones 5-methyltetrahydro-
furan-2-one and 5-heptyltetrahydrofuran-2-one were fully
converted into the corresponding g-lactol 6a,b. 5-Heptyl-
tetrahydrofuran-2-ol 6b was isolated in 83% yield. Notably,
d-decalactone was converted selectively into d-decalactol
(6c), which was isolated in 83% yield. Similarly, with e-
decalactone as the starting substrate, the reduction proceeded
with full conversion, and the hydroxyaldehyde 6d was
obtained selectively in 92% yield.
Figure 2. Molecular structures of the complexes 8a and 9. Thermal
ellipsoids correspond to 50% probability. One molecule of toluene in
8a and hydrogen atoms, except the iron hydride ligands, are omitted
for clarity.
To explore the mechanism of this iron-catalyzed hydro-
silylation, we first investigated the stoichiometric reaction
between 4a and silanes. The reaction of 4a with HSiPh3 under
UV irradiation (350 nm) led to the formation of the complex
[(IMes)Fe(H)(SiPh3)(CO)3] (8a) resulting from the oxidative
ligand (or the phosphine ligand) and the silyl fragment are
located in a trans axial arrangement with a CIMes-Fe-Si angle
À
of 168.50(6)8 (P-Fe-Si, 177.32(2)8). The CNHC Fe bond length
of 1.997(2) ꢀ is similar to that found in the starting complex
À
À
addition of the Si H bond of the silane to unsaturated
4a (1.9988(17) ꢀ). Furthermore, the Fe SiPh3 bond lengths
[(IMes)Fe(CO)3] (7).[21] This complex was fully characterized
by elemental analysis, X-ray diffraction studies,[22] and NMR
spectroscopy: in the 1H NMR spectrum, the hydride exhibited
a well-resolved singlet at d = À8.74 ppm,[23b,25] and in the
(2.3488(9) and 2.3564 (6) ꢀ for 8a and 9, respectively) are
consistent with those described previously.[22a,23] Notably, the
hydride is located in the meridional plane, and the Fe–H
distance of 1.43(2) ꢀ for 8a (1.46(2) ꢀ for 9) is consistent with
an iron–hydride bond.[22a,24]
29Si NMR spectrum,
a singlet at d =+ 28.19 ppm was
observed.[25] The phosphine analogue [(PMe2Ph)Fe(H)-
(SiPh3)(CO)3] (9) was also synthesized. Complex 9 showed
similar spectroscopic features to those of 10a: the hydride
The reaction of 4a with Et2SiH2 in C6D6 led exclusively to
the formation of [(IMes)Fe(H)(CO)3(SiHEt2)] (8b), which
was characterized by NMR spectroscopy (hydride signal at
4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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