Organometallics
Article
Figure 4. (a) Possible sequence of events toward formation of isopropyl propionate. Route 1: reaction of LnMn with −OiPr and subsequent alkyl
transfer. Route 2: reaction of LnMn with Et and subsequent nucleophilic alkoxide attack. (b) Intramolecular carboboration of a
42
́
Mn(alkoxy)carbonyl, as reported by Alvarez et al.
presumably originates from CO available in the solution, which
is liberated upon mixing of the reagents.
NMR and FTIR spectra were recorded (see SI). Addition of
BEt3 to solutions of 1 or 3 in THF did not lead to observable
reactions. Interestingly, Mn diacyl 2 transformed into
monoacyl 1 upon treatment with BEt3 and ultimately formed
KMn(CO)5. This is similar to what was observed by Gladysz
and co-workers, although no organic products were observed
with GC-MS after decomposition to the manganate salt (i.e.,
we did not observe the expected isopropyl propionate ester).37
The combination of potassium isopropoxide with 1 resulted in
formation of dialkoxycarbonyl 2 and Mn-isopropoxo-dimer 3.
Thus, complex 2 appears to originate from the sequential
addition of KOiPr to Mn(CO)5Br, and then to 1. Complexes 2
and 3 did not show any reactivity toward KOiPr. Finally,
KHBEt3 was added to the three complexes to see if they could
sustain formation of Mn formyls. Treatment of 1 with KHBEt3
indeed led to the formation of a new anionic Mn(acyl)-
(formyl) complex, which decomposed to KMn(CO)5 over the
course of approximately a day.37 Compounds 2 and 3 did not
show reactivity toward the hydride reagent, thereby indicating
that the remaining three/four Mn-carbonyl ligands of these
complexes are significantly less reactive. In summary, Mn(I)
carbonyls generally can undergo two sequential nucleophilic
attacks on a carbon monoxide ligand, ultimately leading to
anionic diacyl (or formyl) species. These compounds
frequently are unstable and undergo thermal- or light-induced
decomposition to KMn(CO)5. This decomposition process is
accelerated by the addition of trialkylboranes, although the
underlying mechanism is not completely understood.37
Under identical conditions, the reaction of Mn(CO)5Br with
≥2 equiv of KOiPr results in a mixture of potassium salts of
Mn dialkoxycarbonyl 2, and alkoxide-bridging Mn-μ2-isopro-
poxo-dimer 3 (Figure 3a, details in SI). Compounds 2 and 3
were characterized by 1H/13C NMR, FTIR, and ESI-MS
(Figure 3b). Compound 2 was unstable in solution and fully
degraded over the course of several days. This decomposition
has been observed before for similar Mn/Re complexes35,41
and is presumably induced by both heat and light (although
attempted crystallization at −80 °C in the dark did not
appreciably stop the decomposition). Gratifyingly, crystals
suitable for XRD could be obtained for Mn-isopropoxo-dimer
3.
1
Complex 2 features one CH(CH3)2 septet in the H NMR
spectrum at 4.94 ppm, suggesting that the isopropoxycarbonyl
groups are chemically equivalent (Figure 3b). This is
supported by the single CO stretching band in the FTIR
spectrum at 1610 cm−1. The FTIR spectrum further contains a
medium intensity band at 2062 cm−1, and three strong bands
at 1977, 1955, and 1927 cm−1.35 Similarly to complex 2, the
isopropoxy moieties in 3 are chemically equivalent in solution
and appear as one septet at 4.24 ppm in the 1H NMR
spectrum. The FTIR spectrum features five sharp and intense
bands at 1996, 1986, 1903, 1882, and 1874 cm−1. Single crystal
X-ray structure determination indicated that, in the solid state,
3 exists as a three-dimensional coordination network (Figure
3c). In this arrangement, the Mn and K atoms are 6-
coordinated, and two isopropoxy fragments act as bridging
ligands. The third bridging ligand between the symmetry-
related Mn(I) tricarbonyl centers is provided by KOiPr.
Having identified a number of potentially important
intermediates that provided context for further spectroscopic
investigations, we shifted our focus to stoichiometric reactivity
studies to better understand the observed alkoxycarbonylation
chemistry (Figure 3d). Complexes 1−3 were reacted with 1
We deduced that the observed isopropyl propionate ester
can be formed via one of the two pathways illustrated in Figure
4a. In route 1, the alkoxide fragment is transferred to the Mn
center first, resulting in the formation of a Mn-
(isopropoxycarbonyl) compound similar to 1 or 2. Isopropyl
propionate is liberated upon alkyl transfer from BEt3 to the Mn
complex. Route 2 starts with the alkylation of a carbonyl with
BEt3 and forms a transient propionyl manganese carbonyl
complex. This alkylation is followed by a nucleophilic attack on
the acyl carbon by the alkoxide, ultimately resulting in
1
equiv of BEt3, KOiPr, and KHBEt3, and the resulting H/11B
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Organometallics 2021, 40, 674−681