Synthesis of β-lactones: A highly active and selective catalyst for epoxide carbonylation

Article abstract of DOI:10.1021/ja017434u

A new highly active and selective catalyst for the synthesis of β-lactones from CO and epoxides is reported. The catalyst, [(N,N′-bis(3,5-di-tert-butylsalicylidene) phenylenediamino)Al(THF)₂][Co(CO)₄] ([(salph)Al(THF)₂][Co(CO)₄]) is easily prepared from the corresponding (salph)AlCl and NaCo(CO)₄. At 50 °C and 880 psi of CO, the catalyst (1 mol %) carbonylates epoxides such as propylene oxide, 1-butene oxide, epichlorohydrin, and isobutylene oxide to the lactones β-butyrolactone, β-valerolactone, γ-chloro-β-butyrolactone, and β-methyl-β-butyrolactone in high yield. (R)-Propylene oxide was carbonylated to (R)-β-butyrolactone with retention of stereochemistry. Copyright

Full text of DOI:10.1021/ja017434u

Published on Web 01/24/2002
Synthesis of â-Lactones: A Highly Active and Selective Catalyst for Epoxide
Carbonylation
Yutan D. Y. L. Getzler, Viswanath Mahadevan, Emil B. Lobkovsky, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received October 30, 2001
Poly((R)-â-hydroxybutyrate) (R-PHB, Scheme 1), a thermoplastic
polyester found naturally in some bacteria, shares many of the
physical and mechanical properties of poly(propylene) with the
added feature of biodegradability.1 Despite proven properties and
ready applications, industrial production of R-PHB as a bulk
polymer using biological methods has, to date, proven economically
nonviable.2 Potential alternate synthetic routes to this promising
polymer include Baeyer-Villiger oxidation of an isotactic propy-
lene/carbon monoxide copolymer,3 asymmetric hydrogenation of
the unsaturated polyester from ring opening polymerization (ROP)
of ketene dimer4 and ROP of (R)-â-butyrolactone1a,5 (R-BBL,
Scheme 1). While the first two routes involve postpolymerization
modification of a polymer backbone, a difficult and unreliable task,
the third uses the proven technology of lactone polymerization.
Unfortunately R-BBL is not a readily available commodity6,7 such
as is required for polymer production.
epoxides (Table 1, entry 1), and it exhibits improved selectivity
over Co2(CO)8 based catalysts.15
Scheme 2. Proposed Mechanism of Epoxide Carbonylation
Given the great strength of CO as a ligand for electron-rich late
transition metals and the vital role of the anion in epoxide
carbonylation19 we looked to modification of the cation for
mediation of catalytic activity. Our investigation of simple non-
n
metallic cations such as Bu4N+ and Ph4P+, in combination with
-
Co(CO)4 anions, found them inactive for reaction of CO and
propylene oxide (entries 2, 3). Under the premise that the cation
must activate the epoxide we looked to metal-based cations.
Although the simple complex [Na][Co(CO)4] slowly produces 6%
BBL from CO/propylene oxide (entry 4), it also produces an equal
amount of acetone20 as well as 40% PHB oligomers (2 mol % Co,
16 h). The complex [Cp2Co][Co(CO)4] was inactive for propylene
oxide carbonylation (entry 5). Assuming that the inactivity of this
complex was due to the coordinatively saturated 18e- Cp2Co+
cation, we turned to oxophilic cations capable of ether coordination.
Aluminum-salen complexes were chosen as they are easy to
prepare and modify21 and have previously shown activity as Lewis-
acid catalysts.22
Scheme 1. Poly((R)-â-hydroxybutyrate) from (R)-â-Butyrolactone
R-BBL has been made from ketene and acetaldehyde,8 from rac-
BBL by enzymatic kinetic resolution,9 and from the dimer of ketene
via asymmetric hydrogenation.10 The first two routes suffer
chronically low ee’s and yields, respectively, with hydrogenation
showing the greatest promise of the three. A fourth approach is
the catalytic synthesis of R-BBL from the cheap and abundant
feedstocks, propylene oxide and carbon monoxide.11,12 Herein we
-
report discrete cationic Lewis-acid complexes of Co(CO)4 that
Table 1. Carbonylation of Epoxides
exhibit unprecedented activity and selectivity for epoxide carbo-
nylation. Application of these catalysts with a variety of substrates
is also addressed.
Although metal complexes have been reported for the carbony-
lation of epoxides, yields and selectivities are typically low.13
Mixtures of dicobalt octacarbonyl [Co2(CO)8] and hydroxy-
substituted pyridines are catalytically active for the carbonylation
of a variety of simple epoxides (such as ethylene and propylene
oxide) to produce mixtures of lactones and polyester oligomers,
the ratio of which is dependent on reaction conditions employed.14-16
It is established that neutral donor ligands (L) and [Co2(CO)8],
engage in an equilibrium with [L2Co(CO)3]+[Co(CO)4]-,17 and we
believe this ion pair is active in the epoxide carbonylation chemistry.
In the proposed catalytic cycle (Scheme 2) we envision epoxide
coordination and activation by a Lewis-acidic cation. Backside
entry
catalyst
R
R
time (h)
yield (%)d
1
2
1
2
[PPN][Co(CO)4]/BF3‚Et2Oa
H
Me
24
16
16
16
16
1
77e
0
[nBu4N][Co(CO)4]b
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
3
[Ph4P][Co(CO)4]b
0
4
[Na][Co(CO)4]b
6f
5
[Cp2Co][Co(CO)4]b
0
6
1c
1c
1c
1c
1c
95
95g
>99
73
83h
7
(R)-Me
Et
CH2Cl
Me
1
8
2.5
9.5
1
9
10
Me
a Reference 15; [PPN] ) [Ph3PdNdPPh3]+; 1 mol % Co, 900 psi, 80
°C, DME. b 2 mol % Co, 1000 psi, 80 °C, triglyme. c 1 mol % Co, 880 psi,
50 °C, neat substrate. d Determined by 1H NMR for entries 2-10.
e Reference 15; some R-methyl-â-propiolactone also produced. f Acetone
(6%) and oligomers (40%) also produced. g Product ) R-BBL h R,R-
Dimethyl-â-propiolactone (7%) also produced.
-
attack by Co(CO)4 on the less hindered carbon would give the
ring-opened species B.18 Insertion of CO, ring extrusion and re-
coordination of CO to M′ complete the catalytic cycle. As a
proposed mechanism it has been an important intellectual frame-
work in our catalyst design process (vide infra). Alper and co-
workers have recently demonstrated that the mixture [Ph3PdNd
PPh3]+[Co(CO)4]-/BF3•Et2O is effective for the carbonylation of
The complex [(salph)Al(THF)2][Co(CO)4] (1; Figure 1) is
prepared from the corresponding (salph)AlCl and NaCo(CO)4. The
highly crystalline ruby-red product, which has been fully character-
ized by NMR, IR, and X-ray crystallography, is isolated in 93%
9
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10.1021/ja017434u CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
yield and is stable for prolonged periods under nitrogen. The X-ray
structure of 1 reveals axially bound THF molecules in lieu of the
ion-separated Co(CO)4- anion, testament to its ability to coordinate
cyclic ethers.
Teacher-Scholar Award, a 3M Untenured Faculty Grant, and a Dow
Innovation Recognition Award.
Supporting Information Available: Synthesis and characterization
of 1 and details of epoxide carbonylation (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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1 h (entry 6). Although prior reported systems produce significant
levels of R-methyl-â-propiolactone isomers, acetone and polymeric
side-products in the carbonylation of propylene oxide, we observe
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processed than (R)-PHB;1a thus, an important feature of 1 is that it
carbonylates 1-butene oxide to >99% conversion in 2.5 h (entry
8). This catalyst can also be used to carbonylate epichlorohydrin
to 73% conversion in 9.5 h (entry 9).
The carbonylation of isobutylene oxide reaches 90% conversion
in 1 h, producing a mixture of the two possible regioisomers (entry
10), suggesting different reaction pathways. We propose the primary
sequence to involve nucleophilic attack of the activated epoxide at
the less hindered site by Co(CO)4- to give a â-substituted lactone.
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the epoxide by Lewis-acidic [(salph)Al]+. Subsequent trapping of
the more stable tertiary cation by Co(CO)4- leads to the R-substi-
tuted lactone.
In summary, complex 1 exhibits unprecedented activity and
selectivity for the carbonylation of a variety of epoxides. As opposed
to previous epoxide carbonylation systems this one is composed
of a discrete complex, which will facilitate future mechanistic
studies. Furthermore the reactions may be run neat in substrate
which reduces waste and facilitates purification.25 Given the epoxide
carbonylations presented here we believe catalysts of the general
form [Lewis acid]+[M(CO)x]- can be applied to a broad array of
heterocycle carbonylations. Most important is the utility of this
catalyst in the quick and efficient synthesis, from inexpensive
starting materials, of monomers for the production of poly-
(hydroxyalkanoate)s.
Acknowledgment. We thank Professors Peter Wolczanski and
Barry Carpenter for helpful discussions and Mr. Scott Allen for
the preparation of (R)-propylene oxide. G.W.C. gratefully acknowl-
edges a Packard Foundation Fellowship in Science and Engineering,
a Dreyfus New Faculty Award, an Alfred P. Sloan Research
Fellowship, an Arnold and Mabel Beckman Foundation Young
Investigator Award, a NSF CAREER Award, a Camille Dreyfus
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