Mechanism
Short Description
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Description
Baeyer-Villiger Oxidation The Baeyer-Villiger oxidation, also known as the Baeyer-Villiger rearrangement, was first reported on December 17, 1899 by Adolf von Baeyer and Victor Villiger in Chemische Berichte. It is a popular synthetic tool for the conversion of acyclic ketones to esters and cyclic ketones to lactones, o f which the latter are precursors to hydroxy acids and acyclic diols. Phenols can be obtained from the corresponding aromatic aldehydes. Application of a suitable catalyst enables oxidative ring contraction of cyclohexanone to cyclopentane carboxylic acid, offering an alternative to the Favorskii rearrangement. The original contribution of Baeyer and Villiger referred to the conversion of the cyclic ketones menthone(1) and tetrahydrocarvone(2) to the respective lactones, by monopersulfuric acid, otherwise known as Caro’s acid:
H2SO5
O O
O
(1)
O
H2SO5
O O
(2)
Since then, the utility, regioselectivity and stereospecificity of the reaction has been extended by new transition metal catalysts, zeolite based catalysts, alumina, enzymes and the application of ultrasound. Metachloroperoxybenzoic acid (MCPBA), peroxybenzoic acid (PBA) and trifluoroperoxyacetic acid (TFPAA) are among the most common peracids used. More recent reagent systems include the magnesium salt of monoperoxyphthalic acid (MMPP), sodium perborate, hydrogen peroxide in the presence of boron trifluoride or diselenides. Catalytic Baeyer-Villiger oxidations were feasible with methyltrioxorhenium and hydrogen peroxide i n the ionic liquid [bmim]BF4.. Potassium peroxomonosulfate supported on hydrated silica (‘reincarnated Caro’s acid’) was recently introduced; the reaction is more efficient when carried out in supercritical carbon dioxide. MCPBA preferentially yields the corresponding epoxide in the presence of a double bond, at low temperature, in an inert solvent without the presence of an acid catalyst. Application of bis[trimethylsilyl] peroxide minimizes epoxide formation when an alkene is present. Base catalyzed rearrangements are less common. Mechanism The most accepted mechanism is that proposed by Criegee, or a variation of it. T he salient features of the mechanism are: 1) Retention of stereochemistry stereochemistry by the the migrating group. 2) Migration is concerted with the departure departure of the leaving group. group. The concerted step is rate determining.
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3) Migrating groups with greater electron donating power have correspondingly greater migratory aptitude because of the increased ability to stabilize a positive charge in the transition state. This renders stereoselectivity to the oxidation of unsymmetrical ketones. m
4) Migration is favored when the migrating(R ) group is antiperiplanar to the O-O bond of the leaving group; this i s known as the primary stereoelectronic effect. The antiperiplanar alignment of the lone pair of electrons on oxygen with the migrating group is termed as the secondary stereoelectronic effect. COR' O
primary
H
O O
R
R
m
secondary
5) Electron withdrawing groups on the peroxyacid and peroxide enhance the rate of rearrangement. The overall mechanism can be depicted as Scheme (I):
H
H
O R
O
O R
R
O R
R
R
H R
O H O O R
OH O
O O
H
H
O
O R
R O
R
OR
H
R
O R
Scheme I
Lactone formation may be represented as: H H O
H O
O
H
H O O
O
O
O O
R
H O
O O
Scheme II
2
O
R
OR
Schemes I and II provide general reaction mechanisms for acid catalyzed reactions Scheme III represents the rearrangement of the Criegee intermediate in a cyclical manner R' O R
m
R
O O O H
Scheme III In the case of haloketones, migration tends to occur from the non-halogenated carbon. See additional notes and references for more in formation on Baeyer-Villiger rearrangements occurring through anomalous mechanisms. Examples: O
O
MMPP, NaHCO3
C6H13
(1)
O
MeOH
C6H13
J. Org. Chem (1997), 62, 2633
95%
O
O O2, PhCHO (2)
O
0
Fe2O3, 20 C Angew. Chem. Int. Ed (1998), 37, 1198
92% O
O
O
Na2CO3, H2O2, Ac2O
(3)
))) 6h Chemical Abstracts (1996), 123, 316192j O
84%
O O
MCPBA
(4)
0
CH2Cl2, 20 C Tet. Lett (1977), 31, 2713
Cl
69%
Cunninghamella echinulata
(5)
Cl H
O
O O
Tet. Lett (1997), 38,1195
> 99% ee 31%
3
O O
CPMO
O
(6)
CPMO = cyclopentanone monooxygenase Chem. Commun (1996), 2333
98% ee quantitative
O
O CHMO
(7)
O
Me Me CHMO =cycl ohexanone monooxygenase J. Org.Chem (2003), 68,6222
99% ee 100% conversion
O
O *Engineered e.coli cells
O
(8)
* E. coli cells that overexpress cyclohexanone monooxygenase
J. Org. Chem (2001), 66, 733
O
48%
H2O2 (60%)
O
1 mol % catalyst
O
(9)
CF3CH2OH
J. Org. Chem (2001), 66, 2429
F3C Se F3C
= catalyst 2
4
99%
O
O
H
H MCPBA
H
(10)
H N H Cbz
NaHCO3
N H Cbz
O
CH2Cl2, rt, 30 min
Cl
J. Org. Chem (2002), 67, 3651
Cl
85%, only product
O
O H2O2 (35%), 1 mol % catalyst
O
(11) CF3C6F11, (CH2)2Cl2 0
25 C, 2h 93% Sn[N(SOCF 17)2]4 = catalyst
Tet. Lett (2003), 44, 4977
O CHO
O
OH
OMe
OMe
1%
95%
catalyst, H2O2 (12)
0
MeCN, 80 C, 7 h OMe
87% total conversion Beta-7 zeolite = catalyst SnO2 content = 0 %; Si:Al ratio = 30 (mol/mol)
Journal of Catalysis (2004), 221, 67
(13)
O
MCPBA O
n
CH2Cl2, rt, 4d
O
x
O
y
73% ketone/ester = x/y = 82/18 Macromolecules (2004), 37, 4484
5
O
COOH
catalyst, H2O2 (14)
t-BuOH, 650C, 5h
60% Se)2 catalyst = 0.6 mol% Se)2 Syn. Comm (1999), 29, 2981 5.2 eq. 30% H 2O2
O
O O
5 mol% catalyst
(15)
CH2Cl2, RT, 24 h Ph
Ph
85%
OTf Se
catalyst = 5 mol %
2
Tet. Lett (2005), 46, 8665.
O
(16)
O
hydr-Sio2.KHSO3
O sc CO2 0 250 bar, 40 C
96%
J.Org. Chem (2006), 71, 6432
O
O (17)
PhCHO, O2
O
))) 2h, CCl 4
87.7%
Chemical Eng. Journal (2006), 121, 63
6
(18) O MCPBA, NaHCO3
O
O
CH2Cl2
N
N Ts
OBn
Ts
OBn
73% Tet. Lett (2006), 47, 4865
(19)
COOMe
COOMe TFPAA 0
O
0 C, CH2Cl2, 1h O
H
H
O
(a)
COOMe
O O
H (b)
a: b = 4: 1 75%
Steroids (2007), 72, 466
(20)
CbzHN
COOMe
CbzHN TFPAA
H Me
Ph
0
0 C, CH2Cl2, > 5 h
O
COOMe
H Me
PhO O
75%
J. Org. Chem (2008), 73, 2633.
7
O
O
H2O2, catalyst (10 mol%)
(21)
0
CHCl3, - 40 C, 18h
Ph
O
Ph
99% ee = 88% R catalyst:
X O
O P
O
OH
X X = pyren-1-yl
Angewandte. Chem. Int. Ed (2008), 47, 2840
NaBO3, H2O2 (22)
0
O
HOAc, 60 C O
O
O
O 34%
(R)-(+)-camphor
66%
Tetrahedron : Asymmetry (2008), 19, 796
Additional Notes and References:
An example of reversed regioselectivity was reported by Mikami and Yamanaka:
O
TFPA ( 2.0 eq ) CF3
TFA ( 7.0 eq )
O F3C
O
rt, CH2Cl2, 16h quantitative O O CF3
not observed
Organic Letters (2003), 5, 25, 4803.
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Grein and Crudden published a study of the Baeyer-Villiger reaction of haloketones in the Journal of Organic Chemistry (2006), 71, 861. The reaction mechanism is reviewed in detail.
The Baeyer-Villiger oxidation is similar to the Dakin reaction:
O
O
HO
HO
R
O
R O O
R
H2O, OH
OH
O HO
H
HO
O O H
An interesting departure was found by Adejare’s group; hydride ion migration occurred faster than phenyl ion migration when the phenyl group had halogen substituents, resulting in the formation of carboxylic acid instead of phenol: F
F Br
Br
MCPBA CH2Cl2, reflux
COOH
CHO
(74%)
Journal of Fluorine Chemistry (2000), 105, 107.
A scholarly review of the Baeyer-Villiger oxidation is given by Krow in volume 43 of Organic Reactions (1993). The enantioselectivities of recently isolated Baeyer-Villiger monooxygenases toward alkyl substituted cyclohexanones are reviewed in the Journal of Organic Chemistry (2004), 69,12. Lactone synthesis was accomplished using methyltrioxorhenium/hydrogen peroxide in the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate[bmim]BF4 as reported in Tetrahedron Letters (2003),44, 8991. The magnitude of the preference for antiperiplanar migration over gauche migration is discussed by Radkiewicz-Poutsma in the Journal of Organic Chemistry (2004), 69, 7148. Microwave accelerated Baeyer-Villiger synthesis of lactones was investigated by Ritter: Tetrahedron (2006), 62, 4709. Yamabe and Yamazaki recently discussed the role of hydrogen bonds in the Journal of Organic Chemistry (2007), 72, 3031. The application of zeolite based catalysts and clay based catalysts was reviewed recently by Ruiz and Jimenez-Sanchidrian in Tetrahedron (2008), 64, 2011.
(Copyright: HARINDRAN NAMASIVAYAM, 2002 – 2008)
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