Baeckvall J.-E. (ed.) Modern Oxidation Methods
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Jierry, L., Bou
erat, L., and Taillasson, P.
(2001) Tetrahedron: Asymmetry, 12, 883; (d)
Solladi
e-Cavallo, A., Bou
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Jierry, L. (2001) Eur. J. Org. Chem., 4557; (e)
Solladi
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A. (2003) C. R. Chimie, 6, 603; (f)
Freedman, T.B., Cao, X., Nafie, L.A.,
Solladi
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Bou
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Solladi
e-Cavallo, A., Jierry, L., Norouzi-
Arasi, H., and Tahmassebi, D. (2004) J.
Fluorine Chem., 125, 1371; (h) Solladi
e-
Cavallo, A., Jierry, L., Lupattelli, P.,
Bovicelli, P., and Antonioletti, R. (2004)
Tetrahedron, 60, 11375; (i) Solladi
e-Cavallo,
A., Jierry, L., Klein, A., Schmitt, M., and
Welter, R. (2004) Tetrahedron: Asymmetry,
15, 3891.
38 For ketones 19, 20, 27–29, and related
ketones, see: (a) Matsumoto, K. and
Tomioka, K. (2001) Heterocycles, 54, 615;
(b) Matsumoto, K. and Tomioka, K. (2001)
Chem. Pharm. Bull., 49, 1653; (c)
Matsumoto, K. and Tomioka, K. (2002)
Tetrahedron Lett., 43, 631.
39 For ketone 16 and related ketones, see:
(a) Bortolini, O., Fogagnolo, M., Fantin, G.,
Maietti, S., and Medici, A. (2001)
Tetrahedron: Asymmetry, 12, 1113; (b)
Bortolini, O., Fantin, G., Fogagnolo, M.,
Forlani, R., Maietti, S., and Pedrini, P.
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Bortolini, O., Fantin, G., Fogagnolo, M.,
and Mari, L. (2004) Tetrahedron:
Asymmetry, 15, 3831; (d) Devlin, F.J.,
Stephens, P.J., and Bortolini, O. (2005)
Tetrahedron: Asymmetry, 16, 2653; (e)
Bortolini, O., Fantin, G., Fogagnolo, M.,
and Mari, L. (2006) Tetrahedron, 62, 4482.
40 For ketones 23, 24, and related ketones,
see: Stearman, C.J. and Behar, V. (2002)
Tetrahedron Lett., 43, 1943.
41 For ketone 41, see: Klein, S. and Roberts,
S.M. (2002) J. Chem. Soc., Perkin Trans. 1,
2686.
42 For ketones 53–55, and related ketones,
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(2002) Tetrahedron, 58, 7545; (b) Shing,
T.K.M., Leung, Y.C., and Yeung, K.W.
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Org. Chem., 70, 7279; (e) Shing, T.K.M.,
Luk, T., and Lee, C.M. (2006) Tetrahedron,
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(2009) Tetrahedron: Asymmetry, 20,883.
43 For ketone 6 and related ketones, see: (a)
Chan, W.-K., Yu, W.-Y., Che, C.-M., and
Wong, M.-K. (2003) J. Org. Chem., 68,
6576; (b) Rousseau, C., Christensen, B.,
Petersen, T.E., and Bols, M. (2004) Org.
Biomol. Chem., 2, 3476.
44 For aldehyde 56 and related ketones, see:
Bez, G. and Zhao, C.-G. (2003) Tetrahedron
Lett., 44, 7403.
45 For a recent report on highly
enantioselective epoxidation of silyl enol
ethers with ketone 42 and subsequent
regioselective epoxide opening, see: Lim,
S.M., Hill, N., and Myers, A.G. (2009) J.
Am. Chem. Soc., 131, 5763.
46 Spiro transition state is proposed to be the
major transition state for the epoxidation
with dimethyldioxirane based on the
observation that cis-hexene is epoxidized
7–9 times faster than trans-hexene: (a)
Baumstark, A.L. and McCloskey, C.J.
(1987) Tetrahedron Lett., 28, 3311; (b)
Baumstark, A.L. and Vasquez, P.C. (1988)
J. Org. Chem., 53, 3437.
47 For leading references on computational
studies on transition states for the dioxirane
epoxidation, see: (a) Bach, R.D., Andr
es,J.L.,
Owensby, A.L., Schlegel, H.B., and
McDouall, J.J.W. (1992) J. Am. Chem. Soc.,
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N.C., and Condroski, K.R. (1997) J. Am.
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Singleton, D.A., and W ang, Z. (2005) J. Am.
Chem. Soc., 127, 6679.
48 For a synthesis of chiral c-aryl-
c-butyrolactones using ketone 42, see:
(a)Yoshida, M., Ismail, M.A.-H., Nemoto,
H., and Ihara, M. (1999) Heterocycles, 50,
673; (b) Yoshida, M., Ismail, M.A.-H.,
Nemoto, H., and Ihara, M. (2000) J. Chem.
Soc., Perkin Trans. 1, 2629.
49 Burke, C.P. and Shi, Y. (2009) Organic
Letters, 11, 5150–5153.
50 Hoard, D.W., Moher, E.D., Martinelli,
M.J., and Norman, B.H. (2002) Org. Lett.,
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(2006) ChemBioChem, 7, 54.
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53 Kanada, R.M., Itoh, D., Nagai, M., Niijima,
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57 Morimoto, Y., Iwai, T., and Kinoshita, T.
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58 Morimoto, Y., Nishikawa, Y., and Takaishi,
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59 (a) Morimoto, Y., Takaishi, M., Adachi, N.,
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62 Morimoto, Y., Takaishi, M., Iwai, T.,
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63 For a review, see: Morimoto, Y. (2008) Org.
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64 Bian, J., Van Wingerden, M., and Ready,
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65 Hioki, H., Kanehara, C., Ohnishi, Y.,
Umemori, Y., Sakai, H., Yoshio, S.,
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66 Das, S., Li, L.-S., Abraham, S., Chen, Z.,
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67 Tong, R., Valentine, J.C., McDonald,
F.E., Cao, R., Fang, X., and Hardcastle,
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68 Tong, R. and McDonald, F.E. (2008)
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69 Xiong, Z. and Corey, E.J. (2000) J. Am.
Chem. Soc., 122, 4831.
70 Xiong, Z. and Corey, E.J. (2000) J. Am.
Chem. Soc., 122, 9328.
71 For synthesis and structure revision of
glabrescol, also see: Morimoto, Y., Iwai, T.,
and Kinoshita, T. (2000) J. Am. Chem. Soc.,
122, 7124.
72 Adams, C.M., Ghosh, I., and Kishi, Y.
(2004) Org. Lett., 6, 4723.
73 (a) McDonald, F.E., Wang, X., Do, B.,
and Hardcastle, K.I. (2000) Org. Lett., 2,
2917; (b) McDonald, F.E., Bravo, F.,
Wang, X., Wei, X., Toganoh, M.,
Rodr
ıguez, J.R., Do, B., Neiwert, W.A., and
Hardcastle, K.I. (2002) J. Org. Chem., 67,
2515; (c) Bravo, F., McDonald, F.E.,
Neiwert, W.A., Do, B., and Hardcastle, K.I.
(2003) Org. Lett., 5, 2123; (d) Valentine,
J.C., McDonald, F.E., Neiwert, W.A., and
Hardcastle, K.I. (2005) J. Am. Chem. Soc.,
127, 4586; (e) Tong, R., McDonald, F.E.,
Fang, X., and Hardcastle, K.I. (2007)
Synthesis, 2337.
74 For a review, see: Valentine, J.C. and
McDonald, F.E. (2006) Synlett, 1816.
75 Simpson, G.L., Heffron, T.P., Merino, E.,
and Jamison, T.F. (2006) J. Am. Chem. Soc.,
128, 1056.
76 For SiMe
3
-based strategy for polyether
synthesis, also see: Heffron, T.P. and
Jamison, T.F. (2003) Org. Lett., 5, 2339.
77 Vilotijevic, I. and Jamison, T.F. (2007)
Science, 317, 1189.
78 For additional examples, see: (a) Morten,
C.J. and Jamison, T.F. (2009) J. Am. Chem.
Soc., 131, 6678; (b) Van Dyke, A.R. and
Jamison, T.F. (2009) Angew. Chem. Int. Ed.,
48, 4430.
79 For leading references, see: (a) Lin,
Y.-Y., Risk, M., Ray, S.M., Van Engen,
D., Clardy, J., Golik, J., James, J.C., and
Nakanishi, K. (1981) J. Am. Chem. Soc.,
103, 6773; (b) Shimizu, Y., Chou,
H.-N., Bando, H., Van Duyne, G., and
Clardy, J.C. (1986) J. Am. Chem. Soc.,
108, 514; (c) Pawlak, J., Tempesta, M.S.,
Golik, J., Zagorski, M.G., Lee, M.S.,
Nakanishi, K., Iwashita, T., Gross, M.L.,
and Tomer, K.B. (1987) J. Am. Chem. Soc.,
109, 1144; (d) Nakanishi, K. (1985)
Toxicon, 23, 473.
80 For a leading reference, see: Nicolaou,
K.C. (1996) Angew. Chem. Int. Ed. Engl., 35,
588.
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3 Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric Epoxidation of Alkenes
81 Ager, D.J., Anderson, K., Oblinger, E., Shi,
Y., and VanderRoest, J. (2007) Org. Proc.
Res. Devel., 11, 44.
82 For other synthetic applications of ketone
42, see: (a) Tokiwano, T., Fujiwara, K., and
Murai, A. (2000) Synlett , 335; (b) Bluet, G.
and Campagne, J.-M. (2000) Synlett,221;
(c) Shen, K.-H., Lush, S.-F., Chen, T.-L.,
and Liu, R.-S. (2001) J. Org. Chem., 66,
8106; (d) Guz, N.R., Lorenz, P., and
Stermitz,F.R.(2001)Tetrahedron Lett.,
42, 6491; (e) Olofsson, B. and Somfai, P.
(2002) J. Org. Chem., 67, 8574; ( f)
McDonald,F.E.andWei,X.(2002)Org.
Lett., 4, 593; (g) Kumar, V.S., Aubele, D.L.,
and Floreancig, P.E. (2002) Org. Lett., 4,
2489; (h) Olofsson, B. and Somfai, P.
(2003) J. Org. Chem., 68, 2514; (i)
Madhushaw, R.J., Li, C .-L., Su, H.-L., Hu,
C.-C., Lush, S.-F., and Liu, R.-S. (2003) J.
Org. Chem., 68, 1872; ( j) Smith, A.B. III
and Fox, R.J. (2004) Org. Lett., 6, 1477; (k)
Zhang, Q., Lu, H., R ichard, C., and
Curran,D.P.(2004)J. Am. C hem. Soc.,
126, 36; (l) Halim, R., Brimble, M.A., and
Merten, J. (2005) Org. Lett., 7, 2659; (m)
Kumar,V.S.,Wan,S.,Aubele,D.L.,and
Floreancig, P.F. (2005) Tetrahedron:
Asymmetry, 16, 3570; (n) L ecornu
e, F.,
Paugam, R., and Ollivier,
J. (2005) Eur. J. Org. Ch em., 2589; (o)
Iwasaki,J.,Ito,H.,Nakamura,M.,and
Iguchi, K. (2006) Tetrahedron Lett. , 47,
1483; (p) Curran, D.P., Zhang, Q.,
Richard, C., Lu, H., Gudipati, V., and
Wilcox, C.S. (2006) J. Am. Chem. Soc.,
128, 9561; (q) Wan, S., Gunaydin, H.,
Houk, K.N., and Floreancig, P.E. (2007)
J. Am. Chem. Soc., 129, 7915; (r)
Neighbors, J.D., Mente, N.R., Boss, K.D.,
Zehnder, D.W. II, and Wiemer, D. F.
(2008) Tetrahedron Lett., 49, 516; (s)
Mente, N.R., Neighbors, J.D., and
Wiemer, D.F. (2008) J. Org. Chem., 73
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7963; (t) Chapelat, J., Buss, A., Chougnet,
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10, 5123; (u) Emmanuvel, L. and Sudalai,
A. (2008) Tetrahedron Lett., 49, 5736;
(v) Shichijo, Y., Migita, A., Oguri, H.,
Watanabe, M., Tokiwano, T., Watanabe,
K., and Oikawa, H. (2008) J. Am. Chem.
Soc., 130, 12230; (w) Yu, M. and Snider,
B.B. (2009) Org. Lett., 11, 1031.
83 For an early leading review, see: Dalko, P.I.
and Moisan, L. (2001) Angew. Chem. Int.
Ed., 40, 3726.
84 For leading references, see: (a)Kagan, H.B.
(1999) Chapter 2, in Comprehensive
Asymmetric Catalysis (eds E.N. Jacobsen,
A. Pfaltz, and H. Yamamoto), Springer-
Verlag, Berlin, Heidelberg, Germany;
(b) Ahrendt, K.A., Borths, C.J., and
MacMillan, D.W.C (2000) J. Am. Chem.
Soc., 122, 4243.
References
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115
4
Catalytic Oxidations with Hydrogen Peroxide in
Fluorinated Alcohol Solvents
Albrecht Berkessel
4.1
Introduction
In organic chemistry, oxidations constitute a particularly important and difficult
class of transformations. This is because, in principle, all organic matter (be it the
substrate, the solvent, or the catalyst), in the presence of an oxidant such as hydrogen
peroxide, is only kinetically stable. Thermodynamically, full degradation to carbon
dioxide and water should occur. In other words,for an efficient and selective oxidation
process, two requirements need to be met: (i) substrate oxidation should proceed
efficiently, but only to the point wanted (e.g., thioether to sulfoxide, but not further to
sulfone); and (ii) both the solvent and the catalyst(s) employed should be as stable
as possible in the presence of the oxidant. Fluorinated alcohol solvents [1] are
particularly advantageous in this respect as they are able to activate oxidants such as
hydrogen peroxide toward electrophilic oxidation by (multiple) hydrogen bonding.
Furthermore, and once again by hydrogen bonding, they can alter the reactivity of the
substrates and the products in such a way that multiple oxidations can be selectively
arrested, for example, once the first oxygen atom transfer has occurred. This type of
activity and selectivity enhancement is found, for example, when the sulfoxidation of
thioethers is performed in fluoroalcohol solvents. Fluoroalcohols are highly stable
toward oxidation, in accordance with (ii) above [2, 3]. Furthermore, most substrates
are soluble in or miscible with fluorinated alcohols – another important point with
regard to process efficiency. Scheme 4.1 summarizes a number of readily (i.e.
commercially) available fluorinated alcohols and diols, together with their common
acronyms. Of the nine compounds shown, TFIP and PhTFE are chiral, the other
seven being achiral. With regard to application as solvents in oxidations, trifluoro-
ethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) are employed most
frequently.
This review focuses on the use of hydrogen peroxide as the terminal oxidant. The
virtues of hydrogen peroxide as a readily available and green carrier of active oxygen
have been praised numerous times [4–6]. In the current context, it is particularly
gratifying that hydrogen peroxide is activated toward electrophilic oxidations by
j
117
fluorinated alcohols through multiple hydrogen bonding – a highly synergistic
combination of solvent and oxidant [7, 8]. Nevertheless, it should be noted that
fluorinated alcohols have also proven beneficial as solvents for alkene epoxidations
proceeding via a dioxirane as the active oxidant [9, 10]. The latter is typically generated
from a fluoroketone by treatment with persulfate, in the form of Oxone. In HFIP/
water mixtures, and using fluoroketones as catalysts, the amounts of Oxone and
bicarbonatebuffer required couldbe significantly reduced. Thus, fluorinated alcohols
once again (and significantly) enhance the overall efficiency of the oxidation process.
4.2
Properties of Fluorinated Alcohols
The presence of CF instead of CH next to the COH group confers specific
properties to fluorinated alcohols (Scheme 4.2):
1) Fluorinated alcohols are (mildly) acidic: as exemplified by the pK
a
values of 12.4
for TFE and 9.3 for HFIP, fluorinated alcohols are significantly more acidic than
their nonfluorinated counterparts [11].
2) Fluorinated alcohols are strong hydrogen bond donors: the specific a-values for
TFE and HFIP are 1.51 and 1.96, respectively [12a]. The remarkable strength of
H-bond donation by, for example, HFIP is nicely illustrated by the existence of
its 1: 1-complex with THF, which can be distilled without decomposition [12b].
See Section 4.2.1 for a more detailed look at the hydrogen bond donor features
F
3
C
OH
2,2,2-trifluoroethanol
TFE
1,1,1-trifluoro-2-propanol
TFIP
F
3
CCH
3
OH
1,1,1,3,3,3-hexafluoro-
2-propanol; HFIP
F
3
CCF
3
OH
F
3
CCF
3
OH
CH
3
1,1,1,3,3,3-hexafluoro-
2-methyl-2-propanol;
hexafluoro-tert-butanol; HFTB
F
3
CCF
3
OH
CF
3
perfluoro-tert-butanol
PFTB
CF
3
OH
1-phenyl-2,2,2-trifluoro-
ethanol; PhTFE
CF
3
OH
CF
3
perfluoropinacol
PFP
2-phenyl-1,1,1,3,3,3-
hexafluoro-2-propanol;
PhHFIP
OHHO
F
3
CCF
3
CF
3
F
3
C
OHOH
CF
3
CF
3
F
3
C
F
3
C
1,3-bis(2-hydroxy-1,1,1,3,3,3-
hexafluoropropyl)benzene;
mPh(HFIP)
2
Scheme 4.1
118
j
4 Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents
of fluorinated alcohols and Section 4.3.1.1 for the effects of multiple hydrogen
bonding in epoxidation catalysis.
3) Fluorinated alcohols are poor hydrogen bond acceptors: the specific b-values for
TFE and HFIP are both 0 [13]. For comparison, the hydrogen bond acceptor
constant b of ethanol is 0.77. As a consequence of properties 2 and 3 in the
present list, auto-association of the solvent molecules is less pronounced in
fluorinated alcohols than in nonfluorinated analogs. For example, the dimer-
ization constant for ethanol is 0.89 L mol
1
, whereas that for TFE is 0.65 L mol
1
,
and for HFIP 0.13 L mol
1
[14]. However, as discussed in more detail in
Section 4.2.1, two important additional features need to be taken into account
when nucleophilic/hydrogen bond-accepting substrates are dissolved in fluori-
nated alcohols: First, the presence of hydrogen bond acceptors triggers self-
association/aggregation of the solvent (a sergeant and soldiers effect); second,
the hydrogen bond donor ability of fluorinated alcohol aggregates is enhanced
compared with that of the monomers (H-bond enhanced H-bonding).
4) Fluorinated alcohols are highly polar solvents: With regard to the Reichard-
E
N
T
-value, HFIP ðE
N
T
¼ 1:068Þ even exceeds the scale defined by tetramethylsi-
lane ðE
N
T
¼ 0Þ and water ðE
N
T
¼ 1Þ [15].
F
3
C
OH
2,2,2-trifluoroethanol
TFE
1,1,1,3,3,3-hexafluoro-
2-propanol; HFIP
F
3
CFC
3
OH
boiling point [
o
C]
melting point [
o
C]
p
K
a
dielectric constant ε
polarity, Reichert constant
E
N
T
ionizing power Y
hydrogen-bond donor ability
α
hydrogen-bond acceptor ability β
nucleophilicity constant N
auto-association constant [l•mol
-1
]
73.8 58.6
-43.5 -5
12.4 9.3
26.7 16.7
0.898 1.068
1.8
3.82
1.51 1.96
0
0
-2.78
-4.23
0.65 0.13
Scheme 4.2
4.2 Properties of Fluorinated Alcohols
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119
5) Fluorinated alcohols have high ionizing power: This important property of
fluorinated alcohols is reflected by their Y-values of 1.80 (TFE) and even 3.82
for HFIP [13].
6) Fluorinated alcohols are non-nucleophilic: Compared to, for example, ethanol
with an N-constant of 0, N equals 2.78 for TFE and even 4.23 for HFIP [13].
As a consequence of the above summarized properties of fluorinated alcohols, they
are ideal solvents for the generation of cationic or radical-cationic species or reaction
intermediates. This effect has been exploited numerous times, for example, in the
investigation of organic radical cations [11]. At the same time, it lies at the heart of
the catalysis of alkene epoxidation with hydrogen peroxide and of Baeyer-Villiger-type
oxidations of carbonyl compounds. The mechanisms of the latter two reaction types
are discussed in more detail in Sections 4.3.1.1 (epoxidation) and Section 4.5.1
(Baeyer-Villiger).
4.2.1
A Detailed Look at the Hydrogen Bond Donor Features of HFIP
For understanding the catalytic properties of fluorinated alcohols, it is necessary to
take a closer look at the hydrogen bond donor properties of HFIP and the factors
by which they are influenced [16]. The hydrogen bond donor ability of fluorinated
Figure 4.1 Potential energy (a) and dipole moment (b) of HFIP versus HOCH-dihedral angle in
vacuum (black) and within a PCM (gray).
120
j
4 Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents
alcohols, and in particular HFIP, is mainly dependent on two parameters: (i) the
conformation of the alcohol monomer along the CO bond [16, 17] and (ii) the
cooperative aggregation to hydrogen-bonded alcohol clusters [16].
In a polarizable environment, the absolute minimum structure of HFIP carries the
OH synclinal (sc) or almost synperiplanar (sp) to the adjacent CH (Figure 4.1) [16]. On
the basis of quantum-chemical considerations as well as single-crystal X-ray struc-
tures in which HFIP acts as hydrogen bond donor, HFIP always takes on such a sc
or even sp conformation. In this conformation, the hydrogen bond donor ability of
HFIP is significantly increased (Figures 4.2 and 4.3) [16].
Furthermore, the hydrogen bond donor ability of an HFIP hydroxyl group is
greatly enhanced upon coordination of a second or even third molecule of HFIP
Figure 4.2 Single-crystal X-ray structures of HFIP: (a) view perpendicular to the helix axis, and
(b) view along the helix axis.
O
H
F
3
C
CF
3
H
α
highest H-bond
donor ability
most stable liquid
phase conformer
most stable gas
phase conformer
Figure 4.3 Dependence of the properties of monomeric HFIP on the conformation along the
CO bond.
4.2 Properties of Fluorinated Alcohols
j
121
(Figures 4.4 and 4.5). Aggregation beyond the trime r has no significant additional
effect [16, 18].
Therefore, our mechanistic investigation of the epoxidation of alkenes with
hydrogen peroxide, as summarized in Section 4.3.1.1, will be constrained to reaction
pathways which (i) involve HFIP in an sc or even sp conformation and (ii) to hydrogen-
bonded HFIP aggregates comprising up to four alcohol monomers.
Figure 4.4 LUMO energy (s
OH) (a) and natural charge qH of the hydroxyl proton (b) versus
aggregation state of HFIP.
H
F
3
C O
CF
3
H
O
CF
3
F
3
C
H
O
F
3
C CF
3
n
H-bond donor abilit
y
X
q
H
ε
KS
(σ*
OH
)
Figure 4.5 Aggregation-induced hydrogen bonding enhancement of HFIP.
122
j
4 Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents