Application of Mitsunobu reagents to redox Isomerization of CF 3-containing propargylic alcohols to (E)-α,β-enones

Article abstract of DOI:10.1021/jo102503s

Isomerization of CF₃-containing secondary propargylic alcohols proceeds with excellent E selectivity by treatment with Mitsunobu reagents, tris- (p-methoxyphenyl)phosphine (TMPP), and 1,1′-(azodicarbonyl) dipiperidine (ADDP) in the presence of phenol.

Full text of DOI:10.1021/jo102503s

NOTE
pubs.acs.org/joc
Application of Mitsunobu Reagents to Redox Isomerization of
CF3-Containing Propargylic Alcohols to (E)-r,β-Enones
Yohsuke Watanabe and Takashi Yamazaki*
Division of Applied Chemistry, Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei
184-8588, Japan
S Supporting Information
b
ABSTRACT: Isomerization of CF3-containing secondary propargylic alcohols
proceeds with excellent E selectivity by treatment with Mitsunobu reagents, tris-
(p-methoxyphenyl)phosphine (TMPP), and 1,10-(azodicarbonyl)dipiperidine
(ADDP) in the presence of phenol.
rifluoromethylated compounds have recently attracted sig-
nificant interest in various fields such as medicine, pharma-
Scheme 1. Base-Mediated Redox Isomerization
T
ceuticals, and functional materials due to such unique properties
of the trifluoromethyl group as high electronegativity, electron
density, and hydrophobicity.1 Highly electrophilic β-trifluoro-
methyl R,β-enones 2, as a result of the lower LUMO energy levels
than the corresponding nonfluorinated counterparts,2b have been
utilized as quite important building units for a wide variety of
reactions including Diels-Alder cycloadditions2 and Michael-type
additions.2a,3
In 1949, the first example of redox isomerization of the
γ-hydroxy-R,β-alkynyl ester A to the γ-oxo-R,β-alkenyl ester B
in the presence of Et3N was reported by Nineham and Raphael
for the single substrate of EWG = CO2Me, R = Ph (Scheme 1).4
Koide and Sonye clarified that this isomerization was widely
applicable to materials possessing not only ester functionality but
also phosphate and amide moieties as EWGs in A whose
conversions were promoted by DABCO or NaHCO3.5b-e In
recent years, we have also reported that propargylic alcohols with
a trifluoromethyl (CF3) group at the distal position of the triple
bond (EWG) could be converted to enones by treatment with
Et3N in THF.5f These base-mediated reactions were found to be
limited to only substrates with aryl substituents as R, and the
isomerization of primary and alkyl-substituted secondary pro-
pargylic alcohols usually requires high-cost transition-metal
catalysts.6 We have previously disclosed the synthetic pathway
to ketones 2 by successive Red-Al reduction of 1a, followed by
PDC oxidation (Scheme 2). However, the first step, conversion
of propargylic alcohols 1 to the corresponding allylic counter-
parts 3, sometimes suffered from overreduction of the desired 3
to the difluorinated homoallylic alcohols 4, and separation of
these two compounds was a quite troublesome task.7 While our
above-mentioned process shown in Scheme 1 (EWG = CF3) is
recognized as the alternative protocol to 2, quite unfortunately,
this technique is applicable to only the case for R = Ar as
mentioned above. Herein, we report the first example of the
Scheme 2. Synthetic Route to R,β-Enones 2
redox isomerization of secondary propargylic alcohols 1 to
R,β-enones 2 by treatment with Mitsunobu reagents,8 which is
effective for substrates with not only an aromatic moiety but
also an alkyl group as R without using any transition-metal
catalysts.
In our initial investigation, we found that the reaction of the
model CF3-containing propargylic alcohol 1a with triphenylpho-
sphine, diisopropyl azodicarboxylate (DIAD), and phenol in
Et2O at room temperature afforded R,β-enone 2a in good yield
(Z/E = 75:25, Table 1, entry 1). THF also worked as a solvent
but in a less efficient manner, furnishing 2a in 41% yield (Z/E =
42:58, entry 2). Among the reagent combinations examined,
formation of a number of byproducts was noticed under the
PPh3-DIAD conditions probably by nucleophilic attack of the
Received: December 24, 2010
Published: February 23, 2011
r
2011 American Chemical Society
1957
dx.doi.org/10.1021/jo102503s J. Org. Chem. 2011, 76, 1957–1960
|

The Journal of Organic Chemistry
NOTE
Table 1. Screening of Reagents for Isomerization of the Propargylic Alcohol 1aa
entry
azo compd
phosphine
solvent
yieldb (%)
Z/Eb
1
DIAD
DIAD
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
ADDP
PPh3
PPh3
PPh3
Et2O
THF
THF
THF
THF
THF
68
75:25
42:58
42:58
58:42
48:52
44:56
39:61
38:62
2
41
3
12
4
2-pyridyl-PPh2
14
5
4-MeOC6H4-PPh2
(4-MeOC6H4)3P
40
6
60
7
(4-MeOC6H4)3P
toluene
71
8
(4-MeOC6H4)3P
CH2Cl2
60
N.R.c
9
(4-MeOC6H4)3P
MeCN
10
11
12
13
14d
(4-MeOC6H4)3P
DMF
0
(4-MeOC6H4)3P
THF-H2O (5:1)
THF-toluene (1:1)
THF-toluene (1:1)
THF-toluene (1:1)
N.R.c
51
(4-MeOC6H4)3P
48:52
12:88
<1:>99
(4-MeOC6H4)3P (1.2 equiv)
(4-MeOC6H4)3P (1.2 equiv)
80
98
a The reactions were carried out in a 0.6 mmol scale with 6 mL of a solvent. b Yields and ratios were determined by integration of the 19F NMR peaks of
crude products using PhCF3 as an internal standard. c No reaction. d This reaction was carried out at 66 °C.
Scheme 3. TMPP-Catalyzed Isomerization Experiments
Table 2. Scope of Isomerization of Propargylic Alcohols 1
with Mitsunobu Reagentsa
entry
1
R
temp (°C) time (h)
2
yieldb (%)
1
1a Ph(CH2)2
1b Ph
66
rt
3.0
3.0
3.0
3.0
2.0
3.0
4.0
0.5
1.0
1.0
2a
2b
2c
2d
2e
2f
98
89
94
51
70
71
60
63
0
Huisgen zwitterion to the carbonyl compound 2a formed
(entries 1 and 2).9,10 This is the major reason why we selected
tris(p-methoxyphenyl)phosphine (TMPP) and 1,10-(azodica-
bonyl)dipiperidine (ADDP) as the most effective reagent
system.11 A THF-toluene mixed solvent promoted the reaction
well, affording the least number of byproducts (entry 12). The
best result was eventually obtained by using TMPP (1.2 equiv),
ADDP (1.0 equiv), and PhOH (1.0 equiv) at 66 °C in this solvent
system, which provided the desired R,β-enone 2a in 98% yield
with complete E selectivity (entry 14).
Perfect interconversion of (Z)-2a to the corresponding (E)-
isomer proceeded with a small excess amount of TMPP which
was proved by independent subjection of (Z)-2a to a solution
containing 20 mol % of this phosphine (Scheme 3). The role of
TMPP here seemed to be closely related to the one of Et3N in
our previous report.5f
2
3
1c n-C6H13
66
66
111
111
rt
4
1d BnO(CH2)2
1e c-C6H11
5c
6c
7d
8
1f Ph(MOMO)CH
1g (E)-C3H7CHdCH
1h t-BuCꢀC
1i Ph(CH2)2
2g
2h
2i
rt
9e
66
10f 1j Ph(CH2)2
66
2j
0
a The reaction was carried out in a 0.6 mmol scale with 3 mL each of both
solvents, and the products were obtained in an (E)-exclusive manner
unless otherwise noted. b Isolated yield. c The reaction was conducted in
6 mL of toluene. d Z/E selectivity of the products was 6:94. e 1i possessed
a n-Bu group instead of a CF3 group and was recovered in 99% yield. f 1j
possessed a CO2Et group instead of a CF3 group.
The reaction condition determined above was found to be
generally applicable to a variety of CF3-containing propargylic
alcohols 1, furnishing products 2 in good to excellent yields as
well as with high (E)-selectivity (Table 2). On the other hand,
nonfluorinated materials with a Ph(CH2)2 group as R, and n-Bu
(1i) or CO2Et (1j) moieties instead of a CF3 group failed to
follow this transformation (entries 9 and 10) probably due to the
insufficient activation of the propargylic proton and the side
reaction via nucleophilic 1,4-addition of TMPP or the TMPP-
ADDP zwitterion to the alkyne moiety in the latter case.
To gain mechanistic insight into this reaction, deuterium-
labeling experiments were conducted. Deuterium from the
propargylic position of the alcohol 1b-d was incorporated into
the β-position (77% D) of the R,β-enone 2b-d, not the
1
R-position (0% D) as determined by H NMR (Scheme 4,
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The Journal of Organic Chemistry
NOTE
Scheme 4. Deuterium-Labeling Experiments
Scheme 5. Plausible Mechanism for Isomerization of Pro-
pargylic Alcohols
corresponding anion by attachment of the less anion-stabilizing
amide group as E for TMPP, reducing the likelihood of Michael
addition 2, rather than the case of DIAD with a more electron-
withdrawing ester moiety as E.
eq 1). Moreover, to address a mode of proton migration, an
equimolar mixture of 1b-d and 1k was treated under the same
conditions to exclusively furnish deuterated 2b-d and nondeut-
erated 2k (Scheme 4, eq 2). Furthermore, treatment of a 1:1
mixture of propargylic alcohol 1b and methanol-d4 under the
standard protocol afforded nondeuterated 2b (eq 3) as the sole
product. These results clearly demonstrated the intramolecular
migration of deuterium at the propargylic position of 1b-d to the
β-position of the ketone 2b-d regiospecifically. This phenomen-
on led to expectation that the propargylic or the allenyl anions
would form a tight ion pair with a proton source after abstraction
of D(H).
The following rationalization would be advanced to explain
the product formation (Scheme 5). It is highly possible that
phosphines would first react with azo compounds to form
Huisgen zwitterions Int-1 which were protonated by phenol
and then accepted the attack by propargylic alcohols 1 to furnish
Int-3. Intermediates Int-3 would be further isomerized to allenyl
intermediates Int-4 by quick elimination of D(H) with the aid of
phenoxide. Spontaneous coordination of the resultant PhOD to
the phosphonium cation, followed by recapture of D(H) at the
CF3-attached carbon atom would realize the intramolecular
deuterium migration. While dialkoxytrialkylphosphoranes are
generally and readily transformed into the corresponding
phosphine oxides,12 this would not be the case for Int-4a due
to the stronger two Csp2-O bonds making such conversion
difficult. Instead, Int-4a would experience cleavage of a P-O
bond to give Int-4b, which would be further converted to a
mixture of β-trifluoromethylated R,β-unsaturated ketones (E)-
and (Z)-2, usually the latter preferred as the kinetically controlled
products due to favorable π*CF3-πCdO orbital interaction. This
initial Z-preference would be also explained by orthogonal
orientation of the both π systems, leading to selective proton-
ation of Int-4a from the less hindered enolate face, syn to D.13
Reversible 1,4-addition of TMPP to (Z)-2 as exemplified in
Scheme 3 would allow free rotation around the CR-Cβ axis of
the carbonyl group and would eventually furnish thermodyna-
mically more stable (E)-2 as the sole products. Effectiveness of
the combination of TMPP and ADDP over the usual PPh3 and
DIAD pair would stem from the easier formation of Int-1 by the
more nucleophilic former phosphine and the harder nature of the
In conclusion, we have discovered the novel isomerization of
CF3-containing secondary propargylic alcohols 1 to the corre-
sponding (E)-R,β-unsaturated ketones 2 by treatment with a
combination of tris(p-methoxyphenyl)phosphine, 1,10-(azodi-
carbonyl)dipiperidine, and phenol. Thus, in connection with
our previous method dealing with the same isomerization
reaction limited to the substrates with aromatic substituents at
the propargylic position, the present process successfully solved
such problems, rendering the transformation of 1 to 2 much
more useful and applicable to a wide variety of CF3-containing
propargylic alcohols 1.
’ EXPERIMENTAL SECTION
Substrates 1a,b,14 1c,7 1d,e,14 1f,15 and 1k16 were prepared as
described in the literature.
(E)-1,1,1-Trifluoronon-5-en-2-yn-4-ol (1g). To a solution of
LDA (66 mmol) in THF (100 mL) was added dropwise 2-bromo-3,3,3-
trifuluoropropene (3.0 mL, 30 mmol) at -80 °C. After the mixture was
stirred for 5 min at that temperature, crotonaldehyde (2.17 g, 31 mmol)
in THF (3 mL) was added, and stirring was continued for 1.5 h at -
80 °C. Usual workup and chromatography (n-hexane/AcOEt, 15:1)
afforded 1g (4.61 g, 24 mmol, 80%): colorless oil, Rf = 0.52 (n-hexane/
EtOAc, 4:1); 1H NMR 0.93 (t, J = 7.2 Hz, 3H), 1.44 (sex, J = 7.2 Hz,
2H), 1.95 (d, J = 6.6 Hz, 1H), 2.08 (q, J = 7.2 Hz, 2H), 4.94 (m, 1H), 5.60
(ddt, J = 15.3, 6.6, 1.5 Hz, 1H), 5.93 (dtd, J = 15.0, 6.6, 0.9 Hz, 1H); 13C
NMR 13.6, 21.8, 34.0, 62.3 (q, J = 1.3 Hz), 72.8 (q, J = 52.0 Hz), 86.3
(q, J = 6.2 Hz), 114.1 (q, J = 256.8 Hz), 126.5 (q, J = 1.3 Hz), 136.1;
19F NMR -51.94 (s); IR (neat) 607, 632, 742, 919, 966, 1031, 1145,
1216, 1276, 1459, 1669, 2271, 2877, 2935, 2965, 3320 cm-1; HRMS
(FAB) m/z calcd for C9H12F3O [M þ H]þ 193.0840, found 193.0862.
1,1,1-Trifluorooct-7,7-dimethyl-2,5-octa-2,5-diyn-4-ol
(1h): 45% yield; brown oil; Rf = 0.52 (n-hexane/EtOAc, 4:1); 1H NMR 1.24
(s, 9H), 2.40 (s, br, 1H), 5.20 (q, J = 3.0 Hz, 1H); 13C NMR 22.8, 30.5, 51.7
(q, J =1.2Hz), 70.4 (q, J= 53.3 Hz), 73.8 (q, J= 1.9 Hz), 84.7 (q, J =6.9 Hz),
95.5 (q, J= 1.2 Hz), 114.1 (q, J= 257.4 Hz); 19FNMR-52.15 (d, J=4.8Hz);
IR (neat): 622, 837, 880, 949, 1038, 1082, 1149, 1209, 1277, 1340, 1365, 1458,
1634, 1672, 1716, 2251, 2873, 2932, 2973, 3380 cm-1; HRMS (FAB) m/z
calcd for C10H12F3O [M þ H]þ 205.0840, found 205.0849.
1959
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The Journal of Organic Chemistry
NOTE
Tris(p-methoxyphenyl)phosphine (TMPP).17 n-BuLi (1.6 M
in n-hexane, 20.6 mL, 33 mmol) was added to a THF solution (50 mL)
of p-bromoanisole (6.2 g, 33 mmol) at -50 °C. After 30 min of stirring
at the same temperature, phosphorus trichloride (0.87 mL, 10 mmol)
was added at -50 °C, and the mixture was stirred for 3 h at -10 °C. The
mixture was quenched with saturated aq NH4Cl, and usual workup
followed by recrystallization from EtOH yielded the desired phosphine
TMPP (1.93 g, 5.5 mmol, 55%): white solid, Rf = 0.61; mp 132-134 °C
(e) Ojima, I. Fluorine in Medicinal Chemistry and Chemical Biology;
Wiley: New York, 2009.
(2) (a) Ogoshi, H.; Mizushima, H.; Toi, H.; Aoyama, Y. J. Org. Chem.
1986, 51, 2366–2368. (b) Leuger, J.; Blond, G.; Froehlich, R.; Billard, T.;
Haufe, G.; Langlois, B. R. J. Org. Chem. 2006, 71, 2735–2739.
(3) For recent examples of Michael type reaction of β-trifluoro-
methyl R,β-enones, see:(a) Bariau, A.; Jatoi, W. B.; Calinaud, P.; Troin,
Y.; Canet, J.-L. Eur. J. Org. Chem. 2006, 3421–3433. (b) Esparcieux, C.;
Figueredo, G.; Troin, Y.; Canet, J.-L. Synlett 2008, 1305–1308.
(c) Bheemanapalli, L. N.; Akkinepally, R. R.; Pamulaparthy, S. R. Chem.
Pharm. Bull. 2008, 56, 1342–1348.
1
(n-hexane/EtOAc, 4:1); H NMR 3.79 (s, 9H), 6.84-6.89 (m, 6H),
7.19-7.26 (m, 6H).
General Procedure for Isomerization of Propargylic
Alcohols 1 to R,β-Enones 2d with PhOH in the Presence of
Mitsunobu Reagents. To a THF-toluene (1:1) solution (2 mL) of
the alcohol 1d (0.158 g, 0.60 mmol) were added TMPP (0.256 g,
0.72 mmol), phenol (0.057 g, 0.60 mmol), and 1,10-(azodicarbonyl)-
dipiperidine (ADDP) (0.153 g, 0.60 mmol) at room temperature, and
the mixture was stirred for 3 h at 66 °C. After being cooled to room
temperature, the reaction mixture was passed through a short silica gel
chromatography column (n-hexane/AcOEt, 6:1). The solvent was
removed under reduced pressure, and the residue was purified by silica
gel chromatography (n-hexane/AcOEt, 30:1) to afford 2d (0.0805 g,
0.306 mmol, 51%): colorless oil; Rf = 0.55 (n-hexane/EtOAc, 4:1);
1H NMR (300 MHz, CDCl3) 2.90 (t, J = 6.0 Hz, 2H), 3.80 (t, J = 6.0 Hz,
2H), 4.52 (s, 2H), 6.61 (dq, J = 16.2, 6.3 Hz, 1H), 6.75 (dq, J = 15.9, 1.8
Hz, 1H), 7.26-7.38 (m, 5H); 13C NMR (75.5 MHz, CDCl3) 42.0, 64.9,
73.4, 122.5 (q, J = 269.8 Hz), 127.7, 127.8, 128.5 128.9 (q, J = 35.4 Hz),
134.3 (q, J = 5.6 Hz), 137.9, 196.5; 19F NMR (283 MHz, CDCl3) -
66.55 (d, J = 6.8 Hz); IR (neat) 630, 699, 741, 972, 1029, 1135, 1181,
1276, 1304, 1368, 1455, 1662, 1714, 2869 cm-1; HRMS (FAB) m/z
calcd for C13H14F3O2 [M þ H]þ 259.0946, found 259.0934.
(E)-5,5,5-Trifluoro-1-(methoxymethoxy)-1-phenylpent-3-
en-2-one (2f): 71% yield; colorless oil; Rf = 0.57 (n-hexane/EtOAc,
4:1, v/v); 1H NMR 3.37 (s, 3H), 4.72 (d, J = 6.9 Hz, 1H), 4.75 (d, J = 6.9
Hz, 1H), 5.25 (s, 1H), 6.71 (dq, J = 15.6, 6.6 Hz, 1H), 6.75 (dq, J = 15.6,
2.1 Hz, 1H), 7.32-7.41 (m, 5H); 13C NMR 56.3, 83.2, 95.5, 122.4 (q, J =
267.3 Hz), 127.3, 129.1, 129.2, 129.9 (q, J = 35.3 Hz), 130.1 (q, J = 5.6
Hz), 134.3, 194.5; 19F NMR -66.57 (d, J = 6.8 Hz); IR (neat) 617, 701,
748, 920, 974, 1039, 1135, 1215, 1275, 1307, 1454, 1496, 1656, 1715,
2896, 2954 cm-1; HRMS (FAB) m/z calcd for C13H14F3O3 [M þ H]þ
275.0895, found 275.0860.
(4) Nineham, A. W.; Raphael, R. A. J. Chem. Soc. 1949, 118–121.
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’ ASSOCIATED CONTENT
(12) For examples of the transformation of dialkoxytrialkylpho-
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(15) Watanabe, Y.; Yamazaki, T. J. Fluorine Chem. 2010,
S
Supporting Information. Experimental procedures
b
and characterization data for all new prepared compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tyamazak@cc.tuat.ac.jp.
131, 646–651.
(16) Yamazaki, T.; Yamamoto, T.; Ichihara, R. J. Org. Chem. 1995,
71, 6251–6253.
’ ACKNOWLEDGMENT
(17) Ye, D. Z. B.; Ho, D. G.; Gao, R.; Selke, M. Tetrahedron 2006,
62, 10729–10733.
The generous gift of 2-bromo-3,3,3-trifluoropropene by Tosoh
F-Tech, Inc., is greatly acknowledged.
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