Diastereoselective Mannich reaction of chiral enolates formed by enantioselective conjugate addition of Grignard reagents

Article abstract of DOI:10.1002/ejoc.201101270

Cu-Taniaphos-catalyzed enantioselective addition of Grignard reagents to cyclic enones leads to chiral magnesium enolates. These enolates add to N-protected imines directly, or through in situ transformation to silyl enol ethers. Diastereoselectivity of t

Full text of DOI:10.1002/ejoc.201101270

FULL PAPER
DOI: 10.1002/ejoc.201101270
Diastereoselective Mannich Reaction of Chiral Enolates Formed by
Enantioselective Conjugate Addition of Grignard Reagents
[a]
[a]
ˇ
ˇ
Zuzana Galestoková and Radovan Sebesta*
Keywords: Asymmetric catalysis / Diastereoselectivity / Michael addition / Mannich reaction / Magnesium
Cu–Taniaphos-catalyzed enantioselective addition of Grig-
nard reagents to cyclic enones leads to chiral magnesium
enolates. These enolates add to N-protected imines directly,
or through in situ transformation to silyl enol ethers. Dia-
stereoselectivity of the addition depends on the nitrogen pro-
tecting group of the imine. Diastereoisomers of the resulting
β-amino carbonyl compounds can be separated and are ob-
tained in acceptable yields and in high enantiomeric purities
(up to 99:1er).
metal enolate to a silyl enol ether.[16] Imines bearing elec-
tron-withdrawing groups also react with zinc enolates. Gon-
zales-Gómez and co-workers used Cu–phosphoramidite-
catalyzed addition of dialkylzinc reagents to enones, and
the resulting enolates reacted with chiral sulfinyl imine.[17]
The use of a chiral imine ensured high diastereoselectivity
of enolate addition to the imine. Huang and co-workers
used a phosphoramidite–amide ligand for Et2Zn addition
to acyclic enones. They were able to control the enantio-
selectivity as well as the diastereoselectivity of the reaction
with a chiral catalyst.[18] On the other hand, magnesium
enolates, resulting from the conjugate addition of Grignard
reagents, have been utilized to a much lesser extent.[19] The
synthetic scope and utility of chiral magnesium enolates is
potentially much broader, as there is a large number of
Grignard reagents available, either commercially or though
simple synthetic procedures. Inspired by the work of Fer-
inga and co-workers[20] on Cu–Taniaphos-catalyzed conju-
gate addition of Grignard reagents to enones, we have de-
veloped an extension of this methodology by trapping the
resulting magnesium enolates with imines. The Cu–Tania-
phos complex was able to catalyze the highly enantioselec-
tive conjugate addition of Grignard reagents to cyclohex-2-
enone followed by a one-pot reaction with N-benzylidene-
toluenesulfonamide.[21] With this imine, however, diastereo-
selectivity of the addition was poor. Therefore, we decided
to study this transformation with the aim of addressing the
problem of low diastereoselectivity. In this paper we investi-
gated an array of imines with various N-protecting groups
and also studied the effects of various additives. We have
found an interesting influence of the imine protecting group
on the diastereoselectivity of enolate addition. The study
led to overall improvement of the methodology.
Introduction
Stereoselective catalytic synthesis of complicated mole-
cules possessing several stereogenic centers is still a con-
siderable challenge. Tandem or domino catalytic reactions,
by combining several reactions into a one-pot sequence, try
to minimize the number of necessary synthetic and purifica-
tion operations thus to improve the overall efficiency.[1] Al-
though various tandem or domino reactions are known, the
use of conjugate addition as an initiating reaction seems
particularly appealing. Generation of chiral enolates by
catalytic conjugate addition of organometallic reagents to
enones is a useful and practical method for the preparation
of reactive carbon nucleophiles. Such nucleophilic species
are useful synthetic building blocks and can be added to
various electrophiles.[2] Zinc enolates produced by Cu-cata-
lyzed addition of dialkylzinc reagents to cyclic enones were
successfully trapped by aldehydes,[3] allyl bromide or acet-
ate,[4] alkyl iodide,[5] nitroso compounds,[6] bromine,[7] or
acid anhydride.[8] These enolates also open epoxides[9] or
take part in cross-coupling[10] or cyclopropanation reac-
tions.[11] Chiral metal enolates can also be trapped intramo-
lecularly to form cycles.[12] This was realized by another
conjugate addition[13] or a nucleophilic displacement of a
remote halogen atom.[11] Intramolecular enolate trapping is
also useful for Rh-catalyzed additions, although these reac-
tions typically run without a chiral ligand.[14] Hayashi de-
veloped an intermolecular enantioselective version of rho-
dium enolate trapping in the aldol reaction.[15] Further
transformations can be performed by conversion of the
[a] Department of Organic Chemistry, Faculty of Natural Sciences,
Comenius University,
Mlynska dolina CH-2, 84215 Bratislava, Slovakia, Slovak
Republic
Results and Discussion
The reaction of cyclohex-2-enone (1) with methylmagne-
sium bromide followed by enolate addition to imine 2a af-
Fax: +421-2-60296337
E-mail: radovan.sebesta@fns.uniba.sk
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101270.
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Diastereoselective Mannich Reaction of Chiral Enolates
forded diastereomeric products 3a (Scheme 1). Although the diastereoselectivity for imine 2a and had little effect on
the enantioselectivity, controlled by the chiral Taniaphos li- imines 2b and 2e. Addition of TMSOTf also led to lower
gand, was high (94:6 and 96:4er), the diastereoselectivity yields of products 3, because silyl enol ethers have intrin-
was low (56:44dr). Therefore, we decided to examine the sically lower nucleophilicities than metal enolates. On the
tandem reaction with imines having other nitrogen protect- other hand, with imine 2f, addition of TMSOTf resulted in
ing groups with the aim of improving the diastereoselectiv- a further increase in the diastereoselectivity (up to 7:93dr).
ity. A range of N-protected benzylideneamines 2, either The results of the screening of the nitrogen protecting
commercially available or synthesized according to litera- groups are summarized in Table 1.
ture procedures, were evaluated. We started our investiga-
Table 1. One-pot conjugate addition of MeMgBr to cyclohex-2-en-
tion with previously established optimal reaction conditions
one (1) followed by reaction with imines 2a–f.
(Scheme 1).[21] The reaction with ketone 1 and methylmag-
Entry Imine Additive
Yield of 3[a]
(R,R,S)/(R,R,R)
dr[b]
er[c]
nesium bromide in tBuOMe followed by the addition of
imines in 2-methyltetrahydrofuran (mTHF) afforded a
range of products 3b–f. In all instances, only two major dia-
stereoisomers, (R,R,S)-3 and (R,R,R)-3, were isolated.
1
2
3
4
5
6
7
8
2a
2a
2b
2b
2c
2d
2e
2e
2e
2f
–
35/24
25/11
32/33
32/13
23/24
24/14
23/22
11/9
56:44
67:33
94:6/96:4
98:2/98:2
TMSOTf
–
64:36 96:4/89:11
70/30 93:7/90:10
TMSOTf
–
–
–
63/37
66:34
60:40
60:40
70:30
98:2/97:3
96:4/96:4
97:3/92:8
92:8/97:3
n.d.
TMSOTf
Cu(OTf)2
–
TMSOTf
TIPSCl
9
(11)[d]
18/40 (67)[e]
3/29
10
11
12
22:78 81:19/98:2
2f
7:93
18:82
97:3/98:2
n.d.
2f
n.d.
[a] Isolated yield of pure diastereoisomers. [b] Ratio of (R,R,S)-3/
(R,R,R)-3 determined by 1H NMR (31P NMR for 3f) spectroscopic
analysis of the crude reaction mixture. [c] Enantiomeric purity of
(R,R,S)-3/(R,R,R)-3 determined by enantioselective HPLC (Daicel
chiral columns). [d] Combined isolated yield of both diastereomers.
[e] Yield given in parentheses is the isolated yield of 3f without
separation of diastereomers.
Scheme 1.
The absolute configuration of (R,R,R)-3a was previously
established by X-ray crystallographic analysis.[21] The con-
figurations of products 3b–e were assigned by comparison
of the H,H coupling constants within the series of com-
pounds 3a–e. Coupling constants between COCH and
CHN are consistent within groups of both diastereomers
(R,R,S)-3 (3JH,H = 3.0–4.3 Hz) and (R,R,R)-3 (3JH,H = 5.1–
5.8 Hz). Because of the opposite sense of diastereoselectiv-
ity with imine 2f, the relative configuration of its tandem
reaction product, compound 3f, was further ascertained by
NOESY NMR experiments. Important interactions con-
firming the configurations of both diastereomers (R,R,S)-
3f and (R,R,R)-3f are depicted in Figure 1.
Surprisingly, the addition to the majority of the imines
proceeded with similar results in terms of chemical yield,
diastereoselectivity, and enantioselectivity. A typical exam-
ple is imine 2b, which afforded product 3b in 65% yield
with 2:1dr and 96:4er for (R*,R*,S*)-3b and 89:11er for
(R*,R*,R*)-3b. The reaction with imine 2c was more enan-
tioselective, but product 3c was isolated in lower yield
(Table 1, Entry 5). The same is true also for imines 2d and
2e (Table 1, Entries 6 and 7). Only imine 2f, with a di-
phenylphosphorane protecting group (DPP), behaved dif-
ferently. Imine 2f afforded product 3f with markedly higher
diastereoselectivity (22:78dr). Furthermore, the major dia-
stereomer was (R,R,R)-3f, whereas in all other instances
isomer (R,R,S)-3 prevailed. Isolated yields of the dia-
stereomers of 2f were 18 and 40%. Separation and purifica-
tion of the diastereomers was often difficult and usually led
to some product loss. This was demonstrated by an experi-
ment in which diastereomers (R,R,S)-3f and (R,R,R)-3f
were isolated together in 67% yield (Table 1, Entry 10).
Practicality of the tandem reaction with imine 2f was evalu-
ated also by a reaction on a larger scale (3 mmol), which
resulted in 626 mg (50%) of pure diastereomer (R,R,R)-3f.
Imines without an electron-withdrawing group, such as N-
benzylidene-tert-butylamine or (benzylidene)(trimethylsil-
yl)amine, did not react at all. We also evaluated several
Figure 1. Important NOE interactions confirming relative configu-
ration of product 3f with calculated distances (nm). The lowest-
energy conformers have been optimized by HF/3-21G calculations.
Tosyl-protected imines derived from substituted benzal-
Lewis acidic additives, but without success. In situ transfor- dehydes were also evaluated in the tandem reaction
mation of the magnesium enolate to the silyl enol ether with (Scheme 2). Imines with both electron-withdrawing (i.e., 4a)
trimethylsilyl triflate (TMSOTf) led to a slight increase in and -donating (i.e., 4b) groups afforded tandem reaction
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FULL PAPER
products 5a and 5b, respectively. The diastereoselectivity
and enantioselectivity were slightly lower in comparison to
those obtained with imine 2a. Also in these experiments,
diastereomers (R,R,S)-5a/(R,R,R)-5a and (R,R,S)-5b/
(R,R,R)-5b were separated by column chromatography. An
imine derived from ferrocene carboxaldehyde was unreac-
tive under our reaction conditions, as only starting material
was isolated. Also, a pyridine-derived imine did not afford
the desired product of the tandem reaction.
Scheme 2.
Conjugate addition of ethylmagnesium bromide to cyclic
enones 1, 6, and 7 followed by reaction with imine 2f pro-
ceeded with high enantioselectivity (99:1er) and diastereo-
selectivity with cyclohex-2-enone (1) and cyclohept-2-enone
(7, Scheme 3). However, the chemical yields were somewhat
lower than those obtained with methylmagnesium bromide.
Product 8 of the reaction with cyclopent-2-enone (6) could
not be isolated in pure form, and therefore, its dr was not
determined; its enantiomeric ratio was also low (59:41er).
Figure 2. Calculated (HF/6-31G*) structures of the transition states
of enolate addition to imines (TS1 and TS2 for imine 2b; TS3 and
TS4 for imine 2f).
These eight-membered transition states are less rigid;
therefore, the diastereoselectivities of enolate addition to
imines are only moderate. For sterically less-congested im-
ines, exemplified by 2b with a Ms group, TS1 is preferred
because steric interaction between the cyclohexane ring and
the phenyl group of the imine is smaller than that in TS2.
The preference of TS4 over TS3 seems to be in the more
plausible conformation of imine 2f. Thus, for sulfonate-,
benzoyl-, and Boc-protected imines (i.e., 3a–e), preferential
attack is from the Si face, which leads to major isomers
Scheme 3.
Dependence of the diastereoselection of the Mannich re- (R,R,S)-3a–e, and imine 2f is attacked from its Re face,
action on the nitrogen protecting group is interesting but leading to (R,R,R)-3f as the major product (Figure 2).
not unprecedented.[22] Different stereoselectivity of the
The addition of TMSOTf after the addition of the Grig-
enolate addition to imine 2f in comparison to other imines nard reagent had an interesting effect: a higher diastereo-
led us to investigate possible transition states by computa- selectivity was observed for the reaction of 1 with imine 2f,
tional methods. We assumed that the enolate reacts prefer- although at the expense of chemical yield. With the ad-
entially at its anti side with respect to the methyl group, and dition of TMSOTf, conversion of the Mg-enolate to the si-
then we modeled two possible approaches of the imine. We lyl enol ether takes place, and the reaction likely proceeds
found that the generally proposed six-membered Zimmer- through an open transition state. However, because of at-
man–Traxler cyclic transition states were not possible to op- tractive interaction between the silicon and oxygen atoms
timize and no proper transition states were found. Instead, of the protecting groups, synclinal arrangements of the enol
geometrical optimization led to eight-membered cyclic tran- ether and imine are preferred, and thus similar arguments
sition states, in which Mg was bound to the oxygen atom as with Mg-enolates can be invoked. Figure 3 shows the
of the DPP group or the oxygen atoms of the Ms group transition states leading to the major diastereomers of 3:
(Figure 2).
TS5 to (R,R,S)-3b and TS6 to (R,R,R)-3f.
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Diastereoselective Mannich Reaction of Chiral Enolates
room temperature overnight. The mixture was then quenched with
NH4Cl and extracted with tBuOMe. The combined organic ex-
tracts were concentrated. The crude product was purified by col-
umn chromatography (SiO2; hexane/EtOAc/CH2Cl2 or CH2Cl2/
MeOH). Enantiomeric excess values were determined by HPLC
with a chiral stationary phase.
N-[(2-Methyl-6-oxocyclohexyl)(phenyl)methyl]methanesulfonamide
(3b)
(R,R,S)-3b: White solid. M.p. 185–187 °C (hexane). [α]D = –2.5 (c
= 0.31, CHCl3, 90%ee). 1H NMR (300 MHz, CDCl3): δ = 7.40–
7.27 (m, 5 H, Ph), 6.26 (d, J = 10.5 Hz, 1 H, NH), 4.79 (dd, J =
10.5, 3.1 Hz, 1 H, CH-NH), 2.62 (s, 3 H, SO2-CH3), 2.50 (dd, J =
10.5, 3.1 Hz, 1 H, CH-CO), 2.36–2.11 (m, 3 H, 2 CH2), 2.09–1.90
(m, 2 H, CH2), 1.84–1.63 (m, 1 H, CH), 1.51–1.41 (m, 1 H, CH2)
1.30 (d, J = 6.5 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3):
δ = 213.1 (Cq, CO) 140.6 (Cq, Ph), 128.6 (2 CH, Ph), 127.5 (CH,
Ph), 127.1 (2 CH, Ph), 64.0 (CH), 55.6 (CH), 42.7 (CH2), 42.0
(CH3), 37.3 (CH), 33.6 (CH2), 26.4 (CH2), 20.3 (CH3) ppm. HPLC
(Chiralcel OJ-H, hexane/iPrOH = 85:15, 0.6 mLmin–1, 211 nm): tR
Figure 3. Calculated (HF/6-31G*) structures of the transition states
of TMS-enol ether addition to imines (TS5 for imine 2b; TS6 for
imine 2f).
= 30.3 (major), 25.6 (minor) min. IR (ATR): ν = 3332 (w, NH),
˜
1697 (s, CO), 1316 (s, SO2), 1154 (s, SO2) cm–1. C15H21NO3S
(295.4): calcd. C 60.99, H 7.17, N 4.74; found C 60.84, H 7.07, N
4.63.
Conclusions
Chiral enolates produced by the Cu–Taniaphos-catalyzed
conjugate addition of Grignard reagents to cyclic enones
add diastereoselectively to imines. The resulting β-amino
carbonyl compounds are obtained in satisfactory overall
yields and in high enantiomeric purities (up to 99:1). The
relative configuration of the major diastereomer of the
products can be controlled by the nitrogen protecting group
and these diastereomers can be separated.
(R,R,R)-3b: White solid. M.p. 151–153 °C (hexane). [α]D = +48.8
1
(c 0.31, CHCl3, 62%ee). H NMR (300 MHz, CDCl3): δ = 7.47–
7.43 (m, 2 H, Ph), 7.38–7.29 (m, 3 H, Ph), 6.45 (d, J = 10.2 Hz, 1
H, NH), 4.75 (dd, J = 10.2, 5.2 Hz, 1 H, CH-NH), 2.71 (dd, J =
10.5, 5.2 Hz, 1 H, CH-CO), 2.45 (s, 3 H, SO2-CH3), 2.43–2.22 (m,
2 H, CH2), 2.00–1.89 (m, 1 H, CH2), 1.87–1.77 (m, 1 H, CH2),
1.54–1.42 (m, 3 H, 2 CH2), 1.15 (d, J = 5.9 Hz, 3 H, CH3) ppm.
13C NMR (75 MHz, CDCl3): δ = 213.0 (Cq, CO), 138.3 (Cq, Ph),
129.0 (2 CH, Ph), 128.9 (2 CH, Ph), 128.2 (CH, Ph), 61.9 (CH),
57.4 (CH), 42.1 (CH2), 41.7 (CH3), 35.0 (CH), 33.2 (CH2), 24.0
(CH2), 20.3 (CH3) ppm. HPLC (Chiralpak AD-H, hexane/iPrOH
= 85:15, 0.6 mL min– 1, 211 nm): tR = 26.1 (major), 21.4
Experimental Section
General Methods: All reactions were carried out under an inert
atmosphere of N2. Solvents were dried and purified by standard
methods before use.[23] NMR spectra were recorded with a Varian
(minor) min. IR (ATR): ν = 3286 (w, NH), 1702 (s, CO), 1322 (s,
˜
SO2), 1158 (s, SO2) cm–1. C15H21NO3S (295.4): calcd. C 60.99, H
7.17, N 4.74; found C 61.14, H 7.14, N 4.66.
Mercury plus instrument (300 MHz for H, 75 MHz for 13C) and
1
a Varian NMR System 600 (600 MHz for 1H, 150 MHz for 13C and
242.8 MHz for 31P). Chemical shifts (δ) are given in ppm relative to
tetramethylsilane for 1H NMR and 13C NMR spectroscopy. A uni-
fied chemical shift scale was used for 31P NMR with 85% H3PO4
as the secondary standard (δ = 0.0 ppm, Ξ = 40.4807420). Specific
optical rotations were measured with a Jasco instrument. Flash
chromatography was performed on Merck silica gel 60. Thin-layer
chromatography was performed on Merck TLC-plates silica gel 60,
F-254. Enantiomeric ratios were determined by HPLC with Chi-
ralpak AD-H, OD-H, AS-H, OJ-H, IA (Daicel Chemical Indus-
tries) columns by using hexane/iPrOH as the mobile phase and
detection with a UV detector at 254 and 211 nm. The imines used
in this work were commercially available or prepared according to
the literature.[24]
N-[(2-Methyl-6-oxocyclohexyl)(phenyl)methyl]-P,P-diphenylphos-
phinic Amide (3f)
(R,R,S)-3f: White solid. M.p. 184–186 °C (hexane). [α]D = 0.08 (c
= 0.34, CHCl3, 96%ee). 1H NMR (600 MHz, CDCl3): δ = 7.90–
7.81 (m, 2 H, Ph), 7.69–7.60 (m, 2 H, Ph), 7.52–7.47 (m, 1 H, Ph),
7.46–7.41 (m, 2 H, Ph), 7.38–7.33 (m, 1 H, Ph), 7.30–7.27 (m, 4 H,
Ph), 7.24–7.18 (m, 3 H, Ph), 4.76 (t, J = 11.3 Hz, 1 H, NH), 4.45
(dt, J = 11.3, 4.3 Hz, 1 H, CH-NH), 2.43 (ddd, J = 9.1, 4.3, 0.8 Hz,
1 H, CH-CO), 2.41–2.36 (m, 1 H, CH2), 2.36–2.29 (m, 1 H, CH2),
2.23 (m, 1 H, CH2), 2.02–1.92 (m, 2 H, CH2), 1.84–1.73 (m, 1 H,
CH2), 1.49–1.37 (m, 1 H, CH2), 1.10 (d, J = 6.4 Hz, 3 H, CH3)
ppm. 13C NMR (150 MHz, CDCl3): δ = 213.9 (CO) 143.4 (d, J =
4.5 Hz, CqPh), 133.4 (d, J = 130 Hz, CqPh), 132.5 (d, J = 9.7 Hz,
2 CHPh), 131.7 (d, J = 130 Hz, CqPh),131.7 (overlapped d, 2 CH
with s, CHPh) 131.5 (d, J = 2.8 Hz, CHPh), 128.4 (d, J = 12.5 Hz,
2 CHPh), 128.1 (overlapped d, 2 CH with s, CHPh) 126.9 (s, 2
CHPh), 126.7 (s, CHPh), 64.6 (d, J = 2.8 Hz, CH), 53.5 (CH), 42.5
(CH2), 36.1 (CH), 32.5 (CH2), 25.5 (CH2), 20.6 (CH3) ppm. 31P
Typical Procedure for the Tandem Reaction: Taniaphos (21 mg,
0.031 mmol) and CuCl (2.4 mg, 0.024 mmol) were dissolved in tBu-
OMe (6.0 mL), and the resulting solution was stirred for 30 min at
room temperature. The reaction mixture was then cooled to –60 °C
and cyclohex-2-enone (1; 47 μL, 47 mg, 0.489 mmol) was added.
The solution was stirred for an additional 10 min at –60 °C. Then,
the Grignard reagent (0.76 mmol, Et2O solution) was added over
5 min, and the resulting mixture was stirred for an additional 2 h
at –60 °C. Then, imine 2 (0.328 mmol) dissolved in mTHF (5.0 mL)
was added, and the reaction mixture was allowed to slowly reach
NMR (242.8 MHz, CDCl ): δ = 21.6 ppm. IR (ATR): ν = 3146 (w,
˜
3
NH), 1705 (s, CO), 1186 (s, PO) cm–1. HPLC (Chiralcel OD-H,
hexane/iPrOH = 93:7, 0.7 mLmin–1, 211 nm) tR = 14.8 (major),
18.8 (minor) min. C26H28NO2P (417.5): calcd. C 74.80, H 6.76, N
3.36; found C 74.59, H 6.82, N 3.10.
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9693; b) R. Naasz, L. A. Arnold, A. J. Minnaard, B. L. Fer-
(R,R,R)-3f: White solid. M.p. 189–191 °C (hexane). [α]D = 0.58 (c
= 0.34, CHCl3, 96%ee). 1H NMR (600 MHz, CDCl3): δ = 7.83–
7.77 (m, 2 H, Ph), 7.73–7.66 (m, 2 H, Ph), 7.51–7.36 (m, 4 H, Ph),
7.30 (m, 2 H, Ph), 7.24 (s, 5 H, Ph), 5.13 (t, J = 10.7 Hz, 1 H, NH),
4.41 (dt, J = 11.6, 5.1 Hz, 1 H, CH-NH), 2.87 (dd, J = 10.7, 5.1 Hz,
1 H, CH-CO), 2.31–2.29 (m, 2 H, CH2), 1.93–1.84 (m, 1 H, CH),
1.79–1.72 (m, 1 H, CH2), 1.69–1.36 (m, 3 H, 2 CH2), 1.01 (d, J =
6.4 Hz, 3 H, CH3) ppm. 13C NMR (150 MHz, CDCl3): δ = 213.7
(CO), 140.6 (d, J = 6.6 Hz, CqPh), 133.3 (d, J = 126.4 Hz, CqPh),
132.7 (d, J = 9.5 Hz, 2 CHPh), 132.4 (d, CqPh), 131.7 (d, J =
2.4 Hz, 2 CHPh), 131.6 (d, J = 2.4 Hz, 2 CHPh), 131.5 (d, J =
9.5 Hz, 2 CHPh), 128.7 (s, CHPh), 128.4 (d, J = 12.5 Hz, 2 CHPh),
128.22 (d, J = 12.5 Hz, 2 CHPh), 128.20 (CHPh), 127.2 (CHPh),
63.2 (CH), 54.6 (CH), 42.4 (CH2), 35.3 (CH), 33.1 (CH2), 24.1
(CH2), 20.1 (CH3) ppm. 31P NMR (242.8 MHz, CDCl3): δ =
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Financial support from the Slovak Grant Agency VEGA, grant no.
VEGA 1/0265/09 is gratefully acknowledged. This publication is
the result of the project implementation “Centre of Excellence on
Green Chemistry Methods and Processes (CEGreenI)” supported
by the Research & Development Operational Programme funded
by the European Regional Development Fund. NMR measure-
ments were provided by Slovak State Programme project no.
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Published Online: October 14, 2011
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