Rasta Resin-TBD-Catalyzed γ-Selective Morita-Baylis-Hillman Reactions of α,γ-Disubstituted Allenones

Article abstract of DOI:10.1055/s-0034-1380691

Rasta resin-TBD (RR-TBD) was found to be an efficient organocatalyst for γ-selective Morita-Baylis-Hillman reactions between α,γ-disubstituted allenones and aryl aldehydes. In these reactions the heterogeneous nature of RR-TBD greatly facilitated product isolation since the catalyst could be separated simply by filtration.

Full text of DOI:10.1055/s-0034-1380691

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 1732–1736
letter
1732
S. Ma et al.
Letter
Synlett
Rasta Resin-TBD-Catalyzed γ-Selective Morita–Baylis–Hillman
Reactions of α,γ-Disubstituted Allenones
Shuang Ma
O
O
Yun-Chin Yang
Patrick H. Toy*
O
R1
R1
R3
catalyst (20 mol%)
NMP, r.t. or 50 °C
+
C
H
C
Department of Chemistry, The University of Hong Kong,
Pokfulam Road, Hong Kong, P. R. of China
phtoy@hku.hk
R2
R3
R2
OH
N
catalyst:
R = Me or
N
R
N
rasta resin
Received: 10.03.2015
Accepted after revision: 06.04.2015
Published online: 20.05.2015
polymer-supported nucleophilic phosphine catalysts for
MBH reactions,12 we attempted to use 2 in similar reactions
and settled on allenoate substrates. However, in light of the
report by Selig et al.5 and the fact that Shi and co-workers
originally studied only α-substituted allenones, we changed
our focus to γ-selective MBH reactions of α,γ-disubstituted
allenones catalyzed by 2, and report our results herein.
DOI: 10.1055/s-0034-1380691; Art ID: st-2015-w0173-l
Abstract Rasta resin-TBD (RR-TBD) was found to be an efficient or-
ganocatalyst for γ-selective Morita–Baylis–Hillman reactions between
α,γ-disubstituted allenones and aryl aldehydes. In these reactions the
heterogeneous nature of RR-TBD greatly facilitated product isolation
since the catalyst could be separated simply by filtration.
OH
(A)
EWG
R1
R2
O
nucleophilic catalyst
Key words allenones, Morita–Baylis–Hillman reactions, organocataly-
EWG
+
R1
R2
sis, polymer-supported catalysts, rasta resin
R1 = aryl, alkyl,
heteroaryl
EWG = electron-withdrawing group:
COR, CHO, CN, COOR, PO(OEt)2,
SO2Ph, SO3Ph, SOPh
The Morita–Baylis–Hillman (MBH) reaction is a widely
studied C–C bond-forming transformation between an elec-
tron-withdrawing-group-activated alkene and an electro-
phile, typically an aldehyde or related compound, that is
catalyzed by a nucleophilic organocatalyst.1 Generally MBH
reactions are α-selective, with the new C–C bond formed at
the alkene position adjacent to the activating group
(Scheme 1, A). However, with activated allene substrates,
the new C–C bond can be formed at the γ-position (Scheme
1, B).2 Recently Selig and co-workers have reported exam-
ples of such γ-selective MBH reactions involving allenoates
catalyzed by the organic superbase3 7-methyl-1,5,7-tri-
azabicyclo[4.4.0]dec-5-ene (MTBD, 1, Figure 1 and Scheme
1, C).4–6 Contemporaneous to this research, we were devel-
oping a heterogeneous polystyrene-based rasta resin-sup-
ported analogue of MTBD (RR-TBD, 2, Figure 1) as an or-
ganocatalyst.7–9 Initially we studied the use of 2 in transes-
terification reactions such as biodiesel production,10,11 and
once this research was completed, we next turned our at-
tention to examining the utility of 2 as a nucleophilic cata-
lyst. Since we had extensive prior experience in developing
R
2 = H, COOR,
alkyl
O
(B)
O
R1
O
R1
DMAP (20%)
H
+
C
DMSO, 80 °C
0.5–12 h
C
R2
R2
OH
R1 = Me or Bn
R2 = 4-F, 4-Cl, 2-Cl, 4-Br, 4-NO2, 2,4-Cl2
O
(C)
O
O
R1
R1
R2
OEt
1 (15–30%)
OEt
+
H
C
DMF
C
r.t., 1.5–5 h
R2
R3
R3
OH
R1 = Me, n-Pr, Bn R3 = 4-Cl, H, 2-NO2, 4-NO2, 4-MeO, 2,3-(MeO)2
2 = Me, i-Pr
R
Scheme 1 MBH reaction variations
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1732–1736

1733
S. Ma et al.
Letter
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living radical polymerization initiation groups
heterogeneous core
O
N
N
y
x
N
N
rasta resin
MTBD (1)
R
N
N
R =
flexible functionalized
grafts
N
RR-TBD (2)
Figure 1 MTBD (1), the rasta resin concept, and RR-TBD (2)
Polymer 2 was synthesized as previously reported start-
ing from the initiator functionalized heterogenous core us-
ing a combination of styrene and 4-vinylbenzyl chloride
(5:1 ratio) to install the functionalized grafts by a living
radical polymerization process (Scheme 2, A).10 The benzyl
chloride groups of the grafts were subsequently treated
with deprotonated 1,5,7-triazabicyclo[4.4.0]dec-5-ene to
install the catalytic MTBD group analogues. The allenone
substrates 3A–D were prepared by olefination of in situ
generated ketenes according to a literature procedure, start-
ing from α-bromo ketones 4A,B via phosphonium salt inter-
mediates 5A,B, in moderate overall yields (Scheme 2, B).13,14
With allenones 3A–D in hand, we first investigated the
possibility of their participation in γ-selective MBH reac-
tions catalyzed by 1. In the original report by Shi and co-
workers, DMAP was used as the catalyst in reactions for
which the solvent was DMSO.2 We therefore applied similar
reaction conditions in side-by-side reactions between 3A
and 4-chlorobenzaldehyde (6a) catalyzed by either DMAP
or 1, and observed that the later afforded higher yield of
product 7Aa as a nearly 1:1 mixture of diastereomers than
did the former (Table 1, entries 1 and 2). Changing the sol-
vent to NMP and increasing the amount of allenone 3A rela-
tive to electrophile 6a led to further yield enhancement (Ta-
ble 1, entries 3–5).15 Unfortunately, when a 1:1 ratio of 3A
to 6a was used, the reaction did not go to completion, and
chromatographic purification of 7Aa was required. On the
other hand, using a four-fold excess of the allenone sub-
strate compared to the aldehyde was wasteful, made the
product purification tedious, and resulted in only a slightly
higher yield. Thus, we chose to use a 2:1 ratio of allenone to
aldehyde in the subsequent reactions.
(A)
130 °C, 2 d
+ 5n
+ n
O
N
Cl
O
N
N
N Na
Table 1 MBH Reactions of 3A with 6aa
n
5n
N
O
O
2 (RR-TBD)
THF, 60 °C, 12 h
O
Et
+
Et
catalyst (20 mol%)
Cl
Cl
C
H
C
solvent
Cl
(B)
O
O
3A
6a
OH
O
1. Et3N
Ph3P
R1
R1
7Aa
2. R2CH2COCl
CHCl3, r.t., 4 h
R1
CHCl3, 50 °C
24 h
C
PPh3
Br
Entry
Catalyst
Solvent
3A/6a
Time (h) Combined yield (%)
Br
4
R2
5
3
1
2
3
4
5
DMAP
DMSO
DMSO
NMP
1:1
1:1
1:1
2:1
4:1
6
6
5
4
4
54
67
71
85
89
4A: R1 = Ph
4B: R2 = Et
5A: R1 = Ph
5B: R2 = Et
3A: R1 = Et, R2 = Me (21%)
3B: R1 = Et, R2 = Ph (68%)
3C: R1 = Ph, R2 = Me (49%)
3D: R1 = Ph, R2 = Ph (65%)
1
1
1
1
NMP
Scheme 2 Synthesis of polymer 2 and allenones 3A–D
NMP
a Reaction conditions: allenone 3A, aldehyde 6a (0.4 mmol), catalyst (0.08
mmol), and solvent (1.0 mL) were stirred at r.t. for the indicated time.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1732–1736

1734
S. Ma et al.
Letter
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We next turned our attention to examining the catalyst
1 in a range of γ-selective MBH reactions using the opti-
mized reaction conditions (Table 2). When allenone 3A was
reacted with other electron-withdrawing group substituted
aldehydes 6b,c, high yields of products 7Ab and 7Ac were
obtained in short reaction times (entries 2 and 3). Very high
yield of 7Ad could be obtained even when unactivated
benzaldehyde (6d) was used (entry 4). However, when elec-
tron-rich 4-methoxybenzaldehyde (6e) was used, only rela-
tively low yield of the corresponding product 7Ae was ob-
tained after a prolonged reaction (entry 5). Reactions using
allenone 3B that bears a phenyl group at the γ-position
took longer and afforded lower product yields than did re-
actions with 3A (entries 6 and 7), perhaps due to the steric
hindrance. Phenyl ketone substrates 3C and 3D showed
similar reactivity patterns (entries 8–14). In all cases, the
product was obtained as a nearly 1:1 mixture of diastereo-
mers.
Having established the general utility of 1 as a catalyst
for the reactions of interest, we next used 2 in an identical
set of reactions (Table 2). Since we anticipated that the het-
erogeneous nature of 2 would lead to less efficient reac-
tions, we performed these reactions at 50 °C rather than at
room temperature. Even at this elevated temperature reac-
tion times were much longer using 2 as the catalyst than
those with 1 and product yields were slightly lower (entries
1–14). When a reaction between 3C and 6b was performed
using 2 at room temperature, only 47% yield of 7Cb was ob-
tained after 24 hours, compared to 81% yield of 7Cb after six
hours at 50 °C (entry 15 vs. entry 8). Gratifyingly, when we
Table 2 MBH Reactions of 3 with 6 Catalyzed by 1 or 2a
O
O
O
R1
R1
R3
1 or 2 (20 mol%)
NMP
+
H
C
C
R3
R2
R2
6
OH
3
7
Entry
3
6
R1
R2
R3
7
1
2
Time
(h)
Combined yield Ratio
(%)
Time
(major/minor) (h)
Combined yield Ratio
(%) (major/minor)
1
2
3A
3A
3A
3A
3A
3B
3B
3C
3C
3C
3C
3C
3D
3D
3C
3C
3C
3C
6a
6b
6c
6d
6e
6b
6a
6b
6c
6a
6d
6e
6b
6a
6b
6a
6a
6a
Et
Me
Me
Me
Me
Me
Ph
Cl
7Aa
7Ab
7Ac
7Ad
7Ae
7Bb
7Ba
7Cb
7Cc
7Ca
7Cd
7Ce
7Db
7Da
7Cb
7Ca
7Ca
7Ca
4
1.5
3
85
57:43
50:50
51:49
52:48
56:44
58:42
52:48
53:47
55:45
50:50
59:41
50:50
60:40
52:48
–
24
72
56:44
52:48
50:50
54:46
50:50
55:45
52:48
50:50
51:49
56:44
55:45
54:46
58:42
52:48
50:50
50:50
56:44
-
Et
CN
92
87
87
64
74
66
91
95
89
79
48
74
91
–
12
18
48
60
48
84
6
80
70
50
11
66
19
81
81
81
64
40
69
40
47
33
49
0
3
Et
NO2
H
4
Et
7
5
Et
MeO
CN
Cl
18
9
6
Et
7
Et
Ph
12
0.5
1.5
3
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Me
Ph
CN
NO2
Cl
9
10
12
48
48
20
36
24
24
12
12
10
11
12
13
14
15b
16c
17d
18e
H
5
MeO
CN
Cl
10
6
Ph
9
Me
Me
Me
Me
CN
Cl
–
–
–
–
Cl
–
–
–
Cl
–
–
–
a Reaction conditions: allenone 3 (0.8 mmol), aldehyde 6 (0.4 mmol), 1 or 2 (0.08 mmol), and NMP (1.0 mL) were stirred at r.t. (with 1) or at 50 °C (with 2) for the
indicated time.
b Reaction was carried out at r.t.
c Reaction was carried out using 8 at 50 °C.
d First reuse of 2.
e Second reuse of 2.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1732–1736

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Letter
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used a macroporous polystyrene-supported TBD (PS-TBD, 8)
in which the catalytic groups are located on the interior of a
heterogeneous polymer bead,16,17 much lower product yield
of 7Ca was obtained after a much longer reaction time (en-
try 16 vs. entry 10). This supports the notion that placing
the catalytic groups on flexible grafts makes them more ac-
cessible to the substrate molecules and more efficient com-
pared to having them located on the interior of a polysty-
rene bead, as we have observed in our previous studies.9
Unfortunately, we observed that polymer 2 was not an ef-
fective catalyst when reused (entries 17 and 18), and at this
time the reasons for this are unclear. Reactivation of the
catalytic groups by washing the polymer with a base did
not improve the situation.
In summary, we have found that both 1 and our previ-
ously reported polymer 2 based on the rasta resin architec-
ture are able to effectively catalyze γ-selective MBH reac-
tions between α,γ-disubstituted allenones and aryl alde-
hydes. Superbase 1 was found to be a more efficient catalyst
than the previously used DMAP, and while 2 was not reus-
able in these reactions, it did prove to be a more efficient
catalyst than did a more traditional polystyrene-supported
analogue. Importantly, the heterogeneous nature of 2 did
facilitate product purification when it was used. We are
currently examining other applications for 2 and will report
the results of these studies shortly.
(5) Selig, P.; Turočkin, A.; Raven, W. Synlett 2013, 24, 2535.
(6) For a report regarding the conversion of the alcohol products of
these reactions into the corresponding acetates and carbonates,
see: Selig, P.; Nghiem, T.-L. Synlett 2015, 26, 907.
(7) For a review regarding organic polymer supports in organic
chemistry, see: Lu, J.; Toy, P. H. Chem. Rev. 2009, 109, 815.
(8) For details regarding the rasta resin concept, see: (a) Lindsley, C.
W.; Hodges, J. C.; Filzen, G. F.; Watson, B. M.; Geyer, A. G. J.
Comb. Chem. 2000, 2, 550. (b) McAlpine, S. R.; Lindsley, C. W.;
Hodges, J. C.; Leonard, D. M.; Filzen, G. F. J. Comb. Chem. 2001, 3,
1. (c) Wisnoski, D. D.; Leister, W. H.; Strauss, K. A.; Zhao, Z.;
Lindsley, C. W. Tetrahedron Lett. 2003, 44, 4321. (d) Fournier, D.;
Pascual, S.; Montembault, V.; Haddleton, D. M.; Fontaine, L. J.
Comb. Chem. 2006, 8, 522. (e) Fournier, D.; Pascual, S.;
Montembault, V.; Fontaine, L. J. Polym. Sci. A: Polym. Chem.
2006, 44, 5316. (f) Pawluczyk, J. M.; McClain, R. T.; Denicola, C.;
Mulhearn, J. J.; Rudd, D. J.; Lindsley, C. W. Tetrahedron Lett.
2007, 48, 1497. (g) Chen, G.; Tao, L.; Mantovani, G.; Geng, J.;
Nystroem, D.; Haddleton, D. M. Macromolecules 2007, 40, 7513.
(9) For our previous research using rasta resin, see: (a) Leung, P. S.-
W.; Teng, Y.; Toy, P. H. Synlett 2010, 1997. (b) Leung, P. S.-W.;
Teng, Y.; Toy, P. H. Org. Lett. 2010, 12, 4996. (c) Teng, Y.; Toy, P.
H. Synlett 2011, 551. (d) Lu, J.; Toy, P. H. Synlett 2011, 659.
(e) Teng, Y.; Lu, J.; Toy, P. H. Chem. Asian J. 2012, 7, 351.
(f) Diebold, C.; Lu, J.; Becht, J.-M.; Toy, P. H.; Le Drian, C. Eur. J.
Org. Chem. 2012, 893. (g) Xia, X.; Toy, P. H. Beilstein J. Org. Chem.
2014, 10, 1397. (h) Derible, A.; Yang, Y.-C.; Toy, P. H.; Becht, J.-
M.; Le Drian, C. Tetrahedron Lett. 2014, 55, 4331.
(10) Yang, Y.-C.; Leung, D. Y. C.; Toy, P. H. Synlett 2013, 24, 1870.
(11) For another report regarding the use of RR-TBD as a catalyst,
see: Bonollo, S.; Lanari, D.; Angelini, T.; Pizzo, F.; Marrocchi, A.;
Vaccaro, L. J. Catal. 2012, 285, 216.
(12) (a) Zhao, L.-J.; He, H. S.; Shi, M.; Toy, P. H. J. Comb. Chem. 2004, 6,
680. (b) Zhao, L.-J.; Kwong, C. K.-W.; Shi, M.; Toy, P. H. Tetrahe-
dron 2005, 61, 12026. (c) Teng, W.-D.; Huang, R.; Kwong, C. K.-
W.; Shi, M.; Toy, P. H. J. Org. Chem. 2006, 71, 36. (d) Kwong, C.
K.-W.; Huang, R.; Zhang, M.; Shi, M.; Toy, P. H. Chem. Eur. J. 2007,
13, 2369.
Acknowledgment
This research was supported financially by the University of Hong
Kong and the Research Grants Council of the Hong Kong S. A. R., P. R.
of China (Project No. HKU 705510P).
(13) Lang, R. W.; Hansen, H.-J. Org. Synth. 1984, 62, 202.
(14) General Procedure for the Synthesis of Allenones 3A–D:
α-Bromo ketone 4A or 4B (20.0 mmol), Ph3P (6.29 g, 24.0
mmol), and benzene (100 mL) were added to a 250-mL round-
bottomed flask equipped with a magnetic stirrer and a con-
denser. The reaction flask was immersed in an oil bath, and the
reaction mixture was refluxed for 2 d. After cooling to r.t., the
solvent was removed under reduced pressure to afford the
crude phosphonium salt 5A or 5B as a viscous oil. The crude salt
dissolved in CHCl3 (65 mL) was transferred to a 100-mL round-
bottomed flask equipped with a magnetic stirrer. The reaction
mixture was cooled to 0 °C with an ice-water bath and then
Et3N (6.1 mL, 44 mmol) was added dropwise. The ice-water
bath was removed, and the reaction mixture was then stirred at
r.t. for 3 h. The reaction mixture was cooled to 0 °C and the
appropriate acid chloride (18.0 mmol) was added dropwise.
After 1 h, the reaction mixture was warmed to r.t. and stirred
for a further 10 h. The reaction mixture was transferred to a
separation funnel and H2O (100 mL) was added. The organic
layer was separated and washed with brine (50 mL) and then
dried over MgSO4. The solvent was removed under reduced
pressure to afford a yellow oil which was then purified by silica
gel column chromatography using a mixture of CH2Cl2 and
hexane as the eluent.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1380691.
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References and Notes
(1) For selected reviews regarding MBH reactions and the utility of
MBH products, see: (a) Basavaiah, D.; Reddy, B. S.; Badsara, S. S.
Chem. Rev. 2010, 110, 5447. (b) Basavaiah, D.; Veeraraghavaiah,
G. Chem. Soc. Rev. 2012, 41, 68. (c) Liu, T.-Y.; Xie, M.; Chen, Y.-C.
Chem. Soc. Rev. 2012, 41, 4101. (d) Wei, Y.; Shi, M. Chem. Rev.
2013, 113, 6659.
(2) (a) Zhao, G.-L.; Shi, M. Org. Biomol. Chem. 2005, 3, 3686. (b) Shi,
M.; Guo, Y.-W.; Li, H.-B. Chin. J. Chem. 2007, 25, 828.
(3) (a) Superbases for Organic Synthesis; Ishikawa, T., Ed.; John
Wiley and Sons: Chichester, 2009. (b) Ishikawa, T.; Harwood, L.
M. Synlett 2013, 24, 2507.
(4) For selected examples and reviews regarding the use of MTBD
as a catalyst, see: (a) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.;
Waymouth, R. M. Macromolecules 2010, 43, 2093. (b) Fu, X.; Tan,
C.-H. Chem. Commun. 2011, 47, 8210. (c) Taylor, J. E.; Bull, S. D.;
Williams, J. M. J. Chem. Soc. Rev. 2012, 41, 2109. (d) Selig, P. Syn-
thesis 2013, 45, 703.
4-Methylhepta-4,5-dien-3-one (3A): 1H NMR (400 MHz,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1732–1736

1736
S. Ma et al.
Letter
Synlett
CDCl3): δ = 5.43–5.50 (m, 1 H), 2.59–2.71 (m, 2 H), 1.79 (d, J =
7.3 Hz, 3 H), 1.76 (d, J = 2.7 Hz, 3 H), 1.07 (t, J = 7.3 Hz, 3 H). 13
NMR (101 MHz, CDCl3): δ = 212.29, 202.88, 159.50, 138.13,
91.70, 88.92, 32.10, 13.49, 13.39, 8.99. HRMS: m/z calcd for
C8H12O: 124.0883; found: 124.0881.
(CH3)CCOEt, major + minor], 1.74 [s, 3 H, (CH3)CCHOH, major +
minor], 1.05 (t, J = 7.4 Hz, 3 H, CH2CH3, major + minor). 13C NMR
(major diastereomer, 101 MHz, CDCl3): δ = 208.73 (s, =C=),
202.80 (s, COEt), 140.12 (s, CArCHOH), 133.73 (s, CArCl), 128.62
(s, CArH), 127.67 (s, CArH), 106.43 (s, CCHOH), 105.04 (s, CCOEt),
74.48 (s, CHOH), 32.43 (s, CH2CH3), 8.92 (s, CH2CH3). The signals
for (CH3)CCHOH and (CH3)CCO (14.43, 14.20, 13.81) were not
assigned due to overlapping signals. 13C NMR (minor diastereo-
mer, 101 MHz, CDCl3): δ = 208.67 (s, =C=), 202.86 (s, COEt),
140.09 (s, CArCHOH), 133.80 (s, CArCl), 128.67 (s, CArH), 127.74 (s,
CArH), 106.42 (s, CCHOH), 105.32 (s, CCOEt), 74.39 (s, CHOH),
32.43 (s, CH2CH3), 8.88 (s, CH2CH3). HRMS: m/z calcd for
C
(15) General Procedure for the MBH Reactions: Allenone 3A–D
(0.8 mmol), aldehyde 6a–e (0.4 mmol), NMP (1.0 mL), and 1, 2
or 8 (0.08 mmol) were added to a 10-mL round-bottomed flask
equipped with a magnetic stirrer. The reaction mixture was
stirred either at r.t. (when 1 was used as the catalyst) or at 50 °C
(when 2 or 8 was used as the catalyst) for the reaction times
indicated in Table 2. When 1 was used as the catalyst, solid
NH4Cl (0.006 g, 0.1 mmol) was added to the reaction mixture to
quench the reaction. The reaction mixture was then transferred
to a separation funnel, and then H2O (30 mL) and EtOAc (15 mL)
were added. The organic layer was separated, washed with
brine (30 mL), and dried over MgSO4. The solvent was evapo-
rated under reduced pressure to afford an oil, which was puri-
fied by silica gel column chromatography using a mixture of
EtOAc and hexane as the eluent. When 2 was used as the cata-
lyst, the reaction mixture was merely filtered, and the crude
product was purified by silica gel column chromatography.
7-(4-Chlorophenyl)-7-hydroxy-4,6-dimethylhepta-4,5-dien-
3-one (7Aa): 1H NMR (400 MHz, CDCl3): δ = 7.29–7.37 (m, 4 H,
CHAr, major + minor), 5.26 (s, 1 H, CHOH, major), 5.24 (s, 1 H,
CHOH, minor), 2.50–2.64 (m, 2 H, COCH2CH3, major + minor),
2.43 (br s, 1 H, OH, major), 2.37 (s, 1 H, OH, minor), 1.78 [s, 3 H,
C
15H17Cl1O2: 264.0912; found: 264.0724.
(16) See reference 10 for details and a cartoon illustrating the struc-
ture of 8. The polystyrene support used in 8 is commercially
available and is the typical material used in polystyrene-sup-
ported reagents and catalysts.
(17) For research involving similar polystyrene-supported TBD
reagents and catalysts, see: (a) Fringuelli, F.; Pizzo, F.; Vittoriani,
C.; Vaccaro, L. Chem. Commun. 2004, 2756. (b) Fringuelli, F.;
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1732–1736