Deoxygenation of sulfoxides to sulfides in the presence of zinc catalysts and boranes as reducing reagents

Article abstract of DOI:10.1007/s10562-012-0862-9

In the present study, the zinc-catalyzed deoxygenation of aliphatic and aromatic sulfoxides in the presence of boranes as reducing reagent has been explored. After investigation of different reaction parameters the abilities of catalytic amounts of Zn(OTf)₂ has been demonstrated in the deoxygenation of various sulfoxides. Moreover, various experiments have been performed to shed light on the underlying reaction mechanism.

Full text of DOI:10.1007/s10562-012-0862-9

Catal Lett (2012) 142:1003–1010
DOI 10.1007/s10562-012-0862-9
Deoxygenation of Sulfoxides to Sulfides in the Presence of Zinc
Catalysts and Boranes as Reducing Reagents
Stephan Enthaler
•
•
Sebastian Krackl
Elisabeth Irran Shigeyoshi Inoue
•
Received: 22 May 2012 / Accepted: 15 June 2012 / Published online: 4 July 2012
Springer Science+Business Media, LLC 2012
ꢀ
Abstract In the present study, the zinc-catalyzed deox-
ygenation of aliphatic and aromatic sulfoxides in the
presence of boranes as reducing reagent has been explored.
After investigation of different reaction parameters the
corresponding sulfides. In more detail, the deoxygenation
of sulfur-based compounds is an essential method in bio-
logical processes (dimethylsulfoxide-, biotin-, and methi-
onine sulfoxide reductase) and has some significance in
organic chemistry [32–70]. Recently, we have studied the
deoxygenation of sulfoxides and amides with silanes and
different metal catalysts (e.g., Mo, Fe, Cu, Zn) [71–74]. In
case of zinc excellent yields and chemoselectivities have
been realized under mild reaction conditions. Aside hy-
drosilanes the reduction properties of hydroboranes in the
presence of catalytic amounts of transition metals has been
used in organic chemistry [75–80]. Herein, we report our
investigations on the zinc-catalyzed deoxygenation of
sulfoxides to generate the corresponding sulfides with hy-
droboranes as reductant.
abilities of catalytic amounts of Zn(OTf) has been dem-
2
onstrated in the deoxygenation of various sulfoxides.
Moreover, various experiments have been performed to
shed light on the underlying reaction mechanism.
Keywords Catalysis Á Zinc Á Deoxygenation Á Sulfoxide Á
Sulfide
1
Introduction
During the last decades a remarkable number of protocols
and improvements have been reported for transition metal-
catalyzed reductions; hence nowadays the methodology is
applicable in the synthesis of fine chemicals, pharmaceu-
ticals, agricultural chemicals and natural products [1, 2]. In
recent times attention was drawn to the replacement of
expensive and toxic catalyst metals, which have been
dominated the field, by cheaper and low toxic metals [3, 4].
In this regard, the application of zinc catalysts in combi-
nation with hydrosilanes has been demonstrated to succeed
in the reduction of various functional groups with excellent
yields and selectivities (e.g., C=O, C=N, amides, esters)
2 Results and Discussion
At first the interaction of the zinc source with the sulfoxide
functionality was investigated (Scheme 1). As model sub-
strate p-tolyl sulfoxide (1) was chosen. To a solution of 1
(10 equiv.) in C D was added Zn(OTf) (2, 1.0 equiv.).
6
6
2
The suspension was heated to 80 ꢁC for 1 h, while at 80 ꢁC
a clear solution was obtained. The mixture was slowly
cooled to room temperature, while colorless crystals were
obtained, which were suitable for X-ray measurements.
The solid-state structure of complex 3 has been charac-
terized by single-crystal X-ray diffraction analysis. Ther-
mal ellipsoid plots, selected bond lengths and angles
are shown in Fig. 1. The obtained solid structure of
Zn(OTf) (1) (3) showed an octahedral structure with four
[
5–31]. An attractive model reaction to study the abilities
of catalysts is the deoxygenation of sulfoxides to obtain the
S. Enthaler (&) Á S. Krackl Á E. Irran Á S. Inoue
Department of Chemistry, Cluster of Excellence ‘‘Unifying
Concepts in Catalysis’’, Technische Universit a¨ t Berlin, Straße
des 17. Juni 135/C2, 10623 Berlin, Germany
2
4
sulfoxides which are equatorially coordinated to the zinc
via the oxygen of the sulfoxide functionality, while the
triflate groups are axially coordinated. Recently, we have
e-mail: stephan.enthaler@tu-berlin.de
123

1
004
S. Enthaler et al.
Scheme 1 Investigation of the deoxygenation of sulfoxide 1 with borane 4 in the presence of Zn(OTf)
2
set for C, F, H, O, and S atoms and the LANL2DZ for the Zn
atom [83–86]. The computed geometry is in good agree-
ment with the experimental values. While the HOMO of
Zn(dmso) (OTf) shows mainly the lone pair orbitals of
4
2
oxygen and sulfur atoms, it represents somewhat interaction
between the d-orbital of the zinc atom and the lone pair
orbitals of the oxygen atoms. The S–O bond lengths
˚
(1.5442–1.5480 A) of dimethylsulfoxide moieties in
Zn(dmso) (OTf) are obviously longer than that of ‘‘free’’
2
4
˚
dimethylsulfoxide (1.5114 A). Likewise, the WBI values of
the S–O bonds in dimethylsulfoxide (1.2780) are signifi-
cantly higher than those of dimethylsulfoxide moieties in
Zn(dmso) (OTf) (1.0876–1.0947). These results indicates
4
2
that the S=O bond of dimethylsulfoxide is activated by
coordination to the zinc center (Fig. 2).
To study the effect of activation of the sulfoxide by
complexation IR-measurements were performed with the
free sulfoxide 1 and the isolated complex 3 (Fig. 3). As
pointed out in Fig. 2 a shift of the S–O stretching vibration
was observed. The observed behavior is in agreement with
earlier reported systems and furthermore proves the coor-
dination of the sulfoxide via the oxygen atom to the zinc
center [87].
Fig. 1 Molecular structure of 3. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 50 % probability level.
˚
Selected distances (A): Zn–O1: 2.0552(19), Zn–O2: 2.256 (2),
O1–S1: 1.520(2), O2–S2: 1.459(2)
After investigation of the complexation of the sulfox-
reported a similar motif by using diphenyl sulfoxide as
substrate [72]. In contrast, decreasing the sterical demand
of the sulfoxide, e.g., dimethyl sulfoxide or methyl phenyl
sulfoxide, a different coordination mode was observed [80,
ide 1 to Zn(OTf) , pinacolborane (4, 2.5 equiv. with
2
respect to 1) was added to the mixture in C D
6
6
(Scheme 1). Subsequently, the reaction mixture was
heated for 1 hour to 100 ꢁC and afterwards slowly cooled
to room temperature, while colorless crystals were
obtained. The solid-state structure of complex 5 has been
characterized by single-crystal X-ray diffraction analysis.
Unfortunately, due to disordering the structure was not
fully solved. However, the preliminary solid structure of
Zn(OTf) (1) (5) showed an octahedral structure with four
8
1]. In more detail, the obtained solid structures Zn(sulf-
oxide) (OTf) showed an octahedral complexation of six
6
2
sulfoxides molecules to the cationic zinc center, while the
triflate anions are not coordinated to the metal.
A comparison of the S=O bond length in complex 3
˚ ˚
1.520(2) A] with the free sulfoxide 1 [1.492 (2) A] showed
[
2
6
an elongation of the bond by complexation [82]. Based on
that, the obtained structure 3 can be interpreted as an
intermediate for the activation of sulfoxides and probably it
initiates the subsequent reduction process to obtain the
corresponding sulfide. Moreover, to prove this assumption
theoretical investigations were carried out. DFT calcula-
tions of the model compound Zn(dmso) (OTf) (the tolyl
sulfoxides equatorially coordinated to the zinc via the
oxygen of the sulfoxide functionality and two sulfoxides
in the axial position, while the triflate groups are not
coordinated to the metal. In addition, IR measurements
-
1
revealed a band at 993 cm , which is comparable to the
value for complex 3.
Moreover, the proceeding of the reaction was investi-
4
2
1
groups at the sulfur were replaced by methyl groups,
because of simplicity) were performed at the 6-31G(d) basis
gated by NMR techniques (Fig. 3, 4). In case of H NMR
measurements characteristic signals for the methyl groups
1
23

Deoxygenation of Sulfoxides to Sulfides
1005
Fig. 2 IR-investigations of the
free sulfoxide 1 and the
complex 3
1
Fig. 3 H NMR investigation
of the deoxygenation of
sulfoxide 1 (1.0 equiv.,
0
.72 mmol) with borane 4 (2.5
equiv., 1.8 mmol) in the
presence of Zn(OTf)
5.0 mol %, 0.036 mmol) in
(0.5 mL)
2
(
6 6
C D
of the sulfoxide 1 (1.93 ppm) and the borane 4 (0.95 ppm)
were found after mixing the compounds at room temper-
ature. After heating for 1 h at 100 ꢁC the formation of the
sulfide 5 (1.95 ppm) and the bis-boryloxide O(Bpin)2
interaction of Zn(OTf) with the borane 4 was studied.
2
Zn(OTf) was reacted with 4 (10 equiv.) for 1 h at 80 ꢁC.
2
1
The signals for 2 and 4 were unchanged in H NMR,
19
F
1
1
NMR as well as B NMR. Based on this result, the for-
mation of a zinc-hydride species can be excluded on the
NMR time scale. Noteworthy, the reaction of the sulfoxide
(
0.97 ppm) was observed [88]. The bis-boryloxide
1
O(Bpin) was furthermore characterized by B NMR and
1
2
GC–MS. Afterwards the heating was continued for 23 h
1
Fig. 4). The measured H NMR spectrum showed an
with the borane in the absence of Zn(OTf) resulted in no
2
1
19
(
change of the H NMR or F NMR spectra and product
formation. With regard to the obtained results we excluded
increase of the product. On the other hand the reaction was
1
9
followed by F NMR to investigate the role of zinc(II)
triflate during catalysis. However, no significant change
was observed on the NMR time scale. Afterwards, the
the formation of a L Zn=O species, which was proposed
x
for different metals via the abstraction of the oxygen of the
sulfoxide to yield metal oxides [78].
123

1
006
S. Enthaler et al.
19
Fig. 4 F NMR investigation
of the deoxygenation of
sulfoxide 1 (1.0 equiv.,
0
.72 mmol) with borane 4 (2.5
equiv., 1.8 mmol) in the
presence of Zn(OTf)
5.0 mol %, 0.036 mmol) in
(0.5 mL)
2
(
6 6
C D
The produced sulfide is a potential ligand and can
influence significantly the catalyst activity; hence attempts
were undertaken to synthesize complexes with sulfide
ligands as well as mixed complexes with sulfoxide and
sulfide ligands (Scheme 2). Unfortunately, no suitable
complexes were accessible and crystal structures were only
obtained of the starting material (7) and the complex
Zn(OTf) (thf) (H O) (8) (Fig. 5). The solid-state structure
2
2
2
4
of complex 8 has been characterized by single-crystal
X-ray diffraction analysis. Unfortunately, due to disorder-
ing the structure was not fully refined.
Fig. 5 Molecular structure of 7. Thermal ellipsoids are drawn at the
˚
0 % probability level. Selected distances (A): S1–C1: 1.7769(14),
5
N1–O(1,2): 1.2294(15), N1–C4: 1.4630(17)
Based on the obtained results we assume that the zinc-
catalyst acts as a Lewis-acid (Scheme 3). First, the sulf-
oxide coordinates to the zinc center (intermediate A) thus
an activation of the S=O bond occurs and enhances the
susceptibility of the sulfur for reduction [62, 89]. Subse-
quently, the borane reacts with the intermediate A. An
interaction of the hydride with the sulfur and an interaction
of the boron with the oxygen of the triflate group via a six-
membered transition state is proposed (intermediate B).
Next a sulfonium salt analog is created (intermediate C),
Scheme 2 Investigation of
mixed sulfoxide–sulfide zinc
complexes
1
23

Deoxygenation of Sulfoxides to Sulfides
1007
Scheme 3 Proposed catalytic
cycle for the reduction of
sulfoxides with boranes in the
presence of Zn(OTf)
2
which undergoes elimination of bis-boryloxide O(Bpin) ,
2
3.1.1 Synthesis of Zn(OTf) (1)
2 4
1
hydrogen, and the desired sulfide [80].
After investigation of the reaction mechanism the scope
and limitations of the zinc-catalyzed deoxygenation of
various sulfoxides, including aromatic and aliphatic sulf-
oxides, using boranes as reducing reagent were examined
A mixture of Zn(OTf)
(0.036 mmol) and sulfoxide 1
2
(0.36 mmol) in C
D
6 6
(0.5 mL) was heated to 80 ꢁC until all
compounds were dissolved. Afterwards the temperature was
slowly decreased to room temperature. Colorless crystal
were obtained, which were suitable for X-ray measurements.
Mp: 162 ꢁC; IR (KBr): m = 1,660 w, 1,595 w, 1,452 w,
1,404 w, 1,271 s, 1,207 w, 1,179 m, 1,150 m, 1,119 w,
1,088 w, 1,032 m, 994 m, 814 w, 806 w, 769 w, 706 w,
(
Table 1). At first different boranes were tested in the
deoxygenation of 1. Excellent yields were observed for the
boranes 4 and 9, while with BH only moderate yields of 6
3
were obtained (Table 1, entries 1–3). Moreover, excellent
yields ([99 %) accompanied by excellent chemoselectiv-
ities were achieved for alkyl substituted sulfoxides after
-
1-
650 m, 640 m, 580 w, 518 cm
.2 Single-Crystal X-Ray Structure Determination
Crystals were each mounted on a glass capillary in per-
.
3
1
8 h at 100 ꢁC with a catalyst loading of 5.0 mol % using
borane 9 (Table 1, entries 6–10). To some extent lower
yields were noticed for aryl based sulfoxides (Table 1,
entries 4 and 5).
fluorinated oil and measured in a cold N flow. The data
2
were collected using an Oxford Diffraction Xcalibur S
˚
Sapphire at 150(2) K (MoKa radiation, l = 0.71073 A).
The structures were solved by direct methods and refined
2
on F with the SHELX-97 software package [90]. The
3
Experimental Section
positions of the hydrogen atoms were calculated and con-
sidered isotropically according to a riding model.
3
.1 General
CCDC-882111 (for 7) and 882112 (for 3) contains the sup-
plementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
All compounds were used as received without further
1
purification. H, F, and C NMR spectra were recorded
19
13
1
on a Bruker AFM 200 spectrometer ( H: 200.13 MHz; F:
19
1
88.31 MHz; C: 50.32 MHz) using the proton signals of
3
1
the deuterated solvents as reference. GC–MS measure-
ments were carried out on a Shimadzu GC-2010 gas
chromatograph (30 m Rxi-5 ms column) linked with a
Shimadzu GCMA-QP 2010 Plus mass spectrometer.
3
.3 General Procedure for the Deoxygenation
of Sulfoxides
A pressure tube was charged with an appropriate amount
of Zn(OTf)2 (0.036 mmol, 5.0 mol %), the correspond-
ing sulfoxide (0.72 mmol) and the borane (2.5 equiv.,
1
Hydrogen was not detectable during the 1H NMR studies
123

1
008
S. Enthaler et al.
Table 1 Scope and limitations of the zinc-catalyzed deoxygenation of sulfoxides
2
1
2
1
2
[
b]
Entry
1
Substrate
Borane
Product
Yield [%]
6
>99 (93)
>99 (94)
1
9
2
1
1
6
4
[c]
3
BH
3
•THF 10
6
57
4
5
9
9
11a
85 (81)
1
1
1
12a
89 (83)
1
2
6
7
9
9
13a
14a
>99 (91)
>99 (89)
3
1
4
8
9
9
9
15a
16a
17a
>99 (93)
>99
1
5
[d]
9
1
6
1
0
>99 (95)
1
7
a
b
c
1
Determined by GC methods and H NMR. In brackets the isolated yield is stated
Reaction was performed in THF at 60 ꢁC for 24 h. BH •THF stock solution (*1.0 M in THF)
The product 16a has been characterized only by GC–MS
3
Reaction conditions: Substrate (0.72 mmol), Zn(OTf)
2
(5 mol %), borane (2.5 equiv. 1.80 mmol), toluene (2.0 mL), 18 h, 100 ꢁC
1
3
1
.80 mmol). After addition of toluene (2.0 mL) the reaction
mixture was stirred in a preheated oil bath at 100 ꢁC for
8 h. The mixture was cooled on an ice bath and biphenyl
internal standard) was added. The solution was diluted with
dichloromethane and an aliquot was taken for GC-analysis
30 m Rxi-5 ms column, 40–300 ꢁC). The solvent was
C NMR (CDCl , 50 MHz): d = 137.1, 132.8, 131.3,
3
?
130.1, 21.3 ppm; MS (ESI) m/z = 214 (100, M ), 199 (34),
184 (20), 105 (18), 91 (33), 65 (19); IR (KBr): m = 1,662 w,
1,594 w, 1,493 m, 1,448 w, 1,400 w, 1,381 w, 1,307 w,
1,286 w, 1,118 w, 1,090 m, 1,076 w, 1,039 s, 1,014 m,
970 w, 944 w, 849 w, 816 s, 805 s, 708 w, 635 w, 625 w,
615 w, 534 m, 501 m.
1
(
(
removed and the residue was purified by column chroma-
tography. The analytical properties of the corresponding
sulfides are in agreement with literature data.
3.3.2 Diphenylsulfide (11a)
3
.3.1 Di(p-Methylphenyl)sulfide (6)
1
R = 0.75 (n-hexane:ethyl acetate 1:5); H NMR (CDCl ,
f
3
1
R = 0.79 (n-hexane:ethyl acetate 1:5); H NMR (CDCl ,
13
200 MHz): d = 7.26–7.47 ppm (m, 10H);
C NMR
f
3
2
00 MHz): d = 7.06–7.22 ppm (m, 8H), 2.30 (s, 6H, CH3);
(CDCl , 50 MHz): d = 135.8, 131.0, 129.2, 127.0 ppm;
3
1
23

Deoxygenation of Sulfoxides to Sulfides
1009
?
MS (ESI) m/z = 186 (100, M ), 152 (11), 92 (19), 77 (27),
References
6
5 (20), 51 (43).
1
. Beller M, Bolm C (eds) (2004) Transition metals for organic
synthesis. Wiley-VCH, Weinheim
2. Cornils B, Herrmann WA, Horvath IT, Leitner W, Mecking S,
´
3
.3.3 Di(p-Chlorophenyl)sulfide (12a)
Olivier-Bourbigou H, Vogt D (eds) (2005) Multiphase homoge-
neous catalysis. Wiley-VCH, Weinheim
. Enthaler S, Junge K, Beller M (2008) Angew Chem 120:3363–
3367
. Enthaler S, Junge K, Beller M (2008) Angew Chem Int Ed
47:3317–3321
. Mimoun H, De Saint Laumer JY, Giannini L, Scopelliti R,
Floriani C (1999) J Am Chem Soc 121:6158–6166
. Mimoun H (1999) J Org Chem 64:2582–2589
. Bette V, Mortreux A, Lehmann CW, Carpentier J-F (2003) Chem
Commun 332–333
1
R = 0.67 (n-hexane:ethyl acetate 1:10); H NMR (CDCl ,
f
3
3
4
5
1
3
2
00 MHz): d = 7.23–7.33 ppm (m, 8H);
C NMR
(
CDCl , 50 MHz): d = 133.9, 133.4, 132.3, 129.4 ppm;
3
?
MS (ESI) m/z = 254 (M , not detected), 219 (33), 184
100), 139 (11), 108 (29), 91 (18), 75 (24).
(
6
7
3
.3.4 Dibenzylsulfide (13a)
8
. Bette V, Mortreux A, Ferioli F, Martelli G, Savoia D, Carpentier
J-F (2001) Eur J Org Chem 3040–3045
1
R = 0.74 (n-hexane:ethyl acetate 1:5); H NMR (CDCl ,
f
3
1
3
9. Bette V, Mortreux A, Savoia D, Carpentier J-F (2004) Tetrahe-
dron 60:2837–2842
0. Mastranzo VM, Quinterno L, Anaya de Parrodi C, Juaristi E,
Walsh PJ (2004) Tetrahedron 60:1781–1789
2
00 MHz): d = 7.31–7.37 (m, 10H), 3.65 ppm (s, 4H);
C
NMR (CDCl , 50 MHz): d = 138.0, 128.9, 128.4, 126.9,
3
1
?
5.5 ppm; MS (ESI) m/z = 214 (59, M ), 123 (31), 91
3
1
1
1. Ushio H, Mikami K (2005) Tetrahedron Lett 46:2903–2906
2. Bette V, Mortreux A, Savoia D, Carpentier J-F (2005) Adv Synth
Catal 347:289–302
(
100), 65 (17).
1
3. G e´ rard S, Pressel Y, Riant O (2005) Tetrahedron: Asymmetry
3
.3.5 Dibutylsulfide (14a)
1
4. Park B-M, Mun S, Yun J (2006) Adv Synth Catal 348:1029–1032
6:1889–1891
1
1
H NMR (CDCl , 200 MHz) d = 2.28 (t, 4H, J = 7.20 Hz,
3
15. Bandini M, Melucci M, Piccinelli F, Sinisi R, Tommasi S,
Umani-Ronchi, A (2007) Chem Commun 4519–4521
S(CH CH CH CH ) ), 1.14–1,49 (m, 8H, S(CH CH CH
2
1
CH ) ), 0.75 ppm (t, 6H, J = 7.20 Hz, CH ); C NMR
2
2
2
3 2
2
3
2
1
6. Inagaki T, Yamada Y, Phong LT, Furuta A, Ito J-i, Nishiyama H
2009) Synlett 253–256
7. Gajewy J, Kwit M, Gawro n´ ski J (2009) Adv Synth Catal 351:
055–1063
3
2
3
(
(
(
(
CDCl , 50 MHz) d = 32.1, 32.0, 22.3, 13.8 ppm; MS
3
1
?
ESI) m/z = 146 (M , 40), 103 (15), 90 (31), 61 (97), 56
1
100).
18. Das S, Addis D, Zhou S, Junge K, Beller M (2010) J Am Chem
Soc 132:1770–1771
1
9. Bandini M, Cozzi PG, de Angelis M, Umani-Ronchi A (2000)
Tetrahedron Lett 41:1601–1605
0. Locatelli M, Cozzi PG (2003) Angew Chem Int Ed 42:4928–4930
3
.3.6 Tetrahydrothiophene (15a)
2
1
H NMR (CDCl , 200 MHz): d = 2.42–2.55 (m, 4H),
21. Marinos NA, Enthaler S, Driess M (2010) ChemCatChem 2:
46–853
22. Enthaler S, Eckhardt B, Inoue S, Irran E, Driess M (2010) Chem
Asian J 5:2027–2035
3
1
3
8
1
.36–1.48 ppm (m, 4H); C NMR (CDCl , 50 MHz):
3
?
d = 32.1, 32.0 ppm; MS (ESI) m/z = 87 (13, M ), 59(49),
4
3 (100).
2
3. Enthaler S, Schr o¨ der K, Inoue S, Eckhardt B, Junge K, Beller M,
Driess M (2010) Eur J Org Chem 4893–4901
4. Enthaler S (2011) Catal Lett 141:55–61
5. Someya CI, Inoue S, Irran E, Krackl S, Enthaler S (2011) Eur J
Inorg Chem 2691–2697
6. Enthaler S, Weidauer M, Schr o¨ der F (2012) Tetrahedron Lett
2
2
3
.3.7 2,4-Dithiapentane (16a)
?
MS (ESI) m/z = 110 (14, M ), 108 (99), 61 (100).
2
5
3:882–885
2
2
2
7. Enthaler S, Weidauer M (2012) Catal Lett 142:168–175
8. Enthaler S, Inoue S (2012) Chem Asian J 7:169–175
9. Enthaler S, Weidauer M (2012) Chem Eur J 18:1910–1913
3
.3.8 Phenyl Methylsulfide (17a)
1
R = 0.88 (n-hexane:ethyl acetate 1:5); H NMR (CDCl ,
f
3
30. Someya CI, Inoue S, Krackl S, Irran E, Enthaler S (2012) Eur J
Inorg Chem 1269–1277
1
3
2
00 MHz): d = 7.19–7.35 (m, 5H), 2.53 ppm (s, 3H);
C
3
3
3
1. Haberberger M, Someya CI, Company A, Irran E, Enthaler S
(2012) Catal Lett 142:557–565
2. Hille R, Re e` tey J, Bartlewski-Hof U, Reichenbecher W, Schink B
(1999) FEMS Microbiol Rev 22:489–501
3. Hille R (1996) Chem Rev 96:2757–2816
NMR (CDCl , 50 MHz): d = 138.3, 128.7, 126.5, 124.9,
3
?
5.7 ppm; MS (ESI) m/z = 124 (100, M ), 109 (51), 91
1
(
49), 78 (51), 85 (21).
Acknowledgments Financial support from the Cluster of Excel-
lence ‘‘Unifying Concepts in Catalysis’’ (funded by the Deutsche
Forschungsgemeinschaft and administered by the Technische Uni-
versit a¨ t Berlin) is gratefully acknowledged.
34. Kisker C, Schindelin H, Rees DC (1997) Annu Rev Biochem
66:233–267
35. Enemark JH, Cooney JJA, Wang J-J, Holm RH (2004) Chem Rev
104:1175–1200
123

1
3
010
S. Enthaler et al.
6. McGarrigle EM, Myers EL, Illa O, Shaw MA, Riches SL, Ag-
garwal VK (2007) Chem Rev 107:5841–5883
68. Sousa SCA, Fernandes AC (2009) Tetrahedron Lett 50:6872–
6876
3
3
7. Rickard D, Luther GW III (2007) Chem Rev 107:514–562
8. Nicolas E, Vilaseca M, Giralt E (1995) Tetrahedron 51:5701–
69. Mikami Y, Noujima A, Mitsudome T, Mizugaki T, Jitsukawa K,
Kaneda K (2011) Chem Eur J 17:1768–1772
5
710
70. Cardoso JMS, Royo B (2012) Chem Commun 48:4944–4946
71. Enthaler S (2011) ChemCatChem 3:666–670
72. Enthaler S (2011) Catal Sci Technol 1:104–110
73. Enthaler S, Weidauer M (2011) Catal Lett 141:833–838
74. Krackl S, Company A, Enthaler S, Driess M (2011) ChemCat-
Chem 3:1186–1192
75. Zaidlewicz M, Pakulski MM (2011) In: De Vries JG, Molander
GA, Evans AP (eds) Science of synthesis, stereoselective syn-
thesis, vol 2. Thieme, Stuttgart, pp 59–131
3
4
9. Nicolau KC, Kuombis AE, Synder SA, Simonsen KB (2000)
Angew Chem 112:2629–2633
0. Nicolau KC, Kuombis AE, Synder SA, Simonsen KB (2000)
Angew Chem Int Ed 39:2529–2533
1. Zhu Z, Espenson JH (1995) J Mol Catal A Chem 103:87–94
2. Reis PM, Costa PJ, Rom a˜ o CC, Fernandes JA, Calhorda MJ,
Royo B (2008) Dalton Trans 1727–1733
4
4
4
4
4
4
4
4
4
3. Bahrami K, Khodaei MM, Karimi A (2008) Synthesis 2008,
2
543–2546
4. Khurana JM, Sharma VS, Chacko A (2007) Tetrahedron 63:966–
69
5. Yoo BW, Park MC, Song MS (2007) Synth Commun 37:4079–
083
6. Yoo BW, Song MS, Park MC (2007) Bull Korean Chem Soc
8:171–172
7. Yoo BW, Song MS, Park MC (2007) Synth Commun 37:3089–
093
8. Pandey LK, Pathak U, Rao AN (2007) Synth Commun 37:4105–
109
9. Bahrami K, Khodaei MM, Khedri M (2007) Chem Lett 1324–
325
76. Fernandes C, Rom a˜ o CC (2007) Tetrahedron Lett 48:9176–9179
77. Cabrita I, Sousa SSA, Fernandes AC (2010) Tetrahedron Lett
51:6132–6135
78. Fernandes C, Fernandes JA, Rom a˜ o CC, Veiros LF, Calhorda MJ
(2010) Organometallics 29:5517–5525
79. Fernandes C, Fernandes JA, Paz FAA, Rom a˜ o CC (2008) Dalton
Trans 6686–6688
80. Koshino N, Espenson JH (2003) Inorg Chem 42:5735–5742
81. Persson I (1982) Acta Chem Scand A 36:7–13
9
4
2
3
82. Fuller L, Aitken RA, Ryan BM, Slawin AMZ, Woollins JD
(2009) J Chem Crystallogr 39:407–415
4
83. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Montgomery JA Jr, Vreven T, Kudin KN, Burant
JC, Millam JM, Iyengar SS, Tomasi J, Barone V, Mennucci B,
Cossi M, Scalmani G, Rega N, Petersson GA, Nakatsuji H, Hada
M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nak-
ajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X, Knox JE,
Hratchian HP, Cross JB, Bakken V, Adamo C, Jaramillo J,
Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R,
Pomelli C, Ochterski JW, Ayala PY, Morokuma K, Voth GA,
Salvador P, Dannenberg JJ, Zakrzewski VG, Dapprich S, Daniels
AD, Strain MC, Farkas O, Malick DK, Rabuck AD, Raghava-
chari K, Foresman JB, Ortiz JV, Cui Q, Baboul AG, Clifford S,
Cioslowski J, Stefanov BB, Liu G, Liashenko A, Piskorz P,
Komaromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng
CY, Nanayakkara A, Challacombe M, Gill PMW, Johnson B,
Chen W, Wong MW, Gonzalez C, and Pople JA (2004) Gaussian
03, Revision E.01. Gaussian Inc., Wallingford CT
1
5
5
5
0. Fernandes AC, Rom a˜ o CC (2006) Tetrahedron 62:9650–9654
1. Roy CD, Brown HC (2006) J Chem Res 10:642–644
2. Raju BR, Devi G, Nongpluh YS, Saikia AK (2005) Synlett
3
58–360
5
5
3. Espenson JH (2005) Coord Chem Rev 249:329–341
4. Sanz R, Escribano J, Fern a´ ndez Y, Aguado R, Pedrosa MR,
Arn a´ iz FJ (2004) Synthesis 1629–1632
5. Harrison DJ, Tam NC, Vogels CM, Langler RF, Baker RT,
Decken A, Westcott SA (2004) Tetrahedron Lett 45:8493–8496
6. Kukuskin VY (1995) Coord Chem Rev 139:375–407
7. Yoo BW, Choi KH, Kim DY, Choi KI, Kim JH (2003) Synth
Commun 33:53–57
5
5
5
5
8. Arias J, Newlands CR, Abu-Omar MM (2001) Inorg Chem 40:
185–2192
2
5
6
6
9. Abu-Omar MM, Khan SI (1998) Inorg Chem 37:4979–4985
0. Arterburn JB, Perry MC (1996) Tetrahedron Lett 37:7941–7944
1. Abu-Omar MM, Appelman EH, Espenson JH (1996) Inorg Chem
84. Becke D (1993) J Chem Phys 98:5648–5652
85. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785–789
86. Miehlich B, Savin A, Stoll H, Preuss H (1989) Chem Phys Lett
157:200–206
87. Currier WF, Weber JH (1967) Inorg Chem 6:1539–1543
88. Hawkeswood S, Stephan DW (2005) Dalton Trans 2182–2187
89. Drabowicz J, Mikolajczyk M (1976) Synthesis 527–528
90. Sheldrick GM (1997) SHELXL93 Program for the refinement of
crystal structures. University of G o¨ ttingen, G o¨ ttingen
3
5:7751–7757
6
6
6
2. Kukuskin VY (1990) Russ Chem Rev 59:844–852
3. Madesclaire M (1988) Tetrahedron 44:6537–6551
4. Bryan JC, Stenkamp RE, Tulip TH, Mayer JM (1987) Inorg
Chem 26:2283–2288
5. Cha JS, Kim JE, Kim JD (1985) Tetrahedron Lett 26:6453–6456
6. Brown HC, Ravindran N (1973) Synthesis 42–43
7. Guidon Y, Atkinson JG, Morton HE (1984) J Org Chem 49:
6
6
6
4
538–4540
1
23