Catalytic diboration of unsaturated molecules with platinum(0)-NHC: Selective synthesis of 1,2-dihydroxysulfones

Article abstract of DOI:10.1021/om060666n

Readily available N-heterocyclic platinum complexes provide active catalytic species for 1,2-diboration and anticipate a wide range of applications in alkenes and alkynes. Unprecedented catalytic B - B addition of heteroatom-containing sub-strates provided high selectivity into organosulfur - diboronate esters. Their in situ basic oxidation afforded the desired 1,2-dihydroxysulfones in high yields.

Full text of DOI:10.1021/om060666n

Organometallics 2006, 25, 5829-5831
5829
Catalytic Diboration of Unsaturated Molecules with
Platinum(0)-NHC: Selective Synthesis of 1,2-Dihydroxysulfones
Vanesa Lillo,† Jose Mata,*,‡ Jesu´s Ram´ırez,† Eduardo Peris,‡ and Elena Fernandez*,†
Departamento Qu´ımica F´ısica i Inorga`nica, UniVersitat RoVira i Virgili, Marcel.l´ı Domingo s/n,
43007 Tarragona, Spain and Departamento Qu´ımica Inorga`nica i Orga`nica, UniVersitat Jaume I,
AVenida Vicente Sos Baynat s/n, 12080 Castello´n, Spain
ReceiVed July 26, 2006
Scheme 1
Summary: Readily aVailable N-heterocyclic platinum complexes
proVide actiVe catalytic species for 1,2-diboration and anticipate
a wide range of applications in alkenes and alkynes. Unprece-
dented catalytic B-B addition of heteroatom-containing sub-
strates proVided high selectiVity into organosulfur-diboronate
esters. Their in situ basic oxidation afforded the desired 1,2-
dihydroxysulfones in high yields.
The special electronic and steric properties induced by
N-heterocyclic carbenes (NHCs) are affording new bases for
the preparation of complexes with enhanced catalytic activity,
which now have widespread applications in a large number of
organic reactions.1 In the field of catalyzed diboration reactions,
the success of the process is based on the cleavage of the B-B
bond by low-valent metal centers2 and the kinetic lability of
the resulting bis(boryl) complexes.3 The combination of these
two facts guarantees the activation of tetraalkoxy- and tetraaryl-
oxydiboranes by oxidative addition, despite their relatively high
B-B bond energy,4 and also the B-C reductive elimination to
afford synthetically suitable stereodefined organo-1,2-diborane
compounds.5 The acceptance of the advantageous metal-
promoted 1,2-diboration versus the uncatalyzed reaction6 has
motivated researchers to find a suitable active catalytic system
since Miyaura and Suzuki’s first report.7 Even the asymmetric
version of the reaction has recently acquired special signifi-
cance.8 While all efforts have been focused on metal-phosphine
complexes,9 we recently described a series of NHC-based Ag(I)
and Au(I) complexes that cleanly performed B-B addition to
alkenes.10 Our ongoing research in this field is now aimed at
using a series of Pt(0)-NHC complexes in the 1,2-diboration
of alkynes and alkenes. We point out the first example of B-B
Scheme 2
addition to aryl allylic sulfones to achieve, after oxidative
workup, an alternative method for obtaining 1,2-dihydroxy-
sulfones as valuable intermediates for functionalized cyclic
ethers and lactones11 (Scheme 1).
The NHC-platinum compounds 1-3 were synthesized by
transmetalation from the corresponding silver carbenes, as
shown in Scheme 2. The three complexes were obtained to study
how the electronic nature of the NHC ligand affects the catalytic
performances. Complex 3 has already been reported,12a although
we have changed the preparation procedure. To the best of our
knowledge, this is the first example of a silver transmetalation
to platinum(0). Complexes 1 and 2 were characterized by NMR
* Corresponding author. E-mail: mariaelena.fernandez@urv.net.
† Universitat Rovira i Virgili.
‡Universitat Jaume I.
(1) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291. (b)
Peris, E.; Crabtree, R. H. C. R. Chimie 2003, 6, 33. (c) Peris, E.; Crabtree,
R. H. Coord. Chem. ReV. 2004, 248, 2239. (d) Crudden, C. M.; Allen, D.
P. Coord. Chem. ReV. 2004, 248, 2247.
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T. B. J. Am. Chem. Soc. 1993, 115, 4367. (b) Nguyen, P.; Lesley, G.; Taylor,
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T. B.; Norman, N. C.; Rice, R. C. Main Group Chem. News 1997, 5, 4.
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Chem. Commun. 2005, 3056. (b) Corbera´n, R.; Ram´ırez, J.; Poyatos, M.;
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10.1021/om060666n CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/21/2006

5830 Organometallics, Vol. 25, No. 24, 2006
Table 1. 1,2-Diboration of Alkynes with Pt(0)-NHCa
Notes
entry
Pt(0)-NHC
substrate
diboron
time (h)
conversion (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
2c
2d
2
1
3
2
2
2
2
2
2
2
2
2
PhCCPh
PhCCPh
PhCCPh
PhCCPh
PhCCPh
PhCCPh
p-(CF3)PhCCPhp(CF3)
p-(CF3)PhCCPhp(OMe)
p-(OMe)PhCCPh
PhCCH
B2(pin)2
B2(pin)2
B2(pin)2
B2(pin)2
B2(pin)2
B2(cat)2
B2(cat)2
B2(cat)2
B2(cat)2
B2(pin)2
B2(cat)2
B2(cat)2
B2(cat)2
B2(cat)2
24
24
24
24
24
5
5
5
5
24
8
4
39
60
41
19
>95
>95
50
57
3
PhCCH
99
99
99
99
p(CF3)PhCCH
p(Cl)PhCCH
p(OMe)PhCCPh
8
8
8
b
a Standard conditions: 5 mol % of Pt(0)-NHC catalytic system, alkyne/diboron, 1:1.5, T 25 °C. Conversion on cis-alkenyl bis(boronate) esters, based
1
c
d
on H NMR. Solvent: THF. Solvent: CH3CN. Solvent: toluene.
Table 2. 1,2-Diboration of Alkenes with Pt(0)-NHC and
a
B2(cat)2
conv
(%)b
bis(boryl) alkane
(%)b
Pt(0)-NHC
substrate
PhCHdCH2
PhCHdCH2
PhCHdCH2
PhCHdCH2
PhCHdCH2
p-(F)PhCHdCH2
p-(MeO)PhCHdCH2
vinylcyclohexene
3,3-dimethylbutene
1
2
3
88
97
94
85
42
66
54
58
2c
2d
2
91
85
100
100
67
62
21
11
2
2
2
a Standard conditions: 5 mol % of Pt(0)-NHC catalytic system; alkene/
b
diboron, 1:1.1, T 25 °C reaction time 4 h, solvent THF. Conversion and
1
c
d
selectivity based on H NMR. Solvent: toluene. Solvent: CH3CN.
other imidazolylidenes and electronically similar to phosphines,
as has been observed for other chlorinated-NHC complexes.13
Figure 1. Molecular diagram of 1 (50% probability level, H atoms
omitted). Selected bond lengths (Å) and angles (deg): Pt(1)-C(1)
2.037(6), Pt(1)-C(4) 2.093(7), Pt(1)-C(5) 2.129(6), Cl(1)-C(2)
1.689(7), Cl(2)-C(3) 1.682(7), C(2)-C(3) 1.354(10), C(4)-C(5)
1.407(9), C(6)-C(7) 1.418(9); C(4)-Pt(1)-C(1)-N(2) 78.6(6),
C(6)-Pt(1)-C(1)-N(2) -102.1(6), R 80.3. R ) angle between the
imidazolium ring plane and the xy plane of the metal complex.
The Pt(0)-NHC complex 2 was initially tested in the
diboration of alkynes using diphenylacetylene and phenylacetyl-
ene as model substrates for internal and terminal alkynes,
respectively. As shown in Table 1, THF was the most suitable
solvent for conducting (at room temperature) the clean addition
of bis(pinacolato)diboron, B2(pin)2, to alkynes with the forma-
tion of the cis-alkene bis(boronate) esters (entries 1-3). How-
ever, conversion depends on the electronic properties of the
NHC ligand, and the electron-releasing triazolylidene carbene
ligand in complex 2 was the most suitable, as expected (entries
3-5). Thus, we selected complex 2 for further studies, and the
use of the most reactive bis(catecholato)diboron, B2(cat)2,
allowed quantitative conversions of diphenylacetylene in 5 h
(entry 6).
When the significance of the electronic parameters in this
process was reinforced, we observed that the less electron rich
the diarylalkyne was, the higher the product yield (Table 1,
entries 7-9). This is in agreement with Marder’s observations.14a
Remarkably, complex 2 performed a total conversion on
diborated product, with the less reactive substrate, phenylacetyl-
ene, tolerating even electron-donating and electron-withdrawing
moieties in the substrate (entries 10-14). Conversions were
and elemental analysis. The most significant feature of the
13C{1H} NMR spectrum is the signal at δ 185.5 (1) and 183.5
(2), with coupling constants that confirm Pt binding (1JPt-C
1392 Hz, 1; JPt-C ) 1390 Hz, 2).
)
1
Single crystals of 1 were obtained only by slow evaporation
of a concentrated dichloromethane solution. The molecular
diagram (Figure 1) shows that the coordination is trigonal planar
at the Pt(0) center. The angle of the plane defined by the azole
ring is almost perpendicular to the platinum coordination plane
(R ) 80.3°). The shortest interactions between both ligands arise
from the pseudoaxial methyl groups of the dvtms and the
wingtips of the NHC ligand. The platinum-carbene distance
is in the expected range (Pt-Ccarbene ) 2.037(6) Å). The CdC
bond distance of the terminal vinyl groups provides a direct
measurement of platinum back-bonding in the olefin delocalized
π* orbital. Marko´ et al. have reported a series of similar com-
plexes for which the CdC distances range from 1.42 to 1.49 Å,
depending on the nature of the imidazolylidene ring.12 In our
case, compound 1 has a CdC bond length of 1.354 Å. This is
in accordance with the fact that the electron-donating character
of the carbene is lower than that of other NHC-Pt(0) complexes
reported in the literature,12 because of the introduction of the
two chlorine substituents. In fact, the 4,5-dichloroimidazolyli-
dene carbene can be considered to be topologically similar to
(13) (a) Jackstell, R.; Harkal, S.; Jiao, H. J.; Spannenberg, A.; Borgmann,
C.; Rottger, D.; Nierlich, F.; Elliot, M.; Niven, S.; Cavell, K.; Navarro, O.;
Viciu, M. S.; Nolan, S. P.; Beller, M. Chem.-Eur. J. 2004, 10, 3891-
3900. (b) Oldham, W. J.; Oldham, S. M.; Scott, B. L.; Abney, K. D.; Smith,
W. H.; Costa, D. A. Chem. Commun. 2001, 1348-1349. (c) Viciano, M.;
Mas-Marza´, E.; Sanau´, M.; Peris, E. Organometallics 2006, 25, 3063.
(14) (a) Thomas, R. L.; Souza, F. E. S.; Marder, T. B. J. Chem. Soc.,
Dalton Trans. 2001, 1650-1656. (b) Iverson, C. N.; Smith, M. R., III.
Organometallics 1996, 15, 5155-5165.

Notes
Organometallics, Vol. 25, No. 24, 2006 5831
Table 3. 1,2-Diboration of Aryl Allylic Sulfones with Pt(0)-NHCa
arylallylic sulfone/
diboron
conv
(%)b
bis(boryl)
Pt(0)-NHC
substrate
alkane sulfone (%)b
1
2
3
1
1
1
PhSO2CH2CHdCH2
PhSO2CH2CHdCH2
PhSO2CH2CHdCH2
PhSO2CH2CHdCH2
p-(Me)PhSO2CH2CHdCH2
p-(Cl)PhSO2CH2CHdCH2
1:1.1
1:1.1
1:1.1
1:2
1:2
1:2
90
69
87
100
100
100
75
71
85
90
91
94
a Standard conditions: 5 mol % of Pt(0)-NHC catalytic system; diboron, bis(pinacolato)diboron (B2(pin)2); aryl allylic sulfone/diboron, 1:1.1, T 80 °C,
b
1
reaction time 16 h, Solvent toluene. Conversion and selectivity based on H NMR.
calculated on the basis of the 1H NMR. However, to test whether
the substrate could be pumped off during the sample preparation,
we carried out the same catalytic reaction under d8-THF, and
no changes in the conversion were detected. To broaden the
scope of the catalytic performance of our Pt(0)-NHC com-
plexes, we went on to study alkenes. Complex 2 was found to
be an excellent catalyst precursor that allowed styrene to be
diborated at room temperature within 4 h (Table 2), with THF
being the solvent of choice. However, other alkylborane
byproducts were observed, probably arising from â-H elimina-
tion pathways that compete with the diboration catalytic cycle.9
Electron-poor vinylarenes showed a slight decrease in bis-
(boryl)alkane formation. Extension to aliphatic alkenes was also
possible, but greater percentages of byproducts were formed.
Thus, platinum-carbene-catalyzed diboration improves the
terminal group in the allylic system facilitated the simultaneous
B-B catalytic addition to the CdC bond without any undesired
byproduct formation due to the isomerization of the double bond.
As shown in Table 3, B2(pin)2 was added quantitatively under
optimized conditions (toluene, 80 °C, 16 h), in the presence of
complexes 1-3, and catalytic system 1 proved to be the most
suitable for this transformation. However, small percentages of
monoalkylboranes were observed as a subsequent â-H elimina-
tion competitive pathway. It is interesting to note that the aryl
allylic sulfones could not be diborated with metal-phosphine
catalytic systems based on rhodium or platinum complexes such
as [RhCl(PPh3)3], [Rh(cod)(PPh3)3]BF4, or Pt(PPh3)4. An excess
of diboron leads to total conversion, even with electron-donating
or electron-withdrawing moieties in the substrate.
This study opens up new perspectives for future applications
of metal-mediated synthesis toward chiral nonracemic 1,2-
dihydroxysulfones when asymmetry can be induced through the
chiral N-heterocyclic carbenes, as an alternative to the Sharpless
asymmetric dihydroxylation method or the hydrolytic resolution
of epoxysulfones with Jacobsen’s catalyst. We are also studying
the in situ intramolecular cyclization of bis(boryl)alkyl sulfones
to provide suitable routes for preparing functionalized dihydro-
furans.
B-B addition with Pt(PPh3)4 and [Pt(PPh3)2(C2H4)]16 and is
15
an alternative to the more classical base-free platinum17 and
mono(phosphine)platinum complexes,14 although the latter
provided slightly faster performances.
The Pt-NHC-catalyzed 1,2-diboration reaction makes it
possible for us to use our preliminary findings on alkynes and
alkenes to generate a useful method for preparing bis(boryl)-
alkane sulfones that can be easily converted into 1,2-dihydroxy-
sulfones through in situ oxidation with hydrogen peroxide in a
basic medium. Heteroatom-containing substrates,18 such as aryl
allylic sulfones, have been hydroborated only in a symmetric18b
and asymmetric18c catalytic way to provide, after in situ
oxidation, the branched alcohol as a consequence of the directing
Acknowledgment. The authors are grateful for the financial
support from the CICYT of Spain (CTQ2004-04412/BQU and
CTQ2005-05187/BQU), Generalitat Valenciana (GV05/107) and
Bancaixa-UJI (P1.1B2001-03 and P1.1A2004-05). V.L. thanks
the MEC and J.R. thanks the Generalitat de Catalunya for
fellowships. J.M. thanks the program “Ramon y Cajal”.
effects of the sulfone functional group. This electron-attracting
Supporting Information Available: Experimental procedures
and crystallographic data are available free of charge via the Internet
at http://pubs.acs.org.
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OM060666N
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