Synthesis of biaryl diphosphines via a stepwise regioselective double diels-alder cycloaddition-elimination sequence: Efficient ligands for the palladium-catalyzed amination of aromatic bromides

Article abstract of DOI:10.1021/om9004862

Tropos and atropos biaryl diphosphines have been prepared in a stepwise, highly regioselective double Diels-Alder cycloaddition-elimination sequence between 1,4-bis(diphenylphosphinoyl)buta-l,3-diyne and various 1,3-dienes; the unsymmetrical phosphine 2,6

Full text of DOI:10.1021/om9004862

Organometallics 2009, 28, 5273–5276 5273
DOI: 10.1021/om9004862
Synthesis of Biaryl Diphosphines via a Stepwise Regioselective Double
Diels-Alder Cycloaddition-Elimination Sequence: Efficient Ligands for
the Palladium-Catalyzed Amination of Aromatic Bromides
Simon Doherty,* Catherine H. Smyth,* Ross W. Harrington, and William Clegg
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, U.K.
Received June 9, 2009
Summary: Tropos and atropos biaryl diphosphines have been
prepared in a stepwise, highly regioselective double Diels-
Alder cycloaddition-elimination sequence between 1,4-bis-
(diphenylphosphinoyl)buta-1,3-diyne and various 1,3-dienes;
the unsymmetrical phosphine 2,60-bis(diphenylphosphino)-20-
methoxy-1,10-biphenyl prepared using this approach forms an
efficient catalyst for the amination of a range of aromatic
bromides, giving conversions that rival those obtained with its
BIPHEP counterpart.
1,3-butadiyne and a selection of 1,n-tethered diynes affords
highly substituted biaryl diphosphines,4a and an asymmetric
version of this reaction was subsequently applied to the
synthesis of axially chiral biaryl phosphines and phospho-
nates with exceptionally high enantioselectivity.4b,c Rho-
dium-catalyzed [2 þ 2 þ 2] cycloaddition between oxides
and sulfides of 1-alkynylphosphines and 1,n-tethered diynes
has also been used to prepare bulky biaryl monophosphines5
as well as tetra-ortho-substituted axial chiral biaryl mono-
phosphines6 and P-stereogenic monoaryl phosphines,7 the
latter via rhodium-catalyzed desymmetrization of dialkynyl-
phosphine oxides. Tetra-ortho-substituted biaryl monopho-
sphines have also been prepared via a Diels-Alder cycload-
dition extrusion sequence between 1-alkynylphosphine
oxides and oxygenated 1,3-dienes;8 palladium complexes of
these phosphines are highly efficient catalysts for C-C
and C-N coupling.9 We have now demonstrated that the
corresponding double Diels-Alder cycloaddition-extru-
sion between 1,4-bis(diphenylphosphinoyl)buta-1,3-diyne
and 1,3-dienes occurs in a stepwise, and highly regioselective,
manner to afford symmetrical and unsymmetrical tropos
and atropos biaryl diphosphine oxides, an approach that will
complement existing methodologies and provide access to a
host of new biaryl-based ligands.
By analogy with the Diels-Alder approach developed by
Carter,8 we reasoned that the corresponding cycloaddition-
extrusion between 1,4-bis(diphenylphosphinoyl)buta-1,3-
diyne (1) and 1-methoxy-1,3-cyclohexadiene would afford
the oxide of MeO-BIPHEP (4) (Scheme 1).10 Gratifyingly,
this approach proved successful, and thermolysis of a mix-
ture of 1 and 1-methoxy-1,3-cyclohexadiene at 150 °C for
20 h in a sealed vessel resulted in a highly regioselective
double cycloaddition-extrusion to afford 4 as the sole
product; its identity was established by comparison of the
spectroscopic properties with those reported in the literature
Mono- and bidentate biaryl-based phosphines have
evolved into an exceptionally important class of ligand as a
result of their application in a host of important transition-
metal-catalyzed carbon-carbon and carbon-heteroatom
bond-forming reactions.1 However, the synthesis of these
phosphines is often a nontrivial multistep process;2 two
common approaches involve construction of the biaryl
architecture via Ullmann coupling of a phosphonate-based
aryl halide3a,b and introduction of the phosphino groups via
a palladium- or nickel-catalyzed coupling between a pre-
formed biaryl ditriflate and the corresponding secondary
phosphine.3c Even though these approaches are now well-
established, the demand for more efficient, cost-effective,
and modular methods has led to a number of recent inno-
vative developments in the synthesis of this class of ligand.
In this regard, we have reported that rhodium-catalyzed
[2 þ 2 þ 2] cycloaddition between a phosphine-substituted
*To whom correspondence should be addressed. E-mail: simon.
doherty@newcastle.ac.uk (S.D.).
(1) (a) Shimizu, H.; Nagasaki, I.; Sayo, N.; Saito, T. In Phosphorus
€
Ligands in Asymmetric Catalysis; Borner, A., Ed.; Wiley-VCH: Weinheim.
Germany, 2008; p 207 (b) Li, Y.-M.; Yu, W.-Y.; Chan, A. S. C. Phosphorus
Ligands in Asymmetric Catalysis; Borner, A. ,Ed.; Wiley-VCH: Weinheim,
Germany, 2008; p 260. (c) Seyden-Penne, J. Chiral Auxiliaries and Ligands
in Asymmetric Catalysis: Wiley: New York, 1995. (d) Noyori, R. Asym-
metric Catalysis in Organic Synthesis; Wiley: New York, 1994. Biaryl
monophosphines: (e) Surry, D. S.; Buchwald, S. L. Angew. Chem., Int. Ed.
2008, 47, 6339 and references therein. (f) Martin, R.; Buchwald, S. L. Acc.
Chem. Res. 2008, 41, 1461.
(5) Kondoh, A.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2007,
129, 6996.
(6) Nishida, G.; Noguchi, K.; Hirano, M.; Tanaka, K. Angew. Chem.,
(2) (a) Shimizu, H.; Nagasaki, I.; Saito, T. Tetrahedron2005, 61, 5405.
(b) Berhod, M.; Mignani, G.; Woodward, G.; Lemaire, M. Chem. Rev. 2005,
105, 1801.
(3) (a) Foricher, J.; Heiser, B.; Schmid, R. (Hoffmann-La Roche AG)
PCT Int. Appl. WO 9216535. (b) Cai, D.; Payack, J. F.; Bender, D. R.;
Hughes, D. L.; Verhoeven, T. R.; Reider, P. J. Org. Synth. 1998, 76, 6.
(c) Ager, D. J.; East, M. B.; Esienstadt, A.; Laneman, S. A. Chem. Commun.
1997, 2369.
(4) (a) Doherty, S.; Knight, J. G.; Smyth, C. H.; Harrington, R. W.;
Clegg, W. Org. Lett. 2007, 9, 4925. (b) Doherty, S.; Smyth, C. H.;
Harrington, R. W.; Clegg, W. Organometallics 2008, 27, 4837. (c) Nishida,
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Int. Ed. 2007, 46, 3951.
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(8) For the synthesis of racemic biaryl monophosphines via Diels-
Alder cycloaddition see: (a) Ashburn, B. O.; Carter, R. G. Angew.
Chem., Int. Ed. 2006, 45, 6737. (b) Ashburn, B. O.; Carter, R. G. J. Am.
Chem. Soc. 2007, 129, 9109.
(9) For the synthesis of related diphosphines via Diels-Alder cycload-
dition see: (a) Doherty, S.; Knight, K. J.; Smyth, C. H.; Harrington, R.
W.; Clegg, W. Organometallics 2008, 27, 1679. (b) Doherty, S.; Smyth, C. H.;
Harrington, R. W.; Clegg, W. Organometallics 2009, 28, 888.
€
(10) Schmid, R.; Foricher, J.; Cereghetti, M.; Schonholzer, P. Helv.
Chim. Acta 1991, 74, 370.
r
2009 American Chemical Society
Published on Web 08/18/2009
pubs.acs.org/Organometallics

5274 Organometallics, Vol. 28, No. 17, 2009
Doherty et al.
Scheme 1a
a Legend: (i) 150 °C, 20 h, no solvent; (ii) microwave, toluene, 300 W.
to afford 4 as the sole product. The 31P NMR spectrum of
3 contains two distinct well-separated signals at δ 29.4 and
8.5 characteristic of the biarylphosphine oxide and the
alkynylphosphine oxide fragments, respectively. The identity
of 3 was unequivocally established by a single-crystal X-ray
study, and a perspective view of the molecular structure is
shown in Figure 1. The C(7)-C(8) bond length of 1.2006(19)
ꢀ
˚
A is close to that expected for a carbon-carbon triple bond
and is similar to those found in related monocycloaddition
adducts generated from 1 and substituted cyclopentadiene13
and anthracene14 derivatives. The P(2)-C(7)-C(8) and
C(6)-C(7)-C(8) bond angles of 173.89(13) and 173.82(14)°,
respectively, also lie within the range of 172.25-177.21° for
the corresponding angles in these monoadducts. Here it is
worth noting that the alkynylphosphine oxide adduct 3 could
be used as a template for the synthesis of unsymmetrical
atropos biaryl diphosphines, either by performing a second
cycloaddition-extrusion with a different 1,3-diene or by
conducting a metal-catalyzed asymmetric [2 þ 2 þ 2] cycload-
dition with a 1,n-diyne or an ynenitrile; the latter would
provide direct access to new nonracemic biaryl diphosphines
for use in asymmetric catalysis. In the case of the former
strategy, 3 undergoes a microwave-assisted cycloaddition-
elimination with 1-methoxy-1,3-butadiene in toluene to af-
ford 2,60-bis(diphenylphosphinoyl)-20-methoxy-1,10-biphenyl
(5), which was reduced to the corresponding phosphine, 6, by
heating with an excess of trichlorosilane and Bu3N in xylenes
at 130 °C for 5 h (Scheme 1).15 In this regard, the synthesis of
dissymmetric atropos biaryl diphosphines has only recently
been investigated; the first modular synthesis of this class of
diphosphine was reported by Leroux and Mettler and relied
on a highly regioselective halogen/metal exchange of a com-
mon polybrominated biaryl precursor, but required the use
of a dummy substituent to avoid undesired cyclization to
the “9-phosphafluorene”.16 Our stepwise cycloaddition-
extrusion approach complements the Leroux synthesis, and
together these two strategies will provide access to a host of
Figure 1. Molecular structure of cycloaddition adduct 3. Hydro-
gen atoms and the water molecule of crystallization have been
omitted for clarity. Ellipsoids are at the 40% probability level.
and confirmed by a single-crystal X-ray study.11 The high
regioselectivity can be accounted for by considering the
possible steric interactions between the 1-methoxy substituent
and the diphenylphosphino groups in the transition state for
cycloaddition. As microwave heating has been shown to
reduce reaction times quite significantly compared with the
same reaction conducted under conventional heating,12
a
toluene solution of 1 and 1-methoxy-1,3-cyclohexadiene was
heated witha microwave power of 300 W from room tempera-
ture to 200 °C for 10 min, cooled to room temperature. and
analyzed by 31P NMR spectroscopy; this sequence was
repeated until all the starting material had been consumed.
Near-quantitative formation of 4 was obtained after only five
10 min bursts of microwave irradiation; the yields obtained
under thermal and microwave heating were comparable.
Monitoring of the reaction progress revealed that the cycload-
dition-extrusion occurs in a stepwise manner to initially give
the monoadduct 3, which undergoes a subsequent slower
cycloaddition-extrusion to afford 4. Intermediate 3 was
isolated, characterized by spectroscopic and analytical meth-
ods, and shown to react with 1-methoxy-1,3-cyclohexadiene
(13) Huang, Y.-D.; Yu, H.-T.; Meng, J.-B.; Wang, Y.-M. Chin. J.
Struct. Chem. 2002, 149, 21.
(14) Huang, Y.-D.; Yu, H.-T.; Meng, J.-B.; Metsuura, T.; Wang,
(11) Full details of the X-ray analysis of 4 are provided in the
Supporting Information.
(12) (a) Roberts, B. A.; Strauss, C. R. Acc. Chem. Res. 2005, 38, 653.
(b) Larhed, M.; Moberg, C.; Hallberg, A. Acc. Chem. Res. 2002, 35, 717. (c)
Y.-M. J. Mol. Struct. 2002, 610, 247.
(15) (a) Zhang, Z.; Qian, H.; Longmire, J.; Zhang, X. J. Org. Chem.
2000, 65, 6223. (b) Gorobets, E.; Wheatley, B. M. M.; Hopkins, J. M.;
McDonald, R.; Keay, B. A. Tetrahedron Lett. 2005, 46, 3843.
(16) (a) Leroux, F. R.; Mettler, H. Adv. Synth. Catal. 2007, 349, 323.
(b) Leroux, F. R.; Mettler, H. Syn. Lett. 2006, 766.
€
Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57,
9225. (d) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 46, 6250. (e)
Polshettiwar, V.; Varma, R. S. Acc. Chem. Res. 2008, 41, 629. (f) Leadbeater,
N.; Shoemaker, K. M. Organometallics 2008, 27, 1254.

Note
Organometallics, Vol. 28, No. 17, 2009 5275
Figure 2. Molecular structure of [{2,60-bis(diphenylphosphino)-
20-methoxy-1,10-biphenyl}PdCl2] (7) showing the conformation
of the twist of the biaryl axis. Hydrogen atoms and solvent
molecules have been omitted for clarity. Ellipsoids are at the
40% probability level.
Figure 3. Molecular structure of cycloaddition adduct 12, con-
firming the E stereochemistry of the alkene fragment. Hydrogen
atoms and the water molecule of crystallization have been
omitted for clarity. Ellipsoids are at the 40% probability level.
Scheme 3
Scheme 2
new atropos C1-symmetric biaryl diphosphines. With the
intention of evaluating the efficiency of 6 in the palladium-
catalyzed amination of aryl halides (vide infra), we also
investigated its platinum group metal coordination chemistry.
Dropwise addition of a dichloromethane solution of 6
into a dichloromethane solution of [(cycloocta-1,5-diene)-
PdCl2] resulted in near-quantitative formation of [{2,60-bis-
(diphenylphosphino)-20-methoxy-1,10-biphenyl}PdCl2] (7).
X-ray-quality crystals of 7 were grown by slow diffusion of
n-hexane into a dichloromethane solution at room tempera-
ture; a perspective view of the molecular structure is shown in
Figure 2. The coordination around palladium is somewhat
twisted from regular square planar, with a dihedral angle of
21.7° between the PdP2 and PdCl2 planes, and the natural
bite angle of 93.31(3)° is comparable to those of 92.24(4)
and 92.69(8)° in [(BIPHEP)PdCl2]17 and [(BINAP)PdCl2],18
respectively. The dihedral angle of 80.2° between the least-
squaresplanes of the two aromaticrings of the biaryl fragment
is slightly larger than those of 70.3 and 71.8° found in [(MeO-
BIPHEP)Pd{(R)-(þ)-N,N,dimethyl-R-methylbenzylamine-
C,N}]19 and [(BINAP)PdCl2],18 respectively, giving a more
nearly perpendicular arrangement. The four phenyl rings of
the diphenylphosphino groups are arranged in the familiar
alternating edge-face arrangement.
transformation also occurs in a stepwise manner in that 1
undergoes an initial rapid Diels-Alder cycloaddition-ex-
trusion sequence to give 8, which undergoes a slower second
cycloaddition-extrusion to afford 9 (Scheme 2). Moreover,
microwave-assisted heating of a toluene solution containing
8 and 1-methoxy-1,3-cyclohexadiene results in regioselective
cycloaddition with elimination of ethylene to afford 5 as the
sole product in excellent yield after eight 10 min bursts each
of 300 W.
By direct analogy with the studies described above, we
reasoned that monoadduct 10, the product of a single
cycloaddition between 1 and 1,3-pentadiene, would elimi-
nate hydrogen to afford 11, which would undergo a second
cycloaddition-elimination sequence to give the oxide of
BIPHEMP.21 Surprisingly, though, microwave heating of a
toluene solution of 1 and 1,3-pentadiene at 300 W/200 °C
resulted in a single rapid cycloaddition with immediate
hydrogen transfer to the triple bond of the remaining alky-
nylphosphine oxide to afford 12, as the sole product
(Scheme 3). The identity of 12 was initially based on a
number of distinctive features in the 31P and 1H NMR
spectra as well as a molecular ion peak corresponding to
[M þ H]þ at m/z 519 in the electrospray mass spectrum. Even
though the presence of two signals at δ 32.6 and 24.5 in the
31P NMR spectrum of 12 provided clear evidence that double
cycloaddition had not occurred, both resonances appear
close to the region expected for a biaryl diphosphine oxide,
which suggested that both triple bonds had reacted. A double
By analogy, the Diels-Alder cycloaddition between 1 and
1-methoxy-1,3-butadiene under microwave heating resulted
in elimination of methanol to afford 1,10-bis(diphenylphos-
phinoyl)biphenyl (9) in good yield.20 Not surprisingly, this
(17) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2000,
19, 1567.
(18) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T.; Nishioka, E.;
Yanagi, K.; Moriguchi, K. Organometallics 1993, 12, 4188.
€
(19) Schmid, R.; Foricher, J.; Cereghetti, M.; Schonholzer, P. Helv.
€
Chim. Acta 1991, 74, 370.
(21) (a) Schmid, R.; Cereghetti, M.; Heiser, B.; Schonholzer, P.;
(20) (a) Desponds, O.; Schlosser, M. J. Organomet. Chem. 1996, 507,
257. (b) Vehara, A.; Bailar, J. C. J. Organomet. Chem. 1982, 239, 1.
Hansen, H.-J. Helv. Chim. Acta 1988, 71, 897. (b) Cereghetti, M.; Schmid,
€
R.; Schonholzer, P.; Rageot, A. Tetrahedron Lett. 1996, 30, 5343.

5276 Organometallics, Vol. 28, No. 17, 2009
Doherty et al.
Table 1. Comparative Study of the Palladium-Catalyzed Aryla-
tion of Aniline and n-Hexylamine using Catalyst Generated from
Pd2(dba)3 and either 6 or BIPHEPa
properties of 11 are similar to those of 3 and 8 (Scheme 3).
Thus, conventional heating methods allows for the selective
synthesis of the desired monadduct in good yield.
Since palladium(0) complexes of biaryldiphosphines have
been shown to be effective catalysts for the amination of
aryl bromides with primary amines,24 this transformation
was deemed to be an appropriate benchmark with which
to undertake a comparative study between 6 and BIPHEP.
Our preliminary evaluation focused on the amination of
a range of aryl bromides with aniline and n-hexylamine
using Pd2(dba)3/(phosphine) (1 mol % Pd) in toluene at
80 °C with sodium tert-butoxide as base; full details are
given in Table 1. Gratifyingly, both amines coupled with a
range of electron-poor and electron-rich aryl bromides; the
former gave good conversions as expected, while good con-
versions could also be achieved with the latter more challen-
ging substrates after sufficiently long reactiontimes. Although
this comparison has been limited to a restricted range of
substrate combinations, in the majority of cases Pd(0)/6 either
competed with or outperformed its BIPHEP counterpart.
In conclusion, this note describes the use of a regioselective
stepwise double Diels-Alder cycloaddition-elimination se-
quence to prepare symmetrical and unsymmetrical biaryl
diphosphines. In this regard, it will be straightforward to
diversify the library of available biaryldiphosphines, since
the steric and electronic properties can be modified through
the phosphino groups of the butadiyne-bridged diphosphine,
which is prepared from the dilithiobutadiyne and an appro-
priate chlorophosphine, while the substitution pattern of the
biaryl architecture can varied through a judicious choice of
diene, which is either commercially available or can be
prepared from commercially available reagents. The step-
wise manner of this approach could also lend itself to the
selective synthesis of a much broader range of monosubsti-
tuted pseudo-C2-symmetric atropos biaryl diphosphines,
while an asymmetric [2 þ 2 þ 2] cycloaddition between a
monoadduct and a 1,n-diyne will provide access to new chiral
C1-symmetric biaryl-differentiated diphosphines; precedent
suggests this could occur in exceptionally high ee.4b,4c
Preliminary catalyst testing using the arylation of primary
amines as a benchmark reaction for comparison showed that
Pd(0)/6 rivals its BIPHEP counterpart for the majority of
substrate combinations. The use of Diels-Alder cycloaddi-
tion and extrusion to construct biaryl diphosphines provides
a complementary approach to existing methodologies and is
likely to find applications across the broader areas of ligand
synthesis and catalysis.
entry phosphine
aryl bromide
R0
time (h) yield (%)b
1
2
6
BIPHEP
4-MeC6H4
4-MeC6H4
4-CNC6H4
4-CNC6H4
3,5-Me2-C6H3 C6H5
3,5-Me2-C6H3 C6H5
4-MeOC6H4
4-MeOC6H4
4-CNC6H4
4-CNC6H4
4-t-BuC6H4
4-t-BuC6H4
C6H5
C6H5
C6H5
C6H5
8
8
3
3
3
3
6
6
5
5
7
7
89
82
91
93
87
76
79
73
69
75
92
88
3
4
6
BIPHEP
5
6
6
BIPHEP
7
6
C6H5
C6H5
n-Hex
n-Hex
n-Hex
n-Hex
8
9
BIPHEP
6
10
11
12
BIPHEP
6
BIPHEP
a Reaction conditions: 1.0 mmol of Ar-Br, 1.1 mmol of amine,
0.134 g (1.4 equiv) of NaO-t-Bu, 0.5 mol % of Pd2(dba)3, 1.5 mol %
of 6 or BIPHEP, 1.5 mL of toluene, 80 °C. b Isolated yield. Average of
three runs.
of doublets at δ 6.69 coupled to a multiplet at δ 7.30, each of
intensity 1H and with a JHH value of 17.9 Hz, are char-
3
acteristic of an aryl-substituted trans alkene. The identity of
12 was conclusively established by a single-crystal X-ray
study; a perspective view of the molecular structure is shown
in Figure 3. The structure clearly shows that 12 contains
an arylphosphine oxide, derived from cycloaddition to one
of the triple bonds of 1, and an aryl-substituted trans
vinylphosphine oxide, arising from stereoselective hydrogen
transfer across the adjacent triple bond in the intermediate
cyclohexadiene adduct 10. The C(1)-C(2) bond length of
2
2
ꢀ
˚
1.326(2) A is close to that expected for a C(sp )-C(sp )
ꢀ
˚
˚
double bond (1.34 A) and is similar to that of 1.339 A in an
unoxidized vinyldiphenylphosphine22 (where the double
˚
bond has a Z configuration) and that of 1.334 A for all three
E-configuration substituents of tris(styryl)phosphine oxide,
which has exact crystallographic 3-fold rotation symmetry.23
Evidence for intermediate 10 was initially obtained by
monitoring the thermal reaction between 1 and 1,3-penta-
diene by 31P NMR spectroscopy, which showed the appear-
ance of two resonances at δ 28.3 and 8.2, associated with the
biarylphosphine oxide and alkynylphosphine oxide frag-
ments, respectively, as well as electrospray mass spectro-
metry, which showed the presence of a molecular ion peak at
m/z 519 corresponding to [M þ H]þ. Intermediate 10 was
eventually isolated in high yield after heating the reaction
mixture at 100 °C for 16 h. Microwave heating of a toluene
solution containing 10 at 200 °C using an initial microwave
power of 300 W resulted in near-quantitative conversion
to 12 after five cycles each of 10 min. In stark contrast,
thermolysis of a toluene solution of 10 using conventional
heating methods resulted in hydrogen elimination to afford
11 as the major product together with a minor amount
(ca. 15%) of hydrogen transfer adduct 12; the spectroscopic
Acknowledgment. We gratefully acknowledge the
EPSRC for a studentship (C.H.S.) and an equipment
grant (W.C.) and Johnson Matthey for generous loans
of palladium salts.
Supporting Information Available: Text, a figure, and a table
giving full details of experimental procedures, characterization
data for all new compounds, and details of catalyst testing and
CIF files for compounds 3, 4, 7, and 12 giving details of crystal
data, structure solution and refinement, atomic coordinates,
bond distances, bond angles, and anisotropic displacement
parameters. This material is available free of charge via the
Internet at http://pubs.acs.org.
(22) Taillefer, M.; Cristau, H. J.; Fruchier, A.; Vicente, V. J. Orga-
nomet. Chem. 2001, 624, 307.
(23) Bushuk, B. A.; Bushuk, S. B.; Cherepennikova, N. F.; Douglas,
W. E.; Fukin, G. K.; Grigoviev, I. S.; Klapshina, L. G.; van der Lee, A.;
Semenov, V. V. Mendeleev Commun. 2004, 109.
(24) (a) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc.
1996, 118, 7215. (b) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 2000, 65,
1144.