Tuning the basicity of synergic bimetallic reagents: Switching the regioselectivity of the direct dimetalation of toluene from 2,5- to 3,5-positions

Article abstract of DOI:10.1002/anie.200801158

(Chemical Equation Presented) Meta-meta metalation: Remarkably, toluene can be directly dimanganated or dimagnesiated at the 3,5-positions using bimetallic bases with active Me₃SiCH₂ ligands (see scheme, blue). In contrast, n-butyl l

Full text of DOI:10.1002/anie.200801158

Communications
DOI: 10.1002/anie.200801158
Polymetalation
Tuning the Basicity of Synergic Bimetallic Reagents: Switching the
Regioselectivity of the Direct Dimetalation of Toluene from 2,5- to 3,5-
Positions**
Victoria L. Blair, Luca M. Carrella, William Clegg, Ben Conway, Ross W. Harrington,
Lorna M. Hogg, Jan Klett, Robert E. Mulvey,* Eva Rentschler, and Luca Russo
Organolithium reagents have long enjoyed iconic status
amongst synthetic chemists, in part because of their unrivaled
ability to affect directed metalation (lithiation, lithium–
hydrogen exchange) reactions on arenes, heteroarenes, and
related compounds.[1] If the objective is to generate non-
lithium metal–carbon bonds with subordinate metals having
lower reactivity than lithium, then usually these bonds must
be made indirectly by first forming a lithium–carbon bond and
subsequently carrying out a metathesis reaction with a
subordinate metal salt.[2] Recently, however, there is increas-
ing realization that direct (subordinate) metalation with low-
polarity metals magnesium,[3] zinc,[4] aluminum,[5] or manga-
nese(II)[6] is now achievable with imaginatively composed
bases. Bimetallic cooperativity (mixed-metal synergy) within
the base molecule/mixture[7] is a major contributory factor in
this surprising turnaround. For example, sodium–magnesium
cooperativity in the n-butyl-based compound {Na(nBu)-
sodium manganate base {Na4Mn2(tmp)6(CH2SiMe3)2} to 1,4-
dimanganate benzene to afford the inverse crown
[(tmp)6Na4(1,4-Mn2C6H4)] (2).[11] This result demonstrates
that the concepts of direct metalation and inverse crown
complexation also extend to open-shell transition elements.
Given that the active deprotonating alkyl ligand of our
magnesiate and manganate bases are different (n-butyl and
trimethylsilylmethyl, respectively), for completeness we have
further investigated the reaction of the manganese(II) bases
with toluene. Remarkably, the regioselectivity of the dimeta-
lation (in this case, dimanganation) switches to the 3,5-
(meta,meta) positions. Preparing a new trimethylsilylmethyl-
based magnesiate base and reacting it with toluene reveals the
same surprising 3,5-regioselectivity. Thus, as disclosed herein,
these synergic bases can be tuned to metalate arenes
selectively at different positions depending on the identity
of the basic alkyl component. Furthermore, X-ray crystallo-
graphic studies establish that these unique 3,5-dimetalations
are manifested in a new type of inverse crown, which undergo
electrophilic interception reactions with iodine to give 3, 5-
diiodotoluene.
(tmp)Mg(tmp)}
(tmp = 2,2,6,6-tetramethylpiperidide)
resulted in direct regioselective 2, 5-dimagnesiation of
toluene, manifested in the inverse-crown product
[(tmp)6Na4(2,5-Mg2C6H3CH3)] (1).[8] The tmeda-supported
complex of this base, [(tmeda)Na(nBu)(tmp)Mg(tmp)], mon-
omagnesiates toluene at the 3-position to give [(tmeda)Na-
(tmp)2(3-MgC6H4CH3)] (tmeda = N,N,N’,N’-tetramethylethy-
lenediamine).[9] The bimetallic cooperativity operating in
these reactions prompted their description as alkali-metal-
mediated magnesiations (AMMM).[2a] Knochel also recently
reported[10] direct magnesiation reactions of a series of
polyfunctionalized arenes and heteroarenes using the related
bimetallic base (tmp)2Mg·2LiCl. We recently employed the
Previously, we found no metal dependency in the reac-
tions of the magnesiate base {Na(nBu)(tmp)Mg(tmp)} or
manganate base {Na4Mn2(tmp)6(CH2SiMe3)2} with benzene:
both
produce
benzene-1,4-diide
inverse
crowns
[(tmp)6Na4(1,4-MII C6H4)] (MII = Mg or Mn). Therefore, one
2
would expect the manganate base to also replicate the
magnesiate base in its reaction with toluene to afford
[(tmp)6Na4(2,5-Mn2C6H3CH3)]. Therefore, we applied the
same manganate reaction mixture as that used to dimanga-
nate benzene, BuNa/Htmp/Mn(CH2SiMe3)2 in a 4:6:2 stoi-
chiometry in hexane solution, to one molar equivalent of
toluene (Scheme 1).
The isolated product, in the form of yellow crystalline
needles, was not the one expected, but its 3,5-isomer
[(tmp)6Na4(3,5-Mn2C6H3CH3)] (3). Compound 3 is paramag-
netic, and thus not conductive to a diagnostic NMR spectro-
scopic study, but has been characterized directly by X-ray
crystallography[12] and indirectly by electrophilic interception
(see below). It has a different type of molecular structure
from that of 1: There are two crystallographically distinct but
essentially chemically equivalent molecules in the crystal
structure of 3, so for brevity only one (Figure 1) is discussed
herein. Key features of 1 and 2 are still mimicked in 3. For
[*] V. L. Blair, B. Conway, L. M. Hogg, Dr. J. Klett, Prof. R. E. Mulvey
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde, Glasgow, G1 1XL (UK)
Fax: (+44)141-548-4822
E-mail: r.e.mulvey@strath.ac.uk
L. M. Carrella, Prof. Dr. E. Rentschler
Institut für Anorganische Chemie und Analytische Chemie
Johannes-Gutenberg-Universität Mainz
Duesbergweg 10–14, 55128 Mainz (Germany)
Prof. W. Clegg, Dr. R. W. Harrington, Dr. L. Russo
School of Natural Sciences (Chemistry)
Newcastle University, Newcastle upon Tyne, NE1 7RU (UK)
[**] This research was supported by the UK Engineering and Physical
Science Research Council through grant award nos. GR/T27228/01
and EP/D076889/1.
example,
a
severely puckered twelve-atom [(NaN-
NaNMnN)2] host ring encapsulates a toluenediide guest.
However, a major distinction is in how the host ring and guest
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801158.
6208
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 6208 –6211

Angewandte
Chemie
Figure 2. Superposition of the molecular structures of 1 (gray) and 3
(black), with a least-squares fit of the two aryl rings, showing their
metal–nitrogen host rings around the carbon atoms of their arenediide
guests.
Comparison with 2 shows remarkably similar corresponding
values of Æ 0.085 Š and 2.201(2) Š, despite the different (1,4)
metal substitution pattern on the benzenediide ring, suggest-
ing these inverse crown structures organize about the
Scheme 1. Synthesis of the new inverse crowns 3 (M=Mn) and 4
(M=Mg).
À
optimization of Mn C s bonds. In contrast, the sodium
atoms are situated almost orthogonal to the arene ring
plane (at 84.3 and 75.88, cf. 79.6 and 85.28 in 2) and their
interaction with the p system appears to be compromised
slightly (Na1-C29 2.803(6) Š, Na2-C29A 2.782(6) Š; cf in 2
2.710(2) Š and 2.715(2) Š).
Can the synergic magnesiate base system also be modified
so that it similarly affects 3,5-dideprotonation instead of the
2,5-dideprotonation achieved with the butyl-active base?
Following the recipe that was successful for manganese, we
reacted
a 4:2:6 stoichiometric mixture of BuNa/Mg-
(CH2SiMe3)2/Htmp (equating to Na4Mg2(tmp)6(CH2SiMe3)2
after evolution of 4BuH and 2Me4Si) with one molar
equivalent of toluene (Scheme 1). In this case, as hoped, the
isolated yellow crystalline product was not 1, but its regioiso-
mer [(tmp)6Na4(3,5Mg2C6H3CH3)] (4). The 3,5-aromatic
substitution pattern was immediately discernible from a
1H NMR spectrum, as meta H resonances were absent, but
ortho H and para H resonances in a ratio of 2:1, shifted
downfield (at d = 7.57 and 8.06 ppm, respectively) relative to
those of toluene, were present. X-ray crystallographic stud-
ies[12] established that the molecular structure of 4 (Figure 3)
is analogous to that of 3, though the two crystal structures are
not isomorphous, 4 having only one independent molecule
with exact C2 symmetry. This strong resemblance between
magnesiate 4 and manganate 3 extends to molecular dimen-
sions (corresponding values in 4 to those discussed for 3:
deviation of magnesium atom from arene ring plane
Figure 1. Molecular structure of one of the two independent molecules
of 3, with tmp hydrogen atoms omitted for clarity. Symmetry operator
(C2 axis) A: 2Àx, y, 1= Àz. Selected bond lengths [Š] and angles[8]:
2
Mn1–N1 2.072(5), Mn1–N2 2.082(5), Na1–N2 2.544(5), Na1–N3
2.416(5), Na2–N1A 2.566(6), Na2–N3 2.360(5), Mn1–C29 2.220(6),
Na1–C29 2.803(6), Na2–C29A 2.782(6); N1-Mn1-N2 141.70(19), N1-
Mn1-C29 110.7(2), N2-Mn1-C29 107.45(19), N2-Na1-N3 155.28(19),
N2-Na1-C29 80.75(17), N3-Na1-C29 123.93(18), N1A-Na2-N3
156.6(2), N1A-Na2-C29A 82.50(17), N3-Na2-C29A 119.41(18).
fit together. This can clearly be seen from a superimposition
of 1 and 3 (Figure 2). More of the substituted end of the tolyl
ring is exposed in 3, whereas the host ring adopts a curved,
convex-like posture over the unsubstituted end, in contrast to
its more planar appearance in 1. Chemically this change in
morphology is a consequence of the manganese atoms in 3
bonding to C29/C29A (note the exact molecular C2 axis
running through C28/C31/C32) in place of the hydrogen
atoms abstracted from these 3,5-positions. The manganese
atoms sit coplanar with the arene ring plane (deviations
À
Æ 0.247 Š; Mg1-C2, 2.217(3) Š; angles between Na C
bonds and arene ring plane, 86.1 and 73.08; Na1-C2,
2.801(3) Š, Na2-C2, 2.749(3) Š). Dimetalation of the toluene
ring leads to a noticeable narrowing of the angle at the
metalated, meta carbon atoms (Cl-C2-C3, 112.6(3)8; cf
114.1(6)8 in 3) and concomitant widening at the ortho (C2-
C3-C4, 125.3(3)8; cf 123.5(6)8 in 3) and para carbon atoms
(C2-C1-C2A, 127.1(4)8; 125.9(9)8). A comparison between
the two magnesiate regioisomers 4 and 1 also reveals dimen-
À
À
Æ 0.126 Š) with a Mn1 C29 bond length of 2.220(6) Š.
sional similarities (for example, Mg C in 1, 2.200(2) Š), but
Angew. Chem. Int. Ed. 2008, 47, 6208 –6211
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6209

Communications
Experimental Section
All reactions were carried out under a protective argon atmosphere.
The compound Mg(CH2SiMe3)2 was prepared from the Grignard
reagent (Me3SiCH2)MgCl by manipulation of the Schlenk equilibri-
um by the dioxane precipitation method. The resultant off-white solid
was purified by sublimation at 1758C (10À2 torr) to furnish pure
Mg(CH2SiMe3)2.
3: Htmp (1.2 mL, 6 mmol) was added to a suspension of BuNa
(0.32 g, 4 mmol) in dry n-hexane (45 mL), and the resultant mixture
was allowed to stir at room temperature for 1 h. Mn(CH2SiMe3)2
(0.46 g, 2 mmol) was added to give a yellow/orange solution. Toluene
(0.22 mL, 1 mmol) was then added and the solution was heated to
reflux for 30 min. The bright orange solution was allowed to cool to
room temperature, depositing a crop of yellow crystalline needles
(0.51 g). Removal of the mother liquor with a canula and reduction of
the solvent volume under vacuum allowed a second batch of product
to be isolated as a yellow microcrystalline solid (0.26 g); total yield
68.0%. Elemental analysis calcd (%) for C61H114Mn2N6Na4 (1133.45):
C 64.64, N 7.41, H 10.14; found: C 64.73, N 7.32, H 10.15. M.p. 1858C
(decomp).
Figure 3. Molecular structure of 4, with tmp hydrogen atoms omitted
for clarity. Symmetry operator (C2 axis) A: Àx, y, 1= Àz. Selected bond
2
4: Following the procedure for 3, Mg(CH2SiMe3)2 (0.6 g, 3 mmol)
was added to a mixture of BuNa (0.48 g, 6 mmol) and Htmp (1.6 mL,
9.0 mmol) in n-hexane (20 mL). Toluene (0.16 mL, 1.5 mmol) was
then added and the mixture was heated for 10 min to give a clear
yellow solution. After cooling and filtering, the solution was allowed
to stand for 2 d. A crop of yellow needles (0.64 g, 39.8%) was
lengths [Š] and angles[8]: Mg1–N1 2.058(3), Mg1–N2 2.053(3), Na1–
N1 2.599(3), Na1–N3A 2.400(3), Na2–N2 2.570(2), Na2–N3 2.361(3),
Mg1–C2 2.217(3), Na1–C2 2.801(3), Na2–C2 2.749(3); N1-Mg1-N2
101.09(10), N1-Mg1-C2 108.45(11), N2-Mg1-C2 111.07(11), N1-Na1-
N3A 155.55(10), N1-Na1-C2 79.86(9), N3A-Na1-C2 123.88(9), N2-Na2-
N3 159.88(9), N2-Na2-C2 82.84(9), N3-Na2-C2 117.06(9).
1
obtained. H NMR (400 MHz, C6D6, 208C, TMS): d = 8.06 (s, 1H),
7.57 (s, 2H), 2.14 (s, 3H), 1.92 (m, 2H), 1.68 (m, 4H), 1.53 (s, 12H),
1.52 (s, 12H), 1.42 (s, 12H), 1.37 (s, 12H), 1.31 (s, 12H), 0.87 ppm (s,
12H). Elemental analysis calcd (%) for C61H114Mg2N6Na4 (1072.19):
C 68.33, N 7.82, H 10.72; found: C 68.33, N 7.74, H 11.00. M.p. 1858C
(decomp).
as in the comparison between 2 and 3, the interaction of the
sodium atom with the arene p system appears marginally
À
Reaction of 3 [or crystalline 3] with I2: Iodine in THF (2.8 mL,
2.8 mmol) [3.5 mL, 3.5 mmol] was added to a suspension of 3
(0.7 mmol) [0.5 g, 0.44 mmol] in hexane (20 mL) [10 mL]. After
stirring for 18 h, the mixture was quenched with saturated Na2SO4
solution (5 mL), saturated NH4Cl solution (5 mL), distilled water
(15 mL) and hexane (15 mL). The crude bilayer was filtered through
celite into a separating funnel, with the aqueous layer subsequently
discarded. The organic layer was then washed with distilled water
(15 mL ” 3), dried under anhydrous MgSO4 for 1 h and then filtered
through celite to produce a clear yellow solution. The solvent was
then removed in vacuo and then dissolved in the minimum volume of
hexane. The solution was purified by SiO2 column chromatography
using pure hexane as the eluant to give, after removal of solvent, 3,5-
diiodotoluene as colorless oil (62.5 mg, 26%) [81.2 mg, 54.5%].
Reaction of a crude solution of 4 [or crystalline 4] with I2: A
solution of iodine in THF (4 mL, 4 mmol) [4.8 mL, 4.8 mmol] was
added to a suspension of 4 (0.5 mmol) [0.64 g, 0.6 mmol] in hexane
(20 mL) [10 mL]. The workup, following the procedure used for the
reaction of 3 with I2, afforded 3,5-diiodotoluene as colorless oil
(94.4 mg, 55%) [62.5 mg, 30%].
more efficient in the 2,5-isomer (mean Na C bond length:
À
2.686 Š; in 4, 2.775 Š; angles between Na C bonds and arene
ring plane: in 1, 87.4 and 76.78; in 4 86.1 and 73.08).
Both 3 and 4 were subjected to electrophilic quenching
reactions with iodine. Their 3,5-metal substitution patterns
were confirmed by the formation of 3,5-diiodotoluene after
aqueous workup, with yields of isolated product of 55% and
30%, respectively.
Whereas 4 is diamagnetic, 3 is paramagnetic, and so its
magnetic properties have been explored by variable-temper-
ature magnetization measurements on a powdered sample.[13]
The room temperature value for cT is 8.4 emuKmolÀ1, which
is only slightly lower than the theoretical expectation value of
8.75 emuKmolÀ1 for two uncoupled S = 5/2 centers. With
lowering temperature a decrease in cT is observed only below
50 K, indicating very weak antiferromagnetic exchange
interaction between the two manganese ions. Using the spin
ˆ
ˆ ˆ
Hamiltonian H = À2JS1S2 the susceptibility data were
À1
ˆ
ˆ
simulated satisfactorily, with S1 = S2 = 5/2, J = À0.1 cm
Received: March 10, 2008
Published online: July 10, 2008
and g = 1.97.
In summary, unprecedented polymetalation reactions
have been uncovered in which toluene is directly dimanga-
nated or dimagnesiated at 3,5-positions in a new type of
inverse crown product.[14] The key to unlocking this 3,5-
regioselectivity appears to lie with the trimethylsilymethyl
ligand,[15] in combination with tmp ligands, in the synergic
bimetallic sodium–manganese or sodium–magnesium bases
employed. This surprising finding calls for a screening of a
wide range of bimetallic compounds with different combina-
tions/permutations of ligands as potential synergic bases.
Keywords: alkali metals · inverse crown compounds ·
magnesium · manganese · metalation
.
[1] a) The Directed ortho-Metalation–Cross Coupling Nexus. Syn-
thetic Methodology for Aryl–Aryl and Aryl–Heteroatom–Aryl
Bonds: E. Anctil, V. Snieckus in Metal-Catalyzed Cross-Cou-
pling Reactions, 2nd ed. (Eds.: F. Diederich, A. de Meijere),
2004, pp. 761 – 813; b) The Directed ortho-Metalation Reaction.
A Point of Departure for New Synthetic Aromatic Chemistry“:
6210 www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 6208 –6211

Angewandte
Chemie
C. G. Hartung, V. Snieckus in Modern Arene Chemistry (Ed.: D.
Astruc), Wiley-VCH, Weinheim, 2002, pp. 330 – 367; c) J. Clay-
den, Organolithiums: Selectivity for Synthesis, Pergamon/Elsev-
ier Science Oxford (UK), 2002.
[11] L. M. Carrella, W. Clegg, D. V. Graham, L. M. Hogg, A. R.
Kennedy, J. Klett, R. E. Mulvey, E. Rentschler, L. Russo, Angew.
Chem. 2007, 119, 4746; Angew. Chem. Int. Ed. 2007, 46, 4662.
[12] Crystal data for 3: C61H114Mn2N6Na4, M = 1133.4, monoclinic,
space group P2/c, a = 29.375(6), b = 8.4546(17), c = 29.742(6) Š,
b = 117.14(3)8, V= 6573(2) Š3, Z = 4, 1calcd = 1.145 gcmÀ3, T=
150 K, m(MoKa) = 0.45 mmÀ1 (l = 0.71073 Š); 56250 reflections
measured, 10225 unique, Rint = 0.091; R (F, F2 > 2s) = 0.089, wR
(F2, all data) = 0.204, S (F2) = 1.138, 687 refined parameters,
constrained H atoms, difference map extremes + 0.36 and
À0.59 eŠÀ3. Crystal data for 4: C61H114Mg2N6Na4, M = 1072.2,
monoclinic, space group C2/c, a = 30.194(6), b = 8.4843(17), c =
[2] For examples see: a) S. U. Koschmieder, G. Wilkinson, Poly-
hedron 1991, 10, 135; b) R. A. Bartlett, M. M. Olmstead, P. P.
Power, S. C. Shoner, Organometallics 1988, 7, 1801; c) R. J.
Wehmschulte, P. P. Power, Organometallics 1995, 14, 3264.
[3] a) R. E. Mulvey, Organometallics 2006, 25, 1060; for Group 2
metals in general see, b) M. Westerhausen, Dalton Trans. 2006,
4755.
[4] a) R. E. Mulvey, F. Mongin, M. Uchiyama, Y. Kondo, Angew.
Chem. 2007, 119, 3876; Angew. Chem. Int. Ed. 2007, 46, 3802;
b) J.-M. LꢀHelgoualꢀch, A. Seggio, F. Chevallier, M. Yonehara,
E. Jeanneau, M. Uchiyama, F. Mongin, J. Org. Chem. 2008, 73,
177.
[5] a) M. Uchiyama, H. Naka, Y. Matsumoto, T. Ohwada, J. Am.
Chem. Soc. 2004, 126, 10526; b) B. Conway, E. Hevia, J. Garcia-
lvarez, D. V. Graham, A. R. Kennedy, R. E. Mulvey, Chem.
Commun. 2007, 5241.
[6] a) J. Garcia-lvarez, A. R. Kennedy, J. Klett, R. E. Mulvey,
Angew. Chem. 2007, 119, 1123; Angew. Chem. Int. Ed. 2007, 46,
1105; b) A. R. Kennedy, J. Klett, R. E. Mulvey, S. Newton, D. S.
Wright, Chem. Commun. 2008, 308.
29.403(6) Š, b = 118.85(3)8, V= 6597(3) Š3, Z = 4, 1calcd
=
1.079 gcmÀ3, T= 150 K, m(MoKa) = 0.10 mmÀ1; 15250 reflections
measured, 5665 unique, Rint = 0.059; R (F, F2 > 2s) = 0.068, wR
(F2, all data) = 0.126, S (F2) = 1.071, 344 refined parameters,
constrained H atoms, difference map extremes + 0.20 and
À0.20 eŠÀ3. CCDC-675288 (3) and CCDC-675289 (4) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[13] Magnetic susceptibility data for polycrystalline samples of 3
were collected over the temperature range 2 – 290 K in an
applied magnetic field of 1 T with a SQUID magnetometer
(MPMS-7, Quantum Design). Experimental susceptibility data
were corrected for the underlying diamagnetism using Pascalꢀs
constants. The temperature dependent magnetic contribution of
the glass holder was experimentally determined and subtracted
from the measured susceptibility data. The routine JULIUS was
used for spin Hamiltonian simulations of the data (C. Krebs, E.
Bill, F. Birkelbach, V. Staemmler, unpublished results).
[14] For a recent review about metallacrowns and molecular wheels,
see: G. Mezei, C. M. Zaleski, V. L. Pecoraro, Chem. Rev. 2007,
107, 4933.
[7] The Lochmann–Schlosser superbases, LICKOR, could be
included in this idea of bimetallic cooperativity, though the
partner metal present, potassium, is normally considered to be
more reactive than lithium. See a) L. Lochmann, Eur. J. Inorg.
Chem. 2000, 1115; b) M. Schlosser, Organometallics in Synthesis,
2nd ed., Wiley, Chichester, 2002, chap. 1; c) A. R. Kennedy, J. G.
MacLellan, R. E. Mulvey, Angew. Chem. 2001, 113, 3345;
Angew. Chem. Int. Ed. 2001, 40, 3245.
[8] D. R. Armstrong, A. R. Kennedy, R. E. Mulvey, R. B. Rowlings,
Angew. Chem. 1999, 111, 231; Angew. Chem. Int. Ed. 1999, 38,
131.
[9] P. C. Andrikopoulos, D. R. Armstrong, D. V. Graham, E. Hevia,
A. R. Kennedy, R. E. Mulvey, C. T. OꢀHara, C. Talmard, Angew.
Chem. 2005, 117, 3525; Angew. Chem. Int. Ed. 2005, 44, 3459.
[10] G. C. Clososki, C. J. Rohbogner, P. Knochel, Angew. Chem. 2007,
119, 7825; Angew. Chem. Int. Ed. 2007, 46, 7681.
[15] For seminal papers on the development of this ligand, see
a) M. R. Collier, M. F. Lappert, M. M. Truelock, J. Organomet.
Chem. 1970, 25, C36; b) G. Yagupsky, W. Mowat, A. Shortland,
G. Wilkinson, J. Chem. Soc. D 1970, 1369.
Angew. Chem. Int. Ed. 2008, 47, 6208 –6211
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6211