The methylene-transfer reaction: Synthetic and mechanistic aspects of a unique C-C coupling and C-C bond activation sequence

Article abstract of DOI:10.1021/ja000157r

Oxidative addition of aryl iodides ArI (Ar = (a) C₆H₅, (b) C₆H₄CF₃, (c) C₆H₃(CF₃)₂, (d) C₆H₄CH₃, (e) C₆H₄OCH₃), to the PCP-type complex Rh(PPh₃)[CH₂C₆H(CH₃)₂(CH₂PPh₂)₂] (1), yields the complexes Rh(Ar)[CH₂C₆H(CH₃)₂(CH₂PPh₂)₂](I) (2a-e). Compounds 2a-e undergo intramolecular methylene transfer from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH₂-Ar)[C₆H(CH₃)₂(CH₂PPh₂)₂](I) (3a-e). The methylene transfer, which is a unique sequence of sp²-sp³ C-C bond reductive elimination and sp²-sp³ C-C bond activation, was investigated kinetically (reaction 2a → 3a), yielding the activation parameters ΔH(+) = 17 ± 3 kcal/mol, ΔS(+) = -23 ± 4 eu. The rate-determining step of this reaction is the C-C reductive elimination rather than the C-C activation step. X-ray structural analysis of 2a and 3b demonstrates that the Rh atom is located in the center of a square pyramid with the aryl (2a) and the benzyl (3b) trans to the vacant coordination site. Reaction of the complex Rh(CH₂C₆H₄CF₃)[C₆H₃(CH₂PPh₂)₂](Br) (7c) with carbon nucleophiles (MeLi, PhLi, BzMgCl) leads to a competitive sp²-sp³ and sp³-sp³ C-C coupling, resulting in migration of a methylene or benzylidene into the bis-chelating ring and formation of the corresponding organic products, sp²-sp³ C-C coupling was shown to be kinetically preferred over the sp³-sp³ one, and the more electron-rich the benzyl ligand, the better the migratory aptitude observed. X-ray structural analysis of two benzyl migration products, complexes Rh(PPh₃)[CH(C₆H₄CF₃)C₆H₃(CH₂PPh₂)₂] (11) and Rh(PPh₃)[CH(C₆H₅)C₆H(CH₃)₂(CH₂PPh₂)₂] (16), demonstrates that the rhodium atom is located in the center of a square planar arrangement where the PPh₃ ligand occupies the position trans to the methyne carbon of the benzylidene bridge. The methylene and benzylidene migration reaction is an important transformation for the regeneration of the methylene-donating moiety in the methylene-transfer process.

Full text of DOI:10.1021/ja000157r

J. Am. Chem. Soc. 2000, 122, 7723-7734
7723
The Methylene-Transfer Reaction: Synthetic and Mechanistic Aspects
of a Unique C-C Coupling and C-C Bond Activation Sequence
Revital Cohen,† Milko E. van der Boom,† Linda J. W. Shimon,‡ Haim Rozenberg,‡ and
David Milstein*,†
Contribution from the Departments of Organic Chemistry and Chemical SerVices, The Weizmann Institute
of Science, RehoVot, 76100, Israel
ReceiVed January 14, 2000
Abstract: Oxidative addition of aryl iodides ArI (Ar ) (a) C6H5, (b) C6H4CF3, (c) C6H3(CF3)2, (d) C6H4CH3,
(e) C6H4OCH3), to the PCP-type complex Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2] (1), yields the complexes
Rh(Ar)[CH2C6H(CH3)2(CH2PPh2)2](I) (2a-e). Compounds 2a-e undergo intramolecular methylene transfer
from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH2-
Ar)[C6H(CH3)2(CH2PPh2)2](I) (3a-e). The methylene transfer, which is a unique sequence of sp2-sp3 C-C
bond reductive elimination and sp2-sp3 C-C bond activation, was investigated kinetically (reaction 2a f
3a), yielding the activation parameters ∆Hq ) 17 ( 3 kcal/mol, ∆Sq ) -23 ( 4 eu. The rate-determining step
of this reaction is the C-C reductive elimination rather than the C-C activation step. X-ray structural analysis
of 2a and 3b demonstrates that the Rh atom is located in the center of a square pyramid with the aryl (2a) and
the benzyl (3b) trans to the vacant coordination site. Reaction of the complex Rh(CH2C6H4CF3)[C6H3(CH2-
PPh2)2](Br) (7c) with carbon nucleophiles (MeLi, PhLi, BzMgCl) leads to a competitive sp2-sp3 and sp3-sp3
C-C coupling, resulting in migration of a methylene or benzylidene into the bis-chelating ring and formation
of the corresponding organic products. sp2-sp3 C-C coupling was shown to be kinetically preferred over the
sp3-sp3 one, and the more electron-rich the benzyl ligand, the better the migratory aptitude observed. X-ray
structural analysis of two benzyl migration products, complexes Rh(PPh3)[CH(C6H4CF3)C6H3(CH2PPh2)2] (11)
and Rh(PPh3)[CH(C6H5)C6H(CH3)2(CH2PPh2)2] (16), demonstrates that the rhodium atom is located in the
center of a square planar arrangement where the PPh3 ligand occupies the position trans to the methyne carbon
of the benzylidene bridge. The methylene and benzylidene migration reaction is an important transformation
for the regeneration of the methylene-donating moiety in the methylene-transfer process.
Introduction
Scheme 1
Insertion of transition metal complexes into C-C bonds in
solution is a topic of much current interest1 since it can lead to
the design of new selective and efficient processes for the
utilization of hydrocarbons. The ability to insert a metal into
an unactivated C-C bond raises the possibility that a hydro-
carbon could serve as a source of methylene groups. We have
demonstrated that it is possible to combine selective C-C
cleavage with the activation of other strong bonds to form
products of methylene group transfer (Scheme 1).2 While this
study demonstrated a conceptually new approach toward
hydrocarbon functionalization, the methylene-transfer mecha-
nism was not studied in detail. Transfer of a methylene group
was demonstrated to take place from the metal complex to an
organic moiety, such as benzene, silanes, and disilanes2a as well
as to HCl2b,c and H2.2d On the other hand, the reverse reaction,
in which a methylene is abstracted from an organic moiety and
transferred back to the bis-chelating ligand, regenerating the
“methylene bridge”, is known only for methyl iodide and
remains essentially unexplored.1a,2
* Corresponding author: (e-mail) david.milstein@weizmann.ac.il.
† Department of Organic Chemistry.
‡ Department of Chemical Services.
We report here an intramolecular methylene-transfer process
in which the transferred methylene moiety remains connected
to the metal center, enabling the direct observation and
characterization of several stages in the process involving a
unique combination of C-C reductive elimination and C-C
cleavage reactions. Moreover, we describe here the “reverse”
methylene-transfer reaction, which is an essential transformation
for the regeneration of the active methylene-donating moiety.
(1) For reviews regarding C-C bond activation, see: (a) Rybtchinski,
B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (b) Murakami, M.;
Ito, Y. In Topics in Organometallic Chemistry; Murai, S., Ed.; Springer-
Verlag: New York, 1999; Vol. 3, pp 97-129.
(2) (a) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.; Ben-David,
Y.; Milstein, D. Nature 1994, 370, 42. (b) van der Boom, M. E.; Kraatz,
H.-B.; Ben-David, Y.; Milstein, D. Chem. Commun. 1996, 2167. (c) van
der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.; Milstein, D.
Organometallics 1999, 18, 3873. (d) Liou, S.-Y.; van der Boom, M. E.;
Milstein, D. Chem. Commun. 1998, 687.
10.1021/ja000157r CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/27/2000

7724 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
Scheme 2
consecutive processes can be identified: ArI oxidative addition
to form 2a and methylene transfer to form 3a.
To obtain clean 2a, acceleration of the oxidative addition step
relative to the methylene-transfer process was desired. For this
purpose, complex 1 was dissolved in neat iodobenzene (∼300
equiv) at room temperature, affording a mixture of 2a and 3a
in which the former is the main product (2a:3a ) 10:1).
Complex 2a was further purified by precipitation from a pentane
solution at room temperature. Its NMR spectra were measured
at 10 °C in order to prevent conversion to 3a. The 31P{1H} NMR
spectrum of 2a in C6D6 exhibits a doublet at 26.0 ppm (2JRhP
1
) 127.0 Hz). In H NMR, the bridging methylene group of
ArCH2Rh gives rise to a triplet positioned at 3.31 ppm (3JPH
)
8.0 Hz) which collapses to a broad singlet in H{31P} NMR.
The carbon atom of the methylene appears in 13C{1H} NMR
as a broad doublet at 27.1 ppm (1JRhC ) 17.8 Hz). The structure
of 2a was confirmed by an X-ray structural analysis (see below).
It should be noted that, in the system described here, the
transferred methylene group remains bound to the metal center,
unlike in the previously reported system.2 Remarkably, meth-
ylene transfer proceeds at room temperature, while in the
previously reported system, heating to 100-110 °C was
required.
1
We also describe a pathway in which alkyl moieties other than
the methylene group are transferred to the bis-chelating ring,
extending the scope of the reaction.
Results
Oxidative Addition of ArI and C-C Cleavage. The
methylene-transfer reaction involves several steps, which were
not identified separately. The fact that the reaction proceeded
intermolecularly and under heating hindered the investigation
of the reaction mechanism.2 To obtain mechanistic insight and
investigate the different stages involved in the process, we
sought to synthesize Rh(III) “methylene-bridged” complexes
capable of intramolecular methylene transfer. For this purpose,
various aryl iodides were reacted with the previously reported3
PCP complex 1 (see Experimental Section for the synthesis of
1), resulting in ArI oxidative addition (Scheme 2). Remarkably,
the products of ArI oxidative addition undergo C-C coupling
followed by C-C bond activation under mild conditions,
resulting in intramolecular methylene transfer from the bis-
chelating system of 1 into the Rh-Ar bond. Thus, when 1 was
reacted with an equivalent amount of iodobenzene at room
temperature for 7 days, quantitative formation of two Rh(III)
complexes, 2a and 3a (2a:3a ) 1:6), was observed (Scheme
2). No C-H activation product was detected. Keeping the
mixture at room temperature for 1 week or heating it for 1 h at
60 °C resulted in quantitative conversion of 2a to 3a. Complex
3a was characterized by various NMR techniques. The 31P{1H}
To check the substituent effect on the methylene-transfer
reaction (2 f 3), 1 was treated with aryl iodides bearing
different substituents on the aromatic ring. Complexes 2b-e
were formed and further reacted to give 3b-e (Scheme 2).
Complexes 2b-e and 3b-e are analogous to 2a and 3a,
respectively, as evident from NMR analysis. The structure of
3b was confirmed by X-ray structural analysis (see below).
Expectedly, the rate of the oxidative addition reaction of ArI
was larger as the aryl substituent had a stronger electron-
withdrawing character, following the order ArOCH3 < ArCH3
< ArH < ArCF3.4 Interestingly, the rate of the subsequent
methylene-transfer reaction exhibited an opposite trend, fol-
lowing the order ArCF3 < ArH < ArCH3 < ArOCH3.5 For
example, the following t1/2 values were measured in solution at
room temperature: conversion of 2a to 3a, 1 day; 2b to 3b, 5
days; and 2d to 3d, 12 h.
To further confirm the identity of complexes 3a-e, we
prepared the bromide analogues 7a-c by oxidative addition of
the corresponding benzyl bromides to the previously reported6
complex 6 (Scheme 3) (the synthesis of 6 is described in the
Experimental Section). Reaction of a benzene solution of
complex 6 with 1 equiv of benzyl bromide at room temperature
resulted immediately in quantitative formation of the Rh(III)-
benzyl complexes 7a-c, which were unambiguously character-
ized by various NMR techniques. They exhibit spectroscopic
properties nearly identical to those of their iodide analogous
(3a-c). Treatment of 7a-c with excess of NaH and PPh3
resulted in the formation of the starting complex 6 and the
corresponding toluene derivatives 5a-c.
NMR spectrum of 3a exhibits a doublet at 33.2 ppm (1JRhP
)
129.5 Hz). In 1H NMR, the methylene group of RhCH2Ar gives
rise to a quartet positioned at 4.4 ppm (3JPH ) 2JRhH ) 4.0 Hz)
which collapses to a doublet in 1H{31P} NMR. The methylene
group appears in 13C{1H} NMR as a broad doublet positioned
at 23.2 ppm (1JRhC ) 27.0 Hz). The ipso-carbon of the RhAr
group gives rise to a broad doublet at 173.7 (1JRhC ) 38.0 Hz),
confirming that the rhodium atom is bound directly to the aryl
ring of the PCP ligand. Reaction of 3a with an excess of NaH
and PPh3 yielded the known3a complex 4 and toluene (Scheme
2) as confirmed by NMR and GC/MS, respectively. Thus, two
X-ray Structural Analysis of Complex 2a. Orange platelike
crystals of complex 2a were obtained upon crystallization from
Scheme 3

Methylene Transfer Reaction
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7725
Figure 2. Perspective view (ORTEP) of complex 3b. Hydrogen atoms
are omitted for clarity.
Figure 1. Perspective view (ORTEP) of complex 2a. Hydrogen atoms
are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
3b
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for
2a
Rh(1)-C(20)
Rh(1)-C(11)
2.079(5)
2.049(6)
Rh(1)-P(3)
Rh(1)-P(4)
Rh(1)-I(2)
2.2746(16)
2.2892(17)
2.7303(8)
Rh(2)-C(17)
Rh(2)-C(13)
2.028(4) Rh(2)-P(3)
2.069(4) Rh(2)-P(4)
Rh(2)-I(1)
2.313(2)
2.373(2)
2.745(1)
160.0(1)
C(11)-Rh(1)-C(20) 88.9(2)
P(3)-Rh(1)-P(4) 161.34(6)
C(26)-C(13)-Rh(2) 90.2(3)
P(3)-Rh(2)-P(4)
C(11)-Rh(1)-P(3)
C(11)-Rh(1)-P(4)
C(20)-Rh(1)-P(3)
84.24(17) C(11)-Rh(1)-I(2) 171.36(15)
82.31(17) C(20)-Rh(1)-I(2) 98.99(17)
89.22(17) P(3)-Rh(1)-I(2)
P(4)-Rh(1)-I(2)
C(13)-Rh(2)-P(3)
C(13)-Rh(2)-P(4)
C(17)-Rh(2)-P(3)
85.9(2)
80.1(2)
93.3(2)
C(17)-Rh(2)-I(1) 100.4(2)
C(13)-Rh(2)-I(1) 168.6(2)
92.34(5)
99.08(5)
P(3)-Rh(2)-I(1)
P(4)-Rh(2)-I(1)
95.5(1)
95.4(1)
C(17)-Rh(2)-C(13) 90.9(2)
Cl7 and Rh(CH3)[C6H(CH3)2(CH2N(C2H5)2)(CH2P(t-Bu)2)]Cl8
in which the methyl group is trans to the empty coordination
site. The Rh(1)-P(3), Rh(1)-P(4), and Rh(1)-C(11) bond
lengths in the chelate core of 3b are similar to their counterparts
in the above-mentioned complexes as well as to those in the
hydrido chloride complex Rh(H)[2,6-C6H3(CH2P-t-Bu2)2]Cl.9
Interestingly, the crystal structure of 3b reveals an intramolecular
π-π interaction between the benzyl group, bearing two CF3
electron-withdrawing groups, and one of the phenyl substituents
on the phosphine.10 Reported examples of intramolecular π-π
interaction between two aryl groups in one metal complex are
rare.11
Reactions of Complexes 7c and 3b,e with Carbon Nucleo-
philes. The methylene-transfer reaction described above pro-
ceeds through “methylene donation” from the six-membered
bis-chelating ring to a substrate. Exploring the possibility of
transfer of a methylene group into the bis-chelating ring, going
from a five-membered chelate to a six-membered one, we
studied the reaction of 7c with various carbon nucleophiles.
a pentane solution at room temperature. The single-crystal X-ray
analysis demonstrates that the rhodium atom is located in the
center of a distorted square pyramid. The iodide ligand occupies
the position trans to the benzylic carbon C(13), whereas the
phenyl group is trans to an empty coordination site. The chelate
arene is planar. Although the distance between the metal center
and the aryl carbon, Rh(2)-C(26), is relatively short (2.53 Å),
no evidence for direct interaction between it and the Rh center
was found. To accommodate the chelate, the P-Rh-P angle
(160.0°) is distorted from linearity and the C(26)-C(13)-Rh(2)
bond angle is 90.2°. An ORTEP diagram of a molecule of 2a
is shown in Figure 1. Selected bond lengths and bond angles
are given in Table 1.
X-ray Structural Analysis of Complex 3b. Orange platelike
crystals of complex 3b were obtained upon crystallization from
a pentane solution at room temperature. The single-crystal X-ray
analysis demonstrates that the rhodium atom is located in the
center of a distorted square pyramid. The iodide ligand occupies
the position trans to the ipso-carbon, whereas the benzyl group
is trans to an empty coordination site. The P-Rh-P is distorted
from linearity with an angle of 161.34°. An ORTEP diagram
of a molecule of 3b is shown in Figure 2. Selected bond lengths
and bond angles are given in Table 2. The structure of 3b is
similar to that reported for Rh(CH3)[C6H(CH3)2(CH2P(t-Bu)2)2]-
(7) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am.
Chem. Soc. 1996, 118, 12406.
(8) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.; Milstein, D.
Organometallics 1997, 16, 3981.
(9) Nemeh, S.; Jensen, C.; Binamira-Soriaga, E.; Kaska, W. C. Orga-
nometallics 1983, 2, 1442.
(10) The distance between the two aryl rings is represented by the
distance between two corresponding carbon atoms in the π-π system, for
example: [C(22)‚‚‚C(62) 3.380 Å, C(23)‚‚‚C(63) 3.506 Å]. Similar interac-
tions were observed in intermolecular π-π systems. For example, see: (a)
Ye, B. H.; Chen, X. M.; Xue, G. Q.; Ji, L. N. J. Chem. Soc., Dalton Trans.
1998, 2827. (b) Kampar, E.; Neilands. O. Russ. Chem. ReV. 1986, 55 (4),
334. (c) Swinton, F. L. Molecular Complexes; Elek Science: London,
1974: Vol. 2, p 63. (d) Prout, C. K.; Kamenar, B. Molecular Complexes;
Elek Science: London, 1973; Vol. 1, p 151.
(3) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein. D. Nature
1993, 364, 699. For synthesis details and spectral data see the Experimental
Section. (b) Gozin, M. Ph.D. Thesis, Weizmann Institute of Science, 1995.
(4) (a) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1665. (b)
Hii, K. K. (M).; Thornton-Pett, M.; Jutand, A.; Tooze, R. P. Organometallics
1999, 18, 1887.
(5) Selmeczy, A. D.; Jones, W. D.; Osman, R.; Perutz. R. N. Organo-
metallics 1995, 14, 5677.
(6) Liou, S.-Y.; Gozin, M.; Milstein. D. J. Chem. Soc., Chem. Commun.
1995, 1965. For synthesis details see the Experimental Section.
(11) Takahiko, K.; Kaoru, S.; Kazuhiro, O.; Masaaki, O.; Yoshihisa, M.
Chem. Commun. 1997, 1679.

7726 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
Scheme 4
ppm with JRhP ) 149.0 Hz. 1H NMR shows a multiplet
corresponding to a methyne proton at 5.15 ppm. The corre-
sponding carbon, CH-Rh gives rise in 13C{1H} NMR to a
doublet of doublets positioned at 35.0 ppm (1JRhC ) 40.3 Hz,
2JPC ) 13.5 Hz). The carbon atoms of the two CH2P groups
are inequivalent and appear as doublets at 43.8 and 43.5 ppm
(JPC ) 17.8 and 25.1 Hz, respectively).
Treating a benzene solution of 7c with an equivalent amount
of BzMgCl resulted in the formation of 6 and of a new product
identified as complex 13 (6:13 ) 2:3) (Scheme 4). The
corresponding amounts of the organic products 14 and 9 were
also formed. Formation of the competitive sp2-sp3 C-C
coupling product 11 was not observed. Complex 13 was
characterized by NMR spectroscopy. Reaction of the mixture
(6:13 ) 2:3) with H2 (40 psi) in a Fisher Porter pressure vessel
resulted in the formation of 6 and toluene (Scheme 4). The
spectroscopic properties of 13 are almost identical to those of
11.
Thus, the reactions with carbon nucleophiles result in alkyl
transfer from the incoming reagent into the bis-chelating ring,
in a direction opposite to the methylene-transfer reaction
described above. The resulting organometallic product contains
a six-membered bis-chelating ring rather than the starting five-
membered one, although the latter is generally believed to be
more stable.1a In addition, the reaction of 7c with BzMgCl shows
remarkable selectivity for the migration of only one benzyl
group, originating from the Grignard reagent, rather than a
competitive migration of the two available benzyls. Apparently,
there is a kinetic preference for the more electron rich benzyl
group to migrate into the bis-chelating ring.
To verify that the observed selectivity is due to an electronic
effect on the metal-benzyl bond, the iodide analogue of 7,
bearing an electron-donating OCH3 group, complex 3e, was
treated with 1 equiv of BzMgCl. In contrast to the reaction of
7c with BzMgCl, competitive migration between the two benzyl
groups into the bis-chelating ring took place, as well as their
coupling, resulting in the formation of 4, 15, and 16 (Scheme
5). The preference for the migration of the methoxy-substituted
benzyl over the unsubstituted benzyl indicates that there is an
electronic effect of the substituent on the coupling rate.
Moreover, when complex 3b, bearing two CF3 groups, was
treated with an equivalent amount of BzMgCl, complex 16 was
formed in 96% yield (based on 31P{1H} NMR) (Scheme 5).
No evidence for migration of the other benzyl group was found,
confirming that the more electron rich benzyl has a higher
migratory aptitude. This is consistent with the fact that the
metal-benzyl bond is weakened by the σ-donating substituents.
Remarkably, in this reaction only a minor amount of the sp3-
sp3 C-C coupling product 4 was obtained, in contrast to the
results of the reaction of BzMgCl with the bromide analogue
7c. The structure of complex 16 was confirmed by an X-ray
structural analysis (see below).
Treating a benzene solution of 7c with an equivalent amount
of MeLi in the presence of 1 equiv of PPh3 leads to a
competitive sp2-sp3 and sp3-sp3 C-C coupling reaction,
resulting in the formation of the two known3b complexes 6 and
1
8 (6:8 ) 1:3) as confirmed by H and 31P{1H} NMR and the
two organic products 9 and 10 that were identified by GC/MS
(Scheme 4). Remarkably, migration of a methylene into the bis-
chelating ring takes place to form the Rh(I) complex 8 and the
product of C-H reductive elimination 9, while a competitive
C-C coupling results in complex 6 and compound 10. The
spectroscopic properties of complex 8 are almost identical to
those of its closely related analogue 1. No evidence for a
competitive coupling of the benzyl group with the aryl group,
leading to complex 11, was observed (see below).
Similarly, treating a benzene solution of 7c with 1 equiv of
PhLi in the presence of 1 equiv of PPh3 resulted in the formation
of 6 and the corresponding organic product 12. Interestingly, a
new product, identified as 11, and benzene (6:11 ) 1:1) were
also formed. Complex 11 is undoubtedly the product of a C-C
coupling reaction between the benzyl and the ipso-carbon of
the chelate aryl that is followed by C-H bond activation, PhH
reductive elimination and coordination of the free PPh3 (Scheme
7). Complex 11 was unambiguously characterized by various
NMR techniques and its structure was confirmed by an X-ray
structural analysis (see below). The organic C-C coupling
product 12 was identified by GC/MS. The spectroscopic
properties of 11 are almost identical to those of a similar Rh(I)
complex previously reported by us.6 Since the carbon atom
bound to rhodium is chiral, all three phosphorus atoms are
inequivalent in 31P{1H} NMR. The PPh2 atoms trans to each
other exhibit ddd splitting patterns in 31P{1H} NMR positioned
at 64.6 and 52.7 ppm with JPPtrans ) 259.2 Hz, (JRhP ) 192.8
Hz, JPPcis ) 35.6 Hz, JRhP′ ) 196.0 Hz, and JPP′cis ) 37.3 Hz).
The PPh3 group exhibits an AA′MX pattern centered at 39.5
X-ray Structural Analysis of Complex 11. Orange needle-
like crystals of complex 11 were obtained upon crystallization
from a pentane solution at room temperature. The single-crystal
X-ray structural analysis demonstrates that the rhodium atom
is located in the center of a distorted square planar arrangement.
The PPh3 ligand occupies the position trans to the methyne
carbon C(7). As in 2a, the angle corresponding to the benzylic
“bridge” C(1)-C(7)-Rh(1), was 91.1°. There is no distortion
from aromaticity. The structure of 11 is similar to that of Rh-
[CH2C6H(CH3)2(CH2PPh2)2](PPh3).3 The Rh(1)-P(2), Rh(1)-
P(3), Rh(1)-P(4), and Rh(1)-C(7) bond lengths in the chelate
core of 11 as well as the bond angles C(1)-C(7)-Rh(1), P(2)-

Methylene Transfer Reaction
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7727
Scheme 5
Rh(1)-P(3), P(2)-Rh(1)-P(4), and P(3)-Rh(1)-P(4) are
similar to their counterparts in the above-mentioned complex.
An ORTEP diagram of a molecule of 11 is shown in Figure 3.
Selected bond lengths and bond angles are given in Table 3.
X-ray Structural Analysis of Complex 16. Orange platelike
crystals of complex 16 were obtained upon crystallization from
a pentane solution at room temperature. The single-crystal X-ray
structural analysis demonstrates that the rhodium atom is located
in the center of a distorted square planar arrangement. The PPh3
ligand occupies the position trans to the methyne carbon C(108).
Similarly to 11 and 2a, the angle corresponding to the benzylic
“bridge”, C(108)-C(111)-Rh(1), is 92.71°. There is no distor-
tion from aromaticity. The structure of 16 is almost identical to
that of 11. An ORTEP diagram of a molecule of 16 is shown
in Figure 4. Selected bond lengths and bond angles are given
in Table 4.
mechanism is presented in Scheme 6.12 Interestingly, the rate-
determining step of the overall methylene-transfer reaction (2
f 3) appears to be the C-C reductive elimination rather than
metal insertion into the C-C bond as indicated by the following
considerations: (a) the overall irreversible process for 2a f
3a can be written as
k2
[2a] yk1z [A] y
\z [3a]
(2)
k-1
Since we did not observe accumulation of the postulated
intermediate A (or any other intermediate, Scheme 6), the
steady-state approximation can be applied, leading to the rate
eq 3. Inspection of the structure of intermediate A suggests that
V ) k1k2[2a]/(k-1 + k2)
(3)
Kinetic Study of the Transformation 2a f 3a. To gain
understanding of the methylene-transfer reaction mechanism,
we performed kinetic studies on the transformation of 2a into
3a. PhI oxidative addition to 1 in neat iodobenzene resulted in
a mixture of 2a and 3a (2a:3a ) 10:1). Starting from such a
mixture, the rate of the conversion of 2a to 3a was monitored
by 31P{1H} NMR spectroscopy at various temperatures. Rate
constants were determined by a single-exponential fit (eq 1)
(Figure 5).
the reverse process A f 2a, if it occurs, is expected to be
considerably slower than A f 3a. This is because of the higher
accessibility of the C-C bond to be cleaved in the latter case
and the more favorable transition state (TS-2, Scheme 6)
involving two five-membered chelates (as compared to the six-
membered ones). Thus, the rate equation can be approximated
as ν ) k1[2a]. (b) The rate of the transformation 2 f 3 is
decreased by electron-withdrawing substituents on the benzyl
group, following the order CH3 > H > (CF3)2 (t1/2 ) 2a, 1
day; 2b, 5 days; 2d, 12 h), as expected for a rate-determining
reductive elimination. It has been reported that electron-
withdrawing groups on the aryl decrease the rate of the CAryl-H
reductive elimination, while electron-donating substituents
display the opposite behavior.5 The strong substituent effect
observed in our system is compatible with a rate-determining
C-C reductive elimination. (c) Essentially the same rates are
t
obs
[I] ) [I]oe-k
(1)
The kinetic data is summarized in Table 5. Activation
parameters of 2a f 3a were determined by an Eyring plot
(Figure 6). It appears that the rate-determining step of the
reaction is C-C bond reductive elimination rather than the C-C
activation step, as discussed below.
(12) A reviewer suggested an alternative mechanism involving C-C bond
cleavage in 2 to form an intermediate methylidene complex, which could
then insert into the rhodium-aryl bond to generate complex 3. We believe
that such a sequence does not take place in our system since, as we have
already shown in similar systems,7,8,23 formation of the methylene bridge
(as in 2) is not required for the C-C cleavage step to occur, but rather a
direct insertion of the metal into the CAr-CMe takes place.
Discussion
sp2-sp3 C-C Reductive Elimination. In the reported
system, metal insertion into the C-C bond is most likely
preceded by sp2-sp3 C-C reductive elimination. A plausible

7728 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
Figure 5. Plot of ln[2a] vs time for the transformation of 2a to 3a in
C6D6 at 313 (×), 318 (2), 323 (9), and 338 K ([).
Figure 3. Perspective view (ORTEP) of complex 11. Hydrogen atoms
are omitted for clarity.
Figure 6. Eyring plot for the transformation of 2a to 3a in C6D6.
in organometallic chemistry, often representing the product-
forming step in a number of important stoichiometric and
catalytic reactions.15 However, it has not been studied exten-
sively since relatively few reductive elimination reactions
proceed from stable starting materials to stable products.16,17
In particular, examples of mechanistic investigation of C-C
reductive elimination from Rh(III) are extremely scarce.15e,18
We are aware of only one kinetic investigation of C-C reductive
elimination from Rh; in that report, the reductive elimination
process is induced by oxidation.18
The 2a f 3a transformation allowed a kinetic study of the
non-oxidatively induced C-C reductive elimination process.
Figure 4. Perspective view (ORTEP) of complex 16. Hydrogen atoms
are omitted for clarity.
(13) Rybtchinski, B.; Milstein, D. J. Am. Chem. Soc. 1999, 121, 4528.
(14) Gandelman, M.; Vigalok, A.; Konstantinovski, L.; Milstein. D.
Submitted for publication.
obtained in the presence or absence of 1 equiv of PPh3. Since
a side equilibrium between the phosphine and intermediate A
is expected, which would reduce the concentration of A, a
retardation effect might have resulted had the C-C activation
step been rate determining. Coordinative unsaturation is a prime
requirement in C-C activation by Rh(I).1a,13 (d) Single-step
metal insertion into a C-C bond in the Rh(CH3)[C6H(CH3)2-
(CH2N(C2H5)2)(CH2P(t-Bu)2)]Cl system, has been shown to
occur at temperatures as low as -70 °C. The resulting reaction
rates are higher by orders of magnitude than the ones obtained
in the 2 f 3 transformation.14
(15) (a) Atwood, J. D. Inorganic and Organometallic Reaction Mech-
anisms, 2nd ed.; VCH Publishers: New York, 1997; p 168 and references
therein. (b) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem.
Soc. 1998, 120, 2843. (c) Hill, G. S.; Puddephatt, R. J. Organometallics
1998, 17, 1478. (d) Mearcone, J. E.; Moloy, K. G. J. Am. Chem. Soc. 1998,
120, 8527. (e) Hahn. C.; Spiegler, M.; Herdtweck, E.; Taube, R. Eur. J.
Inorg. Chem. 1999, 435. (f) Huang, J.; Haar, C. M.; Nolan, S. P.
Organometallics 1999, 18, 297.
(16) Balazs, A. C.; Johnson, K. H.; Whitesides, G. M. Inorg. Chem. 1982,
21, 2162.
(17) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem.
Chem. Soc. Jpn. 1981, 54, 1857.
The reductive elimination reaction is a key transformation
(18) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 56.

Methylene Transfer Reaction
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7729
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for
11
Scheme 6
Rh(1)-C(7)
C(8)-C(7)
C(1)-C(7)
2.180(5)
1.497(7)
1.488(7)
Rh(1)-P(4)
Rh(1)-P(3)
Rh(1)-P(2)
2.2723(13)
2.3168(14)
2.3169(14)
C(1)-C(7)-Rh(1) 91.1(3)
P(3)-Rh(1)-P(4) 100.12(5)
C(7)-Rh(1)-P(3) 83.34(14)
P(3)-Rh(1)-P(2) 155.69(5)
C(7)-Rh(1)-P(4) 174.19(14) C(1)-C(7)-C(8)
121.7(4)
C(7)-Rh(1)-P(2) 79.54(14)
P(4)-Rh(1)-P(2) 98.46(5)
Rh(1)-C(7)-C(8) 121.2(4)
Table 4. Selected Bond Lengths (Å) and Bond Angles (deg) for
16
Rh(1)-C(111)
C(111)-C(112)
C(111)-C(108)
2.173(3)
1.507(4)
1.476(4)
Rh(1)-P(3)
Rh(1)-P(4)
Rh(1)-P(2)
2.2710(8)
2.2971(7)
2.3163(7)
99.28(3)
C(108)-C(111)-Rh(1) 92.71(17) P(3)-Rh(1)-P(4)
C(111)-Rh(1)-P(3)
C(111)-Rh(1)-P(4)
C(111)-Rh(1)-P(2)
P(4)-Rh(1)-P(2)
174.51(8) P(3)-Rh(1)-P(2)
100.67(3)
78.56(7)
82.79(7)
155.11(3)
C(108)-C(111)-C(112) 122.8(2)
Rh(1)-C(111)-C(112) 123.75(19)
Table 5. Kinetic Data for 2a f 3a Transformationa
T (K)
k2 × 10-5 (s-1
)
T (K)
k2 × 10-5 (s-1
)
313
318
7.0
323
333
22.3
39.3
12.5
a∆Hq ) 17 ( 3 kcal/mol; ∆Sq ) -23 ( 4 eu; ∆Gq ) 24 ( 4
313
kcal/mol.
strong C-C bonds at mild conditions.7,8 Here, not only does
the C-C cleavage process take place at room temperature but
it is not the rate-determining step of the overall methylene-
transfer reaction. Moreover, this is a unique example of a C-C
cleavage process in which a metal inserts into a relatively
sterically hindered C-C bond of a bulky benzyl group. In other
reported C-C cleavage processes occurring at room tempera-
ture, the metal inserts into an Ar-CH3 bond.7,8 Metal insertion
into an Ar-CH2CH3 bond takes place only under heating.23
The activation of the strong C-C bond most probably
proceeds through formation of a reactive, unsaturated 14e
intermediate, which is stabilized by the relatively large iodide
ligand. In the presence of PPh3, coordination of it to A might
take place reversibly, generating B in low concentration (Scheme
6).
It should be noted that the C-C cleavage product 3 is the
thermodynamic product in the 2 f 3 transformation, even when
the non-chelating aryl ring bears the electron-withdrawing CF3
groups (complexes 2b,c), which are expected to strengthen the
Rh-Ar bond by an inductive effect. Seemingly, the dominant
contribution to the relative thermodynamic stability of com-
plexes 2 and 3 is the higher stability of the five- vs the six-
membered bis-chelating ring.
Because of the inherent difficulties in studying C-C bond
oxidative addition directly, most of the insight into C-C
cleavage reactions has come from studying the microscopic
reverse reaction, namely, C-C bond reductive elimination.15a,c,22
The C-C activation and reductive elimination reactions reported
here are both intramolecular and the C-C bonds involved in
these reactions are of the same hybridization character, namely,
sp2-sp3 Ar-Bz bonds. Thus, the sp2-sp3 C-C bond activation
transition state should resemble the one for C-C bond reductive
elimination in the 2 f 3 transformation based on the similarity
of the bonds that are cleaved and formed (Scheme 6). The
activation parameters found for the C-C reductive elimination
The observation of a substantially negative entropy of activation
(∆Sq ) -23 ( 4 eu) for 2a f 3a suggests an organized
nonpolar three-centered transition state (TS-1, Scheme 6).19,20
The ∆Hq value of 17 ( 3 kcal/mol indicates that an early
transition state is involved. Activation parameters obtained in
our system are similar to the ones obtained for reductive
elimination of CH3CH3 from Pd(IV)21 and of ArAr from Pt-
(II).19
Notably, the reductive elimination process involved in the
transformation 2 f 3 takes place under mild conditions (room
temperature). The relatively low electron density on the Rh
center, due to phenyl (as compared to alkyl) substituents on
the phosphine ligand, together with coordinative unsaturation,
should favor C-C bond reductive elimination in complex 2.22
A contributing factor to the facility of the reductive elimination
may also be the favorable conversion of the six-membered bis-
chelating system in 2a-e to the five-membered one in 3a-e.
C-C Bond Activation Process. C-C bond activation with
the PCP-type complexes 1, bearing phenyl groups on the
diphosphine ligand, has been shown to take place upon heating
to 100-110 °C.2a,3,6 Remarkably, direct insertion into a strong
sp2-sp3 C-C bond can take place eVen at room temperature
when complex 1 is treated with an aryl iodide. High electron
density together with coordinative unsaturation should favor
C-C oxidative addition.15a,22 In the system studied here, the
phenyl phosphines impart a considerably lower electron density
on the metal center in comparison to tert-butylphosphines in
the PCP systems that have been shown to insert directly into
(19) Braterman, P. S.; Cross, R. J.; Young, G. B. J. Chem. Soc., Dalton
Trans. 1977, 1892.
(20) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933.
(21) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J.
D. Organometallics 1988, 7, 1363.
(22) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organo-Transition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987; p 322.
(23) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Gozin, M.;
Milstein, D. J. Am. Chem. Soc. 1998, 120, 13415.

7730 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
reaction indicate a nonpolar three-centered transition state (see
above). Therefore, the transition state involved in the adjacent
sp2-sp3 C-C oxidative addition is most probably of the same
character. Such a transition state was shown to be involved in
C-C bond activation in the t-Bu PCP and PCN ligand
systems7,8,24 and seems to be involved in the case of the less
basic phenyl phosphines as well.
Scheme 7
Alkyl Migration into the Bis-Chelating Ring. Competitive
C-C Coupling Reactions. Reaction of complex 7c with various
carbon nucleophiles results in migration of an alkyl group into
the bis-chelating ring (Schemes 4 and 5). This reaction is
significant for the regeneration of the methylene “bridge” in
the PCP-Rh complex that is active in the methylene-transfer
reaction. Moreover, complexes 11, 13, and 15 can potentially
be utilized for transferring benzylidene groups to other organic
moieties. Migration of alkyl groups to the chelating ring in
similar systems is reported in the case of Rh, Ir,25 and Ru 26
PCP systems and cationic Pt NCN systems.27 However, the
migration mechanisms are different from the one reported here
(1,2 sigmatropic shift in the cationic Rh and Ir PCP and Pt NCN
systems and migratory insertion to vinylidene in the Ru PCP
system).
After substitution of the bromide in 7c with the carbon
nucleophile, most probably a 5-coordinated unsaturated Rh(III)
complex bearing three σ-Rh-C bonds is obtained and C-C
reductive elimination may occur in three directions (Scheme
7): (a) coupling of the incoming alkyl with the BzCF3 ligand
generating an unsaturated Rh(I) complex that coordinates PPh3
to form complex 6; (b) coupling of the BzCF3 ligand with the
chelate ipso-carbon followed by a C-H activation process and
alkyl H reductive elimination generating the bridged complex
11; (c) coupling of the incoming alkyl with the chelate ipso-
carbon that in the case of the Me and the Bz ligands is followed
by C-H activation and BzCF3-H reductive elimination generat-
ing complexes 8 and 13, respectively. In the case of the Ph
ligand, coupling with the ipso-carbon cannot be continued
productively by a subsequent C-H activation and reductive
elimination process, but it might take place reversibly.
The reactions of 7c with various carbon nucleophiles lead to
the formation of the kinetic products, which are trapped by a
nonreversible C-C or C-H reductive elimination reaction that
forms the 4-coordinated 16e Rh(I) product (after coordination
of PPh3). The ratios between the complexes formed in the
reactions are the reflection of the coupling aptitudes of the
σ-bound carbon atoms.
there is no evidence for the formation of 11, the product of the
competitive sp2-sp3 BzCF3-Cipso coupling.
When comparing the two possibilities for sp2-sp3 C-C
coupling, breaking of a weaker or less sterically hindered bond
should be preferred. It is conceivable that electron-withdrawing
substituents on the benzyl group strengthen the Rh-CH2-Ar
bond. Thus, the observed migratory aptitudes of the benzyl
groups can be explained in terms of the Rh-C bond strength.
The clear preference for the migration of the BzOCH3 over the
unsubstituted benzyl in the reaction of 3e with BzMgCl supports
this hypothesis.
In the reaction of 7c with PhLi, two competitive sp2-sp3
C-C coupling reactions take place. Notably, there is no
preference for coupling of the Ph-BzCF3 over the coupling of
the Cipso-BzCF3, although in the latter case, the Rh-C bond
of the PCP chelate is broken. The carbon nucleophile most likely
approaches the rhodium center from the direction of the vacant
coordination site of 7c, i.e., trans to the BzCF3 ligand. In that
case, an isomerization process prior to the Ph-BzCF3 coupling
would be necessary, to bring the two ligands into cis arrange-
ment while the BzCF3 group and the ipso-carbon are already
in cis configuration. This restriction might slow the BzCF3-
Ph coupling process with respect to the BzCF3-Cipso one,
resulting in the observed 1:1 ratio between the two products.
Such an isomerization process would be required in the reactions
of 7c with other carbon nucleophiles as well, which can also
explain the preferred migrations of alkyls into the ring rather
than their coupling with the other available ligand.
In the reaction of 7c with alkyl nucleophiles (MeLi, BzMgCl),
there is a clear preference of sp2-sp3 over sp3-sp3 C-C
coupling as indicated by the products ratio (8 or 13 vs 6)
(Scheme 4), although the sp2-sp3 C-C coupling involves
breaking of a chelate C-Rh bond.28 Remarkably, in both cases,
(24) (a) van der Boom, M. E.; Ben-David, Y.; Milstein, D. J. Am. Chem.
Soc. 1999, 121, 6652. (b) van der Boom, M. E.; Ben-David, Y.; Milstein,
D. Chem. Commun. 1998, 917.
(25) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 895.
(26) Lee, H. M.; Yao, J.; Jia, G. Organometallics 1997, 16, 3927.
(27) (a) Albrecht, M.; Gossage, R. A.; Spek, A. L.; van Koten, G. J.
Am. Chem. Soc. 1999, 121, 11898. (b) Grove, D. M.; van Koten, G.;
Louwen, J. N.; Noltes, J. G.; Spek, A. L.; Ubbels, H. J. C. J. Am. Chem.
Soc. 1982, 104, 6609. (c) van Koten, G.; Timmer, K.; Noltes, J. G.; Spek,
A. L. J. Chem, Soc., Chem. Commun. 1978, 250.
(28) sp2-sp3 C-C bond reductive elimination was shown to be preferred
over an sp3-sp3 one: (a) Loar, M. K.; Stille, J. K. J. Am. Chem. Soc. 1981,
103, 4174. (b) Goddard, W. A., III. J. Am. Chem. Soc. 1986, 108, 6115. (c)
Calhorda, M. J.; Brown, J. M.; Cooley, N. A. Organometallics 1991, 10,
1431. (d) Thompson, J. S.; Atwood, J. D. Organometallics 1991, 10, 3525.
(e) Kruis, D.; Markies, B. A.; Canty, A. J.; Boersma, J.; van Koten, G. J.
Organometall. Chem. 1997, 532, 235.
In conclusion, a unique intramolecular methylene-transfer
system, combining consecutive C-C reductive elimination and
C-C activation processes was discovered and investigated from
both mechanistic and synthetic points of view. The methylene
transfer was shown to occur at room temperature regardless of

Methylene Transfer Reaction
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7731
3.37 (b s, 4H, CH2P), 1.88 (s, 3H, ArCH3), 1.78 (distorted d, J ) 1.8
Hz, 6H, ArCH3). 13C{1H} NMR (CDCl3) δ: 138.4 (d, 1JPC ) 17.2 Hz,
Ar), 135.8 (t, 3JPC ) 3.8 Hz, Ar), 134.6 (t, JPC ) 3.6 Hz, Ar), 133.1 (d,
2JPC ) 18.8 Hz, Ar), 131.6 (m, JPC ) 6.0 Hz, JPC ) 2.7 Hz, Ar), 129.9
electronic or steric effects. Intermediates that until now were
only postulated were identified and characterized, allowing the
kinetic investigation of the reaction. The rate-determining step
was found to be the C-C reductive elimination rather than the
C-C activation, and the observed activation parameters indicate
a nonpolar three-centered transition state.
4
3
(t, JPC ) 2.2 Hz, Ar), 128.57 (s, Ar), 128.2 (d, JPC ) 6.7 Hz, Ar),
30.0 (d, 1JPC ) 18.0 Hz, CH2P), 20.3 (dt, JPC ) 2.6 Hz, JPC ) 1.3 Hz,
4
ArCH3), 16.7 (t, JPC ) 7.0 Hz, ArCH3).
The methylene transfer takes place also in the reverse
direction, i.e., into the bis-chelating ring system. This process
is kinetically controlled (by a subsequent elimination process),
serving as a key transformation for the regeneration of the
methylene “bridge” in the methylene-transfer process. The
migration of benzyl group into the complex’s bis-chelating ring
is selective and is dominated by the electronic properties of the
migrating group.
Synthesis of r,r′-Diphenylphosphino-m-xylene (DPPX). A solu-
tion of BuLi in hexanes (95 mL, 1.6 M, 152 mmol) was added at -40
°C to a solution of HPPh2 (25.67 g, 138 mmol) in THF (250 mL).
After the addition was complete, the reaction mixture was allowed to
warm to 0 °C. It was then cooled to -70 °C, and a solution of R,R′-
dichloro-m-xylene (10.5 g, 60 mmol) in THF (300 mL) was added to
it dropwise. The reaction mixture was allowed to warm to room
temperature and stirred overnight, followed by refluxing for 1 h. The
solvent was removed in vacuo, and the solid residue was dissolved in
ether (400 mL). H2O (20 mL) was added to the solution, the organic
layer was separated, dried over Na2SO4, filtered, and concentrated to
30 mL. Pentane (50 mL) was added to the concentrated etherial solution
of the product, and the vessel was cooled at -30 °C for overnight.
The resulting white crystals were filtered, washed with 3 × 10 mL of
cold pentane, and dried in a vacuum, yielding 27.3 g (96%) of the
DPPX ligand.
Experimental Section
General Procedures. All experiments with metal complexes and
phosphine ligands were carried out under an atmosphere of purified
nitrogen in a Vacuum Atmospheres glovebox equipped with a MO 40-2
inert gas purifier or using standard Schlenk techniques. All solvents
were reagent grade or better. All nondeuterated solvents were refluxed
over sodium/benzophenone ketyl and distilled under argon atmosphere.
Deuterated solvents were dried over 4-Å molecular sieves. All the
solvents were degassed with argon and kept in the glovebox over 4-Å
molecular sieves. Commercially available reagents were used as
received. [Rh(COE)2Cl]2 (COE ) cyclooctene) was prepared according
to literature procedures.29 Complex 1 was previously reported3 but
synthetic details were not given.
Characterization of DPPX. 31P{1H} NMR (CDCl3) δ: -10.32 (s).
1H NMR (CDCl3) δ: 7.90 (b s, 8H, Ar), 7.84 (b s, 12H, Ar), 7.5
(distorted t, 3JHH ) 7.5 Hz, 1H, Ar), 7.44 (s, 1H, Ar), 7.3 (distorted d,
3JHH ) 7.4 Hz, 2H, Ar), 3.86 (s, 4H, CH2P). 13C{1H} NMR (CDCl3)
δ: 138.9 (d, 1JPC ) 15.4 Hz, Ar), 137.9 (dd, 2JPC ) 8.1 Hz, 4JPC ) 1.2
2
3
Hz, Ar), 133.5 (d, JPC ) 18.5 Hz, Ar), 131.1 (t, JPC ) 6.9 Hz, Ar),
3
129.24 (s, Ar), 128.9 (d, JPC ) 6.6 Hz, Ar), 128.7 (b s, Ar), 127.6
(dd, 3JPC ) 6.9 Hz, 5JPC ) 2.3 Hz, Ar), 36.5 (d, 1JPC ) 15.9 Hz, CH2P).
Synthesis of 1. A solution of DPPM ligand (86 mg, 0.167 mmol)
and PPh3 (44 mg, 0.167 mmol) in THF (20 mL) was added dropwise
at room temperature to a solution of [Rh(COE)2Cl]2 (60 mg, 0.084
mmol) in THF (20 mL). The reaction mixture was stirred for 12 h at
70 °C producing Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2](H)(Cl) and 1 (
in 3:7 ratio by 31P{1H} NMR, respectively). The solution was
evaporated to dryness and dissolved in benzene (50 mL). MeLi (50
µL, 1.0 M, 0.050 mmol) was added to the solution, showing an
immediate change in color from yellow to dark brown and a complete
conversion to 1 based on 31P{1H} NMR. The solution was filtered and
dried under vacuum to give 133 mg (87% yield) of the product 1.
Characterization of Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2](H)(Cl):
1H, 13C{1H} NMR and 31P NMR spectra were recorded at 400, 100,
and 162 MHz, respectively, using a Bruker AMX-400 NMR spectrom-
eter. All the NMR measurements were performed in C6D6, unless
1
otherwise specified. H NMR and 13C{1H} NMR chemical shifts (δ)
are reported in ppm downfield from tetramethylsilane. 1H NMR
chemical shifts are referenced to the residual hydrogen signal of the
deuterated solvent (7.15 ppm benzene). In 13C{1H} NMR measurements,
the signal of C6D6 (128.00 ppm) was used as a reference. 31P NMR
chemical shifts are reported in ppm downfield from H3PO4 and
referenced to an external 85% solution of phosphoric acid in D2O. 13C-
1H heteronuclear correlation was measured using a 5-mm inverse
probehead with a z-gradient coil. Screw-cap 5-mm NMR tubes were
used in the NMR follow-up experiments. Abbreviations used in the
description of NMR data: Ar, aryl; Ar′, nonchelate aryl; b, broad; s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; v, virtual.
Electrospray (ES) mass spectrometry was performed using a MicroMass
LCZ detector 4000 with CV of 43 V, temperature of 150 °C, and EE
of 4.2 V. GC/MS was performed using a Saturn 2000 Varian instrument
with SPB-5 Supelco column of 30 m × 0.25 mm and a temperature
program of 15 °C/min, 30-240 °C. Elemental analyses were performed
at the Hebrew University of Jerusalem.
1
2
31P{1H} NMR (THF-d8): δ 69.4 (dd, JRhP ) 136.0 Hz, JPP ) 11.4
1
1
Hz, 2P, PPh2), 27.7 (dt, JRhP ) 99.5 Hz, 1P, PPh3). H NMR (THF-
d8): δ 8.5 (m, Ar), 7.54 (t, J ) 7.3 Hz, Ar), 7.45 (t, J ) 7.3 Hz, Ar),
7.37 (b s, Ar), 7.16 (b t, J ) 7.8 Hz, Ar), 6.94 (t, J ) 7.4 Hz, Ar), 6.90
(d, J ) 6.0 Hz, Ar), 6.83 (t, J ) 7.5 Hz, Ar), 6.59 (b s, Ar), 4.1 (dvt,
left part of ABq, 2JHH ) 13.9 Hz, 2JPH ) 4.0 Hz, 2H, CH2P), 3.4 (b d,
2
right part of ABq, JHH ) 13.9 Hz, 2H, CH2P), 2.9 (m, J ) 10.4 Hz,
J ) 9.1 Hz, 2H, CH2Rh), 2.42 (s, 6H, ArCH3), -20.3 (tt, J ) 12.9 Hz,
J ) 3.2 Hz, 1H, HRh); (C6D6): δ 8.58 (m, Ar), 7.76 (m, Ar), 7.19 (m,
Ar), 6.90 (m, Ar), 6.81 (t, J ) 6.4 Hz, Ar), 6.66 (m, Ar), 6.55 (m, Ar),
6.39 (b s, Ar), 6.05 (b t, J ) 7.9 Hz, Ar), 3.94 (dvt, left part of ABq,
2JHH ) 13.9 Hz, 2JPH ) 4.2 Hz, 2H, CH2P), 3.40 (q, J ) 10.5 Hz, J )
9.2 Hz, 2H, CH2Rh), 3.34 (b d, J ) 14.4 Hz, 2H, CH2P), 2.36 (s, 6H,
ArCH3), -20.1 (tt, J ) 12.9 Hz, J ) 3.7 Hz, 1H, HRh). 13C{1H} NMR
(THF-d8): δ 154.2 (dq, 3JPC ) 7.2 Hz, J ) 1.3 Hz, Ar), 141.3 (vt, 2JPC
) 17.6 Hz, Ar), 137.4 (b d, 1JPC ) 33.0 Hz, Ar), 136.4 (t, 3JPC ) 13.7
Hz, Ar), 136.11 (b s, Ar), 135.4 (d, J ) 10.3 Hz, Ar), 134.8 (b d, J )
8.8 Hz, Ar), 132.7 (d, J ) 9.5 Hz, Ar), 132.2 (dd, J ) 12.8 Hz, J )
1.7 Hz, Ar), 131.5 (vt, J ) 4.4 Hz, Ar), 132 (unresolved aromatic
peaks), 128.3 (dt, J ) 42.4 Hz, Ar), 127.5 (m, Ar), 125.0 (d, J ) 2.3
Hz, Ar), 39.9 (vt, 1JPC ) 13.4 Hz, CH2P), 19.87 (s, ArCH3), 16.9 (ddt,
2JPC,trans ) 55.8 Hz, J ) 11.5 Hz, J ) 4.5 Hz, CH2Rh). IR (film): 2078
Synthesis of 2,4-Bis(diphenylphosphinomethyl)mesitylene (DPPM).
A solution of BuLi in hexanes (57 mL, 1.6 M, 91.2 mmol) was added
at -40 °C to a solution of HPPh2 (15.0 g, 80.6 mmol) in THF (150
mL). After the addition was complete, the reaction mixture was allowed
to warm to 0 °C. It was cooled to -70 °C, and a solution of 2,4-bis-
(chloromethyl)-mesitylene (8.2 g, 37.8 mmol) in THF (200 mL) was
added to it dropwise. The reaction mixture was allowed to warm to
room temperature and stirred overnight, followed by refluxing for 1 h.
The solvent was removed in vacuo, and the solid residue was dissolved
in ether (300 mL). H2O (15 mL) was added to the solution; the organic
layer was separated, dried over Na2SO4, filtered. and concentrated to
20 mL. Pentane (50 mL) was added to the concentrated etherial solution
of the product, and the vessel was cooled at -30 °C for overnight.
The resulting white crystals were isolated by filtration, washed with 3
× 10 mL of cold pentane, and dried in a vacuum, yielding 17.4 g (89%)
of the DPPM ligand.
cm-1 (nHRh).
1
Characterization of 1. 31P{1H} NMR (C6D6): δ 66.7 (dd, JRhP
)
2
1
Characterization of DPPM. 31P{1H} NMR (CDCl3) δ: -16.31 (s).
192.6 Hz, JPP ) 31.6 Hz, 2P, PPh2), 40.2 (dt, JRhP ) 154.1 Hz, 1P,
1
1H NMR (CDCl3) δ: 7.42-7.29 (m, 20H, Ar), 6.63 (b s, 1H, Ar),
PPh3). H NMR (C6D6): δ 7.9 (m, 4H, aromatic), 7.2-7.1 (m, 14H,
aromatic), 6.8-6.4 (m, 18H, aromatic), 3.9 (ddvt, left part of ABq,
(29) Herde, J. L.; Senoff, C.V. Inorg. Nucl. Chem. Lett. 1971, 7, 1029.
2JHH ) 13.8 Hz, 2JPH ) 4.7 Hz, 4JPH ) 1.2 Hz, 2H, CH2P), 3.44 (right

7732 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
2
2
part of ABq, JHH ) 13.8 Hz, 2H, CH2P), 2.7 (m, JRhH ) 2.3 Hz,
3JPH,cis ) 5.8 Hz, 3JPH,trans ) 11.9 Hz, 2H, CH2Rh), 2.30 (s, 6H, ArCH3).
13C{1H} NMR (C6D6): δ 145.0 (m, 3JPC ) 6.7 Hz, ArC), 140.5 (vtm,
(d, 1JRhC ) 38.0 Hz, Cipso), 39.5 (t, 2JPC ) 13.5 Hz, ArCH2P), 23.2 (d,
1JRhC ) 27.0 Hz, Ar′CH2Rh), 22.7 (s, ArCH3). The assignment was
confirmed by a 13C-DEPT experiment. Anal. Calcd: C, 59.87; H, 4.66.
Found: C, 60.31: H, 5.23.
2JPC ) 14.1 Hz, J ) 2.4 Hz, ArC), 138.9 (dvt, 1JPC ) 31.3 Hz, 3JPC
)
2.5 Hz, ArCPPh3), 137.5 (t, 3JPC ) 10.7 Hz, ArCPPh2), 135.0 (vt, 2JPC
Reaction of 1 with 1-Iodo-3,5-bis(trifluoromethyl)benzene. Syn-
thesis of 2b and 3b. 1-Iodo-3,5-bis(trifluoromethyl)benzene (15 µL,
0.085 mmol) was added to a benzene-d6 solution (0.5 mL) of 1 (15
mg, 0.017 mmol). The mixture was allowed to stay at room temperature
for 48 h, resulting in color change from orange to purple-red. 31P{1H}
NMR revealed quantitative formation of two products, 2b and 3b (2b:
3b ) 4:1) and an equivalent amount of PPh3. Complex 2b was too
unstable for isolation and was characterized in solution. Upon heating
of the mixture of 2b and 3b at 70 °C for 2 h, complex 2b was
quantitatively converted into 3b. Precipitation from pentane gave a
purple-red solid in 75% yield. Complex 3b is soluble in benzene and
2
) 7.7 Hz, ArCHPPh3), 134.2 (d, JPC ) 12.9 Hz, ArCHPPh2), 131.6
(vt, 3JPC ) 5.2 Hz, ArC), 130.69 (s, ArCHPPh3), 129.34 (s, ArCHPPh2),
3
3
127.6 (vt, JPC ) 14.9 Hz, ArCHPPh3), 127.1 (m, JPC ) 8.8 Hz,
ArCHPPh2), 121.29 (s, ArCH), 36.9 (vt, 1JPC ) 11.4 Hz, CH2P), 20.09
2
(s, ArCH3), 17.4 (dm, JPC,trans ) 39.4 Hz, ArCH2Rh).
Synthesis of 6. A solution of DPPX ligand (95 mg, 0.200 mmol)
and PPh3 (53 mg, 0.200 mmol) in 20 mL of THF was added dropwise
to a solution of [RhCl(COE)2]2 (72 mg, 0.100 mmol) in 20 mL of THF
at room temperature. The reaction mixture was stirred for 12 h at 80
°C to form Rh(PPh3)[C6H3(CH2PPh2)2](H)(Cl). The volatiles of the
solution were evaporated, the solid was redissolved in THF (30 mL),
and 100 equiv of NaH (240 g, 10 mmol) was added to the solution.
The reaction mixture was stirred for 24 h at room temperature, showing
the formation of 6 by 31P{1H} NMR. The solution was evaporated, the
remaining solid was dissolved in benzene (30 mL), and the solution
was filtered off and dried under high vacuum to give 146 mg (83%
yield) of the product 6.
THF.
1
Characterization of 2b. 31P{1H} NMR (C6D6): δ 23.2 (d, JRhP
)
118.9 Hz). 1H NMR (C6D6): δ 8.36 (m, 4H, ArH), 7.47 (b s, 2H, ArH),
7.28 (m, 8H, ArH), 7.11 (m, 4H, ArH), 6.84 (m, 2H, ArH), 6.74 (m,
2
4H, ArH), 3.55 (b d, left part of ABq, JHH ) 14.6 Hz 2H, ArCH2P),
3.23 (vt, 3JHP ) 8.1 Hz, 2H, ArH2Rh), 3.01 (dt, right part of ABq, 2JPH
) 4.5 Hz 2H, ArCH2P), 1.93 (s, 6H, ArCH3). 19F{1H} NMR (C6D6):
δ -62.20 (s, CF3). Selected signals for 13C{1H} NMR (C6D6): δ 153.7
(d, 2JRhC ) 44.6 Hz, Ar′Cipso), 28.6 (t, 1JPC ) 13.1 Hz, CH2P), 28.5 (b
Characterization of Rh(PPh3)[C6H3(CH2PPh2)2](H)(Cl). 31P{1H}
NMR (THF): δ 47.0 (dd, 1JRhP ) 111.1 Hz, 2JPP ) 24.3 Hz, 2P, PPh2),
1
1
1
d, JPC ) 17.7 Hz, CH2Rh), 19.5 (s, CH3Ar). The assignment was
19.8 (dt, JRhP ) 82.8 Hz, 1P, PPh3). H NMR (CD2Cl2): δ 7.5-6.8
confirmed by a 13C-DEPT.
2
2
(m, 38H, aromatic), 4.55 (dvt, left part of ABq, JHH ) 15.2 Hz, JPH
2
Characterization of 3b. 31P{1H} NMR (C6D6): δ 33.1 (d, JRhP
)
1
) 3.8 Hz, 2H, CH2P), 3.74 (dvt, right part of ABq, JHH ) 15.2 Hz,
2JPH ) 4.6 Hz, 2H, CH2P), -16.9 (m (ddt), J ) 12.8 Hz, J ) 12.3 Hz,
1JRhH ) 22.7 Hz, 1H, HRh). 13C{1H}NMR (CD2Cl2): δ 166.7 (ddt,
124.7 Hz). 1H NMR (C6D6): δ 6.7-8.2 (m, 24H, ArH), 3.95 (vq, 3JHP
) 3.9 Hz, 2H, Ar′CH2Rh), 3.88 (dt, left part of ABq, 2JHH ) 17.4 Hz,
2JPH ) 5.3 Hz, 2H, ArCH2P), 3.41 (b d, right part of ABq, 2H, ArCH2P),
2.24 (s, 6H, ArCH3). 19F{1H} NMR (C6D6): δ -62.45 (s, CF3). Selected
signals for 13C{1H} NMR (C6D6): δ 172.2 (d, 2JRhC ) 40.2 Hz, Cipso),
2JPC,trans ) 99.3 Hz,2JPC,cis ) 3.6 Hz, JRhC ) 25.8 Hz, Cipso), 144.6
1
(dvt, 2JPC ) 8.3 Hz, 2JPC ) 1.5 Hz, ArC), 135.5 (m), 134.4 (d, 2JPC
)
11.2 Hz), 133.9 (t, JPC ) 5.2 Hz), 133.6 (t, JPC ) 5.2 Hz), 130.0 (d,
JPC ) 7.3 Hz), 129.2 (d, J ) 1.8 Hz), 128.82 (b s), 128.2 (dt, J ) 5.9
Hz, J ) 4.7 Hz), 127.8 (d, J ) 8.9 Hz), 124.40 (s), 122.3 (dt, J ) 8.8
1
1
39.3 (t, JPC ) 15.1 Hz, CH2P), 22.39 (s, CH3Ar), 18.23 (b d, JPC
)
29.2 Hz, CH2Rh). The assignment was confirmed by a 13C-DEPT and
C-H correlation experiments. Anal. Calcd: C, 53.88; H, 3.79. Found:
C, 54.21; H, 4.09.
1
3
Hz, J ) 4.7 Hz), 47.5 (ddvt, JPC ) 16.8 Hz, JPC ) 7.5 Hz, J ) 2.2
Hz, CH2P). IR (Film): 2107 cm-1 (nRhH).
Characterization of 6. 31P{1H} NMR (C6D6): δ 50.7 (dd, JRhP
)
1
Reaction of 1 with p-(Trifluoromethyl)iodobenzene. Synthesis of
2c and 3c. p-(Trifluoromethyl)iodobenzene (13 µL, 0.085 mmol) was
added to 0.5 mL of a benzene-d6 solution of 1 (15 mg, 0.017 mmol).
The mixture was allowed to stay at room temperature for 48 h, resulting
in a color change from orange to purple-red. 31P{1H} NMR revealed
quantitative formation of two products, 2c and 3c (2c:3c ) 4:1), and
an equivalent amount of PPh3. Complex 2c was too unstable for
isolation and was characterized in solution. Upon heating at 70 °C for
2 h, complex 2c was quantitatively converted into 3c. Precipitation
from pentane gave complex 3c as a purple-red solid in 75% yield.
2
1
161.6 Hz, JPP ) 30.6 Hz, 2P, PPh2), 38.9 (dt, JRhP ) 121.6 Hz, 1P,
1
PPh3). H NMR (C6D6): δ 7.6-7.5 (m, 14H, aromatic), 6.9-6.7 (m,
21H, aromatic), 6.34 (b s, 1H, ArH), 3.94 (vt, 2JPH ) 3.1 Hz, 4H, CH2P).
13C{1H} NMR (C6D6): δ 178.4 (ddt, 2JPC,trans ) 78.8 Hz, 2JPC,cis ) 7.7
1
2
2
Hz, JRhC ) 31.9 Hz, Cipso), 148.3 (ddvt, JPC ) 11.2 Hz, JPC ) 2.3
3
1
3
Hz, JPC ) 1 Hz, Ar), 139.5 (dt, JPC ) 30.0 Hz, JPC ) 2.2 Hz, Ar),
3
1
2
138.0 (td, JPC ) 16.8 Hz, JPC ) 1.7 Hz, Ar), 134.7 (d, JPC ) 13.5
Hz, Ar), 133.6 (dt, 2JPC ) 6.2 Hz, Ar), 121.5 (dvt, 3JPC ) 9.7 Hz, 4JPC
3
) 2.8 Hz, Ar), 128.55 (s, Ar), 128.6 (d, JPC ) 7.4 Hz, Ar), 128.4 (d,
4JPC ) 1.5 Hz, Ar), 127.4 (d, JPC ) 8.8 Hz, Ar), 124.1 (s, Ar), 49.9
3
Complex 3c is soluble in benzene and THF.
1
3
1
Characterization of 2c. 31P{1H} NMR (C6D6): δ 24.7 (d, JRhP
)
(ddvt, JPC ) 13.7 Hz, JPC ) 7.7 Hz, J ) 2.8 Hz, CH2P).
123.0 Hz). 1H NMR (C6D6): δ 6.5-7.9 (m, 25H, ArH), 3.57 (b d, left
Reaction of 1 with Iodobenzene. Synthesis of 2a and 3a. Complex
1 (25 mg, 0.028 mmol) was dissolved in 1 mL (4.9 mmol, 300 equiv)
of iodobenzene and stirred for 4 h at room temperature, resulting in a
color change from brown to red. 31P{1H} NMR revealed quantitative
formation of 2a and 3a (2a:3a ) 10:1) and an equivalent amount of
PPh3. The iodobenzene solution was concentrated and the products were
precipitated from cold pentane giving a red solid of 2a and 3a mixture
(2a:3a ) 10:1) in 70% yield. Upon heating at 70 °C for 2 h, complex
2a underwent quantitative conversion into 3a. Complex 3a is soluble
2
3
part of ABq, JHH ) 14.5 Hz, 2H, ArCH2P), 3.18 (t, JHP ) 8.6 Hz,
2H, ArCH2Rh), 3.14 (dt, right part of ABq, 2JPH ) 4.6 Hz 2H, ArCH2P),
2.03 (s, 6H, ArCH3). 19F{1H} NMR (C6D6): δ -61.20 (s, 3F, CF3).
Selected signals for 13C{1H} NMR (C6D6): δ 158.3 (d, JRhC ) 40.2
2
1
1
Hz, Ar′Cipso), 29.3 (t, JPC ) 13.1 Hz, CH2P), 27.9 (b d, JPC ) 17.1
Hz, CH2Rh), 19.9 (s, CH3Ar).
Characterization of 3c. 31P{1H} NMR (C6D6): δ 32.6 (d, JRhP
)
1
126.5 Hz). 1H NMR (C6D6): δ 6.5-7.9 (m, 25H ArH), 4.13 (vq, 3JHP
) 3.5 Hz, 2H, Ar′CH2Rh), 3.82 (dt, left part of ABq, 2JHH ) 17.0 Hz,
2JPH ) 5.6 Hz, 2H, ArCH2P), 3.31 (b d, right part of ABq, 2H, ArCH2P),
2.21 (s, 6H, ArCH3). 19F{1H} NMR (C6D6): δ -62.08 (s, 3F, CF3).
in benzene and THF.
Characterization of 2a. 31P{1H} NMR (C6D6): δ 26.0 (d, JRhP
)
1
127.0 Hz). 1H NMR (C6D6): δ 7.7-6.5 (m, 26H, ArH), 3.58 (b d, left
2
3
2
part of ABq, JHH ) 14.5 Hz, 2H, ArCH2P), 3.31 (vt, JPH ) 8.0 Hz,
Selected signals for 13C{1H} NMR (C6D6): δ 172.8 (d, JRhC ) 40.2
2
2H, ArCH2Rh), 3.25 (dt, right part of ABq, JPH ) 4.6 Hz, ArCH2P),
Hz, Cipso), 39.3 (t, 1JPC ) 13.1 Hz, CH2P), 22.7 (s, CH3Ar), 20.3 (b d,
1JPC ) 26.2 Hz, CH2Rh). Anal. Calcd: C, 56.65; H, 4.19. Found: C,
57.29; H, 4.65.
2.05 (s, 6H, 2 CH3Ar). Selected signals for 13C{1H} NMR (C6D6): δ
2
1
29.6 (t, JPC ) 13.0 Hz, ArCH2P), 27.1 (d, JRhC ) 17.8 Hz, ArCH2-
Rh), 20.91 (s, ArCH3). The assignment was confirmed by a 13C-DEPT
Reaction of 1 with p-Iodotoluene. Synthesis of 2d and 3d.
p-Iodotoluene (18.6 mg, 0.085 mmol) was added to a benzene-d6
solution (0.5 mL) of 1 (15 mg, 0.017 mmol). The mixture was allowed
to stay at room temperature for 48 h, resulting in a color change from
orange to dark red. 31P{1H} NMR revealed quantitative formation of
two products, 2d and 3d (2d:3d ) 1: 4), and an equivalent amount of
PPh3. Complex 2d was too unstable for isolation and was characterized
experiment.
Characterization of 3a. 31P{1H} NMR (C6D6): δ 33.2 (d, JRhP
)
1
129.5 Hz). 1H NMR (C6D6): δ 8.5-6.5 (m, 26H, ArH), 4.4 (vq, 2JRhH
2
) 4.0 Hz, 2H, Ar′CH2Rh), 3.9 (dt, left part of ABq, JHH ) 17.0 Hz,
2JHP ) 5.0 Hz, 2H, CH2P), 3.4 (b d, right part of ABq, 2H, CH2P), 2.3
(s, 6H, ArCH3). Selected signals for 13C{1H} NMR (C6D6): δ 173.7

Methylene Transfer Reaction
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7733
in solution. Upon heating at 70 °C for 30 min, complex 2d was
quantitatively converted into 3d. Complex 3d is soluble in benzene
and THF and moderately soluble in pentane. Due to the similar
solubility of the complex and PPh3 in organic solvents they could not
2H, ArCH2P). 19F{1H} NMR (C6D6): δ -62.45 (s, CF3). Selected
2
signals for13C{1H} NMR (C6D6): δ 167.9 (d, JRhC ) 30.2 Hz, Cipso),
39.7 (t, 1JPC ) 15.1 Hz, CH2P), 17.5 (d, 1JPC ) 30.2 Hz, CH2Rh). The
assignment was confirmed by a 13C-DEPT experiment.
be separated by fractional crystallization.
Synthesis of 7c. p-(Trifluoromethyl)benzyl bromide (2.8 µL, 0.018
mmol) was added to a benzene-d6 solution (0.5 mL) of 6 (15 mg, 0.018
mmol) at room temperature, resulting in a color change from yellow
to dark brown. 31P{1H} NMR revealed quantitative formation of the
oxidative addition product 7c and equivalent amount of PPh3. It was
1
Characterization of 2d. 31P{1H} NMR (C6D6): δ 25.8 (d, JRhP
)
127.5 Hz). 1H NMR (C6D6): δ 8.0-6.7 (m, 25H, ArH), 3.66 (b d, left
2
3
part of ABq, JHH ) 14.4 Hz, 2H, ArCH2P), 3.38 (t, JHP ) 8.5 Hz,
2H, ArCH2Rh), 3.34 (dt, right part of ABq,2JPH ) 4.5 Hz, 2H, ArCH2P),
2.24 (s, 3H, tolArCH3), 2.12 (s, 6H, ArCH3). Selected signals for
impossible to separate the compound from PPh3.
1
13C{1H} NMR (C6D6): δ 29.9 (t, JPC ) 16.1 Hz, CH2P), 27.0 (b d,
1
Characterization of 7c. 31P{1H} NMR (C6D6): δ 27.2 (d, JRhP
)
1JPC ) 18.8 Hz, CH2Rh), 20.2 (s, tolCH3), 19.8 (s, CH3Ar).
127.8 Hz). 1H NMR (C6D6): δ 6.5-8.2 (m, 27H, ArH), 4.1 (b q, 2JPH
Characterization of 3d. 31P{1H} NMR (C6D6): δ 32.9 (d, JRhP
)
1
2
) 3.4 Hz, 2H, Ar′CH2Rh), 3.6 (dt, left part of ABq, JHH ) 17.0 Hz,
131.2 Hz). 1H NMR (C6D6): δ 8.3-6.2 (m, 25H, ArH), 4.33 (vq, 3JHP
) 3.5 Hz, 2H, Ar′CH2Rh), 3.87 (dt, left part of ABq, 2JHH ) 17.0 Hz,
2JPH ) 5.3 Hz, 2H, ArCH2P), 3.45 (b d, right part of ABq, 2H, ArCH2P),
2.26 (s, 6H, ArCH3), 1.64 (s, 3H, tolCH3). Selected signals for 13C{1H}
NMR (C6D6): δ 173.7 (d, 2JRhC ) 30.2 Hz, Cipso), 39.6 (t, 1JPC ) 14.1
Hz, CH2P), 23.8 (b d, 1JPC ) 26.2 Hz, CH2Rh), 22.7 (s, CH3Ar), 21.8
(s, tolCH3).
2JPH ) 5.0 Hz, 2H, ArCH2P), 3.4 (dt, right part of ABq, 2JPH ) 4.0 Hz,
2H, ArCH2P). 19F{1H} NMR (C6D6): δ -62.37 (s, CF3). Selected
2
signals for13C{1H} NMR (C6D6): δ 168.2 (d, JRhC ) 36.1 Hz, Cipso),
39.7 (t, 1JPC ) 14.5 Hz, CH2P), 19.5 (d, 1JPC ) 25.2 Hz, CH2Rh). The
assignment was confirmed by a 13C-DEPT and a 13C-1H correlation
experiment.
Reaction of 7c with MeLi. MeLi (1.0 M solution in THF/cumene,
18 µL, 0.018 mmol) was added to a benzene-d6 solution (0.5 mL) of
7c (15 mg, 0.018 mmol) at room temperature in the presence of 1 equiv
of PPh3, resulting immediately in color change from dark red to orange.
31P{1H} NMR revealed quantitative formation of the two known3b
complexes 6 and 8 (6:8 ) 1: 3) and two organic compounds 9 and 10,
Reaction of 1 with p-Iodoanisole. Synthesis of 2e and 3e.
p-Iodoanisole (58 mg, 0.25 mmol) was added to a benzene-d6 solution
(0.5 mL) of 1 (22 mg, 0.025 mmol). The mixture was allowed to stay
at room temperature for 60 h, resulting in color change from orange to
dark red. 31P{1H} NMR revealed quantitative formation of two products,
2e and 3e (2e:3e ) 1:4), and an equivalent amount of PPh3. Complex
2e was too unstable for isolation and was characterized in solution.
Upon heating at 70 °C for 20 min, complex 2e was quantitatively
converted into 3e. Complex 3e is soluble in benzene and THF and
which were identified by a GC/MS technique. GC/MS of 9, M.+
)
174. GC/MS of 10, M.+ ) 160 fragmentation pattern confirms the
identity of the compounds.
Reaction of 7c with PhLi. PhLi (1.8 M solution in cyclohexanes-
ether, 10 µL, 0.018 mmol) was added to a benzene-d6 solution (0.5
mL) of 7c (15 mg, 0.018 mmol) at room temperature in the presence
of 1 equiv of PPh3, resulting immediately in color change from dark to
light red. 31P{1H} NMR revealed quantitative formation of two products,
6 and 11 (6:11 ) 1:1). GC/MS revealed the formation of 12. Complex
11 could not been isolated from 6 and was characterized in a mixture
(6:11 ) 1:1).
moderately soluble in pentane.
Characterization of 2e. 31P{1H} NMR (C6D6): δ 25.0 (d, JRhP
)
1
1
126.2 Hz). H NMR (C6D6): δ 5.9-8.2 (m, 25H, ArH), 3.60 (dt, left
2
3
part of ABq, JHH ) 14.6 Hz, JHP ) 4.5 Hz, 2H, ArCH2P), 3.39 (s,
3H, ArOCH3), 3.27 (vt, 3JHP ) 7.1 Hz, 2H, ArCH2Rh), 3.23 (dt, right
part of ABq, 2JPH ) 4.5 Hz, 2H, ArCH2P), 2.05 (s, 6H, ArCH3). Selected
signals for 13C{1H} NMR (C6D6): δ 54.9 (s, ArOCH3), 29.8 (t, 1JPC
)
13.0 Hz, CH2P), 26.9 (b d, 1JPC ) 17.7 Hz, CH2Rh), 19.8 (s, CH3Ar).
Characterization of 11. 31P{1H} NMR (C6D6): δ 64.6 (ddd, left part
Characterization of 3e. 31P{1H} NMR (C6D6): δ 33.3 (d, JRhP
)
1
2
1
2
of ABq JPPtrans ) 259.2 Hz, JRhP ) 192.8 Hz, JPPcis ) 35.6 Hz, 1P,
132.3 Hz). 1H NMR (C6D6): δ 8.32 (m, 4H, ArH) 7.36 (m, 4H, ArH),
7.11 (m, 8H, ArH), 6.74 (m, 7H, ArH), 6.06 (m, 2H, ArH), 4.32 (vq,
3JHP ) 3.4 Hz, 2H, Ar′CH2Rh), 3.88 (dt, left part of ABq 2JHH ) 17.1
1
2
PPh2), 52.7 (ddd, right part of Abq, JRhP ) 196.0 Hz, JPPcis ) 37.3
1
2
Hz, 1P, PPh2), 39.5 (2 dd, JRhP ) 149.0 Hz, JPPcis ) 37.3, 35.6 Hz,
1
1P, PPh3). H NMR (C6D6): δ 8.0-6.6 (m, 42H, ArH), 5.15 (b m,
2
1H, Ar2CHRh), 3.77 (dd, 2JHH ) 16.0 Hz, 2JPH ) 3.0 Hz, 1H, ArCH2P),
Hz, JPH ) 5.3 Hz, 2H, ArCH2P), 3.44 (b d, right part of ABq, 2H,
2
ArCH2P), 3.09 (s, 3H, ArOCH3), 2.25 (s, 6H, ArCH3). Selected signals
3.68 (m, 1H, ArCH2P), 3.48 (m, 1H, ArCH2P), 3.17 (b t, 1H, JHH
)
for 13C{1H} NMR (C6D6): δ 173.6 (d, 2JRhC ) 40.2 Hz, Cipso), 54.4 (s,
12.5 Hz, ArCH2P). Selected signals for 13C{1H} NMR (C6D6): δ 156.2
1
1
2
1
ArOCH3), 39.5 (t, JPC ) 16.1 Hz, CH2P), 23.9 (b d, JPC ) 28.2 Hz,
CH2Rh), 22.6 (s, CH3Ar). The NMR assignment was confirmed by a
13C-DEPT and 13C-1H correlation experiments. Anal. Calcd: C, 59.17;
H, 4.73. Found: C, 58.12; H, 4.75.
(dm, JRhC ) 8.1 Hz, Ar′CCHRh), 43.8 (d, JPC ) 17.8 Hz, CH2P),
1
1
43.5 (d, JPC ) 25.1 Hz, CH2P), 35.0 (dm, JRhC ) 40.3 Hz, CHRh).
The assignment was confirmed by a 31P-31P and a 13C-1H correlation
experiment. GC/MS for 12, M.+) 236. The fragmentation pattern
confirmed the identity of the compound.
Synthesis of 7a. Benzyl bromide (2.1 µL, 0.018 mmol) was added
to a benzene-d6 solution (0.5 mL) of 6 (15 mg, 0.018 mmol) at room
Reaction of 7c with BzMgCl. BzMgCl (1.0 M solution in THF, 18
µL, 0.018 mmol) was added to a benzene-d6 solution (0.5 mL) of 7c
(15 mg, 0.018 mmol) at room temperature in the presence of 1 equiv
of PPh3, resulting immediately in color change from dark to light red.
31P{1H} NMR revealed quantitative formation of two products, 6 and
13 (6:13 ) 2:3). GC/MS revealed the formation of the organic product
9. Complex 13 could not isolated from 6 and was characterized in a
mixture (6:13 ) 2:3).
1
temperature, resulting in a color change from yellow to dark red. H
and 31P{1H} NMR analysis of the product solution showed the
quantitative formation of 7a and PPh3. It was impossible to separate
the compound from PPh3.
Characterization of 7a. 31P{1H} NMR (C6D6): δ 29.6 (d, JRhP
)
1
1
130 Hz). H NMR (C6D6): δ 6.4-8.3 (m, 28H, aromatics), 4.28 (vq,
2JPH ) 3.5 Hz, 2H, Ar′CH2Rh), 3.63 (left part of ABq, JHH ) 16.7
2
1
Hz, JHP ) 4.5 Hz, 2H, ArCH2P), 3.40 (b d, right part of ABq,1JHP
)
Characterization of 13. 31P{1H} NMR (C6D6): δ 66.5 (ddd, left part
4.2 Hz, 2H, ArCH2P). Selected signals for 13C{1H} NMR (C6D6): δ
169.1 (d, 2JRhC ) 37.0 Hz, Cipso), 39.9 (t, 1JPC ) 15.0 Hz, CH2P), 22.2
2
1
2
of Abq, JPPtrans ) 253.8 Hz, JRhP ) 198.6 Hz, JPPcis ) 32.5 Hz, 1P,
1
2
PPh2), 54.6 (ddd, right part of Abq, JRhP ) 200.0 Hz, JPPcis ) 36.3
1
1
2
(d, JPC ) 27.2 Hz, CH2Rh).
Hz, 1P, PPh2), 38.4 (2 dd, JRhP ) 146.3 Hz, JPPcis ) 32.5, 36.3 Hz,
1
Synthesis of 7b. 3,5-Bis(trifluoromethyl)benzyl bromide (3.3 µL,
0.018 mmol) was added to a benzene-d6 solution (0.5 mL) of 6 (15
mg, 0.018 mmol) at room temperature, resulting in a color change from
yellow to dark red. 31P{1H} NMR revealed quantitative formation of
the oxidative addition product 7b and equivalent amount of PPh3. It
1P, PPh3). H NMR (C6D6): δ 8.0-6.6 (m, 43H, ArH), 5.29 (b m,
2
1H, Ar2CHRh), 4.08 (b d, JHH ) 13.0 Hz, 1H, ArCH2P), 3.71 (dd,
2JHH ) 14.1 Hz, 2JPH ) 10.0 Hz, 1H, ArCH2P), 3.44 (dd, 2JHH ) 14.1
2
2
Hz, JPH ) 5.2 Hz, 1H, ArCH2P), 3.21 (b t, JHH ) 13.0 Hz, 1H,
ArCH2P). Selected signals for 13C{1H} NMR (C6D6): δ 152.0 (dm,
2JRhC ) 8.0 Hz, Ar′CCHRh), 44.3 (d, 1JPC ) 15.4 Hz, CH2P), 43.8 (d,
was impossible to separate the compound from PPh3.
Characterization of 7b. 31P{1H} NMR (C6D6): δ 28.5 (d, JRhP
)
1JPC ) 25.5 Hz, CH2P), 34.9 (dm, JRhC ) 42.5 Hz, CHRh). The
1
1
125.1 Hz). 1H NMR (C6D6): δ 6.5-8.2 (m, 26H, ArH), 3.9 (b q, 2JPH
assignment was confirmed by a 13C-DEPT and a 13C-1H 2D correlation
experiment. GC/MS of 9, Μ.+ ) 160. The fragmentation pattern
confirmed the identity of the compound.
2
) 3.8 Hz, 2H, Ar′CH2Rh), 3.6 (dt, left part of ABq, JHH ) 17.1 Hz,
2JHP ) 5.5 Hz, 2H, ArCH2P), 3.4 (dt, right part of ABq, 2JHP ) 3.9 Hz,

7734 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Cohen et al.
Reaction of 3e with BzMgCl. BzMgCl (2.0 M solution in THF,
4.5 µL, 0.009 mmol) was added to a benzene-d6 solution (0.5 mL) of
3e (8 mg, 0.009 mmol) in the presence of 1 equiv of PPh3 resulting
immediately in color change from dark red to orange. 31P{1H} NMR
revealed quantitative formation of three products, 4, 15, and 16 (4:15:
16 ) 1:10:4). GC/MS revealed the formation of the organic products
18 and 19. Complexes 15 and 16 could not been separated from the
solution and were characterized in a mixture (4:15:16 ) 1:10:4).
Characterization of 15. 31P{1H} NMR (C6D6): δ 70.95 (ddd, left
part of ABq, 2JPPtrans ) 254.5 Hz, 1JRhP ) 200.1 Hz, 2JPPcis ) 31.3 Hz,
X-ray Analysis of the Structure of 2a. Complex 2a was crystallized
from a pentane solution at room temperature.
Crystal data: C41H38IP2Rh, orange, plates, 0.2 × 0.05 × 0.05 mm3,
monoclinic, P2(1)/c (No. 14), a ) 14.4630(5) Å, b ) 17.4720(8) Å, c
) 22.0250(6) Å, â ) 141.684(3)°, from 20 degrees of data, T ) 120
K, V ) 3450.7(3) Å3, Z ) 4, Fw ) 822.46, Dc ) 1.583 Mg/m3, µ )
1.511 mm-1
.
Data collection and treatment: Nonius Kappa CCD diffractometer,
Mo KR (λ ) 0.710 73 Å), 11 817 reflections collected, 0ehe16,
0eke19, -24ele15, frame scan width 1.0°, scan speed 1°/60 s, typical
peak mosaicity 0.6°, 4833 independent reflections (Rint ) 0.039). Data
were processed with Denzo-Scalepack.
1
2
1P, PPh2), 56.4 (ddd, right part of Abq, JRhP ) 202.1 Hz, JPPcis
)
34.9 Hz, 1P, PPh2), 37.95 (2 dd, 1JRhP ) 145.1 Hz, 2JPPcis ) 31.4, 34.8
Hz, 1P, PPh3). 1H NMR (C6D6): δ 8.0-6.6 (m, aromatics, 49H), 5.26
2
Solution and refinement: The structure was solved by direct methods
with SHELXS-97. Full-matrix least-squares refinement was based on
F2 with SHELXL-97. Idealized hydrogen were placed and refined in a
riding mode; 444 parameters with 0 restraints, final R1 ) 0.029 (based
on F2) for data with I > 2σ(I) and R1 ) 0.0374 on all 4833 reflections,
(b m, 1H, Ar2CHRh), 4.01 (b d, JHH ) 14.4 Hz, 1H, ArCH2P), 3.64
2
(b t, JHH ) 13.1 Hz, 1H, ArCH2P), 3.50 (s, 3H, ArOCH3), 3.25 (m,
2H, ArCH2P), 2.43 (s, 6H, ArCH3). ES-MS: M1+ 987 m/z.
Characterization of 16. 31P{1H} NMR (C6D6): δ 70.4 (ddd, left part
2
1
2
of ABq, JPPtrans ) 256.8 Hz, JRhP ) 199.9 Hz, JPPcis ) 31.7 Hz, 1P,
goodness of fit on F2 ) 0.947, largest electron density 0.583 e Å-3
.
1
2
PPh2), 55.8 (ddd, right part of ABq, JRhP ) 200.8 Hz, JPPcis ) 35.3
1
2
Hz, 1P, PPh2), 38.4 (2 dd, JRhP ) 145.8 Hz, JPPcis ) 31.7, 34.9 Hz,
X-ray Analysis of the Structure of 11. Complex 11 was crystallized
from a pentane solution at room temperature.
1
1P, PPh3). H NMR (C6D6): δ 8.0-6.6 (m, aromatics, 47H), 5.30 (b
2
m, 1H, Ar2CHRh), 4.00 (m, 2H, ArCH2P), 3.58 (b t, JHH ) 13.1 Hz,
Crystal data: C58H46F3P3Rh, orange, needles, 0.2 × 0.1 × 0.1 mm3,
monoclinic, P2(1)/c (No. 14), a ) 17.067(1) Å, b ) 17.379(1) Å, c )
15.905(1) Å, â ) 104.508(10)°, from 20 degrees of data, T ) 120 K,
V ) 4567.1(5) Å3, Z ) 4, Fw ) 995.77, Dc ) 1.448 Mg/m3, µ )
1H, ArCH2P), 3.26 (dd, 2JHH ) 14.6 Hz, 2JPH ) 4.9 Hz, 1H, ArCH2P),
2.42 (s, 6H, ArCH3). ES-MS: M1+ ) 957 m/z. GC/MS of 18, Μ.+
)
122. GC/MS of 19, Μ.+ ) 91. The fragmentation pattern confirmed
the identity of the compounds.
0.532 mm-1
.
Reaction of 3b with BzMgCl. BzMgCl (2.0 M solution in THF,
4.5 µL, 0.009 mmol) was added to a benzene-d6 solution (0.5 mL) of
3b (8 mg, 0.009 mmol) at room temperature in the presence of 1 equiv
of PPh3, resulting immediately in color change from dark red to orange.
31P{1H} NMR revealed quantitative formation of two products, 16 and
4 (16:4 ) 24:1). GC/MS revealed the formation of the organic product
21. GC/MS of 21, Μ.+ ) 228. The fragmentation pattern confirmed
the identity of the compound. ES-MS, M1+ ) 957 m/z.
Data collection and treatment: Nonius Kappa CCD diffractometer,
Mo KR (λ ) 0.710 73 Å), 39 645 reflections collected, 0ehe17,
0eke18, -16ele16, frame scan width 1.3°, scan speed 1°/244 s,
typical peak mosaicity 0.6°, 5526 independent reflections (Rint ) 0.085).
Data were processed with Denzo-Scalepack.
Solution and refinement: The structure was solved by direct methods
with SHELXS-97. Full-matrix least-squares refinement was based on
F2 with SHELXL-97. Idealized hydrogen were placed and refined in a
riding mode; 586 parameters with 0 restraints, final R1 ) 0.0416 (based
on F2) for data with I > 2σ(I) and R1 ) 0.0636 on all 5526 reflections,
X-ray Analysis of the Structure of 3b. Complex 3b was crystallized
from a pentane solution at room temperature.
Crystal data: 2C43H36P2F6RhI, orange, rectangular plate, 0.2 × 0.1
× 0.01 mm3, monoclinic, P2(1)/c (No. 14), a ) 29.732(6) Å, b )
13.089(3) Å, c ) 20.270(4) Å, â ) 100.13(3)°, with 2 independent
molecules in the au, from 10 degrees of data, T ) 120 K, V ) 7765(3)
goodness of fit on F2 ) 0.947, largest electron density 0.583 e Å-3
.
NMR Follow-Up Experiments. NMR Measurements and Data
Processing. The processes were monitored by 31P{1H} NMR. The
spectra were measured using 5-mm screw-cap NMR tubes, which were
inserted into the NMR spectrometer probe preset to the desired
temperature. The monitoring program was started after 2-3 min of
temperature equilibration and included periodical FID acquisition at
constant intervals. The delay was varied in different experiments from
15 to 45 min, depending on experimental conditions. The reactions
were monitored until completed. The spectra were processed (Fourier
transform, baseline and phase correction, integration) using an automatic
program. Raw data from the spectra were processed using the Microsoft
Excel 98 program on a Macintosh personal computer.
Å3, Z ) 4, Fw ) 1916.94, Dc ) 1.640 Mg/m3, µ ) 1.377 mm-1
.
Data collection and treatment: Nonius Kappa CCD diffractometer,
Mo KR, graphite monochromator (λ ) 0.710 73 Å), 31 277 reflections
collected, -31eh e31, -14eke0, 0ele21, frame scan width 0.3°,
scan speed 1°/90 s, typical peak mosaicity 1.0°, 11 595 independent
reflections (Rint ) 0.034). Data were processed with Denzo-Scalepack.
Solution and refinement: The structure was solved by direct methods
(SHELXS-97). Full-matrix least-squares refinement was based on F2
(SHELXL-97). Idealized hydrogens were placed and refined in a riding
mode; 947 parameters with 0 restraints, final R1 ) 0.0434 (based on
F2) for data with I > 2σ(I) and R1 ) 0.0601 for all data based on 9308
reflections, goodness of fit on F2 ) 1.030, largest electron density 0.864
Transformation of 2a to 3a. In a typical experiment, 0.5 mL of a
C6D6 solution of 2a (25 mg, 0.028 mmol) was added to an NMR tube.
The tube was inserted into the preheated probe. Follow-up measure-
ments were performed at 40, 45, 50, and 65 °C.
e Å-3
.
X-ray Analysis of the Structure of 16. Complex 16 was crystallized
from a pentane solution at room temperature.
Crystal data: C59H52P3Rh, orange, plate, 0.1 × 0.1 × 0.05 mm3,
triclinic, P1bar (No. 2), a ) 10.4480(4) Å, b ) 13.3430(6) Å, c )
16.9060(7) Å, R ) 81.105(2)°, â ) 82.732(2)°, γ ) 84.391(2)°, from
20 degrees of data, T ) 120 K, V ) 2302.41(7) Å3, Z ) 2, Fw )
Acknowledgment. We thank Boris Rybtchinski for valuable
discussions. This work was supported by the U.SsIsrael
Binational Science Foundation, Jerusalem, Israel, and by the
MINERVA Foundation, Munich, Germany. D.M. is the holder
of the Israel Matz professorial chair of organic chemistry.
956.83, Dc ) 1.380 Mg/m3, µ ) 0.515 mm-1
.
Data collection and treatment: Nonius Kappa CCD diffractometer,
Mo KR (λ ) 0.710 73 Å), 12 390 reflections collected, -10ehe12,
-16eke15, -20ele20, frame scan width 1.0°, scan speed 1°/90 s,
typical peak mosaicity 1.0°, 8948 independent reflections (Rint ) 0.062).
Data were processed with Denzo-Scalepack.
Supporting Information Available: Tables of crystal data
and structure refinement, atomic coordinates, bond lengths and
angles, anisotropic displacement parameters, and hydrogen atom
coordinates for complexes 2a, 3b, 11, and 16 (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Solution and refinement: The structure was solved by direct methods
with SHELXS-97. Full-matrix least-squares refinement was based on
F2 with SHELXL-97. Idealized hydrogen were placed and refined in a
riding mode; 574 parameters with 0 restraints, final R1 ) 0.0391 (based
on F2) for data with I > 2σ(I) and R1 ) 0.0497 on 8943 reflections,
goodness of fit on F2 ) 1.061, largest electron density 1.137 e Å-3
.
JA000157R