Carbon-carbon bond activation by Rhodium(I) in solution. Comparison of sp2-sp3 vs sp3-sp3 C-C, C-H vs C-C, and Ar-CH3 vs Ar-CH2CH3 activation

Article abstract of DOI:10.1021/ja982345b

Reaction of [RhCIL₂]₂ (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6(CH₂P(t)Bu₂)₂C₆H₃} (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp³-sp³ ArCH₂-CH₃ bond. The identity of complex Rh(Et){2,6-(CH₂P(t)Bu₂)₂C₆H₃}Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η(I)-N₂){2,6-(CH₂P(t)Bu₂)₂C₆H₃} (7) with EtI. It is possible to direct the bond activation process toward the benzylic C-H bonds of the aryl- alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ((t)Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH₂-CH₃ bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH₂CH₃ bond activation process. Thus, aryl-C activation by Rh(I) is kinetically preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH₂CH₃ bond, followed by β-H elimination, may be preferred over sp²-sp³ C-C activation of an Ar-CH₃ group.

Full text of DOI:10.1021/ja982345b

J. Am. Chem. Soc. 1998, 120, 13415-13421
13415
Carbon-Carbon Bond Activation by Rhodium(I) in Solution.
Comparison of sp2-sp3 vs sp3-sp3 C-C, C-H vs C-C, and
Ar-CH3 vs Ar-CH2CH3 Activation
Milko E. van der Boom, Shyh-Yeon Liou, Yehoshoa Ben-David, Michael Gozin, and
David Milstein*
Contribution from the Weizmann Institute of Science, Department of Organic Chemistry,
RehoVot 76100, Israel
ReceiVed July 6, 1998
Abstract: Reaction of [RhClL2]2 (L ) cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-
(CH2PtBu2)2C6H3} (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction
proceeds with no intermediacy of activation of the weaker sp3-sp3 ArCH2-CH3 bond. The identity of complex
Rh(Et){2,6-(CH2PtBu2)2C6H3}Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the
new Rh(η1-N2){2,6-(CH2PtBu2)2C6H3} (7) with EtI. It is possible to direct the bond activation process toward
the benzylic C-H bonds of the aryl-alkyl group by choice of the Rh(I) precursor, of the substituents on the
phosphorus atoms (tBu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to
the product of insertion into the ArCH2-CH3 bond (had it taken place) was prepared and shown not to be an
intermediate in the Ar-CH2CH3 bond activation process. Thus, aryl-C activation by Rh(I) is kinetically
preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH2CH3 bond,
followed by â-H elimination, may be preferred over sp2-sp3 C-C activation of an Ar-CH3 group.
Introduction
reactions.21-29 Recently, we observed catalytic activation of a
strong C-C single bond28 and an unprecedented oxidative
addition of a C-C bond of a fluorinated organic substrate.29
We report here a system that allows the direct observation of
an oxidative addition of a strong sp2-sp3 Ar-Et bond to Rh-
(I), affording a new unsaturated Rh(III)-ethyl complex, which
upon heating undergoes slow â-H elimination. We also
qualitatively compare sp2-sp3 vs sp3-sp3 C-C, C-H vs C-C,
and Ar-CH3 vs Ar-CH2CH3 carbon-carbon bond activation.
Part of this work has been communicated.25
The insertion of transition metal complexes into C-C bonds
in solution is a topic of much current interest.1-20 We have
reported transition metal insertion into strong C-C single bonds
in solution and details related to the mechanism of these
* To whom correspondence should be addressed. Fax: +972-8-9344142.
E-mail: comilst@wiccmail.weizmann.ac.il.
(1) Crabtree, R. H. Chem. ReV. 1985, 85, 245.
(2) For a review, see: Rybtchinski, B.; Milstein, D. Angew. Chem. Int.
Ed., in press.
(3) Suggs, J. W.; Jun, C.-H. J. Chem. Soc., Chem. Commun. 1985, 92.
(4) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc. 1994, 116, 3647.
(5) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Jones, W. D.
Organometallics 1997, 16, 2016.
(6) Fujimura, F.; Aoki, S.; Nakamura, E. J. Org. Chem. 1991, 56, 2809.
(7) Suzuki, H.; Takaya, Y.; Takemori, T. J. Am. Chem. Soc. 1994, 116,
10779.
(8) Lu, Z.; Jun, C.-H.; de Gala, S. R.; Sigalas, M.; Eisenstein, O.;
Crabtree, R. H. Organometallics 1995, 14, 1168.
(9) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc.
1998, 120, 2843.
(10) Huffman, M. A.; Liebeskind, L. S. J. Am. Chem. Soc. 1991, 113,
2771.
Results and Discussion
Preparation of Substrates. The new aryl-ethyl phosphines
1 and 2 were prepared in order to explore the possibility of
competitive activation of unstrained sp2-sp3 and sp3-sp3 C-C
single bonds. The synthesis of substrates 1 and 2 is similar to
that of other PCP-based ligands.26,30-33 It consists of lithiation
(21) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993,
699.
(22) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.; Ben-David,
Y.; Milstein, D. Nature 1994, 370.
(23) Gandelman, M.; Vigalok, A.; Shimon, L. J. W.; Milstein, D.
Organometallics 1997, 16, 3981.
(11) Nicholls, J. C.; Spencer, J. L. Organometallics 1994, 13, 1781.
(12) Murakami, M.; Amii, H.; Shigeto, K.; Ito, Y. J. Am. Chem. Soc.
1996, 118, 8285.
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1997, 119, 9307.
(14) Crabtree, R. H.; Dion, R. P.; Gibboni, D. J.; McGrath, D. V.; Holt,
E. M. J. Am. Chem. Soc. 1986, 108, 7222.
(24) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117,
9774.
(25) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Chem. Soc., Chem. Commun.
1995, 1965.
(15) Kang, J. W.; Moseley, K.; Maitlis, P. M. J. Am. Chem. Soc. 1969,
91, 5970.
(16) Eilbracht, P. Chem. Ber. 1976, 109, 1429.
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1974, 1324.
(26) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am.
Chem. Soc. 1996, 118, 12406.
(27) van der Boom, M. E.; Kraatz, H.-B.; Ben-David, Y.; Milstein, D.
J. Chem. Soc., Chem. Commun. 1996, 2167.
(28) Liou, S.-Y.; van der Boom, M. E.; Milstein, D. Chem. Commun.
1998, 687.
(19) Hemond, R. C.; Hughes, R. P.; Locker, H. B. Organometallics 1986,
5, 2391.
(20) Yeh, W.-Y.; Hsu, S. C. N.; Peng, S.-M.; Lee, G. H. Organometallics
1998, 17, 2477.
(29) van der Boom, M. E.; Ben-David, Y.; Milstein, D. Chem. Commun.
1998, 917.
(30) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020.
10.1021/ja982345b CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/10/1998

13416 J. Am. Chem. Soc., Vol. 120, No. 51, 1998
Van der Boom et al.
Scheme 1
Scheme 3
Scheme 4
Scheme 2
of 2-bromo-1,3-dimethylbenzene, coupling with ethyl bromide,
bromination, and reaction with HPtBu2 or LiPPh2 (Scheme 1).
Compounds 1 and 2 were obtained as white powders and were
1
characterized by H, 31P, and 13C NMR.
Ar-CH2CH3 vs ArCH2-CH3 Activation with 1. Reaction
of the alkene complex [RhClL2]2 (L ) ethylene or cyclooctene)
with 2 equiv of 1 in toluene at 120 °C (5 min in a sealed tube)
resulted in quantitative formation of the new pentacoordinated
Rh(III)-ethyl complex 3 by selective oxidative addition of the
strong sp2-sp3 C-C bond (compare bond dissociation energy
(BDE) of Ph-Et ) 96.3 ( 1 kcal/mol; Scheme 2).34 Complex
3 was characterized by various NMR techniques (vide infra).
Thermolysis of 3 in toluene at 120 °C overnight resulted in the
quantitative formation of ethylene and the known Rh(III)-
hydride complex 4,30,35 which was unambiguously identified
(I)-dinitrogen complex 7 and ethane (Scheme 3).37 The ethane
was collected by standard vacuum line techniques and analyzed
by GC. The air-sensitive complex 7 was characterized by 1H,
31P, and 13C NMR and IR. It exhibits spectroscopic properties
similar to those of its iridium analogue (vide infra).38 No THF
coordination was observed in the 1H NMR, probably as a result
of the bulky tert-butyl substituents on the phosphorus atoms.32,39
η1-N2 binding to similar tert-butyl PCP-Rh(I) complexes
competes favorably with CO2 and even with ethylene.39 Treat-
ment of 7 with 1 equiv of EtI in toluene or dioxane at room
temperature led to the selective formation of 6 in excellent yield
(∼95%). The reaction was completed within 10 min, and no
intermediates were observed by 1H and 31P NMR. As observed
for 3, thermolysis of the product solution at 120 °C overnight
resulted in the quantitative formation of the iodide analogue of
1
by H and 31P NMR and by comparison with an authentic
sample. The ethylene was observed by 1H NMR and was
collected by standard vacuum line techniques and identified and
quantified by GC. Using deuterated toluene, no Rh(III)-D
formation was observed by 2H NMR, indicating that the solvent
does not contribute to the Rh(III)-H formed. Oxidative
addition of a strong Ar-CH3 single bond to Rh(I) in solution
was observed in our group with other PCP-type sys-
tems.21,22,24-26,28,29 Very recently, we observed metal insertion
into the strong sp2-sp3 Ar-O bond of an aryl ether 5,32,36 which
probably occurs via an (unobserved) oxidative addition product
Rh(OMe){2,6-(CH2PtBu2)2C6H3}Cl (A) followed by â-hydride
elimination to give formaldehyde and 4 (Scheme 2).
1
1
4 (by H, 31P, and 13C NMR) and ethylene (by H NMR and
GC).35
Mechanistically, coordination of 1 to the metal center is likely
to precede the Ar-C bond cleavage step (Scheme 4, B).
Coordination of both phosphine arms to the metal center was
postulated for the Ar-H,30 Ar-OMe (5, Scheme 2),32,36 Ar-
29
CH3 (8, Scheme 5),26 and Ar-CF3 analogues of 1 with
To confirm the identity of 3, we prepared the iodide analogue
6 by EtI oxidative addition to the new dinitrogen complex 7.
Reaction of 3 with an excess of NaH in THF under nitrogen at
room temperature led to the quantitative formation of the Rh-
rhodium(I) and iridium(I) and was observed for similar sub-
strates with platinum(II)40 and ruthenium(II).41 Performing the
reaction (1 f 3) at room temperature resulted in the formation
1
of oligomers, as indicated by H and 31P NMR spectroscopy,
(31) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997,
16, 3786.
(32) van der Boom, M. E.; Liou, S.-Y.; Shimon, L. J. W.; Ben-David,
Y.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 6531.
(33) Weisman, A.; Gozin, M.; Kraatz, H. B.; Milstein, D. Inorg. Chem.
1996, 35, 1792.
(34) Egger, K. W.; Cocks, A. T. HelV. Chim. Acta 1973, 56, 1516.
(35) Details about this â-H elimination process will be reported in a
forthcoming paper.
(36) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Vigalok, A.;
Milstein, D. Angew. Chem., Int. Ed. Engl. 1997, 36, 625.
(37) This compound was probably observed by Kaska et al. in a
nonselective dehydrochlorination reaction of 4 with NaN(SiMe3)2.43
(38) Rybtchinski, B. M.Sc. Thesis, The Weizmann Institute of Science,
Rehovot, Israel, 1996.
(39) Vigalok, A.; Ben-David, Y.; Milstein, D. Organometallics 1996,
15, 1839.
(40) van der Boom, M. E.; Gozin, M.; Ben-David, Y.; Shimon, L. J.
W.; Frolow, F.; Kraatz, H. B.; Milstein, D. Inorg. Chem. 1996, 35, 7068.
(41) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.;
van Koten, G. J. Am. Chem. Soc. 1997, 119, 11317.

C-C Bond ActiVation by Rh(I) in Solution
J. Am. Chem. Soc., Vol. 120, No. 51, 1998 13417
Scheme 5
Figure 1. NOE interactions detected for 3.
phosphorus atoms are magnetically equivalent and coordinated
to a Rh(III) center. The structure of 3 is fully supported by 1H
and 13C NMR. For instance, the Rh-CH2CH3 group appears
in the 13C{1H} NMR spectrum as a double triplet at δ 16.83,
with 1JRhC ) 29.1 Hz and cis 2JPC ) 4.9 Hz. The Rh-CH2CH3
group appears as a doublet at δ 23.61, with 2JRhC ) 1.4 Hz. In
which probably collapsed to monomeric species upon heating.
Former studies in our group have shown that coordination of
similar di-tert-butyl phosphines such as 8 to Rh(I) and Ir(I)
complexes occurs at room temperature and controls the overall
rate in the activation of an Ar-CH3 bond (Scheme 5).26
Likewise, substitution of the olefin (cyclooctene or ethylene)
by 1 is probably slow relative to the bond activation. No
complexes resulting from C-H or sp3-sp3 C-C activation (D,
C; Scheme 4) were observed in the reaction of 1 with [RhClL2]2
1
the H NMR spectrum, this alkyl group appears as a double
triplet at δ 1.24 (3H), with 3JRhH ) 2.3 Hz and 3JHH ) 7.2 Hz,
and as a multiplet at δ 2.40 (2H), with 2JRhH ) 2.8 Hz, 3JHH
)
7.2 Hz, and cis 3JPH ) 7.1 Hz. From the NOESY spectrum, it
1
is possible to unambiguously assign the H NMR resonances
1
(L ) cyclooctene or ethylene) by H and 31P NMR.
for 3 and to show that the structure in solution has the ethyl
group cis to the aryl group (Figure 1; NOE cross-peaks are seen
between Ha and tBua, between Hb and the CH2CH3, and between
tBub and the CH2CH3). In the 1H NMR spectrum of 6, the CH3
and CH2 resonances of the ethyl ligand appear at δ 0.89 and
2.38, respectively, and two sets of resonances are seen for the
protons of the tBu groups and the CH2 “arms” of the PCP ligand.
When the RhCH2CH3 protons of 6 were irradiated, selective
enhancement of the RhCH2CH3 signal was observed. A doublet
is observed in the 31P{1H} NMR spectrum of 6 at δ 52.75, with
1JRhP ) 122.5 Hz. The similarities in the 13C NMR chemical
shifts of the ipso-carbons to those of analogous rhodium aryl
halide complexes where the X-ray structure is known, Rh(H)-
{2,6-(CH2PtBu2)2C6H3}Cl (4) (δ 166.85),43 Rh(Me){2,6-(CH2Pt-
Bu2)2-3,5-(CH3)2-C6H}Cl (10) (δ 168.80),26 Rh(Me){2-(CH2Pt-
Bu2)-6-(CH2N(C2H5)2-3,5-(CH3)2-C6H}Cl (δ 170.04),23 and 3
(δ 167.57), indicate that the aryl group is directly bound to the
metal center trans to the halide ligand. It is known that 13C
NMR spectroscopy is a sensitive tool for analyzing electronic
trends in aryl-bound complexes.33,44 Thus, for 3 and 6, the
strongest trans director, the ethyl ligand, is trans to the vacant
coordination site, in agreement with crystal structures of
analogous square-pyramidal PCP- and PCN-type rhodium(III)
and iridium(III) complexes.23,26,38,43,45,46 Theoretical studies
predict that the square-pyramidal geometry is preferred for five-
coordinated d6 complexes.42 The air-sensitive dinitrogen com-
plex 7 is unequivocally characterized by 1H, 31P{1H}, and
To evaluate whether activation of the weaker sp3-sp3 C-C
bond (∆BDE{Ph-CH2CH3 - PhCH2-CH3} ) 24.5 kcal/
mol)34,42 is an intermediate step in the observed sp2-sp3 C-C
activation process or perhaps a reversible parallel process (C,
Scheme 4), [RhCl(C8H14)2]2 (C8H14 ) cyclooctene) was reacted
with 2 equiv of 1 in C6D6 under H2 (20 psi) at 120 °C for 16
h in a Fischer Porter pressure vessel. Analysis of the reaction
1
solution by H and 31P NMR showed exclusive formation of
the known Rh(III)-hydride complex 4,30 which was identical
to an authentic sample. GC analysis of the gas phase showed
formation of ethane (∼90%). Only traces of methane were
observed (<4%), providing evidence that sp3-sp3 C-C cleav-
age is not involved either on the reaction coordinate or as a
side equilibrium. A pathway involving stepwise sp3-sp3, sp2-
sp3 C-C bond cleavage generating 2 equiv of CH4 and 4 would
have been thermodynamically more favorable.25,34 Thus, in this
system, sp2-sp3 C-C bond activation is kinetically preferred
over that of a sp3-sp3 C-C bond.
The possibility of a side equilibrium involving C-H activa-
tion (D, Scheme 4) cannot be ruled out. However, under
appropriate conditions, C-C activation can be kinetically and
thermodynamically more favorable than C-H activation.23,26
Rhodium insertion into an Ar-CH3 bond becomes more
favorable as compared with insertion into an ArCH2-H bond
when higher electron density is involved.24 The reaction of
[RhCl(C2H4)2]2 with ligand 8, having electronic and steric
properties similar to those of 1, resulted in competitive C-H
and C-C oxidative addition at room temperature (Scheme 5).26
In a slower process, 9 undergoes C-H reductive elimination,
followed by C-C oxidative addition, to give 10 quantitatively.
It is expected that, in the Ar-Et system, 1, having only two
benzylic C-H bonds and considering more steric hindrance
imposed by the methyl group, the C-C cleavage is even more
competitive with C-H activation.
1
13C{1H} NMR and IR.37 The H NMR spectrum shows two
t
1:2:1 triplet resonances for the Bu and benzylic protons at δ
1.26 and 3.15 (JPH ) 6.3 and 3.7 Hz), respectively, which
collapse into singlets upon phosphorus decoupling. The 31P-
{1H} spectrum exhibits a doublet resonance at δ 81.01 (1JRhP
) 157.9 Hz), indicative of two magnetically equivalent phos-
phorus nuclei coordinated to a Rh(I) center. The Rh-η1-N2
moiety of 7 exhibits a characteristic band in the IR spectrum at
Identification of Complexes 3, 6, and 7. Complexes 3 and
6 were unequivocally characterized by various NMR techniques
and exhibit nearly identical spectroscopic properties. Assign-
(43) Nemeh, S.; Jensen, C.; Binamira-Soriaga, E.; Kaska, W. C.
Organometallics 1983, 2, 1442.
(44) Van de Kuil, L. A.; Luitjes, H.; Groves, D. M.; Zwikker, J. W.;
Van der Linden, J. G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth,
W.; van Koten, G. Organometallics 1994, 13, 468.
(45) Crocker, C. J.; Errington, R. J.; McDonald, W. S.; Odell, K. J.;
Shaw, B. L. J. Chem. Soc., Chem. Commun. 1979, 498.
(46) Crocker, C. J.; Empsall, H. D.; Errington, R. J.; Hyde, E. M.;
McDonald, W. S.; Markham, R. J.; Norton, M. C.; Shaw, B. L.; Weeks, B.
J. Chem. Soc., Dalton Trans. 1982, 1217.
1
ments in the H and 13C{1H} NMR spectra were made using
1
1H{31P}, H-1H COSY, NOESY, and 13C-DEPT-135 NMR.
In the 31P{1H} NMR spectrum of 3, one doublet resonance
appears at δ 54.99 with 1JRhP ) 123.8 Hz, indicating that both
(42) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058.

13418 J. Am. Chem. Soc., Vol. 120, No. 51, 1998
Van der Boom et al.
Scheme 6
Scheme 7
ν ) 2133 cm-1. This frequency is close to those observed for
other PCP-type Rh(I) complexes: Rh(η1-N2){HC(CH2CH2Pt-
Bu2)2},39 Rh(η1-N2){CH3C(CH2CH2PtBu2)2},47 Rh(η1-N2){1-O-
2,6-(CH2PtBu2)2-4-CH3-C6H2},32 and it is indicative of an “end-
on” coordinated dinitrogen complex.48,49 Very recently, the
X-ray structures of [Ir{2,6-(CH2PtBu2)2C6H3}]2-(µ-N2),50 [Rh-
{1-O-2,6-(CH2PtBu2)2-4-CH3-C6H2}]2-(µ-N2),32 and Rh(η1-
N2){CH3(CH2CH2PtBu2)2]}43 were reported. The N2 ligand of
7 is readily displaced by CO, affording the carbonyl complex
Rh(CO){2,6-(CH2PtBu2)2C6H3}, with 1H and 31P NMR and IR
spectra identical to those reported in the literature.30
activation, forming the thermally stable 15 (up to 150 °C),24
while reaction of 2 with ClRh(PPh3)3 (at 120 °C in toluene or
benzene in a sealed vessel) results in C-C activation to give
16 and ethylene (91%; Scheme 7), which were identified and
1
quantified by H, 31P, and 13C NMR, IR, and X-ray analysis
(vide infra) and by GC analysis of the gas phase. Using
deuterated solvents, no Rh(III)-D formation was observed by
2H NMR. Very recently, we observed a similar phenomenon
with Ni(II).51 Exclusive benzylic C-H activation occurs upon
heating of the iPr analogue of 14 with a stoichiometric amount
C-H vs C-C Activation. Interestingly, it is possible to
direct the bond activation process toward the benzylic protons
of the ethyl group by choice of the rhodium(I) precursor and
the substituents on the phosphorus atoms (Scheme 6). As
recently communicated,25 reaction of HRh(PPh3)4 or PhRh-
(PPh3)3 with 2 yields the chiral product of C-H activation 12
(Scheme 6), which is analogous to the unobserved D (Scheme
4). Reaction of 12 with H2 (20 psi) at 120 °C resulted in
formation of ethane (95% by GC) and 13, which was unam-
i
of NiI2 in ethanol, whereas the Pr analogue of 1 and 2 favors
Ar-Et activation. Thus, de-ethylation of an arene, which is
presumably driven by the â-H elimination process and ethylene
release, occurs more readily than Ar-Me cleavage in a similar
system.
Ar-CH2CH3 vs ArCH2-CH3 Activation with 2.
A
1
biguously identified by H, 31P, and 13C NMR and by com-
consecutive sp3-sp3, sp2-sp3 C-C bond activation pathway
of 2 and ClRh(PPh3)3 under H2, generating CH4 and 16, would
most probably involve the intermediacy of a species such as
15 (Scheme 7). Treatment of a THF solution of 15 with H2
(20 psi) at 80 °C resulted in the formation of methane (>90%
by GC) and 17, which is analogous to 16. Dehydrochlorination
of 17 with MeLi or excess KOtBu in THF or dioxane resulted
in the formation of the known Rh(I) complex Rh{2,6-(CH2-
PPh2)2-3,5-(CH3)2-C6H}(PPh3) as judged by 1H and 31P NMR.21
Performing the reaction of 2 with ClRh(PPh3)3 under H2 (20
psi) resulted in the formation of ethane and 16,25 which has
been fully characterized by X-ray analysis (Figure 2).52,53 Only
traces of methane were observed (<4%), indicating that the
metal center activates only the sp2-sp3 C-C bond in the
presence of H2. Apparently, intermediates analogous to 15 are
not involved.
parison with an authentic sample.25 The mechanism of the
hydrogenolysis of 12 is probably similar to the one reported
for the hydrogenolysis of an analogous benzylic Rh(I) com-
plex.21 Interestingly, thermolysis of 12 at 120 °C in toluene in
the absence of H2 resulted in the quantitative formation of
ethylene and 13. We believe that 12 undergoes (reversible) â-H
elimination, giving a Rh(I)-H species E. Metal insertion into
the sp2-sp2 Ar-C bond followed by C-H elimination and
phosphine coordination affords the observed ethylene and 13.
It is known that [Rh(η1-N2){CH3C(CH2CH2PtBu2)2}] undergoes
reversible â-hydride elimination upon N2 dissociation.47 While
the activation of the C-C bond of biphenylene is known,4,5,8,20
activation of an unstrained sp2-sp2 C-C bond in solution has
not been reported. Regardless of the mechanism involved, in
both systems 1 and 2, exclusive sp2-sp3 C-C bond cleavage
with Rh(I) was observed, showing that this selective bond
activation process can take place with significantly different
electron density and bulk at the metal center. In system 2,
benzylic C-H activation is kinetically favored over C-C
activation.
The new benzylic Rh(III)-methyl complex 19 was prepared
in order to evaluate unambiguously whether activation of the
sp3-sp3 C-C bond of 2 occurs in the absence of H2. Complex
19 is the iodide analogue of the expected product of Rh(I)
insertion into the ArCH2-CH3 bond of 2 and ClRh(PPh3)3.
Treatment of a THF solution of 15 with excess Et3N or KOtBu
resulted in the formation of the known complex 18 (>90 and
1
40% yield by H and 31P NMR, respectively), which can be
obtained quantitatively by reaction of 14 with HRh(PPh3)4 or
PhRh(PPh3)3 in THF at room temperature.21 Oxidative addition
of CH3I to 18 in toluene at room temperature in a sealed tube
leads to exclusive formation of 19 and PPh3, which were
Ar-CH2CH3 vs Ar-CH3 Activation. Interestingly, reac-
tion of 14 with ClRh(PPh3)3 results in quantitative C-H
1
1
characterized by H, 31P, and 13C NMR and FD-MS. The H
NMR shows clearly the presence of the ArCH2Rh and Rh-
CH3 moieties, which appear at δ 2.25 (dt, 3JPH ) 8.3 Hz, 2JRhH
(47) Vigalok, A.; Kraatz, H.-B.; Konstantinovsky, L.; Milstein, D. Chem.
Eur. J. 1997, 3, 253.
(48) Busetto, C.; Dalfonso, A.; Maspero, F.; Perego, G.; Zazzetta, A. J.
Chem. Soc., Dalton Trans 1977, 1828.
(49) Thorn, D. L.; Tulip, T. H.; Ibers, J. A. J. Chem. Soc., Dalton Trans.
1979, 2022.
(51) van der Boom, M. E.; Ben-David, Y.; Milstein, D., manuscript in
preparation.
(52) Gozin, M. Ph.D. Thesis, The Weizmann Institute of Science,
Rehovot, Israel, 1995.
(50) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics 1998,
17, 1.
(53) A similar structure of a 3,5-lutidine-based PCP Rh(I) complex was
reported recently.33

C-C Bond ActiVation by Rh(I) in Solution
J. Am. Chem. Soc., Vol. 120, No. 51, 1998 13419
center. sp3-sp3 C-C bond activation is not involved, either
as a side equilibrium or as an intermediate process, even in the
presence of H2, which could have driven the overall process to
a thermodynamically more favorable consecutive sp3-sp3, sp2-
sp3 C-C bond activation process forming methane.25,34 Thus,
the reason for the preference of ethane (in the presence of H2)
or ethylene formation is kinetic. The iodide analogue 19 of
the expected product of metal insertion into the sp3-sp3 C-C
bond was prepared and shown not to be an intermediate in the
sp2-sp3 Ar-C bond activation process. Although the Ar-CH2-
CH3 bond is substantially stronger than the ArCH2-CH3 bond
(compare BDE values of Ph-CH2CH3 ) 96.3 ( 1 kcal/mol,
PhCH2-CH3 ) 71.8 ( 1 kcal/mol),34,42 selective insertion into
the Ar-C bond takes place, indicating that this process is
product controlled, and it is likely that BDE(Ar-M + M-CH2-
CH3) - BDE(ArCH2-M + M-CH3) > ∼20 kcal/mol. The
Ar-Rh bond is known to be quite strong.54,55 The chelate ring
size formed may also play a role, although both the five- and
six-membered chelates are very stable. The higher accessibility
and lower directionality of the sp2-sp3 vs the sp3-sp3 C-C
bond are kinetic factors that undoubtedly favor activation of
the Ar-C bond. Although kinetic studies were not performed
here, a mechanism involving a nonpolar three-centered transition
state is likely to be operative for the Rh(I) insertion into the
Ar-CH2CH3 bond, as shown for the direct Rh(I) insertion into
the Ar-CH3 bond of 8.26 The remarkably stable unsaturated
Rh(III)-CH2CH3 complexes 3 and 6, obtained by selective
insertion of Rh(I) into the Ar-C bond or by oxidative addition
of EtI to a dinitrogen complex 7, are isostructural with
previously postulated Rh(III)-OCH3 species A involved in
Ar-O bond activation.32,36 It is noteworthy that sp2-sp3 C-O
activation by Rh(I) in 5 is kinetically preferred over sp3-sp3
C-O bond activation, while Pd(II) exclusively activates the
adjacent sp3-sp3 C-O bond.32,36 Our observations show that
cleavage of an Ar-CH2CH3 bond, followed by â-H elimination,
may be preferred over sp2-sp3 C-C activation of an Ar-CH3
group. Thus, systems might be designed to selectively activate
unstrained C-C bonds of substrates having â-Hs, while similar
compoundssbut lacking â-H’ssundergo exclusively C-H
activation.
Figure 2. ORTEP view of 16. Selected bond lengths (Å): Rh(1)-
Cl(1) ) 2.514(1); Rh(1)-P(1) ) 2.400(1); Rh(1)-P(2) ) 2.322(1);
Rh(1)-P(3) ) 2.319(1); Rh(1)-C(1) ) 2.090(4). Selected bond angles
(deg): P(1)-Rh(1)-P(2) ) 99.92(4); P(1)-Rh(1)-P(3) ) 98.75(4);
P(2)-Rh(1)-P(3) ) 157.19(4); P(1)-Rh(1)-C(1) ) 177.9(1); Cl(1)-
Rh(1)-P(1) ) 90.62(4); Cl(1)-Rh(1)-P(2) ) 88.37(4); Cl(1)-Rh-
(1)-P(3) ) 104.43(4); Cl(1)-Rh(1)-C(1) ) 91.3(1); Rh(1)-C(1)-
C(2) ) 120.7(3); Rh(1)-C(1)-C(6) 121.8(3).
Scheme 8
) 1.1 Hz, 2H) and δ 1.65 (vq, 3JPH ) 4.5 Hz, 2JRhH ) 3.1 Hz,
3H), respectively. The 31P{1H} spectrum exhibits a doublet
resonance at δ 28.9 (1JRhP ) 127.5 Hz), indicative of two
magnetically equivalent phosphorus nuclei coordinated to a Rh-
(III) center. The 13C{1H} NMR shows clearly the presence of
the Rh(III)-CH3 moiety at δ 7.90 (bd, 1JRhC ) 30.8 Hz) and in
the 13C-DEPT-135 NMR, a positive signal is observed, indica-
tive of an odd number of protons. 13CH3I was used as well to
Experimental Section
General Procedures. All reactions were carried out under nitrogen
in a Vacuum Atmospheres glovebox (DC-882) equipped with a
recirculation (MO-40) “Dri Train” or under argon using standard
Schlenk techniques. Oxygen levels (<2 ppm) were monitored with
Et2Zn (1 M solution in hexane, Aldrich), and water levels (<2 ppm)
were monitored with TiCl4 (neat, BDH chemicals). Solvents were
reagent grade or better, dried, distilled, and degassed before introduction
into the glovebox, where they were stored over activated 4 Å molecular
sieves. Deuterated solvents were purchased from Aldrich and were
degassed and stored over 4 Å activated molecular sieves in the
glovebox. [RhClL2]2 (L ) cyclooctene or ethylene) was prepared by
a published procedure.56,57 Reaction flasks were washed with deionized
water followed by acetone and then oven-dried prior to use. GC
analyses were performed on a Varian 3300 gas chromatograph equipped
with a molecular sieve column.
1
unambiguously assign this moiety in the H and 13C NMR.
Complex 19 is stable in solution at room temperature for 24 h
but decomposes slowly upon heating of the product solution at
70 °C (in a sealed tube). However, compounds indicative of
Ar-C bond cleavage such as 20 are not observed. Thus,
cleavage of the ArCH2-CH3 bond is most probably not involved
either on the reaction coordinate or as a side equilibrium in the
thermolysis of 12 (Scheme 6) or in the reaction of 2 with ClRh-
(PPh3)3 (Scheme 7).
Spectroscopic Analysis. The 1H, 31P{1H}, and 13C{1H} NMR
spectra were recorded at 400.19, 161.9, and 100.6 MHz, respective-
ly, on a Bruker AMX 400 NMR spectrometer. 1H, 31P{1H}, and 13C-
{1H} spectra were also recorded at 250.17, 101.3, and 62.9 MHz,
Summary and Conclusions
We observed directly metal insertion into a strong Ar-CH2-
CH3 bond prior to â-H elimination and have shown that sp2-
sp3 C-C bond activation in this system is kinetically preferable
to the unobserved sp3-sp3 C-C bond activation with rhodium-
(I), regardless of the bulk and the electron density at the metal
(54) Martinho-Simoes, J. A.; Beauchamp, J. L. Chem. ReV. 1990, 90,
629.
(55) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650.
(56) Herde´, J. L.; Senoff, C. V. Inorg. Nucl. Chem. Lett. 1971, 1029.
(57) Cramer, R. Inorg. Chem. 1962, 1, 722.

13420 J. Am. Chem. Soc., Vol. 120, No. 51, 1998
Van der Boom et al.
respectively, on a Bruker DPX 250 NMR spectrometer. All chemical
shifts (δ) are reported in ppm and coupling constants (J) are in hertz.
The 1H and 13C NMR chemical shifts are relative to tetramethylsilane;
the resonance of the residual protons of the solvent was used as an
internal standard h1 (δ 7.15 benzene; 7.26 chloroform; 7.09 toluene)
and all-d solvent peaks (δ 128.0 benzene; 77.0 chloroform; 20.4
toluene), respectively. 31P NMR chemical shifts are relative to 85%
H3PO4 in D2O at δ 0.0 (external reference), with shifts downfield of
the reference considered positive. Assignments in the 1H and 13C{1H}
NMR were made using H{31P}, H-1H COSY, and 13C-DEPT-135
NMR. All measurements were carried out at 298 K. Ph3PO was used
as an internal standard for integration. IR spectra were recorded as
films between NaCl plates on a Nicolet 510 FT spectrometer.
Formation and Thermolysis of Rh(Et){2,6-(CH2PtBu2)2C6H3}Cl
(2). A C6D6 solution (1 mL) of 1 (30 mg, 0.71 mmol) and [RhClL2]2
(L ) C2H4 or C8H14) (25 mg, 0.035 mmol) was loaded into a 5-mm
screwcap NMR tube and heated for 5 min at 120 °C. The resulting
deep red solution was analyzed by 1H, 1H{31P}, 1H-1H COSY, NOESY,
31P{1H}, 13C{1H}, and 13C-DEPT-135 NMR, showing the quantitative
formation of 2 and C8H14 or C2H4. Continued heating for 16 h resulted
in the quantitative formation of the known Rh(H){2,6-(CH2PtBu2)2C6H3}-
Cl (4) and ethylene, as judged by 1H and 31P NMR and GC.30,35
Addition of an authentic sample to the solution resulted in overlap of
1
1
1
signals in H and 31P NMR. The reaction was also performed in a
sidearm flask to allow quantitative analysis of the gas phase by GC.
3
1H NMR (C6D6): δ 1.17 (vt, JPH ) 6.0 Hz, 18H, C(CH3)3), 1.24 (dt,
3
3
Formation of 1 and 2. (a) Synthesis of 2-Ethyl-1,3-dimethyl-
benzene. A solution of 2-bromo-1,3-dimethylbenzene (18.1 g, 97.5
mmol) in ether (20 mL) was added dropwise to a cold (0 °C)
n-butyllithium solution (1.6 M in hexane, 80 mL) in a 250-mL three-
necked round-bottom flask equipped with an argon inlet, dropping
funnel, and magnetic bar. The resulting reaction mixture was refluxed
overnight, cooled to room temperature, and filtered. The residue was
washed with cold pentane (3 × 25 mL) and dried in a vacuum, affording
2-lithio-1,3-dimethylbenzene as a white solid in quantitative yield (11
g). A solution of ethyl bromide (21.3 g, 195 mmol) in THF (50 mL)
was added dropwise to a stirred suspension of 2-lithio-1,3-dimethyl-
benzene (11 g, 97.5 mmol) in THF (150 mL) at - 60 °C in a 500-mL
Schlenk flask. The reaction mixture was warmed to room temperature
and stirred overnight. The THF was removed by rotary evaporation,
and the residue was dissolved in CH2Cl2 (300 mL), washed with water
(3 × 100 mL), and dried again. Distillation (90-92 °C/0.20 mmHg)
afforded a colorless oil (4.6 g, 35%). 1H NMR (CDCl3): δ 6.88 (s,
3JRhH ) 2.3 Hz, JHH ) 7.2 Hz, 3H, RhCH2CH3), 1.35 (vt, JPH ) 6.0
2
3
3
Hz, 18H, C(CH3)3), 2.40 (m, JRhH ) 2.8 Hz, JHH ) 7.2 Hz, JPH
)
2
7.1 Hz, 2H, RhCH2CH3), 3.07 (ABq, ∆AB ) 34.0 Hz, JHH ) 17.6
Hz, JPH ) 3.5 Hz, 4H, CH2P), 7.0 (m, 3H, ArH). 13C{1H} NMR
2
1
2
(C6D6): δ 16.83 (dt, JRhC ) 29.1 Hz, JPC ) 4.9 Hz, RhCH2CH3),
23.61 (d, JRhC ) 1.4 Hz, RhCH2CH3), 29.71 (vt, JPC ) 2.2 Hz,
2
2
2
2
C(CH3)3) 31.10 (vt, JPC ) 2.3 Hz, C(CH3)3), 33.01 (dvt, JRhC ) 2.7
Hz, 1JPC ) 8.9 Hz, CH2P), 36.49 (dvt, 2JRhC ) 1.6 Hz, 1JPC ) 6.6 Hz,
1
C(CH3)3), 36.76 (vt, JPC ) 7.4 Hz, C(CH3)3), 122.88 (dt, JRhC ) 1.1
Hz, JPC ) 8.6 Hz, Cmeta), 130.30 (s, Cpara), 150.16 (dt, 2JRhC ) 1.0 Hz,
2JPC ) 8.7 Hz, Cortho), 167.57 (dt, JRhC ) 35.8 Hz, JPC ) 1.9 Hz,
1
2
1
C
ipso). 31P{1H} NMR (C6D6): δ 54.99 (d, JRhP ) 123.8 Hz, 2P).
Formation and Thermolysis of Rh(Et){2,6-(CH2PtBu2)2C6H3}I
(6). EtI (3 mg, 0.019 mmol) was added to a yellow C6D6 solution (1
mL) of 5 (10 mg, 0.019 mmol). The red reaction solution was loaded
1
1
into a 5-mm screwcap NMR tube and analyzed by H, H-1H NOE,
1H{31P}, and 31P{1H} NMR. The reaction was completed within 10
min, and no intermediate compounds were observed. Continued heating
for 16 h resulted in the formation of Rh(H){2,6-(CH2PtBu2)2C6H3}I
and ethylene (∼95%), as judged by 1H and 31P NMR and GC analysis
of the solution.35 The reaction was also performed in a sidearm flask
to allow quantitative analysis of the gas phase by GC. 1H NMR
3
3H, ArH), 2.55 (q, JHH ) 7.5 Hz, 2H, CH2CH3), 2.21 (s, 6H, CH3),
1.01 (t, 3JHH ) 7.6 Hz, 3H, CH2CH3). 13C{1H} NMR (CDCl3): δ 141.5,
136.3, 128.7, 126.0 (all s, Ar), 23.2 (s, CH2CH3), 20.1 (s, CH3), 13.8
(s, CH2CH3).
(b) Synthesis of 2-Ethyl-1,3-dibromomethylbenzene. A mixture
of 2-ethyl-1,3-methylbenzene (3.6 g, 27 mmol), NBS (9.6 g, 54 mmol),
and AIBN (∼0.1 g) in CCl4 (150 mL) was refluxed for 5 h in a 500-
mL three-necked round-bottom flask equipped with an argon inlet and
condenser. After filtration, the solution was washed with water (3 ×
25 mL), treated with MgSO4, filtered, and concentrated by rotary
evaporation. The residue was stored overnight at - 20 °C, affording
a white lacrimatory solid which was filtered, washed with cold
cyclohexane (3 × 25 mL), and dried in a vacuum (7.2 g, 91%). This
product was further purified by distillation (110 °C/0.16 mmHg) and
by column chromatography using hexane as eluent (3.1 g, 39%). 1H
3
3
(C6D6): δ 0.89 (dt, JRhH ) 2.4 Hz, JHH ) 7.2 Hz, 3H, RhCH2CH3),
1.16 (vt, 2JPH ) 6.1 Hz, 18H, C(CH3)3), 1.42 (vt, 3JPH ) 6.3 Hz, 18H,
C(CH3)3), 2.38 (m, 2JRhH ) 2.7 Hz, 2JHH ) 7.2 Hz, 3JPH ) 6.1 Hz, 2H,
2
2
RhCH2CH3), 3.12 (ABq, ∆AB ) 9.2 Hz, JHH ) 17.8 Hz, JPH ) 3.7
Hz, 4H, CH2P), 7.1 (m, 3H, ArH). 31P{1H} NMR (C6D6): δ 52.63 (d,
1JRhP ) 122.5 Hz). FD-MS: (M+ - I) 524.8.
Formation of Rh(η1-N2){2,6-(CH2PtBu2)2C6H3} (7). To a solution
of 3 (40 mg, 0.071 mmol) in THF (5 mL) was added excess of NaH
(35 mg, 1.3 mmol). The suspension was stirred for 24 h at room
temperature. The mixture was filtered and dried under vacuum. The
resulting solid was dissolved in benzene (10 mL), and the solution was
filtered again. Complex 7 was obtained as a yellow air-sensitive solid
after evaporation of the benzene. Passing CO through a benzene (3
mL) solution of 7 for 15 min resulted in quantitative formation of Rh-
(CO){2,6-(CH2PtBu2)2C6H3}, as judged by 1H and 31P NMR and IR.30
3
3
NMR (CDCl3): δ 7.21 (d, JHH ) 7.6 Hz, 2H, ArH), 7.05 (t, JHH
)
3
7.6 Hz, 1H, ArH), 4.43 (s, 4H, CH2Br), 2.80 (q, JHH ) 7.6 Hz, 2H,
CH2CH3), 1.21 (t, JHH ) 7.6 Hz, 3H, CH2CH3). 13C NMR (CDCl3):
3
δ 142.4, 136.8, 131.9, 127.1 (all s, Ar), 31.7 (s, CH2Br), 22.0 (s, CH2-
CH3), 15.7 (s, CH2CH3).
(c) Phosphination. For 1: The phosphination with HPtBu2 to afford
1 was done according to literature procedures.26,30,32 31P{1H}
(CDCl3): δ - 31.5 (s). 1H NMR (CDCl3): δ 7.25 (m, 2H, ArH),
6.90 (t, 1H, ArH), 3.10 (q, 2H, 3JHH ) 7.4 Hz, CH2CH3), 2.82 (s, 4H,
CH2P), 1.41 (t, 3H, 3JHH ) 7.5 Hz, CH2CH3), 1.06 (s, 36H, C(CH3)3).
For 2: A solution of 2-ethyl-1,3-bis(bromomethyl)benzene (1.8 g, 6.0
mmol) in THF (120 mL) was added dropwise to a cold THF solution
(-78 °C, 30 mL) of LiPPh2 (2.4 g, 13 mmol) in a 250-mL three-necked
round-bottom flask equipped with an argon inlet and a dropping funnel.
The solution was warmed to room temperature and stirred overnight.
The reaction mixture was concentrated by rotary evaporation, toluene
was added (100 mL), and the solution was filtered and concentrated
again. The residue was dissolved in CHCl3 (100 mL), filtered, and
concentrated by rotary evaporation. The residue was recrystallized from
pentane to afford a white solid 2 (2.8 g, 92%). 31P{1H} (CDCl3): δ
-11.7 (s). 1H NMR (CDCl3): δ 7.6-6.8 (m, 23H, ArH), 3.38 (s, 4H,
CH2P), 2.95 (q, 3JHH ) 7.5 Hz, 2H, CH2CH3), 1.34 (t, 3JHH ) 7.5 Hz,
3H, CH2CH3). 13C{1H} NMR (CDCl3): δ 141.5-125.5 (Ar), 34.0 (d,
1JPC ) 16.8 Hz, CH2P), 22.8 (t, 4JPC ) 4.8 Hz, CH2CH3), 15.3 (t, 5JPC
) 1.8 Hz, CH2CH3).
2
1H NMR (C6D6): δ 1.26 (vt, JPH ) 6.3 Hz, 36H, C(CH3)3), 3.15 (vt,
2JPH ) 3.7 Hz, 4H, CH2P), 7.1 (m, 3H, ArH). 13C{1H} NMR (C6D6):
δ 35.18 (vt, 2JPC ) 6.6 Hz, C(CH3)3), 36.36 (dvt, 2JRhC ) 6.6 Hz, 1JPC
) 10.1 Hz), 120.73 (t, JPC ) 9.8 Hz, Ar), 123.57 (s, Ar), 128.53 (s,
Ar), 157.79 (dt, JRhC ) 3.6 Hz, JPC ) 12.8 Hz, Ar). 31P{1H} NMR
(C6D6): δ 81.01 (d, 1JRhP ) 157.9 Hz). IR (film): ν ) 2133 cm-1, (s,
NtN).
Hydrogenolysis of the Ar-C Bond in 15. A THF solution (30
mL) of 15 (60 mg, 0.066 mmol) was loaded into a 90 cm3 Fischer
Porter pressure vessel, pressurized with H2 (20 psi), and heated for 12
h at 80 °C. The gas phase was removed using a vacuum line and
analyzed quantitatively by GC. The reaction mixture was concentrated
to 5 mL. Addition of cold pentane (20 mL) resulted in precipitation
of an orange powder, which was filtered off and dried under high
vacuum to give >90% yield of 17. The analogous complex 16, lacking
the two methyl substituents in the 3 and 5 positions of the aromatic
ring, has similar spectroscopic features.25 31P{1H} NMR (THF): δ
46.6 (dd, 1JRhP ) 111.1 Hz, 2JPP ) 24.5 Hz, 2P), 19.8 (dt, 1JRhP ) 82.2
Hz, 1P). Addition of MeLi (1.2 equiv) or KOtBu (5 equiv) to the

C-C Bond ActiVation by Rh(I) in Solution
J. Am. Chem. Soc., Vol. 120, No. 51, 1998 13421
Table 1. Crystallographic Data for Complex 16
product solution at room temperature resulted in quantitative formation
of Rh{2,6-(CH2PPh2)2-3,5-(CH3)2-C6H}(PPh3), as judged by 1H and 31
NMR.
P
formula
fw
space group
C50H43P3RhCl‚(CD3)2CO
880.79
P21/n
Thermolysis of Complex 12. A toluene-d8 solution (1 mL) of 12
(10 mg) was loaded into a 5-mm screwcap NMR tube and heated for
3 days at 120 °C. 1H and 31P NMR analysis showed the quantitative
formation of 13.25 Addition of an authentic sample to the solution
resulted in overlap of signals in 1H and 31P NMR. Ethylene was
observed by 1H NMR and GC. The reaction was also performed in a
sidearm flask to allow quantitative analysis of the gas phase by GC.
a, Å
b, Å
c, Å
16.052(5)
20.434(5)
14.519(2)
104.43(2)
4612.1(9)
1.34
â, deg
V, Å3
Dcalcd, g cm-3
T, K
298
Z
4
5.61
6637
4667
0.031
0.039
Formation and Thermolysis of Complex 19. To a toluene-d8
solution (1 mL) of 1821 (10 mg, 0.024 mmol) was injected 3 µL (0.048
mmol) of CH3I (or 13CH3I) by a microsyringe at room temperature. 1H
and 31P NMR analysis of the reaction mixture after 30 min showed
exclusive formation of 19 and PPh3. The red solution was dried under
vacuum to remove excess of CH3I, and the resulting red oil was
redissolved in toluene-d8 (1 mL). The product was not separated from
PPh3. Complex 19 is stable at room temperature in solution but slowly
decomposes at 70 °C (24 h for ∼90% decomposition). Compounds
indicative of Ar-C bond cleavage were not observed. 1H NMR
(toluene-d8): δ 7.4-6.5 (m, ArH of 19 and PPh3), 3.3 (left part of
µ(Mo KR), cm-1
no. of unique reflections
no. of reflections with I > 3σ(I)
R
Rw
refinement process. Refinement proceeded to convergence by minimiz-
ing the function ∑w(|Fo| - |Fc|)2. A final difference Fourier synthesis
map showed several peaks less than 0.3 e/Å3 scattered about the unit
cell without a significant feature. The discrepancy indices R )
∑||Fo| - |Fc||/∑||Fo| and Rw ) [∑w(|Fo| - |Fc|)2/∑w(|Fo| ]1/2 are
2
2
2
presented with other crystallographic data in Table 1. An ORTEP view,
selected bond angles, and distances of the molecular structure are shown
in Figure 2.
ABq, JHH ) 13.7 Hz, JPH ) 3.8 Hz, 2H, CH2P), 2.7 (right part of
ABq, 2JHH ) 13.7 Hz, 2JPH ) 4.4 Hz, 2H, CH2P), 2.25 (td, 3JPH ) 8.3
2
3
2
Hz, JRhH ) 1.1 Hz, 2H, ArCH2Rh), 1.65 (m, JPH ) 4.5 Hz, JRhH
)
3.1 Hz, 3H, RhCH3), 1.54 (s, 6H, ArCH3). 31P{1H} NMR (toluene-
d8): δ 28.9 (d, 1JRhP ) 127.5 Hz, 19), -4.71 (bs, PPh3). 13C{1H} NMR
(toluene-d8): δ 139-126 (CAr of 19 and PPh3), 27.71 (vt, 1JPC ) 12.2
Hz, CH2P), 22.41 (bd, 1JRhC ) 15.8 Hz, ArCH2Rh), 19.45 (s, ArCH3),
7.90 (bd, 1JRhC ) 30.8 Hz, RhCH3). FD-MS: m/z ) 760 (M+), correct
isotope pattern.
Acknowledgment. This work was supported by the U.S.-
Israel Binational Science Foundation, Jerusalem, Israel, and by
the Minerva Foundation, Munich, Germany. D.M. is the holder
of the Israel Matz Professorial Chair in Organic Chemistry. We
thank Dr. S. Cohen (the Hebrew University of Jerusalem,
Jerusalem, Israel) for performing the X-ray structural analysis.
X-ray Crystal Structure Determination of Complex 16. A crystal
was analyzed (at 298 K) on a PW1100/20 Philips four-circle computer-
controlled diffractometer, Mo KR (λ ) 0.710 69 Å) radiation with a
graphite crystal monochromator in the incident beam. The unit cell
dimensions were obtained by a least-squares fit of 24 centered
reflections in the range of 11 < θ < 14°. Intensity data were collected
using the ω-2θ technique to a maximum 2θ of 46°. The scan width,
∆ω, for each reflection was 1.00 + 0.35 tan θ, with a scan speed of
2.1 deg/min. Background measurements were made for a total of 20
s at both limits of each scan. Three standard reflections were monitored
every 60 min. No systematic variations in intensities were found.
Intensities were corrected for Lorentz and polarization effects. All non-
hydrogen atoms were found by using the SHELXS-80 direct method
analysis.58 After several cycles of refinements,59 the positions of the
hydrogen atoms were calculated, except HRh, and added to the
Supporting Information Available: Tables of crystal-
lographic data and structure refinement, atomic coordinates,
bond lengths and angles anistropic displacement parameters, and
hydrogen atom coordinates for 16 (11 pages, print/PDF). See
any current masthead page for ordering information and Web
access instructions.
JA982345B
(58) Sheldrick, G. M. SHELXTL, An integrated system for solving,
refining and displaying crystal structures from diffraction data; University
of Go¨ttingen, Germany, 1980.
(59) All crystallographic computing was done on a VAX computer at
the Hebrew University of Jerusalem, Israel, using TEXSAN structure
analysis software.