Novel 1,2-rearrangement of porphyrinatorhodium(III) alkyls: Cis β- hydride elimination/olefin metal-hydride insertion pathway [4]

Full text of DOI:10.1021/ja9808548

9686
J. Am. Chem. Soc. 1998, 120, 9686-9687
Scheme 1. 1,2-Alkyl Rearrangement of Rh(bocp)(alkyl)
Novel 1,2-Rearrangement of
Porphyrinatorhodium(III) Alkyls: Cis â-Hydride
Elimination/Olefin Metal-Hydride Insertion
Pathway
Kin Wah Mak and Kin Shing Chan*
Department of Chemistry
The Chinese UniVersity of Hong Kong
Shatin, Hong Kong
Special AdministratiVe Region, China
ReceiVed March 16, 1998
Alkyl 1,2-rearrangements of alkylmetal complexes (eq 1) play
a crucial role in organometallic chemistry on both grounds of
transition-metal catalysis and bioinorganic chemistry. It is a
governing factor in determining the regioselectivity of the products
formed in transition-metal promoted catalysis.1-4 Furthermore,
alkyl 1,2-rearrangement is important in bioinorganic chemistry
due to its potential relevance to the coenzyme B12 dependent 1,2-
rearrangements.5-9
Scheme 2. Synthesis of Porphyrinatorhodium(III) Alkyls
Alkyl 1,2-rearrangements have been reported,10-12 and were
proposed to undergo a stepwise â-hydride elimination and metal-
hydride olefin reinsertion mechanism.10b,11b,d Alkyl 1,2-rearrange-
ments in metal complexes with macrocyclic ligands were rare
due to the unavailability of mutual cis coordination sites.13 We
now report that the Rh(bocp)CH2CH2Ph 1c14 (Scheme 1) under-
goes reversible thermal 1,2-alkyl rearrangement via a stepwise
cis â-hydride elimination/olefin Rh-H insertion pathway.15
The electron-deficient porphyrin ligand, H2(bocp) 3, was
synthesized from H2(btpp)14 by octachlorination of H2(btpp) 4 at
the â positions in a procedure similar to that reported by
Dolphin.16 3 was then metalated with RhCl3‚xH2O in refluxing
PhCN to give Rh(bocp)Cl(PhCN) 5 in 94%. Complex 1c was
then obtained in 85% yield by the reductive alkylation of 5 with
NaBH4/BrCH2CH2Ph (Scheme 2).17
The novel 1,2-alkyl rearrangment was observed upon heating
a solution of 1c in benzene-d6 (25 mM) at 80 °C for 10 h to form
Rh(bocp)CH(CH3)C6H5 2c in 87%. The structure of 2c was
confirmed by independent synthesis. Monitoring the reaction at
80 ( 0.2 °C by 1H NMR spectroscopy yielded a first-order
dependency on [1c] with kobs estimated to be (1.4 ( 0.1) × 10-4
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry, 2nd ed.; University
Science Book: Mill Valley, CA, 1987.
s-1
.
The isomerization was found to be reversible. 2c gave an
equilibrating mixture of 1c and 2c upon heating at 80 °C for 48
h. The equilibrium constant was estimated roughly to be 35 with
the secondary alkyl complex being the major isomer, which
(2) Lazzaroni, R.; Settambolo, R.; Uccello-Barretta, G. Organometallics
1995, 14, 4644.
(3) Horiuchi, T.; Shirakawa, E.; Nozake, K.; Takaya, H. Organometallics
1997, 16, 2981.
(4) LaPointe, A, M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906.
corresponded to a free energy difference of about 10.5 kJ mol-1
.
(5) Dolphin, D., Ed. B12; Wiley: New York, 1982; Vols. 1 and 2.
(6) Halpern, J. Science 1985, 227, 869.
The driving force for the isomerization into the sterically more
bulky secondary isomer probably resulted from the presence of
the slightly electron-withdrawing phenyl group in stabilizing the
secondary Rh-C bond through bond polarization.10,18-20
(7) Finke, R. G. In Molecular Mechanisms of Bioorganic Processes;
Bleasdale, C., Golding, B. T., Eds.; The Royal Society of Chemistry:
Cambridge, England, 1990; pp 245-290.
(8) Pratt, J. M. In Metal Ions in Biological Systems; Sigel, H., Sigel, A.,
Eds.; Marcel Dekker: New York, 1993; Vol. 29; pp 229-280.
(9) Marzilli, L. G. In Bioinorganic Catalysis; Reedijk, J. Ed.; Marcel
Dekker: New York, 1993; pp 227-260.
Thermolysis of the 13C-labeled complex Rh(bocp)*CH2CH2C6H5
1c* 21 gave Rh(bocp)CH(*CH3)Ph 2c* with the 13C-labeled atom
migrated to the R-methyl group. Therefore, rhodium atom rather
than phenyl group migrated in the isomerization (Scheme 1).
Cis coordination sites are apparently absent in complex 1c and
â-hydride elimination is therefore presumably hindered. It
prompted us to investigate the possible radical involvement via
(10) (a) Reger, D. L.; Garza, D. G.; Baxter, J. C. Organometallics 1990,
9, 873. (b) Reger, D. L.; Garza, D. G.; Lebioda, L. Organometallics 1992,
11, 4285. (c) Reger, D. L.; McElligott, P. J. J. Organomet. Chem. 1981, 216,
C12. (d) Reger, D. L.; Culbertson, E. C. Inorg. Chem. 1977, 16, 3104.
(11) (a) Bennett, M. A.; Charles, R. J. Am. Chem. Soc. 1972, 94, 666. (b)
Bennett, M. A.; Charles, R.; Mitchell, T. R. B. J. Am. Chem. Soc. 1978, 100,
2737. (c) Bennett, M. A.; Jeffery, J. C. Inorg. Chem. 1980, 19, 3763. (d)
Bennett, M. A.; Crisp, G. T. Organometallics 1986, 5, 1792; 1800.
(12) Tamaki, A.; Magennis, S. A.; Kochi, J. K. J. Am. Chem. Soc. 1974,
96, 6140.
(16) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30,
251.
(17) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc.
1975, 97, 6461.
(18) (a) Settambolo, R.; Pucci, S.; Bertozzi, S.; Lazzaroni, R. J. Organomet.
Chem. 1995, 489, C50. (b) Settambolo, R.; Caiazzo, A.; Lazzaroni, R. J.
Organomet. Chem. 1996, 506, 337.
(13) Del Rossi, K. J.; Wayland, B. B. J. Am. Chem. Soc. 1985, 107, 7941.
(14) bocp ) 5,10,15,20-tetrakis-(4′-tert-butylphenyl)-2,3,7,8,12,13,17,18-
octachloroporphyrinate. btpp ) 5,10,15,20-tetrakis-(4′-tert-butylphenyl)por-
phyrinate. oep ) 2,3,7,8,12,13,17,18-octaethylporphyrinate.
(15) Mak, K. W.; Leung, Y. B.; Wang, R.-J.; Mak, T. C. W.; Chan, K. S.
Book of Abstracts, 210th American Chemical Society National Meeting,
Chicago, IL, Fall 1995; American Chemical Society: Washington, DC, 1995;
INOR 629.
(19) (a) Blau, R. J.; Espenson, J. H.; Bakac. A. Inorg. Chem. 1984, 23,
3526. (b) Kirker, G. W.; Bakac, A.; Espenson, J. H, J. Am. Chem. Soc. 1982,
104, 1249.
(20) Halpern, J. Acc. Chem. Res. 1982, 15, 238.
S0002-7863(98)00854-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/02/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9687
Rh-C bond homolysis. 1c [9.5 mM] was found to react much
slower in the presence of nBu3SnH [95 mM] at 80 °C to give the
apparent radical trapping adduct ethylbenzene in 14 d with 93%
yield. This slower rate of trapping argued against the involvement
of radical intermediate.
The rate of alkyl rearrangement of 1c was found to be retarded
to one-eighth in the presence of added pyridine (5 equiv) as the
sixth coordinating axial ligand. Coordination of pyridine to Rh
was confirmed by 1H NMR and UV-visible spectrophotometric
titrations. Titration of 1c in anaerobic benzene with pyridine
between 15 and 35 °C confirmed a 1:1 complex of Rh-pyridine
was formed with the ∆H° and ∆S° determined to be -63 ( 8 kJ
mol-1 and -140 ( 30 J mol-1 K-1 respectively.25 The Rh-
pyridine binding constant (log K) at 80 °C was estimated to be
1.52. The retardation of alkyl rearrangement by axial ligand
coordination may be attributed to the coordination saturation
caused by ligand coordination which hindered â-hydride elimina-
tion.
A basic requirement for â-hydride elimination is the availability
of vacant cis coordination sites, which is apparently lacking in
the Rh(bocp)alkyl complexes. Nonetheless, â-hydride elimination
reactions of organocobalamins,27 organocobalt complexes with
coplanar N4 donor corrin ligands, and agostic interaction28 in
metalloporphryin have been reported. Furthermore, crystal
structures of metalloporphyrin complexes, having two mutually
cis coordinations on the same axial side, are known.29 The above-
mentioned mechanistic evidences support the stepwise â-hydride
elimination/Rh-H olefin reinsertion as a plausible mechanism
for the alkyl 1,2-rearrangement. It implied the possibility of axial
cis coordination on organorhodium porphyrin complexes. This
study may serve as a model for the possible metal involvement
in vitamin B12 dependent 1,2-rearrangement.5
The Eyring plot22 of the rearrangement of 1c studied over the
temperature range from 75 to 90 °C yields the ∆Hqobs and ∆Soqbs
of the 1,2-rearrangement to be 53 ( 3 kJ mol-1 and -171 ( 9
J mol-1 K-1, respectively. Compared with a typical rhodium-
carbon bond dissociation energy of 187 kJ mol-1 as in Rh(oep)-
CH(Bu)OH,14,23 the small ∆Hoqbs measured precludes Rh-C
homolysis being the rate-determining step. Furthermore, the
extremely negative ∆Hqobs supports a structurally organized
transition state, contrary to homolytic fission of the Rh-C bond.
On the basis of the measured activation parameters, a â-hydride
elimination/Rh-hydride olefin insertion mechanism is proposed
for the rearrangement.
The rates of the alkyl rearrangement were found to increase
with electron-rich para-substituted phenylethyl moieties. The
increased rate with electron-donating para-substituted phenylethyl
(kobs/10-4s-1: NO2 1a, 0.10; Cl, 1b 0.35; H, 1c, 1.4; Me, 1d, 1.8;
OMe, 1e, 3.7) supported the concept that a positive charge is
developing at the benzylic carbon. Therefore, the benzylic
hydrogen likely migrated as a hydride.
The rate of 1,2-rearrangement of 1c also exhibited a kinetic
isotope effect of 5.6 when the two benzylic hydrogen atoms were
replaced by deuterium atoms. It established that the benzylic
C-H bond cleavage occurred before or at the rate-determining
step of the 1,2-rearrangement.
The intermediacy of olefin was established by exchange
experiments.10b 1c underwent slow alkyl exchanges with excess
p-nitrostyrene to give Rh(bocp)CH2CH2C6H4NO2 1a. No re-
arrangement product was observed probably due to slow re-
arrangement (observed half-life about 20 h) (eq 2).
In summary, the novel reversible 1,2-alkyl rearrangement was
observed with the Rh(bocp)CH2CH2Ph complex. Mechanistic
studies have shown that the rearrangement proceeds through
rhodium migration via a stepwise cis â-hydride elimination/Rh-H
olefin insertion pathway. Further studies of this 1,2-rearrange-
ments are in progress.
Acknowledgment. We thank the Research Grants Council of Hong
Kong for financial support (RGC Ref. No. CU96702) and the purchase
of high-field NMR spectrometers.
Supporting Information Available: Experimental procedures and
spectroscopic data for selected compounds (8 pages, print/PDF). See
any current masthead page for ordering information and Web access
instructions.
Independently synthesized Rh(bocp)H 824 was found to react
readily with styrene to give complex 1c in 88% yield, which
further confirmed the intermediacy of Rh-H and olefin in the
1,2-rearrangement.
JA9808548
(25) Perkampus, H.-H. UV-Vis Spectroscopy and Its Applications (English
ed.); Springer-Verlag: Berlin Heidelberg, 1992; pp 158-162.
(26) Ng, F. T. T.; Rempei, G. L. J. Am. Chem. Soc. 1982, 104, 621.
(27) (a) Grate, J. H.; Schrauzer, G. N. J. Am. Chem. Soc. 1979, 101, 4601.
(b) Schrauzer, G. N.; Grate. J. H. J. Am. Chem. Soc. 1981, 103, 541.
(28) Collman, J. P.; Chien, A. S.; Eberspacher, T. A.; Brauman, J. I. J.
Am. Chem. Soc. 1998, 120, 425.
(29) (a) Yang. C.-H.; Dazugan, S. J.; Goedken, V. L. J. Chem. Soc., Chem.
Commun. 1985, 1425. (b) Yi, G.-B.; Khan, M. A.; Richter-Addo, G. B. Inorg.
Chem. 1995, 34, 5703.
(21) Orfanopoulos, M.; Smonou, I.; Foote, C. S. J. Am. Chem. Soc. 1990,
112, 3607.
(22) (a) ln(k/T) ) -∆Hq/RT + ln(k/h) + ∆Sq/R. (b) Espenson, J. H.
Chemical Kinetics and Reaction Mechanism, 2nd ed.; McGraw-Hill: New
York, 1995; pp 156-160.
(23) (a) Martinho Simo˜es, J. A.; Beauchamp, J. L. Chem. ReV. 1990, 90,
629. (b) Wayland, B. B. Polyhedron, 1988, 7, 1545.
(24) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986,
25, 4039.