1,2-Rearrangements of β-nitrogen-substituted (porphyrinato) rhodium(III) ethyls

Article abstract of DOI:10.1021/om0501475

(Porphyrinato)rhodium(III) ethyls containing β-nitrogen substituents have been showed to undergo clean, thermal 1,2-rearrangements into the α-nitrogen-substituted rhodium ethyls. The rates and the equilibrium positions were found to be dependent on the el

Full text of DOI:10.1021/om0501475

Organometallics 2005, 24, 2561-2563
2561
Communications
1,2-Rearrangements of â-Nitrogen-Substituted
(Porphyrinato)rhodium(III) Ethyls
Siu Kwan Yeung and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, China
Received March 2, 2005
Scheme 1. Reaction Pathway via 2-Aminoethyl
Metal Complexes
Summary: (Porphyrinato)rhodium(III) ethyls containing
â-nitrogen substituents have been showed to undergo
clean, thermal 1,2-rearrangements into the R-nitrogen-
substituted rhodium ethyls. The rates and the equilib-
rium positions were found to be dependent on the
electronic nature of the N substituents. Complexes 1a-
3a were characterized by X-ray crystallography.
rhodium(III) complexes undergo 1,2-rearrangement
through a stepwise â-hydride elimination/metal hydride
insertion pathway.10 The rates approaching the equi-
libria and the equilibrium constants are often dependent
on the nature of the alkyl substituents. Expanding the
studies into metal alkyls containing a heteroatom such
as nitrogen at the â-position will likely aid in further
understanding 1,2-rearrangements which may bear
potential relevance to B12 chemistry.
1,2-Rearrangements of alkylmetal complexes, in which
the bonding position of the metal interchanges with an
adjacent hydrogen atom, play crucial roles in bioinor-
ganic chemistry1-4 and organometallic chemistry5-8 (eq
1). The rate of this isomerization and the equilibrium
In basic organometallic chemistry, alkyl transition-
metal complexes containing â-nitrogen substituents may
(1) undergo relatively facile 1,2-rearrangements of the
alkyl groups via â-hydride elimination or (2) form
metal-olefin complexes via â-elimination of the amino
group, depending on the alkyl ligands and reaction
conditions (Scheme 1). 2-Aminoalkyl transition-metal
complexes have been reported11 and proposed to be
intermediates in the catalytic amination of olefins.
These intermediates usually undergo â-hydride elimi-
nation to yield enamines or imines.12-14 On the other
hand, â-amino elimination of amine to regenerate the
cationic metal olefin complex has also been reported.15
The propensity increases with increasing stability of the
nitrogen-substituted group.16,17 The effect of substitu-
ents at nitrogen of the â-aminoethyl ligands is not well-
known in the partition between â-amino elimination or
1,2-rearrangement pathways via â-hydride elimination.
We have successfully synthesized four electronically
different â-nitrogen-substituted18 rhodium porphyrins
constant are very important in determining the rates
and regioselectivity in a variety of processes.
In bioinorganic chemistry, the mechanism of 1,2-
rearrangements catalyzed by coenzyme B12 has been a
subject of much interest.3 Although radicals are com-
monly accepted to be formed during the enzymatic
process, the nature of the rearrangement process still
remains a subject of much discussion.9 Alkyl 1,2-
rearragnements bear potential relevance to the mecha-
nistic studies of the coenzyme B12 dependent rearrange-
ments. Metal complexes with macrocyclic ligands are
widely used vitamin B12 coenzyme models. We have
previously reported that (porphyrinato)phenylethyl-
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
(1) Dolphin, D., Ed. B12; Wiley: New York, 1982; Vols. 1 and 2.
(2) Banerjee, R. Chemistry and Biochemistry of B12; Wiley-Inter-
science: New York, 1999.
(10) (a) Mak, K. W.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686-
9687. (b) Mak, K. W.; Xue, F.; Mak, T. C. W.; Chan, K. S. J. Chem.
Soc., Dalton Trans. 1999, 3333-3334.
(3) (a) Halpern, J. Science 1985, 227, 869-875. (b) Wollowitz, S.;
Halpern, J. J. Am. Chem. Soc. 1988, 110, 3112-3120.
(4) Toraya, T. Chem. Rev. 2003, 103, 2095-2127.
(5) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry, 2nd
ed.; University Science Books: Mill Valley, CA, 1987.
(6) Parshall, G. W. Homogeneous Catalysis; Wiley-Interscience: New
York, 1980.
(11) De Renzi, A.; Di Blasio, B.; Morelli, G.; Vitagliano, A. Inorg.
Chim. Acta 1982, 63, 233-241.
(12) Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Chem. Commun.
2003, 16, 2060-2061.
(13) Hirai, H.; Sawai, H.; Makishima, S. Bull. Chem. Soc. Jpn. 1970,
43, 1148-1153.
(14) Utsunomiya, M.; Kuwano, R.; Kawatsura, M.; Hartwig, J. F.
J. Am. Chem. Soc. 2003, 125, 5608-5609.
(7) Henrici-Olive, G.; Olive, S. The Chemistry of the Catalyzed
Hydrogenation of Carbon Monoxide; Springer-Verlag: New York, 1984.
(8) Heck, R. F.; Breslow, D. S. J. Am. Chem. Soc. 1963, 85, 2779-
2782.
(9) (a) He, M.; Dowd, P. J. Am. Chem. Soc. 1998, 120, 1133-1137.
(b) He, M.; Dowd, P. J. Am. Chem. Soc. 1996, 118, 711.
(15) Brunet, J.-J.; Cadena, M.; Chu, N. C.; Diallo, O.; Jacob, K.;
Mothes, E. Organometallics 2004, 23, 1264-1268.
(16) (a) Steinborn, D.; Sedlak, U.; Taube, R. Z. Anorg. Allg. Chem.
1982, 492, 103-112. (b) Steinborn, D. Angew. Chem., Int. Ed. Engl.
1992, 31, 401-421.
(17) Seul, J. M.; Park, S. Dalton 2002, 1153-1158.
10.1021/om0501475 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/28/2005

2562 Organometallics, Vol. 24, No. 11, 2005
Communications
Table 1. Result of 1,2-Rearrangements of
â-Nitrogen-Substituted
Scheme 2. Two Potential Reaction Pathways for
Rh(por)CH2CH2NR1R2
(Porphyrinato)rhodium(III) Ethyls
isomeric
complex temp/°C timea 106kobs b/s-1 ratio (2°/1°) yieldc/%
1a
1b
2a
3a
4
5a
6a
7a
120
120
90
31 days
29 days
18 h
2.9
8
86
88
85
69
0.36
∼7
95.0
100
19
80
11 h
∼992
in 23% yield in 7 days. It is likely formed from hydro-
metalation of Rh(ttp)H, generated via â-hydride elimi-
nation of 4a, with the Rh(ttp)-ethylene complex, pro-
duced also via parallel â-amine elimination of 4a
(Scheme 2).
Complex 5a, with a more electron rich porphyrin, also
underwent thermal 1,2-rearrangement at 120 °C in 53
days to give 5b. The rate of thermal rearrangement of
5a was 15 times slower than that of 1a, which may be
accounted for by the slight increase of Rh-C bond
strength in 5a with an electron-donating porphyrin.20
The â-aminoethyl complex 6a decomposed to give a
small amount of the secondary isomer 6b, and no
Rh(tdbpp)CH2CH2(tdbpp)Rh was observed. The dimeth-
ylamino complex 7a rearranged rapidly and completely
to 7b at 60 °C in 5 min.
The mechanism of the 1,2-rearrangements is consis-
tent with a â-hydride elimination/metal hydride rein-
sertion pathway. A cis vacant coordination site is known
to be necessary for â-hydride elimination to occur.21
Complex 1a remained stable at 120 °C for 1 month in
the presence of added pyridine (1 equiv). The rearrange-
ment was therefore inhibited by pyridine coordination.10a
Existence of a free olefin intermediate was confirmed
by exchange experiments with styrene. Heating the
N-pyrrolidonyl ethyl complex 2a in the presence of
styrene (15 equiv) at 80 °C for 4 days in benzene-d6 led
to the slow formation of the exchange product Rh(ttp)-
CH2CH2Ph. The coproduct 1-vinyl-2-pyrrolidine was
formed in 21% yield (eq 3). No secondary alkyl complex
80
3 h
d
6
4
e
120
80
53 days
45 h
0.59
39
31
81
60
5 min
a Time to achieve equilibrium. b Estimated from the first-order
rate fit. For blank entries, the rate was too fast to measure or
there was extensive decomposition. c Total yield of two isomers.
d Only Rh(ttp)CH2CH2(ttp)Rh was observed in 23% yield. e Only
complex 7b was observed.
in order to examine their thermal behavior. â-N-
substituted ethylrhodium complexes of Rh(ttp)CH2CH2X
(ttp ) 5,10,15,20-tetratolylporphyrinate; X ) N-phthal-
imido (1a; 70%), N-pyrrolidonyl (2a; 50%), dibenzyl-
amino (3a; 39%), amino (4a; 67%)) have been prepared
by reduction of Rh(ttp)Cl with NaBH4 followed by
subsequent alkylation with the corresponding 2-N-
substituted ethyl bromides or aziridine.19 Rh(ttp)CH2-
CH2NMe2 could not be prepared, as it is thermally
unstable at room temperature. More stable â-N-sub-
stituted ethylrhodium complexes, Rh(tdbpp)CH2CH2X
(tdbpp ) 5,10,15,20-meso-tetrakis(3,5-di-tert-butylphen-
yl)porphyrinate; X ) N-phthalimido (5a; 65%), amino
(6a; 65%), dimethylamino (7a; 68%)), were prepared by
using more electron rich porphyrins.
Complexes 1a-3a underwent smooth thermal 1,2-
rearrangements in anaerobic benzene-d6. The secondary
complexes 1b-3b were formed cleanly (eq 2). The
was detected by 1H NMR spectroscopy. Complex 3a also
yielded Rh(ttp)CH2CH2Ph in the presence of styrene.
However, no dibenzylvinylamine was detected, presum-
ably due to its extensive oligomerization (eq 4).22
isomerization was found to be reversible (Table 1). The
backward reaction starting from 1b gave a similar
equilibrated mixture of 1a and 1b upon heating at 120
°C for 29 days. Even the good leaving group N-phthal-
imido in 1a did not undergo â-amido elimination in the
presence of pyridine. Complex 4a was thermally un-
stable and decomposed rapidly at 60 °C in 2 h, with the
dirhodium complex Rh(ttp)CH2CH2(ttp)Rh (8) observed
Therefore, the rearrangement goes through a Rh hy-
dride/olefin reinsertion pathway.
The complexes 1a-3a were further characterized by
single-crystal X-ray analyses. X-ray crystal data of
complexes showed that the â-carbons on the pyrrole
(20) Martinho, S. J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629-
688.
(21) (a) Reger, D. L.; Garza, D. G.; Lebioda, L. J. Am. Chem. Soc.
1991, 10, 902-906. (b) Reger, D. L.; Garza, D. G.; Lebioda, L.
Organometallics 1992, 11, 4285-4292. (c) Reger, D. L.; Ding, Y.; Garza,
D. G.; Lebioda, L. J. Organomet. Chem. 1993, 452, 263-270.
(22) Schwan, A. L.; Refvik, M. D. Tetrahedron Lett. 1993, 34, 4901-
4904.
(18) Isaacs, N. S. Physical Organic Chemistry; Longman: England,
1987.
(19) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem.
Soc. 1975, 97, 6461-6466.

Communications
Organometallics, Vol. 24, No. 11, 2005 2563
rings are displaced alternatively above and below the
least-squares plane of the 24-atom porphyrin core23 (see
Figure 4 in the Supporting Information). The distortion
of the macrocycle is a result of unfavorable steric
interactions between the alkyl ligand and the porphyrin
core. These nonplanar porphyrin structures may facili-
tate the â-hydride elimination through conformational
change to allow cis coordination.
The rate of isomerization increased with a more
electron-donating group at the nitrogen atom (Table 1).
The rates of the 1,2-rearrangements followed the order
(X) dimethylamino . dibenzylamino . N-pyrrolidonyl
> N-phthalimido. The reactivity trend is consistent with
the â-hydride elimination pathway in forming a car-
bonium ion like intermediate which is stabilized by a
more electron donating group.21,24
N-pyrrolidonyl > N-phthalimido. The relative stability
of isomeric rhodium alkyls has been reported to depend
on the polarity of the metal-carbon bond.20,21 The
strength of a metal-carbon bond is enhanced by the
inductive electron-withdrawing N substituent at the
R-carbon.20 Notably, 3a and 7a completely isomerized
to the secondary isomer, which may be further stabilized
by the possible coordination of an amino group.
In summary, we have shown that a series of elec-
tronically different â-nitrogen-substituted rhodium por-
phyrins underwent clean thermal 1,2-rearrangements
into the R-N-substituted isomer. The mechanism of the
reaction is consistent with a â-hydride elimination/metal
hydride insertion pathway.
Acknowledgment. We thank H. S. Chan for the
X-ray crystallographic determination and the Research
Grants Council of the HKSAR, China (Grant No. CUHK
425/01P), for financial support.
The isomeric ratios depend on the electronic effect at
the nitrogen atom. The sterically more crowded second-
ary complexes were obtained as the major isomers. The
positions of the isomeric ratios followed the order (X,
secondary/primary) dimethylamino . dibenzylamino .
Supporting Information Available: Text, tables and
figures giving experimental and spectroscopic details for new
compounds and crystallographic data for complexes 1a-3a;
crystallographic data are also given as CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
(23) Linder, D. P.; Rodgers, K. R.; Banister, J.; Wyllie, G. R. A.;
Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 2004, 126, 14136-
14148.
(24) (a) Bennett, M. A.; Charles, R.; Mitchell, T. R. B. J. Am. Chem.
Soc. 1978, 100, 2737-2743. (b) Bennett, M. A.; Crisp, G. T. Organo-
metallics 1986, 5, 1792-1800; 1800-1807.
OM0501475