Nonradical trapping pathway for reactions of nitroxides with rhodium porphyrin alkyls bearing β-hydrogens and subsequent carbon-carbon bond activation

Article abstract of DOI:10.1021/om0201718

A novel nitroxide-induced hydrogen atom abstraction and β-elimination of rhodium porphyrin alkyls was observed. The subsequent carbon-carbon bond activation of methyl-substituted nitroxides by the rhodium(II) porphyrin radical yielded rhodium porphyrin methyl complexes. A nonradical trapping pathway for reactions of 2,2,6,6-tetramethylpiperidinoxy (TEMPO) nitroxide with rhodium porphyrin alkyls was identified.

Full text of DOI:10.1021/om0201718

2362
Organometallics 2002, 21, 2362-2364
Non r a d ica l Tr a p p in g P a th w a y for Rea ction s of
Nitr oxid es w ith Rh od iu m P or p h yr in Alk yls Bea r in g
â-Hyd r ogen s a n d Su bsequ en t Ca r bon -Ca r bon Bon d
Activa tion
Kin Wah Mak, Siu Kwan Yeung, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin,
New Territories, Hong Kong
Received February 27, 2002
Summary: A novel nitroxide-induced hydrogen atom
abstraction and â-elimination of rhodium porphyrin
alkyls has been observed. Subsequent carbon-carbon
bond activation of methyl-substituted nitroxides by the
rhodium(II) porphyrin radical yielded rhodium porphy-
rin methyl complexes.
2,2,6,6-Tetramethylpiperidinoxy (TEMPO) and re-
lated nitroxide radicals have been extensively used as
efficient radical traps in organic, bioorganic, and orga-
nometallic chemistry. The estimations of bond dissocia-
tion energies of metal-alkyl bonds by kinetic methods
using nitroxides as radical traps have been successfully
accomplished in vitamin B12 and related models,1,2
cyclopentadienylmetal alkyl complexes,3 and ruthenium
porphyrin alkyl complexes.4,5 These radicals, however,
can behave as reagents rather than as innocuous traps
in the reactions with organometallics. An interesting
comment was made by J ames and Dolphin in the report
on Ru(oep)(CH3)2 and Ru(oep)(C6H5)2 reactivity (oep )
octaethylporphyrin dianion): “the rate of decomposition
of Ru(oep)(CH3)2 was found to be dependent on the
concentration of TEMPO which also appears to react
with Ru(oep)CH3”.5 Nitroxides are well-known ligands
for a variety of transition-metal complexes.6,7 TEMPO
has been reported to react with the porphyrin Ru(oep)-
CH3 to yield Ru(oep)CO.7 We have also reported recently
that rhodium(II) porphyrin also reacts with nitroxides
in a carbon-carbon bond activation (CCA) to yield
methylrhodium porphyrin.8 In the course of investigat-
ing the mechanism of 1,2-rearrangements of rhodium
F igu r e 1. Structures of rhodium porphyrins.
porphyrin alkyls,9 we have observed that TEMPO did
not act as an innocuous radical trap but reacted directly
with rhodium porphyrin alkyls bearing â-hydrogens via
a novel â-hydrogen abstraction/elimination process to
generate rhodium porphyrin radicals, which then un-
derwent aliphatic CCA with nitroxides.
The results of the thermal reactions of TEMPO and
rhodium porphyrin alkyls are summarized in Table 1
(Figure 1, eq 1). Rh(bocp)CH3 (3a ) was formed unex-
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
(1) (a) Halpern, J . Polyhedron 1998, 7, 1483-1490. (b) Ng, F. t. T.;
Rempel, G. L.; Mancuso, C.; Halpern, J . Organometallics 1990, 9,
2762-2772.
pectedly from the thermolysis of 3 with TEMPO (5
equiv). Rh(bocp)CH2CH2Ph was consumed within the
first 1 h of the reaction without any Rh(bocp)CH3
formed, as determined by TLC analysis. The rate of
disappearance of 3 was therefore found to be much
faster than that of 1,2-rearrangement into Rh(bocp)CH-
(Ch3)Ph (10 h at 80 °C).9a Then, after a total reaction
time of 7 h, Rh(bocp)CH3 was formed in 45% isolated
yield. Increasing the amount of TEMPO to 15 equiv did
not increase the rate of disappearance of 3 but acceler-
ated the rate of the formation and the yield of product
3a (90 min and 81%). Therefore, a stable rhodium
(2) Koenig, T. W.; Hay, B. P.; Finke, R. G. Polyhedron 1988, 7, 1499-
1516.
(3) Mancuso, C.; Halpern, J . J . Organomet. Chem. 1992, 428, C8-
C11.
(4) Collman, J . P.; McElwee, L.; Brothers, P. J .; Rose, R. J . Am.
Chem. Soc. 1986, 108, 1332-1333.
(5) Ke, M.; Rettig, S. J .; J ames, B. R.; Dolphin, D. J . Chem. Soc.,
Chem. Commun. 1987, 1110-1111.
(6) (a) Felthouse, T. R.; Dong, T.-Y.; Hendrickson, D. N.; Shieh, H.-
S.; Thompson, M. R. J . Am. Chem. Soc. 1986, 108, 8201-8214. (b)
Cogne, A.; Grand. A.; Rey, P. J . Am. Chem. Soc. 1987, 109, 7927-
7929. (c) More, J . K.; More, K. M.; Eaton, G. R.; Eaton, S. S. Pure Appl.
Chem. 1990, 62, 241-246. (d) More, K. M.; Eaton, G. R.; Eaton, S. S.;
Hankovszky, O. H.; Hideg, K. Inorg. Chem. 1989, 28, 1734-1743.
(7) Selyer, J . W.; Fanwick, P. E.; Leidner, C. R. Inorg. Chem. 1992,
31, 699-3700.
(9) (a) Mak, K. W.; Chan, K. S. J . Am. Chem. Soc. 1998, 120, 9686-
9687. (b) Mak, K. W.; Chan, K. S. J . Chem. Soc., Dalton Trans. 1999,
3333-3334.
(8) Tse, M. K.; Chan, K. S. J . Chem. Soc., Dalton Trans. 2001, 510-
511.
10.1021/om0201718 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/16/2002

Communications
Organometallics, Vol. 21, No. 12, 2002 2363
Ta ble 1. Su m m a r y of Th er m olysis of (P or p h yr in a to)r h od iu m Alk yls w ith TEMP O
time for
disappearance
of complex
total
reacn
time
amt of
TEMPO (equiv)
Rh(por)CH3;
yield, %
complex
temp (°C)
Rh(ttp)CH2CH2Ph (1)
Rh(bttp)CH2CH2Ph (2)
Rh(bocp)CH2CH2Ph (3)
Rh(bocp)CH2CH2Ph (3)
Rh(bocp)CH(CH3)Ph (4)
Rh(bocp)CH2CH2CN (5)
Rh(ttp)CH2CH3 (6)
5
5
5
15
5
10
10
10
80
80
80
48 h
48 h
6 days
6 days
7 h
1a ; 70b
2a ; 25b
3a ; 45b
3a ; 81a
15 min
15 min
3 days
12 h
80
90 min
3 days
24 h
120
120
120
120
3a ; 70b
3a ; 86b
1a ; 10d (51e)
1a ; 74b
NAc
14 daysc
7 days
Rh(ttp)CH2CH2CH3 (7)
7 days
Isolated yield from column chromatography. Estimated yields from 1H NMR integration. c Only 19% of starting material converted.
Based on 100% of starting material. e Based on 19% starting material converted.
a
b
d
porphyrin intermediate likely formed. More electron rich
porphyrin rhodium alkyls such as 1 and 2 reacted with
excess TEMPO similarly. The reactivity appeared to
decrease with the increasing strength of the â-C-H
bond (3 vs 5, 1 vs 6 and 7).
Sch em e 1. P r op osed Mech a n ism for th e
F or m a tion of Rh (p or )CH3 fr om Rh (p or )CH2CH2R
To elucidate the mechanism of the formation of Rh-
(por)CH3 complexes, the 13C-enriched phenylethyl rhod-
ium complex *3 was reacted with TEMPO (eq 2). The
methyl ligand of 3a formed from the reaction was found
to be unenriched with 13C and appeared as a broad
1
singlet at -4.84 ppm in the H NMR spectrum of the
1
reaction mixture without any J C-H coupling observed.
This ruled out the possibility that the methyl ligand
came from the R-carbon of the phenylethyl ligand of 3.
From the proton-coupled 13C NMR spectrum of the
reaction mixture of *3 with TEMPO, two enhanced
1
triplets at 73.7 and 113.8 ppm with J C-H values equal
Mechanism A is disfavored on the basis of the unlikely
homolysis of the rather strong Rh-C bond at 120 °C
(BDE ≈ 60 kcal/mol)12,13 as well as the absence of
PhCH2CH2-TEMPO in the presence of an excess of the
efficient spin trap TEMPO. Though mechanism B is
apparently supported by the established intermediate
of Rh(por)H, generated via â-hydride elimination in the
1,2-rearrangements of Rh(por)CH2CH2R,9 the rates of
disappearance of the starting materials were, however,
much faster in the presence of TEMPO. In the absence
of TEMPO, the 1,2-rearrangements of Rh(bocp)CH2CH2-
Ph and Rh(ttp)CH2CH2Ph took 109a and 144 h,9b re-
spectively. Therefore, Rh(por)CH2CH2Ph must react
with TEMPO directly. Furthermore, Rh(bocp)CH2CH2-
CN did not undergo any significant 1,2-rearrangement
after heating in benzene at 150 °C for 11 days. No Rh-
(bocp)H was formed, as no rearrangement was observed.
However, it reacted with TEMPO at 120 °C within 2
days to give 3a . Therefore, the presence of Rh(por)H is
ruled out. Mechanism C remains the most probable.
Abstraction of the fairly weak C-H bond R to Ph, CN,
and Me groups in these rhodium complexes is energeti-
cally feasible at the reaction temperature of 80-120 °C,
and the reaction rates increased as the C-H bond
strengths decreased (BDE(PhCH2-H) ) 88 kcal/mol,13
to 148 and 157 Hz, respectively, were observed. At least
two products were therefore formed. The resonance at
113.8 ppm was assigned to the terminal sp2 carbon of
1
styrene and was quantified in 20% yield by H NMR
1
spectroscopy (cf. δ(C6H5CHdCH2) 112.3 ppm, J C-H
-
(CH2dCH2) ) 156 Hz in CDCl3).10a However, the signal
at 73.7 ppm could not be unambiguously assigned and
is unlikely to be TEMPO-*CH2CH2Ph.10b Therefore, the
CH2-CH2Ph fragment remained intact and the Rh-C
bond was cleaved in the reaction.
The reaction of of Rh(por)CH2CH2R with TEMPO to
form Rh(por)CH3 appears to involve a two-step process
with the formation of a rhodium porphyrin radical
intermediate. The activation of the reaction by the
coordination of TEMPO to Rh(por)CH2CH2R is also
possible. The first step likely involves the generation of
the rhodium(II) porphyrin radicals, and the second step
is possibly the CCA of TEMPO with RhII(por) radical
to produce Rh(por)CH3.8 Three mechanistic possibilities
exist for the generation of Rh(por) radicals (Scheme 1):
(A) homolysis of a rhodium-alkyl bond, (B) â-hydride
elimination of Rh(por)CH2CH2R to Rh(por)H-hydrogen
abstraction with TEMPO,11 and (C) â-hydrogen abstrac-
tion of Rh(por)CH2CH2R with TEMPO-Rh-C homoly-
sis.
(10) (a) Pretsch, E.; Seibl, J .; Simon, W.; Clerc, T. Tables of Spectral
Data for Structure Determination of Organic Compounds; Springer-
Verlag: Berlin, Heidelberg, 1989; pp C172-C173, C225. (b) Patel, V.
F.; Pattenden, G. J . Chem. Soc., Perkin Trans. 1 1990, 2703-2708.
(11) Chan, K. S.; Leung, Y.-B. Inorg. Chem. 1994, 33, 3187.
(12) (a) Wayland, B. B. Polyhedron 1988, 7, 1545-1555. (b) Way-
land, B. B.; Ba, S.; Sherry, A. J . Am. Chem. Soc. 1991, 113, 5305-
5311.
(13) Li, G.; Zhang, F. F.; Pi, N.; Chen, H. L.; Zhang, S. Y.; Chan, K.
S. Chem. Lett. 2001, 284-285.

2364 Organometallics, Vol. 21, No. 12, 2002
Communications
BDE(Me(H)(CN)C-H) ) 90 kcal/mol,14 BDE(Me(Me)-
CH2-H) ) 95 kcal/mol,14 BDE(MeCH2-H) ) 98 kcal/
mol,14 BDE(TEMPO-H) ) 70 kcal/mol15). The hydrogen
abstraction step may likely be synchronous with the
homolysis of the Rh-C bond to generate a Rh(por)
radical.
ium porphyrin alkyls bearing â-hydrogens and subse-
quent carbon-carbon bond activation via rhodium
porphyrin radicals. The innocuous nature of TEMPO as
a radical trap in the determination of metal alkyls of
organometallics, especially stronger ones, needs to be
viewed cautiously.
In summary, we have identifed a nonradical trapping
pathway for reactions of TEMPO nitroxide with rhod-
Ack n ow led gm en t. We thank the Research Grants
Council of Hong Kong of the SAR of China for financial
support (No. CUHK 4202/99p).
(14) McMillen. D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
33, 493-532.
(15) Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J . Am. Chem.
Soc. 1973, 95, 8610-8614.
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