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the reaction intermediates spectroscopically and proposed a reac-
tion mechanism. The porphyrin complex of iron, which is the most
abundant transition metal, was the most effective among those
tested. This reaction system forms a carbon radical under mild
conditions (r.t., dark). FeI(Por)ꢀ, a highly reduced form of iron
porphyrin, was confirmed to be formed in this reaction. Thus, the
reaction enables both N2O consumption and synthesis of coupling
products, some of which would be useful for organic synthesis.
Since N2O may become the dominant ozone-depleting substance
emitted in the 21st century,16 the reaction system described here
might prove useful as a green process to decrease emissions of N2O,
perhaps in conjunction with large-scale synthetic applications.
This work was partly supported by Grants-in-Aid for Scientific
Research on Priority Areas (No. 19028054, 20037056, ‘‘Chemistry
of Concerto Catalysis’’) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Scheme 2 Proposed mechanism of the reductive coupling of alkenes with the
N2O–Fe(Por)–NaBH4 system.
Notes and references
It is known that both FeIII(Por)(OMe)2 and FeIII(Por)OMe are
formed in the presence of a 10-fold excess of MeOꢀ over
FeIII(Por)Cl in toluene–MeOH.13 Either FeIII(Por)(OMe)2 or
FeIII(Por)OMe would be reduced to FeII(Por) with NaBH4 and
a small amount of H–FeII(Por)ꢀ is presumably also formed
because NaBH4 can transfer both a hydride and one electron
to Fe(Por). Formation of H–FeII(Por)ꢀ has also been proposed in
electrochemical reduction of H2O catalysed by Fe(TPP).14 The
alkenyl group is inserted into the Fe–H bond to afford the
s-alkyl-Fe(Por) complex initially. The Fe–C bond of tert-alkyl-
Fe(Por) is homolytically cleaved to give the tert-alkyl radical and
FeI(Por)ꢀ at room temperature due to the higher stability of the
tert-alkyl radical as well as steric repulsion. The two radicals
afford the homo-coupling product, and FeI(Por)ꢀ reduces N2O
to N2 and H2O, regenerating FeIII(Por). Kojo et al. suggested that
NaBH4 and iron porphyrin form the hydrogen radical (Hꢁ).15
Hꢁ, which is formed by homolysis of the H–Fe bond of the
intermediate, attacks the alkene to afford the same radical as
that formed in Scheme 2. However, this alternative pathway is
considered unlikely because the UV-Vis spectrum of Fe(TPP)Cl
with NaBH4 in the absence of alkene under Ar showed only
FeII(TPP) species, not FeI(TPP). The role of ROꢀ has not been
clarified, but it may stabilise FeI(Por)ꢀ species against degradation
through protonation of the basic pyrrole nitrogens and/or facilitate
Fe–C bond cleavage by acting as an axial ligand.
1 For reviews: (a) M. Costas, Coord. Chem. Rev., 2011, 255, 2912–2932;
(b) B. J. Anding and L. K. Woo, in Handbook of Porphyrin Science,
ed. K. M. Kadish, K. M. Smith and R. Guilard, World Scientific, 2012,
ch. 100, vol. 21, pp. 145–319.
2 (a) B. Meunier, A. Robert, G. Pratviel and J. Bernadou, in The
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000, ch. 31, vol. 4,
pp. 119–187; (b) J. T. Groves, K. Shalyaev and J. Lee, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, San Diego, 2000, ch. 27, vol. 4, pp. 17–40.
3 R. Ito, N. Umezawa and T. Higuchi, J. Am. Chem. Soc., 2005, 127,
834–835, and references therein.
4 (a) J. T. Groves and J. S. Roman, J. Am. Chem. Soc., 1995, 117, 5594–5595;
(b) H. Tanaka, K. Hashimoto, K. Suzuki, Y. Kitaichi, M. Sato, T. Ikeno
and T. Yamada, Bull. Chem. Soc. Jpn., 2004, 77, 1905–1914.
5 I. Tabushi and N. Koga, J. Am. Chem. Soc., 1979, 101, 6456–6458.
6 Y. Aoyama, T. Watanabe, H. Onda and H. Ogoshi, Tetrahedron Lett.,
1983, 24, 1183–1186.
7 T. Okamoto and S. Oka, J. Org. Chem., 1984, 49, 1589–1594.
8 (a) T. Santa, T. Mori and M. Hirobe, Chem. Pharm. Bull., 1985, 33,
2175–2178; (b) T. Mori, T. Santa, T. Higuchi, T. Mashino and
M. Hirobe, Chem. Pharm. Bull., 1993, 41, 292–295.
9 N2O gas that we used had 99.999% warranted purity (Showa Denko
K. K.).
10 D. Lexa, J. Mispelter and J.-M. Saveant, J. Am. Chem. Soc., 1981, 103,
´
6806–6812.
11 G. S. Srivatsa, D. T. Sawyer, N. J. Boldt and D. F. Bocian, Inorg.
Chem., 1985, 24, 2123–2125.
12 M. Bayachou, L. Elkbir and P. J. Farmer, Inorg. Chem., 2000, 39,
289–293.
13 T. Otsuka, T. Ohya and M. Sato, Inorg. Chem., 1984, 23, 1777–1779.
14 I. Bhugun, D. Lexa and J.-M. Saveant, J. Am. Chem. Soc., 1996, 118,
´
3982–3983.
We have found a novel N2O reduction-coupled alkene dimer-
ization catalysed by metalloporphyrins under mild conditions (at
ordinary pressure and room temperature). We carefully examined
15 S. Kojo and S. Sano, J. Chem. Soc., Perkin Trans. 1, 1981, 2864–2870.
16 A. R. Ravishandra, J. S. Daniel and R. W. Portmann, Science, 2009,
326, 123–125.
c
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