Communication
Green Chemistry
addition reaction lies in the tautomeric equilibrium of the of Tokyo, the Japan Science and Technology Agency (JST), and
pentavalent (1) and trivalent (I) phosphorus compounds13 the Ministry of Education, Culture, Sports, Science and Tech-
(vide infra), and the poor reactivity of SPOs other than 1a could nology (MEXT), Japan.
be rationalized based on this equilibrium. Under ambient
temperature, electron-rich SPOs exist almost exclusively in the
pentavalent tautomeric form (1). Thus, photooxidation of the
P-lone pair of I cannot occur and would prevent the hydrophos-
Notes and references
phinylation reaction under visible light irradiation. On the
other hand, electron-poor substituents stabilize the trivalent
tautomer I, but may not react because the P-lone pair may be
insufficiently electron-rich to undergo photooxidation, or the
resulting phosphinoyl radical may not be electrophilic enough
to attack the alkene.
A tentative mechanism for the rhodamine B-catalyzed
hydrophosphinylation under visible light irradiation is pro-
posed (Scheme 2).
1 L. D. Quin, A Guide to Organophosphorus Chemistry, Wiley-
Interscience, New York, 2000.
2 For recent reviews on the addition of SPOs to alkenes,
please see: (a) Q. Xu and L.-H. Han, J. Organomet. Chem.,
2011, 696, 130; (b) L. Coudray and J.-L. Montchamp,
Eur. J. Org. Chem., 2008, 3601; (c) D. Enders, A. Saint-Dizier,
M.-I. Lannou and A. Lenzen, Eur. J. Org. Chem., 2006, 29;
(d) J.-L. Montchamp, J. Organomet. Chem., 2005, 690, 2388.
3 For representative examples, please see: (a) C. Midrier,
M. Lantsoght, J.-N. Volle, J.-L. Pirat, D. Virieux and
C. V. Stevens, Tetrahedron Lett., 2011, 52, 6693;
(b) L.-B. Han and C.-Q. Zhao, J. Org. Chem., 2005, 70,
10121; (c) C. M. Jessop, A. F. Parsons, A. Routledge
and D. Irvine, Tetrahedron Lett., 2003, 44, 479;
(d) T. Bunlaksananusorn and P. Knochel, Tetrahedron Lett.,
2002, 43, 5817; (e) D. Semenzin, G. Etemad-Moghadam,
D. Albouy, O. Diallo and M. Koenig, J. Org. Chem., 1997, 62,
2414.
4 For representative examples, please see: (a) C. Petit,
F. Fécourt and J.-L. Montchamp, Adv. Synth. Catal., 2011,
353, 1883; (b) M. O. Shulyupin, M. A. Kazankova and
I. P. Beletskaya, Org. Lett., 2002, 4, 761; (c) S. Deprèle and
J.-L. Montchamp, J. Am. Chem. Soc., 2002, 124, 9386;
(d) L.-B. Han, F. Mirzaei, C.-Q. Zhao and M. Tanaka, J. Am.
Chem. Soc., 2000, 122, 5407.
In the photoredox cycle, the photoexcited rhodamine B
(PC*)14 is expected to facilitate the single-electron oxidation of
the phosphinous acid I to generate the corresponding radical
cation II.15 Upon deprotonation, phosphinoyl radical III would
be formed and undergo anti-Markovnikov addition to alkene 2
to furnish radical addition intermediate IV. Chain propagation
by the hydrogen abstraction of SPO 1 or its trivalent tautomer I
with radical intermediate IV would regenerate phosphinoyl
radical III and provide the desired hydrophosphinylation
product 3. As for the reduced photocatalyst PC•⊖, it may be
quenched by trace amounts of oxygen gas in the solvent to
regenerate rhodamine B (PC) and close the photoredox cycle.16
In summary, we describe a mild protocol for the hydro-
phosphinylation of SPOs with unactivated alkenes in the presence
of rhodamine B under visible light irradiation. The reactions
proceed smoothly, with a remarkably low catalyst loading of an
inexpensive and commercially available organic dye, by utiliz-
ing minimal amounts of isopropanol as a green solvent to
provide tertiary phosphine oxides in moderate to excellent
yields. These results demonstrate the power of visible-light
photocatalysis to activate electron-rich heteroatoms, and we
anticipate the design of a similar environmentally benign
radical-type addition reaction by utilizing this strategy.
5 (a) H. K. Lenker, M. E. Richard, K. P. Reese, A. F. Carter,
J. D. Zawisky, E. F. Winter, T. W. Bergeron, K. S. Guydon
and R. A. Stockland Jr., J. Org. Chem., 2012, 77, 1378;
(b) T. Hirai and L.-B. Han, Org. Lett., 2007, 9, 53;
(c) R. A. Stockland Jr., R. I. Taylor, L. E. Thompson and
P. B. Patel, Org. Lett., 2005, 7, 851.
6 S. Kawaguchi, A. Nomoto, M. Sonoda and A. Ogawa, Tetra-
hedron Lett., 2009, 50, 624.
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS), the Global COE Program (Chemistry Innovation
through Cooperation of Science and Engineering), The University
7 For reviews on visible-light photocatalysis, please see:
(a) J. Xuan and W.-J. Xiao, Angew. Chem., Int. Ed., 2012, 51,
6828; (b) L. Shi and W. Xia, Chem. Soc. Rev., 2012, 41, 7687;
(c) M. A. Ischay and T. P. Yoon, Eur. J. Org. Chem., 2012,
3359; (d) J. W. Tucker and C. R. J. Stephenson, J. Org.
Chem., 2012, 77, 1617; (e) N. Hoffmann, ChemSusChem,
2012, 5, 352; (f) D. Ravelli and M. Fagnoni, ChemCatChem,
2012, 4, 169; (g) J. M. R. Narayanam and C. R. J. Stephenson,
Chem. Soc. Rev., 2011, 40, 102; (h) T. P. Yoon, M. A. Ischay
and J. Du, Nat. Chem., 2010, 2, 527.
8 For representative examples, please see: (a) G. Zhao,
C. Yang, L. Guo, H. Sun, C. Chen and W. Xia, Chem.
Commun., 2012, 48, 2337; (b) D. B. Freeman, L. Furst,
A. G. Condie and C. R. J. Stephenson, Org. Lett., 2012, 14,
94; (c) Y. Pan, S. Wang, C. W. Kee, E. Dubuisson, Y. Yang,
K. P. Loh and C.-H. Tan, Green Chem., 2011, 13, 3341;
Scheme 2 The assumed reaction mechanism.
Green Chem.
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