1220
Can. J. Chem. Vol. 83, 2005
Scheme 1.
[27] H2Ru2O8Q2 → QRuO4 + QRuO4H2
H
R
C
H
O
R
C
H
H
O
O
k1
Conclusions
R
O Ru
O
O
R
:
Ru
O
k-1
O
OQ
OQ
The reaction between alcohols and tetrapropylammonium
perruthenate exhibits intense autocatalysis, making it an ex-
cellent reagent for synthetic purposes.
The product responsible for the autocatalytic nature of the
reaction appears to be ruthenium dioxide.
The presence of moisture decreases the efficiency of the
catalyst.
The rate law for the reaction under both noncatalytic and
catalytic conditions is first order in the alcohol and second
order in the oxidant.
H
O
R
O
O
O
R
C
H
O
H
k2
:
R
C
H
O
Ru
Ru
O
Ru
O
R
O Ru
O
O
k--2
O
O
OQ
OQ
OQ
OQ
O
H
H
O
O
R
O
O
R
k3
Ru
O
Ru
O
R
C
H
O
Ru
O
Ru
O
+
O
C
R
H
O
OQ OQ
O
O
OQ
OQ
O
The rate of reaction is much greater for 2-propanol than
for THF, suggesting that the reaction involves formation of
an intermediate ruthenate ester.
The oxidation of cyclobutanol to cyclobutanone by TPAP
indicates that the reaction is a two-electron process.
Primary deuterium kinetic isotope effects are observed for
the oxidation of both 2-propanol-2-d and 2-propanol-O-d by
TPAP.
(31, 32). However, the reactions of oxidants, such as chro-
mic acid (H2CrO4), that are initiated by ester formation, pro-
ceed more rapidly when alcohols are the reductants (33).
The observation that cyclobutanol is oxidized to
cyclobutanone indicates that the oxidative process is a two-
electron transfer. Previous work has established that hydro-
gen atom transfer from cyclobutanol produces a free radical
that opens very rapidly to give acyclic products (34–36). The
oxidation of cyclobutanol to cyclobutanone, therefore, indi-
cates that QRuO4 is a two-electron oxidant.
The impact of substituent effects on the reaction rates is
not large.
In addition, the low sensitivity of the reaction to
substituent changes displayed in the Taft plots (Figs. 14 and
15), suggests a cyclic process in which there is little charge
built up on the α carbon in the transition state. A reaction se-
quence, consistent with these observations, is presented in
Scheme 1.
References
1. W.P. Griffith and S.V. Ley. Aldrichimica Acta, 23, 13 (1990).
2. W.P. Griffith. Chem. Soc. Rev. 179 (1992).
3. S.V. Ley, J. Norman, W.P. Griffith, and S.P. Marsden. Synthe-
sis, 634 (1994).
The rate law derived from the reaction sequence in
Scheme 1 is consistent with the observed isotope effects
since the rate constants for O–D cleavage (k1) and C–D
cleavage (k3) are both in the numerator of the derived rate
law, as indicated in eq. [25].
4. R. Bloch and C. Brillet. Synlett, 829 (1990).
5. S. Kamlage, M. Sefkow, B.L. Paul-Zobel, and M.G. Peter. J.
Chem. Soc. Chem. Commun. 331 (2001).
6. K.R. Guertin and A.S. Kende. Tetrahedron Lett. 34, 5369
(1993).
7. Y. Tokuhaga, M. Ihara, and K. Fukumoto. J. Chem. Soc.
Perkin Trans. 1, 207 (1997).
8. M.H. Yates. Tetrahedron Lett. 38, 2813 (1997).
9. V. Farmer and T. Welton. Green Chem. 4, 97 (2002).
10. Y.M. Ahn, G.G. Vander Velde, and G.I. Georg. J. Org. Chem.
67, 7140 (2002).
11. LC. Dias, L.G. de Oliverira, and M.A. de Sousa. Org. Lett. 5,
265 (2003).
12. C.K. Acosta, P.N. Rao, and H.K. Kim. Steroids, 58, 205 (1993).
13. I.J. Rosenstein and T.A. Tynan. Synth. Commun. 30, 1447
(2000).
14. I.E. Marko, A. Gautier, M. Tsukszaki, A. Llobet, E.
Plantalech-Mir, C.J. Urch, and S.M. Brown. Angew. Chem.
Int. Ed. Engl. 38, 1960 (1970).
15. S.V. Ley, A. Madin, and N.J.T. Monck. Tetrahedron Lett. 34,
7479 (1998).
[25] Rate = (k1k2k3[R2CHOH][QRuO4]2)/(k–1k–2
+ k–1k3 + k2k3[QRuO4])
Kinetic studies cannot be used to investigate reactions occur-
ring after the transition state; however, the observed 1:1
stoichiometry allows some speculation with respect to the
fate of the co-product (H2Ru2O8Q2). If this product (contain-
ing two ruthenium atoms, both with an average oxidation
state of +6) was reduced to ruthenium(IV) by reaction with
the solvent, the stoichiometry would be, ROH:QRuO4 = 1:2.
On the other had, if H2Ru2O8Q2 was reduced by 2 mol of 2-
propanol, as in eq. [26], the stoichiometry would be,
ROH:QRuO4 = 3:2. Both of these possibilities, therefore, ap-
pear to be ruled out.
[26] 2R2CHOH + H2Ru2O8Q2 → 2R2CO + 2QRuO4H3
16. D. Diez-Martin, P. Grice, H.C. Kolb, S.V. Ley, and A. Madin.
Synlett, 326 (1990).
One possibility that results in a 1:1 stoichiometry is the
suggestion that H2Ru2O8Q2 could dissociate into two differ-
ent products, one with ruthenium in an oxidation state of +5
(H2RuO4Q) and the other, an oxidation state of +7 (QRuO4),
as in eq. [27]. Since a ruthenium(V) species would be ex-
pected to be very reactive, it would likely be reduced to ru-
thenium(IV) by reaction with the solvent. The overall result
would be a 1:1 stoichiometry.
17. N.J. Anthony, A. Armstron, S.V. Ley, and A. Madin. Tetrahe-
dron Lett. 30, 3209 (1989).
18. A.C. Dengel, W.P. Griffith, and R.A. Hudson. Transition Met.
Chem. (Weinheim, Ger.), 10, 98 (1985).
19. D.G. Lee, Z. Wang, and W.D. Chandler. J. Org. Chem. 57,
3276 (1992).
20. D.R. Lide (Editor). Handbook of chemistry and physics. 75th
ed. CRC Press, Boca Raton, Fla. 1994.
© 2005 NRC Canada