Toward nonoxidative routes to oxygenated organics: Stereospecific deoxydehydration of diols and polyols to alkenes and allylic alcohols catalyzed by the metal oxo complex (C5Me5)ReO3

Full text of DOI:10.1021/ja9620604

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
9448
J. Am. Chem. Soc. 1996, 118, 9448-9449
Scheme 1. Catalytic Cycle for Diol Deoxydehydration to
Toward Nonoxidative Routes to Oxygenated
Organics: Stereospecific Deoxydehydration of Diols
and Polyols to Alkenes and Allylic Alcohols
Catalyzed by the Metal Oxo Complex (C5Me5)ReO3
Alkenesa
Gerald K. Cook and Mark A. Andrews*
Chemistry Department
BrookhaVen National Laboratory
Upton, New York 11973-5000
ReceiVed June 19, 1996
Our research program is exploring nonoxidatiVe routes to
oxygenated organic compounds starting from renewable biomass
carbohydrates.1,2 Such processes offer significant environmental
advantages over conventional approaches based on hydrocarbon
oxidation. The latter pose multiple liabilities, ranging from
petrochemical extraction and shipping hazards to the copro-
duction of substantial waste streams due to recalcitrant selectiv-
ity problems. Carbohydrates, with one oxygen atom per carbon,
are actually too oxygen-rich relative to many important oxygen-
ated organics. Removal of carbohydrate hydroxyl groups would
therefore be useful, particularly if a different reactive functional
group could be introduced. This has now been accomplished
via a new catalytic reaction, the stereospecific deoxydehydration
of diols and polyols to alkenes and allylic alcohols. Various
stoichiometric reactions effecting the same net transformation,
diol didehydroxylation, have already proven valuable in syn-
thetic organic chemistry.3,4
a Cp*ReVO2 is in equilibrium with its µ-oxo-bridged dimer
[Cp*ReV(O)(µ-O)]2, either of which may participate in the catalytic
cycle.
Reports of alkene extrusion from metal diolate complexes5-8
(e.g., Cp*Re(O)(ethanediolate)) inspired us to design a catalytic
cycle incorporating this key reaction (Scheme 1, 9-12 o’clock
positions). A potential means to complete such a cycle (Scheme
1, 12-3 and 3-9 o’clock positions) was provided by the
Cp*Re(O)(diolate) syntheses developed by Gable,9 employing
reactions of diols with an equilibrium mixture10 of Cp*ReVO2
and its dimer [Cp*ReV(O)(µ-O)]2, prepared in situ by reduction
of Cp*ReO3.11,12 Successful implementation of the conceptual
catalytic cycle was demonstrated by quantitative conversion of
phenyl-1,2-ethanediol (PedH2) into styrene by treatment with
triphenylphosphine in the presence of Cp*ReO3 in chloroben-
zene (55 turnovers/Re; eq 1, Figure 1A).13,14
Figure 1. Catalytic reaction profiles for deoxydehydration of phenyl-
1,2-ethanediol by PPh3. [PPh3:PedH2:Re ) 68:57:1, [Cp*ReO3] ) 0.65
mM, T ) 90 °C.]
Reactions conducted in more coordinating solvents, such as
tetrahydrofuran (THF) or N-methyl-2-pyrrolidinone (NMP),
were much less successful, catalysis ceasing after less than 10
turnovers (Figure 1B). Monitoring of the catalytic reaction in
THF-d8 by 1H NMR revealed the formation of a new, unidenti-
fied Cp*Re-containing species. Stoichiometric control reactions
indicate that this results from the instability of [Cp*ReV(O)(µ-
O)]2/Cp*ReVO2 in the presence of excess PPh3 at 90 °C toward
over-reduction to a catalytically inactive Re complex (denoted
“Re(III)”, Scheme 1, center), as evidenced by formation of the
same new Cp* peak observed in the catalytic reactions and
roughly 1 equiv of Ph3PO.15 Reduction of ReV to ReIII by PPh3
has precedent in other Re oxo chemistry.16 While this result
explains the probable cause of catalyst deactivation, it does not
account for the large solvent effect, since “Re(III)” formation
is also observed to occur in benzene-d6 at a comparable rate
(k5/3) under stoichiometric conditions (k5/3(THF)/k5/3(C6D6) ≈
2).
(1)
(1) Andrews, M. A.; Klaeren, S. A. J. Am. Chem. Soc. 1989, 111, 4131-
4133.
(2) Andrews, M. A.; Klaeren, S. A.; Gould, G. L. In Carbohydrates as
Organic Raw Materials II; Descotes, G., Ed.; VCH: New York, 1993; pp
3-25.
(3) Block, E. Org. React. 1984, 30, 457-566.
(4) Larock, R. C. ComprehensiVe Organic Transformations: A Guide
to Functional Group Preparations; VCH: New York, 1989; pp 155-156.
(5) Schrock, R. R.; Pedersen, S. F.; Churchill, M. R.; Ziller, J. W.
Organometallics 1984, 3, 1574-1583.
(6) Herrmann, W. A.; Marz, D.; Herdtweck, E.; Scha¨fer, A.; Wagner,
W.; Kneuper, H.-J. Angew. Chem., Int. Ed. Engl. 1987, 26, 462-464.
(7) Gable, K. P.; Juliette, J. J. J. J. Am. Chem. Soc. 1996, 118, 2625-
2633 and references therein.
(8) Zhu, Z.; Al-Ajlouni, A. M.; Espenson, J. H. Inorg. Chem. 1996, 35,
1408-1409.
(9) Gable, K. P. Organometallics 1994, 13, 2486-2488.
(10) Gable, K. P.; Juliette, J. J. J.; Gartman, M. A. Organometallics 1995,
14, 3138-3140.
(11) Herrmann, W. A.; Serrano, R.; Ku¨sthardt, U.; Guggolz, E.; Nuber,
B.; Zeigler, M. L. J. Organomet. Chem. 1985, 287, 329-344.
(12) Herrmann, W. A.; Flo¨el, M.; Kulpe, J.; Felixberger, J. K.;
Herdtweck, E. J. Organomet. Chem. 1988, 355, 297-313.
(13) Standard conditions for PedH2 reactions: [Cp*ReO3] ) 0.65 mM,
PPh3:PedH2:Re ) 68:57:1, with PPh3 added last.
(14) No products from other types of reactions were observed that would
correspond to those reported for MeReO3-catalyzed reactions of simple
alcohols in the absence of phosphine: Zhu, Z.; Espenson, J. H. J. Org.
Chem. 1996, 61, 324-328.
Under catalytic conditions, however, the extent of “Re(III)”
formation is a function of the partitioning of the Cp*ReV dioxo
complex(es) between over-reduction (k5/3) and “ketalization” by
the diol substrate to yield Cp*ReVO(diolate) (kket, Scheme 1,
(15) Definitive characterization of this species or its oxidation state has
not been undertaken at this point since small amounts of other species are
also formed and integration is made difficult by near overlap of some of
1
the Cp* peaks in the H NMR (see Supporting Information).
(16) Conry, R. R.; Mayer, J. M. Inorg. Chem. 1990, 29, 4862-4867.
S0002-7863(96)02060-4 CCC: $12.00 © 1996 American Chemical Society

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 39, 1996 9449
3-9 o’clock positions).17 This partition ratio was determined
experimentally (see Supporting Information): kket/k5/3 ≈ 60 for
benzene-d6 and kket/k5/3 ≈ 2 for THF-d8. The striking solvent
dependence is therefore primarily the result of inhibition of diol
ketalization by donor/hydrogen-bonding solvents (kket(C6D6)/
also conducted preliminary studies of unprotected alditols where
regiospecificity is a significant consideration. In prior work,
we have shown that good to excellent regioselectivity is
achievable in the complexation of alditols to (Ph2PCH2CH2-
CH2PPh2)PtII (e.g., 89:11 for 1,2- vs 2,3-coordination of
erythritol).21 Catalytic deoxydehydration of this tetritol under
biphasic conditions in chlorobenzene gave the products expected
from 1,2- and 2,3-coordination to Re, 3-butene-1,2-diol and cis-
2-butene-1,4-diol, respectively, in an 85:15 ratio. The major
product observed, however, was the fully deoxygenated product
butadiene (∼80%, eq 3).
k
ket(THF) ≈ 15).
The above analysis predicts that catalyst deactivation by
“Re(III)” formation can be diminished by either accelerating
ketalization or slowing down over-reduction. The former has
been accomplished by the addition of p-toluenesulfonic acid
(pTSA) as a cocatalyst.9 Reaction of PedH2 under standard
conditions13 in THF in the presence of pTSA (11 equiv/Re)
results in 91% conversion to styrene in just over 13 h (Figure
1C). The rate of styrene formation is virtually independent of
the decreasing concentrations of both PedH2 and PPh3 over the
course of this reaction, indicating that the rate-limiting step does
not depend upon either reactant. This is consistent with
extrusion of styrene from Cp*ReO(Ped) being rate limiting, with
kext ) 1.3(1) × 10-3 s-1 (4.8 turnovers/h) in both THF and
NMP in the presence of pTSA at 90 °C. This is in good
agreement with the value of 2.1 × 10-3 s-1 for styrene extrusion
from isolated Cp*ReO(Ped), derived from activation parameters
reported by Gable and Juliette.18
(3)
This illustrates a limitation inherent in the use of a biphasic
carbohydrate/catalyst reaction mixture, presently imposed by
catalyst lifetime problems in homogeneous solutions in NMP.
The initial deoxydehydration products, in this case the two
butenediols, should have much greater solubility in chloroben-
zene than the parent polyol; thus multiple, sequential deoxy-
dehydration is favored. Determination of the actual regiose-
lectivity of erythritol deoxydehydration therefore depends on
identifying the source of the butadiene. Control experiments
show that 3-butene-1,2-diol is readily deoxydehydrated to
butadiene by Cp*ReO3 in the presence of PPh3, while there
appears to be no direct deoxydehydration of the 1,4-diol isomer.
Unfortunately, cis-2-butene-1,4-diol is also observed to isomer-
ize to 3-butene-1,2-diol under the catalysis conditions, and
therefore it is not possible to determine the true regioselectivity
in this system. The major product of xylitol deoxydehydration
under similar conditions appears to be 2,4-pentadiene-1-ol.
In summary, the present catalytic method for net didehy-
droxylation of diols is potentially competitive in many cases
with the standard Corey-Winter approach,22 treatment of the
diol thionocarbonate with phosphite reagents at elevated tem-
peratures. Other metal diolate complexes are known to extrude
alkene at room temperature;5,8,23 hence we believe that it should
be possible to develop related catalytic cycles for net diol
didehydroxylation that function under milder conditions, com-
parable to the best current stoichiometric methods.24 Finally,
catalytic cycles utilizing more inexpensive, environmentally
friendly reductants, such as hydrogen or carbon monoxide,
would facilitate bulk carbohydrate deoxydehydration reactions
to produce desirable oxygenated organics.
The alternative method of avoiding catalyst deactivation by
over-reduction is to decrease k5/3. This can be accomplished
by utilization of a less active reductant than PPh3, as demon-
strated by deoxydehydration of PedH2 under standard conditions
in either benzene or THF using tris(perfluorophenyl)phosphine,
P(C6F5)3, in place of PPh3. The rate of product formation in
these reactions is identical to that observed in the THF/PPh3/
pTSA system.
The prototype carbohydrate, glycerol, is also cleanly deoxy-
dehydrated by PPh3 to the corresponding alkene, allyl alcohol
(67 turnovers in 23 h), in a biphasic, saturated solution in
chlorobenzene at 125 °C without pTSA.19 The reaction
proceeds without significant loss in catalyst activity over the
course of the reaction, exhibiting zero-order behavior consistent
with an extrusion rate constant from Cp*ReO(glycerolate) of
7.6(4) × 10-4 s-1 (2.7 turnovers/h). The same reaction
conducted homogeneously in NMP in the presence of pTSA (6
equiv/Re) led to gradual catalyst deactivation, allyl alcohol
production reaching only 28 of the 68 maximum possible
turnovers. The mechanism responsible for catalyst deactivation
under these reaction conditions is not yet understood, since
changes in the amount of acid- or diol-to-phosphine ratio had
little effect and use of P(C6F5)3 in place of PPh3 in the absence
of acid gave very few turnovers.
Catalytic deoxydehydration of the protected alditol 1,2:5,6-
diisopropylidene-D-mannitol proceeds stereospecifically, as
expected,6,18,20 yielding the corresponding trans-alkene (48
turnovers, 86% yield based on diol consumed, eq 2). We have
Acknowledgment. The authors thank Drs. Seth Brown, Morris
Bullock, and George Gould for helpful discussions. This research was
carried out at Brookhaven National Laboratory under contract DE-
AC02-76CH00016 with the U.S. Department of Energy and supported
by its Division of Chemical Sciences, Office of Basic Energy Sciences.
Supporting Information Available: Full experimental details (7
pages). See any current masthead page for ordering and Internet access
instructions.
(2)
JA9620604
(17) This is true regardless of whether over-reduction or ketalization
occurs by direct reaction of the µ-oxo dimer Cp*2Re2O4 or via the
equilibrium concentration of its monomer Cp*ReO2.
(18) Gable, K. P.; Juliette, J. J. J. J. Am. Chem. Soc. 1995, 117, 955-
962.
(21) Andrews, M. A.; Voss, E. J.; Gould, G. L.; Klooster, W. T.; Koetzle,
T. F. J. Am. Chem. Soc. 1994, 116, 5730-5740.
(22) Corey, E. J.; Winter, R. A. E. J. Am. Chem. Soc. 1963, 85, 2677-
2678.
(19) Typical conditions for carbohydrate reactions: [Cp*ReO3] ) 0.65
mM, PPh3:alditol:Re ) 68:100:1, with PPh3 added last (see Supporting
Information for details).
(23) Sharpless, K. B.; Umbreit, M. A.; Nieh, M. T.; Flood, T. C. J. Am.
Chem. Soc. 1972, 94, 6538-6540.
(24) Corey, E. J.; Hopkins, P. B. Tetrahedron Lett. 1982, 23, 1979-
1982.
(20) Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116, 833-839.