posted 12-05-1999 09:14 PM         

"Catalytic Transfer Hydrogenation. In recent years a few labatories
have started to employ catalytic transfer hydrogenation (CTH) [3].
This is a safe and simple operation in which the catalyst and hydrogen
gas are replaced with a catalyst and a hydrogen donor such as
cyclohexane [4], hydrazine [5], formic acid [6], ammonium formate [7],
cyclohexdiene [8], and phosphinic acid [9], sodium hypophosphite [10].
This type of hydrogenation is usually conducted in flasks fitted with
a magnetic stirrer and a reflux condenser. Ethyl alcohol is a widely
used solvent for CTH. Recently we [11] have demonstrated that
catalytic transfer hydrogenation can be conducted very rapidly and in
essentially quanitative yield inside an unmodified domestic microwave
oven. ...

Mircowave-Assisted Reactions. Two pioneering papers [12] appeared in
1986 on remarkable acceleration of many organic reactions upon
irradition with mircowaves (2450 MHz). Since then a number of
labatories, inculding our own, have been studying mircowave-assisted
chemical syntheses [13]. ...

...

... We prefer to conduct experiments in open vessels in inexpensive,
unmodified, domestic mircowave ovens. A wide variety of compounds
have been synthesized using our mircowave-induced organic reaction
enhancement (MORE) chenistry techniques [15].

These techniques are also very convenient for rapid and safe catalytic
transfer hydrogenation experiments.

MORE Chemistry Techniques. We have devolpedan unconvential
experimential set up for conducting organic reactions to take
advantage of the special nature of mircowave energy.

Erlenmeyer flasks or beakers with loose covers are preferred reaction
vessels for ambient pressure reactions in unmodified domestic
mircowave ovens. The upper parts of these vessels remain cool since
glass is transparent to mircowaves. The solvent (mircowave transfer
agent) should be polar and with a suitably high boiling point at least
20 or 30 [degrees] higher than the projected reaction tempature.

Domestic mircowave ovens produce 2450 MHz radition at a rate that is
controlled to a moderate degree by an "on-off" cycle. For finer
control of the mircowave energy input to small-scale reaction mixtures,
it is convenient to use a "heat sink"- a beaker of water placed next
to the reaction vessels inside the oven. This heat sink with
appropriate amounts of water captures a significant amount of the
mircowave energy thereby reducing the energy supplied to the reaction
mixture.

Since mircowave energy is absorbed by all of the polar molecules at
the same time, no stirrer is required for reaction mixtures in shallow
layers. ...

Mircowave-Assisted CTH. Ethylene glycol (bp 198 C) or more
eco-friendly 1,3-proanediol, bp 210-212 C as the solvent and ammonium
formate as the hydrogen donor form an excellent combination for
catalytic transfer hydrogenation under mircowave irradition. Hydrazine
hydrate was used as the hydrogen donor in a few cases but ammonium
formate proved to be more convenient. Most of our studies on CTH
reactions have been conducted with Pd/C (10%) catalyst. A few
experiments were catalyzed with Ra/Ni catalyst. ...

Reduction and Hydrolysis of [beta]-Lactams. ...

...

... We [17] obsevred eariler that, in the presense of a large excess
of Raney Nickel catalyst and hydrogen, 3-methoxy-1,4-diphenyl-2-
azetidinone under went [beta]-lactam scission to provide a small
amount of the anilide of [aplha]-methoxy-[beta]-phenylproponic acid
(Scheme 5). However under milder conditions, cleaveage of the
[beta]-lactam ring does not occur.

Recently we [11] have studied microwave assisted catalyitc transfer
hydrogenlysis at 12-130 C using 10% Pd/C as the catalyst. Rapid
scission of 4-phenyl-2-azetidinones was observed in every case. The
N-benzyl group of the [beta]-lactam [9] was not hydrogenolyzed, but
the O-Bn group at C-3 was converted to an OH group; alkenes (11, 13,
and 15) were reduced to alkyl groups (scheme 7). The reduction product
was obtained in high yield and in a few minutes. It is useful to
note that under these conditions Ra/Ni did not cause cleave of the
[beta]-lactam ring in 3 (See scheme 2). "

[paraphrase of scheme 7]
"Scheme 7 ... [where _ is a spacer, Bn = C6H5-CH2, Ph = C6H5,
MWI* = mircowave irradition]

BnO-HC---CH-Ph HCO2NH4, 10% Pd/C
_____|___| ------------------->
___O=C---N-R Ethlyene Glycol, MWI* - 80-83%
_______9

HO-HC---CH2-PH
____|___|
__O=C---NH-R
_____10
"
"In summary, we have devised safe, rapid, and effiecent techniques
for conducting catalytic hyogenation and hydrogenolysis useing just
beakers and flasks and unmodified domestic microwave ovens. ...

...

General Procedure for CTH Reactions. ... The reaction vessel should
be a beaker or Erlenmeyer flask of fairly large size. A beaker of
water should be be placed near this reaction vessel to serve as a
"heat sink" to provide a finer control on the amount of mircowave
energy input into the hydrogenation mixture. Water absorbs mircowave
energy very efficently and thereby reduces the amount of energy
absorbed by the reaction mixture. The approxiamate amount of water to
be used can be determined by a trail run involving only the solvent
and without the catalyst.

The desired tempature of the solvent should rise to 110-120 C in about
3 min. The catalyst should be quickly introduced into the reaction
vessel and covered with the solvent (such as ethylene glycol,
bp 198 C) and made into a slurry by gently swirling motion of the
beaker or the conical flask. The compound to be reduced is dissolved
in the solvent (ethlyene glycol or 1,3-propanediol) and then added to
the reaction vessel. The hydrogen donor (such as ammonium formate)
is added now. Mircowave irradition for the predetermained peroid of
time to reach a tempature of 110-130 C should be applied. The
mircowave oven door should be opened, and the tempature of the reaction
mixture shloud be checked to be in the desired range.

The oven door should be closed and irraditionwith the mircowave should
be resumed for another 3-4 min. The mircowave oven should be switched
off, and the reaction vessel removed from the oven. Careful
decantation of the reaction mixture after cooling followed by the
addition of glycol to the reaction vessel would preserve the catalyst
for the next experiment.

It is customary in our labatory to place a beaker cover or filter
funnel on top of the reaction vessel to prevent any accidential
spillage. Since glass is nearly transparent to mircowaves, the upper
parts of the beaker or flask serves as a condenser for any small
amounts of vapor formed. ... We have observed that the optimal ratio
of the catalyst (10% Pd/C) to substrate is 0.3:1 by wieght for each
reducilble group. Five equivalents of ammonium formate for each
reducible group gave good results."

[3] "Synthesis and Biological Activity", 1994, Chapter 2, page 11, (b)
Tetrahedron Lett. 1994, 35, (c) Synthesis 1988, 91

[4] J. Org. Chem. 1990, 45, 2010, (b) Synthesis 1977, 685, (c) J.
Chem. Rev. 1974, 74, 567, (d) J. Chem. Soc. 1954, 3544.

[5] Synthesis 1978, 751, (b) S. Chem. Rev. 1965, 65, 51.

[6] J Chem. Soc. Chem. Commun. 1987, 1329. (b) J. Org. Chem. 1979, 44,
3442.

[7] Synthesis 1980, 929. (b) Synthesis 1987, 53, (c) J. Org. Chem.
1986, 11, 1930, (d) Synthesis 1986, 133, (e) S. Helv. Chim. Acta. 1985,
68, 745, (f) J Ind. Chem. Soc. 1998, 75, 690.

[8] Tetraherdon Lett. 1992, 33, 2299, (b) J. Org. Chem. 1978, 43, 4194.

[9] Tetrahedron 1978, 34, 313, (b) J Chem. Res. (S) 1977, 117.

[10] J. Chem. Soc. Perkin Trans. 1 1993, 529.

[11] Snylett 1993, 575, (b) J. Org. Chem. 1991, 56, 6968

[12] Tetrahedron Lett. 1986, 27, 279, (b) Tetraherdon Lett. `986, 27,
4945

[13] Tetraherdon Lett. 1995, 51, 10403

[15] Chemtech 1997, 27, 18, (b) Hetrocycles 1997, 44, 405, (c) Res.
Chem. Intermed. 1994, 20, 1, (d) Tetrahedron Lett. 1992, 33, 3603

[17] J. Org Chem. 1974, 39, 2877