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C h a p t e r 5T e c h n i q u e s fo r c o n d u c t i n g c h
e m i c a lr e a c t i o n s u n d e r m i c r o w a v ei r r a d i
a t i o n .Tw o p ione e r ing wor ks tha t we r e pub l i she d a
lm o s t 20 - ye a rs a go [ 12 ,13 ] de sc r ibe d se v -e r a l o
r ga n ic syn the se s tha t we r e c om ple te d in h igh y ie ld
whe n c onduc te d in dom e s t i cmicrowave ovens in sea led g
lass or Tef lon vesse ls . S ince the n d i f fe rent techn iques
havebe e n de ve lope d a nd the y a r e p r e se n te d he r e in
o r de r to a l low r e a de r s to c hoose the m os ta ppr opr i a
t e t e c hn ique s f o r the i r ne e ds .
The s im ple s t m e thod f o r c onduc t ing m ic r owa ve - a
ss i s t e d r e a c t ions invo lve s i r r a d ia t ionof r e a c
ta n t s in a n ope n ve sse l. Du r ing the r e a c t ion , r e a
c ta n t s a r e in a so lu t ion o f a po la rh i g h -b o i li n
g s o lv e n t so t h a t t h e t e m p e r a t u r e o f t h e r e
a c t i o n m i x t u r e d o e s n o t r e ac h t h ebo i l ing po
in t o f the so lven t . S uc h a m e tho d wa s t e r m e d "M ic
r owa ve - Or ga n ic R e a c t ionE n h a n c e m e n t " ( M O R
E ) [ 4 9 ] . D e s p i t e t h e co n v e ni en c e, a d i s a d v
a n t a g e o f t h e M O R Ete c hn ique c ons i s t s i n the l
im i t a t ion to h igh - bo i l ing po la r so lve n t s suc h a s
DM S O, DM F ,N- m e thy lm or pho l ine , d ig lym e e tc . Th i s
a pp r oa c h h a s be e n a d a p te d to lowe r - bo il ing so
l-vents (e .g . to luene) and re f lux condi t ions , but , for
reac t ions a t re f lux , microwave cavi t ies( r e a c to r s )
ha ve to be m od i f i e d , e. g. by m a k ing a sh ie lde d op e
n ing to p r e ve n t the l e a k -a ge o f m ic r owa ve s, t h r
ou gh whic h the r e a c t ion ve sse l ha s to be c on ne c te d
to a n up r igh tc onde nse r , d r opp ing f unne l , ne u t r a l
ga s in l e t, e t c. A nu m be r o f r e flux sys t e m s ha ve be
e nde ve lope d in a n e f f o r t t o use so lve n t s in m ic r
owa ve - a ss i s t e d syn the s i s w i thou t the r i sk o fe xp
los ion. Th e m o s t im po r ta n t f e a tu r e o f suc h r e a c
to r s i s t o p r e ve n t f l a m m a ble va por se sc a p ing f
rom a m ic rowa ve c a v i ty ( a pp l i c a to r ) , i n wh ic h
the g e ne r a t ion o f e l e ct r ic a r c sc r e a te d f r om
in te r a c t ion o f m ic r owa ve s wi th suc h m a te r i a l s
a s sm a l l p i e c e s o f m e ta l s ,g r a ph i t e , e t c c
ons t i tu t e s a po te n t i a l h a z a r d o f a n e xp los ion
.
As wa s m e n t ion e d p r e v ious ly , p r e s su r i z e d c
ond i t ions ha ve be e n r e p or t e d f o r m ic r owa ve -a ss
i s t e d o r ga n ic syn the s i s [ 50 ] . The r e a c t ions we
r e c a r r i e d ou t a t a dom e s t i c m ic r owa veove n a nd
c om m e r c ia l ly a va i l a b le sc r e w- up p r e s su r e ve
sse l s m a de o f Te f lon , P ET a ndP E E K , w h i ch a re b o
t h m i c ro w a v e t r a n s p a r e n t ( F ig . 5 .1 ). I t h
as t o b e p o i n t e d o u t t h a tr e a c t ions und e r suc h
c ond i t ions ha ve to be s t r i c t ly c on t r o l l e d w i th
a n e f fi ci en t powe r f ee d -
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40 C H A P T E R 5. T E C H N I Q U E S F O R C O N D U C T IN G
R E A C T I O N S
ba c k be c a use a ny ve sse l c a n be c om e ove r - p r e s
su r i z e d r e su l t ing in a da nge r o f e xp los ions .F o r
t u n a t e l y , m o s t m i c ro w a v e r e a c t o rs a r e n o
w a d a y s e q u i p p e d w i t h a d i re c t t e m p e r a t u
r ea nd p r e s su r e c on t r o l , wh ic h in tu r n a vo ids fa
i lu r e s owing to the r m a l a nd p r e s su r e r un a w a ysof
c he m ic a l r e ac t ions . Th i s s t r a t e g y ha s be e n
suc ce ss fu l ly a pp l i e d to a g r e a t nu m be r o fsyn the
se s , b u t one ne e ds to ke e p in m in d t ha t i t a lwa ys ge
ne r a te s a r i sk o f e xp los ions i ft h e e q u i p m e n t i
s n o t o p e r a t e d p r o p e r l y .
F igu r e 5 .1 . R e a c t ion ve sse ls f o r pe r f o r m ing
c he m ic a l r e a c t ions und e r p r e s su r i z e d c on d i
-t ions . R e pr in te d wi th the pe r m iss ion f r om [ 3 ]
.
M ic r owa ve he a t in g ha s p r ove n to be o f be ne f i t
pa r t i c u la r ly f o r r e a c t ions u nd e r "d r y"m ed ia
(e .g . , so lvent - f ree condi t ion s) in open vesse l sys te m
s ( i . e. , in the a bsen ce of a so l-v e n t , o n so l id s u p
p o r t w i t h o r w i t h o u t c a t a l y s t s) [ 4 ]. R e a c
t i o n s u n d e r " d r y " c o n d i t io n swe r e o r ig ina l
ly de ve lope d in the l a t e 1980 's [ 51 ], bu t so lve n tl e
ss sys t e m s u nde r m ic rowa vec ond i t ions o ff er se ve r a
l a dd i t ion a l a dv a n ta ge s . Th e a bse nc e o f so lve n
t re duc e s th e r i sko f e xp los ions whe n the r e a c t ion t
a ke s p la c e in a c lose d ve ssel . M or e ove r , a p r o t i
c d ipo la rso lve n t s w i th h igh bo i l ing po in t s a r e e
xpe ns ive a nd d i f f i c u l t t o r e m ove f r om the r e a c
t ionm i x t u r e s . D u r i n g m i c r ow a v e i n d u c t i o
n o f r e a c ti o n s u n d e r d r y c o n d i ti o n s , t h e r
e a c t a n t sa ds o r be d on th e su r f a c e of a lum ina , s
i l ic a ge l, c la y , a nd o the r m ine r a l supp or t s a bso
r b m i -c r owa ve s wh e r e a s the sup po r t d oe s no t , a
nd t r a nsm iss ion o f m ic r owa ve s is no t r e s t r i c t e
d .M or e ove r , m ic r owa ve s c a n in t e r a c t d i r e c t
ly w i th r e a ge n t s a nd , the r e f o r e , c a n m or e e f
f i -c i e n t ly d r ive c he m ic a l r e ac t ions . Th e poss
ib le a c c e le r a t ions o f suc h r e a c t ions a r e e xpe c
te d
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41
t o b e o p t i m a l s in ce t h ey a re n o t m o d era t e o
r im p ed ed b y s o lv en ts . Co n s eq u en t l y , s u chs u p
p o r t ed r eag en t s e f f ic ien tl y i n d u ce r eac t i o n
s u n d er s a fe an d s i m p l e co n d i t io n s ev en w i t
hdom es t ic microwave ovens ins tead of expens ive specia l ized
com m ercia l microwave sys tems .Apply ing so lven t - f ree condi
t ions , th ree genera l poss ib i l i t i es can be rea l i zed
[52] :
9 r eag en t s u p p o r t ed o n m i n e ra l o x i d es an d
/o r g rap h i t e9 so l id - l iqu id so lven t - f ree phase t
rans fer ca ta lys i s (PTC)9 n ea t r eac t i o n
Aluminas , s i l i cas , c lays , zeo l i t es o r g raphi te
can be se lec ted as ac id ic , bas ic o r neu t ra ls u p p o r t
s , d ep en d i n g o n t h e t y p e o f r eac ti o n s . A l u m
i n a a lo n e can ac t a s a b as e t o ward sa ac i d co m p o u
n d s b u t i f a s t ro n g b as e i s n eces s a ry , KF o n a l
u m i n a can i o n i ze ca rb o nac i ds u p t o p K a - 3 5. On t
h e o t h e r h an d , m o n t m o r i l lo n i t e s ( cl ay s ) s
u ch a s K1 0 o r KS Foffer ac id i t i es very comparab le to s t
rong minera l ac ids (e .g . , n i t r i c and su l fur ic ac ids
,r e s p ec t i v e l y ) . R eac t i o n s a re s i m p l y p e r
fo rm ed b y m i x i n g r eag en t s an d a m i n e ra l s u p p o
r tin the absence of any organ ic so lven t fo l lowed by microwave
i r rad ia t ion [52] .
Phase- t rans fer ca ta ly t i c (PTC) condi t ions , which are
speci f i c fo r an ion ic reac t ions (andanion ic ac t iva t ion)
are per fec t ly wel l t a i lo red for microwave ac t iva t ion ,
because af ter ionex ch an g e b e t ween a s u b s t r a t e an d
ca t a l y s t , t h e r e s u l t i n g n u c l eo p h i l i c i o
n p a i r i s a h i g h l ypolar species especia l ly p rone to in
terac t ion wi th microwaves [32] . Eventual ly , the mix tureof
nea t reage n ts in an ope n vesse l can l ead to a reac t ion u
nde r microwave condi t ionsprov ided tha t one of the reagents i s
l iqu id o r a low mel t ing so l id tha t couples wel l wi thm i c
ro wav es. On t h e o t h e r h an d , ev en a s m a l l am o u n t
o f a g o o d m i c ro wav e ab s o rb e r(e.g,. H20, DMF; fo r ex
am p l e , s ee Tab le 1 .3 ) ad d ed t o r eac t i o n m i x t u
res t h a t co n s is to f s u b s t r a t e s t h a t d o n o t ab
s o rb m i c ro wav es in t h e s o li d s t a t e can i n i t ia t
e an i n c reas e o fr e a c ti o n m i x t u r e t e m p e r a t u
r e a n d t h e n c h e m ic a l r e a ct io n .
In genera l , reac t ions under so lven t - f ree condi t ions
per formed in a dense media suf ferd i f f i cu l t i e s w i t h h
ea t t r an s fe r t h ro u g h t h e r eac t i o n m ed i u m an d
h o m o g en o u s h ea t i n g u n -der convent ional condi t ions
. S ince microwave i r rad ia t ion i s a mea ns of vo lum et r ic
heat ingo f m a t e r i a l s , t em p e ra t u re is m o re u n i
fo rm l y d i s t r i b u t ed u n d e r m i c ro wav e co n d i ti
o n s . Im -p ro v em en t o f t em p e ra t u re h o m o g en e i
t y an d h e a t i n g r a t e i m p l ie s , in t u rn , f a s t e
r r eac t io n san d le ss d eg rad a t i o n o f t h e f i na l p
ro d u c t s . Fo r t h e p u rp o s e o f t h i s b o o k an d t
ak i n g i n t oacco u n t s o m e s a fe t y p recau t i o n s , a
l l t h e ex p e r i m en t s p e r fo rm ed an d d es c r i b ed h
e re h av ebeen car r i ed ou t in open vesse l s , somet ime under
ref lux condi t ions . Therefore , four reac-t i o n s y s t em s
were ch o s en t o ca r ry o u t t h e s y n t h es i s u n d er m
i c ro wav e co n d i t i o n s , an d t h ereact ions are g rouped
accord ing to these sys tems , i .e . :
9 r eac t i o n s i n h o m o g en o u s m ed i a9 r eac t io n
s o f r eag en t s s u p p o r t ed o n m i n e ra l s u p p o r t
s9 so l id - l iqu id so lven t - f ree phase t rans fer ca ta lys
i s (PTC) reac t ions9 r eac t io n s o f n ea t r eag en t s
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