Valor presente Neto

Page la The R éaction of Bromodurene w ith Base oa a« „ « « 101 2a R eactions of Other 2,6-D im ethylaryl H alides w ith

Potassium t-B utoxide « » a « <, « o a » « , a <> 115

(a) Bromo-m-xylene , oo oo oa oo » 117

(b) Bromomesitylene oo oa », oo o, « 119

(c) ChlorodureiiG o o « a a o « , « 122

(d) lododurene » » , » « a , » *. » a ,, » 123

(e) Bromoisodurene » »a ,a c* a» <. « , 123

(f) Bromopentamethylbenaene a» oa . 124

3o R eactions of 2 ,6-D im ethylaryl H alides w ith Potassium

t-B utoxide in Cyclohexene o » « » a « * a <, » « 125 4o The Thermal Decom positions of o-Halophenol S a lts «a , 129

5a C onclusions , «« o» »» o» «« «a a, «a a 139

—oCO—

As d escrib ed in th e In tro d u c tio n , bases such as sodamide do not re a c t w ith a ry l h a lid e s to giv e arynes when both ortho p o s itio n s

are blocked* The s ta rtin g p o in t of th is in v e s tig a tio n was the

observation th a t bromodurene elim in ated iiydrogen bromide on re a c tio n w ith potassium t-b u to x id e at high tem peratures. The mechanism of

th is re a c tio n was obviously of i n t e r e s t, r a is in g , fo r example, th e p o ssib le p a r tic ip a tio n of 1,4-dehydrobenzenes.

1. The R eaction of Bromodurene w ith Base

The re a c tio n of bromodurene w ith potassium t-butoxide was found to give durene as the major pro d u ct. Higher b o ilin g compounds were

also formed which wore id e n tif ie d as th e h ih e n zy ls ( l ) . (2) and (3) These r e s u lts are summarised below.

Br

Br

TT _{CH CH}

OH CH.

10^

The presence of h ib en zy ls among th e products suggests the p a r tic ip a tio n of benzyl ra d ic a ls in the rea ctio n . A p o ssib le

Scheme

Br

Bu "0

!

Br

re a c tio n so hone involving such in te rm ed ia tes i s o u tlin ed in

Scheme I . In th is scheme th e i n i t i a l step? i? in th e re a c tio n i s m e ta lla tio n by th e base to give a benzyl anion? follow ed by lo s s of bromide ion? ste p ii^ to give th e carbene (5)» Such in te rm ed ia tes may have e ith e r a t r i p l e t or a s in g le t s tr u c tu re . However i f a

s in g le t carbene s tru c tu re is involved then one might expect a ry l t- b u ty l e th e rs or diphonylm ethanes to be formed by re a c tio n of the d ip o la r resonance form (9) w ith other anions in the system . Such products wore n o t is o la te d or d e te c te d in re a c tio n s w ith bromodurene

03u

ArCH,

A precedent fo r a t r i p l e t carbene s tru c tu re i s provided by 103

iDenzocyclopropenes?

104 -

X - 3CH. R

He oHtainod e . s . r . sp e c tra stro n g ly suggestive of a t r i p l e t in te r ­ m ediate 5 and he p o stu la te d resonance w ith the d ir a d ic a l (lO) which

ho was ah le to tr a p w ith butadiene R CH CH R (10) CH •> (2 isom ers)

Gloss was unable to o b tain oorresp)onding a d d itio n products w ith simple o le f in s9 and he th e re fo re p o stu la te d a tw o-step

mechanism fo r th e a d d itio n to b u tad ien e; involving a second; re s o n a n c e -s ta b ilis e d ; d ir a d io a l. A sim ila r ex p lan atio n could account fo r th e absence of a d d itio n products when bromodurene and potassium t-b u to x id e were allow ed to re a c t in cyclohexene in th is in v e s tig a tio n .

G lo ss's experim ents were c a rrie d oui; p h o to ly tic a lly ? however^ and under such co n d itio n s a d ir a d ic a l might ho more fe a s ib le than m d cr th e therm al co n d itio n s of the experim ents d escrib ed in th is t h e s i s . An a lte r n a tiv e ; and perhaps more ac ce p ta b le m e c h a n ism involves re a c tio n of the carbene; in the t r i p l e t s ta te , w ith another m olecule of bromodurene, to give a benzyl ra d ic a l d ir e c tly , as was

form ulated in Scheme lo

This i s then assumed to r e a c t by d im é risatio n or cro ss-co u p lin g to give th e observed p ro d u cts.

In Scheme I , durene i s assumed to a r is e by th e tr a n s f e r of hydrogen from bromodurene to th e benzyl ra d ic a l (?)? stop iv .

Although tr a n s f e r s of th is type between benzyl r a d ic a ls and ohloro- toluene occur to th e ex ten t of about 15^ a t V~((f as d iscu ssed in the In tro d u c tio n , and th e high tem peratures a t which the re a c tio n w ith bromodurene i s c a rrie d out would favour the tra n s fe r re a c tio n ,

i t seems doubtful w hether a l l th e durene formed could a ris e by th is means.

- 106

ÎXL a lto r n a tiv o rou te to durene i s th e r e fo r e p o stu la te d in Scheme I I , in v o lv in g n u c le o p h ilic a tta c k on bromine by th e b a se , p a r a lle lin g B u n n ett' s o b ser v a tio n s on th e d e io d in a tio n of iodoben-

zenes by potassium t-b u to x id e in dim ethyl su lph oxid e 50 OBu

> _{+ Bu'^OBr}

Scheme I I

> b ib e n z y ls (a s in Scheme I)

Support fo r th e form ation of durene by t h is rou te in p r e fe r ­ ence to th a t o u tlin e d in Scheme I i s obtain ed from th e r e a c tio n of bromodurene w ith sodam ide. Under th e same c o n d itio n s as th o se used in the r e a c tio n o f bromodurene w ith potassium t-b u to x id e , th e

y ie ld of dureno i s reduced to l6 in/lOO m, of b a se , approxim ately h a lf th a t ob tain ed w ith potassium t-b u t oxide, w h ile th e y ie ld of b ib o n z y ls i s in c re a se d s l i g h t l y . I t i s reason ab le to exp ect a d iffe r e n c e in r e a c t iv it y between t-b u to x id e ion and amide io n in n u c le o p h ilic a tta ck on th e h alogen, th u s lea d in g to d iffe r e n c e s in

the y ie ld of durene. On th e other hand, sin c e the form ation of phenyl anion s from a r y l h a lid e s and sodamide i s known to occur under c o n d itio n s in which no r e a c tio n occu rs between bromodurene

17

and sodam ide, and s in c e th e removal o f an proton by lith iu m cyclohoxylam ide from to lu en e i s found to bo about one hundred tim es f a s te r than the rem oval of a proton from th e arom atic ring.^^^ the form ation of th e b en zyl anion i s presumably not ra te -d e to rm in in g , and d iffe r e n c e s in th e b a s ic it y of t-b u to x id e and amide io n s w i l l not a f f e c t th e produ cts formed from i t ,

A se r io u s o b je ctio n to Scheme I I , however, i s th e absence of id e n t if ia b le products a ssig n a b le to th e precursory t-b u ty l

hypobrornite, which should be formed to the e x te n t of 35 rn/lOO m. The only lo w -b o ilin g product is o la t e d in r e a c tio n o f bromodurene w ith potassium t-b u t oxid e was t-b u t a n d (74 m/lOO m, ) . A cetone, which might be exp ected to a r is e from th e decom p osition of t-b u ty l hypobrornite, was absent ( g . l . c . ) , and other p o s s ib le decom position p rodu cts such as methane or m ethyl bromide were not d e te c te d e ith e r

’.Sclieme I I I

B u'O

CHgBr