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Preparation, Properties, and Uses of Benzeneboronic Acid ROBERT M. WASHBURN, ERNEST LEVENS, CHARLES F. ALBRIGHT, FRANKLIN A. BILLIG, and E. S. CERNAK

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Research Department, American Potash & Chemical Corp., Whittier, Calif.

The purpose of this study was the development of a commercially practical process for the preparation of benzeneboronic acid based on known laboratory methods. The reaction of boron trifluoride with phenylmagnesium bromide was investigated briefly. The process finally developed, involving the reaction of phenylmagnesium bromide a n d methyl borate to give yields of up to 100% of benzeneboronic a c i d , was shown to be of general applicability by the preparation of p-chlorobenzene- a n d 1-naphthaleneboronic acids in high yields. This paper presents: the effect of process variables on the yield of benzeneboronic a c i d ; a unifying discussion of the important chemical a n d physical aspects of the process; new physical a n d chemical data concerned with the preparation and properties of benzeneboronic a c i d ; and a discussion of the reactions of benzeneboronic acid a n d its immediate derivatives.

T h e nomenclature f o r b o r o n compounds is i n a state of flux a t t h e present t i m e . T h e f o l l o w i n g l i s t o f b o r o n c o m p o u n d s discussed i n t h i s p a p e r i n d i c a t e s t h e n o m e n c l a t u r e u s e d . T h e r e a d e r i s r e f e r r e d t o t h e f o l l o w i n g references f o r b o r o n n o m e n c l a t u r e (8, 64, 65, 68, 69, 72). C H B(OH) C HôB(OCH )2 CeHsBCls (CeHs^BOH (CeHs^BOCEU (CeH ) B (CeH )4B"Li e

5

2

3

e

B

5

8

+

Benzeneboronic acid D i m e t h y l benzeneboronate Benzeneboronyl dichloride Dibenzeneborinic acid M e t h y l dibenzeneborinate Triphenylborane L i t h i u m tetraphenylborohydride

B a c k g r o u n d . B e n z e n e b o r o n i c a c i d w a s first r e p o r t e d b y M i c h a e l i s a n d B e c k e r (58, 59), w h o p r e p a r e d b e n z e n e b o r o n y l d i c h l o r i d e b y h e a t i n g b o r o n t r i c h l o r i d e a n d d i p h e n y l m e r c u r y a t 180° t o 2 0 0 ° C . i n a sealed t u b e ( E q u a t i o n 1 ) . B e n z e n e b o r o n y l dichloride was f o u n d t o h y d r o l y z e easily, g i v i n g benzeneboronic a c i d ( E q u a t i o n 2 ) .

102

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

WASHBURN, LEVENS, ALBRIGHT, BILLIG, AND CERNAK-BENZENEBORONIC ACID

Hg

+ BC1

BC1

3

2

103

HgCl

+

(1)

OH BC1

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2

+

2H 0 2

2HC1

(2)

M i c h a e l i s a n d B e c k e r r e p o r t e d the m e l t i n g p o i n t of b e n z e n e b o r o n i c a c i d t o b e 2 0 4 ° C , b u t l a t e r , M i c h a e l i s a n d B e h r e n s (60) c h a n g e d t h i s t o 2 1 6 ° C . D i p h e n y l m e r c u r y has been s h o w n t o r e a c t s m o o t h l y w i t h b o r o n t r i b r o m i d e u n d e r reflux t o give b e n z e n e b o r o n y l d i b r o m i d e a n d d i b e n z e n e b o r i n y l b r o m i d e (31, 61). K h o t i n s k y a n d M e l a m e d (85) were t h e first t o describe t h e p r e p a r a t i o n o f b o r o n i c a c i d s f r o m G r i g n a r d reagents a n d b o r a t e esters. T h e y r e p o r t e d t h a t e t h y l , n - p r o p y l , i s o b u t y l , a n d i s o a m y l b o r a t e s g a v e a b o u t 5 0 % y i e l d s o f b o r o n i c acids o r t h e i r esters, w h e n t h e a l k y l ester was a d d e d t o t h e G r i g n a r d reagent a t 0 ° C . T h e y also c l a i m e d t h e f o r m a t i o n o f some b y - p r o d u c t t o l u e n e w h e n m e t h y l b o r a t e w a s u s e d , b u t t h i s c o u l d n o t b e c o n f i r m e d b y G i l m a n a n d V e r n o n (27). K r a u s e a n d N i t s c h e (39, Ifi) i n v e s t i g a t e d t h e r e a c t i o n o f b o r o n t r i f l u o r i d e a n d G r i g n a r d reagents. T h e y f o u n d t h a t a n excess o f p h e n y l m a g n e s i u m b r o m i d e g a v e t r i p h e n y l b o r a n e , b u t a n excess o f b o r o n t r i f l u o r i d e g a v e a m i x t u r e o f t r i p h e n y l b o r a n e , dibenzeneborinyl fluoride, a n d benzeneboronyl difluoride. T h e difluoride yielded t h e boronic acid when hydrolyzed. G i l m a n a n d V e r n o n (27) i n v e s t i g a t e d t h e p r e p a r a t i o n o f b e n z e n e b o r o n i c a c i d u s i n g t h e p r o c e d u r e s o f K h o t i n s k y a n d M e l a m e d (35). T r i p h e n y l b o r a t e a n d p h e n y l m a g ­ nesium bromide gave 4 0 % of phenol, 16.4% of benzeneboronic acid, a n d 1.3% of b i p h e n y l . T h e y reported a n 8 6 % y i e l d of the boronic acid f r o m the reaction of a 0.25-mole r u n o f m e t h y l b o r a t e a n d p h e n y l m a g n e s i u m b r o m i d e , b u t w i t h m o r e c o n ­ c e n t r a t e d s o l u t i o n s t h e y i e l d was r e d u c e d t o 5 8 % a n d a 3 0 % y i e l d o f benzene w a s o b ­ t a i n e d . T h e benzene w a s s u p p o s e d l y o b t a i n e d b y h y d r o l y s i s o f u n r e a c t e d G r i g n a r d reagent. K ô n i g a n d S c h a r r n b e c k (38) r e p o r t e d t h e p r e p a r a t i o n o f b e n z e n e b o r o n i c a c i d a n d d i b e n z e n e b o r i n i c a c i d b y a d d i n g i s o b u t y l b o r a t e t o t h e G r i g n a r d reagent a t 0 ° C . S e a m a n a n d J o h n s o n (71) c o u l d n o t r e p e a t t h e w o r k o f G i l m a n a n d V e r n o n . T h e y s t a t e d t h a t t h e m e t h y l b o r a t e m u s t b e free of m e t h a n o l " s i n c e t h e l a t t e r c a u s e d a m a r k e d d i m i n u t i o n of the y i e l d , a p p a r e n t l y greater t h a n could be accounted for o n t h e basis o f t h e G r i g n a r d d e s t r o y e d . " F u r t h e r m o r e , these a u t h o r s r e p o r t e d t h a t a n i n v e r s e G r i g n a r d r e a c t i o n g a v e b e t t e r y i e l d s . S u b s e q u e n t l y , B e a n a n d J o h n s o n (8) were able t o increase t h e y i e l d o f b e n z e n e b o r o n i c a c i d c o n s i d e r a b l y b y u s i n g t h e m o r e e a s i l y p u r i f i e d b u t y l b o r a t e a n d effecting t h e r e a c t i o n a t — 6 0 ° C . I n several runs yields r a n g e d f r o m 50 t o 6 0 % . M o r e c o n c e n t r a t e d s o l u t i o n s gave l o w e r y i e l d s (42 t o 4 7 % ) . B r a n c h a n d his c o w o r k e r s (5, 6, 8, 23, 88, 89), K u i v i l a a n d his c o w o r k e r s (41-49), a n d o t h e r s (9, 74, 76, 79) h a v e u s e d t h e p r o c e d u r e o f B e a n a n d J o h n s o n f o r t h e p r e p ­ a r a t i o n o f b e n z e n e b o r o n i c a c i d a n d o t h e r a r e n e b o r o n i c a c i d s . T h e y i e l d s were g e n e r a l l y about 5 0 % . D u r i n g a study of the p r e p a r a t i o n a n d physical properties of benzeneboronic acid, H u t t o (81) o b t a i n e d y i e l d s o f a b o u t 4 0 % b y a d d i n g p h e n y l m a g n e s i u m b r o m i d e t o m e t h y l b o r a t e c o o l e d w i t h a n i c e b a t h . H u t t o also c h e c k e d t h e p r o c e d u r e s o f K h o ­ t i n s k y a n d M e l a m e d (35) a n d o b t a i n e d 1 0 . 5 % y i e l d ; t h e p r o c e d u r e of K r a u s e a n d N i t s c h e (39, 40) g a v e a b o u t 1 0 % y i e l d . M e l ' n i k o v (55) u s e d t h e p r o c e d u r e o f K ô n i g a n d S c h a r r n b e c k (38), b u t L e t s i n g e r a n d S k o o g (52) p r e f e r r e d a n i n v e r s e p r o c e d u r e a n d l o w t e m p e r a t u r e s . A n o t h e r s y n t h e t i c a p p r o a c h , w h i c h a p p a r e n t l y gives l o w e r y i e l d s , i n v o l v e s t h e r e a c t i o n o f p h e n y l l i t h i u m w i t h a b o r a t e ester. B r i n d l e y , G e r r a r d , a n d L a p p e r t (9)

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

ADVANCES IN CHEMISTRY SERIES

104

reported a 3 6 % y i e l d of benzeneboronic acid using b u t y l borate a t — 8 0 ° C . Letsinger a n d S k o o g (52) r e p o r t e d a 4 1 % y i e l d o f b u t y l d i b e n z e n e b o r i n a t e a n d a 1 3 % y i e l d o f e t h y l e n e g l y c o l b e n z e n e b o r o n a t e f r o m t h e r e a c t i o n o f 2 moles o f p h e n y l l i t h i u m w i t h b u t y l ethylene glycol borate. Terminology. T h e f o l l o w i n g t e r m s are u s e d i n t h e D i s c u s s i o n a n d E x p e r i m e n t a l sections o f t h i s p a p e r : = R e a c t a n t s a d d e d as r a p i d l y as possible w h i l e k e e p i n g the r e a c ­ t i o n t e m p e r a t u r e c o n s t a n t (0.33 h o u r o r less t o a d d 3.00 m o l e s ) Slow reaction = R e a c t a n t s a d d e d s l o w l y (0.75 t o 4.0 h o u r s t o a d d 3.00 m o l e s ) 3 - M o l e r e a c t i o n = T h r e e moles of G r i g n a r d (as 3 . 0 M s o l u t i o n ) + 3 moles o f m e t h y l b o r a t e i n 1500 m l . o f e t h e r

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Fast reaction

Figure 1.

Equipment for study of methyl borate a n d phenylmagne­ sium bromide reaction A. Methyl borate addition buret β. Stirring motor C. Grignard addition buret D. Thermometer E. Morton flask

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

105

WASHBURN, LEVENS, ALBRIGHT, BILLIG, AND CERNAK-BENZENEBORONIC ACID

2 - M o l e r e a c t i o n = T w o m o l e s o f e a c h r e a c t a n t b u t t h e same t o t a l v o l u m e ( d i ­ l u t e d w i t h e t h e r ) as a 3-mole r e a c t i o n 1 - M o l e r e a c t i o n = O n e m o l e o f e a c h r e a c t a n t b u t t h e s a m e t o t a l v o l u m e as a 3-mole reaction

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Experimental Reagents. T h e ether used was M a l l i n c k r o d t A . R . grade, stored over s o d i u m . P h e n y l m a g n e s i u m b r o m i d e w a s o b t a i n e d f r o m A r a p a h o e as a 3 . 0 M s o l u t i o n i n d i e t h y l e t h e r . D i f f e r e n t b a t c h e s , a n a l y z e d b y t h e m e t h o d o f G i l m a n (26), v a r i e d b e t w e e n 3.0 a n d 3 . 2 M . T h e m a t e r i a l was u s e d as r e c e i v e d ( b a s e d o n t h e a n a l y s i s ) o r d i l u t e d w i t h e t h e r as necessary. B o r o n t r i f l u o r i d e w a s o b t a i n e d f r o m M a t h e s o n . B o r o n t r i f l u o r i d e - e t h e r a t e w a s p r e p a r e d b y p a s s i n g t h e gas i n t o c o o l e d e t h e r u n t i l t h e w e i g h t i n d i c a t e d t h e c o r r e c t a m o u n t of b o r o n t r i f l u o r i d e h a d been a b s o r b e d . M e t h y l borate was commercial anhydrous m a t e r i a l f r o m A m e r i c a n P o t a s h & C h e m i c a l C o r p . I t was f o u n d b y a n a l y s i s t o c o n t a i n 9 9 . 0 % o f ester. T h e m e t h y l b o r a t e w a s p u r i f i e d b e f o r e use b y f r a c t i o n a t i o n i n a 3 0 - p l a t e c o l u m n . p - C h l o r o p h e n y l m a g n e s i u m b r o m i d e w a s prepared from p-bromochlorobenzene ; 1-naphthylmagnesium bromide was prepared f r o m 1 - b r o m o n a p h t h a l e n e i n e t h e r - b e n z e n e s o l v e n t (26). E q u i p m e n t . T h e e q u i p m e n t u s e d is s h o w n i n F i g u r e 1. F o r s m a l l r u n s ( 1 - l i t e r flask) the stirrer used was a L a b l i n e S t i r - O - V a c ( f r o m L a b l i n e , I n c . , 217 N o r t h D e s P l a i n e s S t . , C h i c a g o 6, 111.) s t i r r i n g a t a b o u t 5000 r . p . m . F o r l a r g e r r u n s ( 5 - l i t e r flask), a P r e m i e r , 1-inch d i a m e t e r , D u p l e x D i s p e r s a t o r ( o b t a i n e d f r o m P r e m i e r M i l l C o r p . , G e n e v a , Ν . Y . ) s t i r r i n g a t a b o u t 7500 r . p . m . w a s u s e d . Reaction of B o r o n Trifluoride Etherate a n d Phenylmagnesium Bromide. T h e r e a c t i o n of b o r o n t r i f l u o r i d e a n d p h e n y l m a g n e s i u m b r o m i d e ( E q u a t i o n 3 ) w a s b r i e f l y i n v e s t i g a t e d u s i n g t h e p r o c e d u r e of K r a u s e a n d N i t s c h e (40). I n e a c h e x p e r i m e n t t h e solids f u m e d o n first e x p o s u r e t o a i r i n d i c a t i n g t h e presence of t r i p h e n y l b o r a n e ; t h e

(3) b o r o n c o n t e n t of t h e p r o d u c t , i s o l a t e d i n 0 t o 4 6 % y i e l d , g e n e r a l l y c o r r e s p o n d e d m o s t closely t o d i b e n z e n e b o r i n i c a c i d o r i t s a n h y d r i d e . T h e s e r e s u l t s a r e i n a c c o r d w i t h t h e o b s e r v a t i o n s of H u t t o (31). T h i s l i n e of i n v e s t i g a t i o n was d r o p p e d i n f a v o r o f t h e m o r e e a s i l y h a n d l e d r e a c t i o n w i t h m e t h y l borate. Reaction of Phenylmagnesium Bromide and M e t h y l Borate. T h e p r e p a r a t i o n o f benzeneboronic acid f r o m phenylmagnesium bromide and m e t h y l borate c a n be repre­ sented b y E q u a t i o n s 4, 5, a n d 6. Reaction.

(4)

Hydrolysis.

+ 3CH OH + Mg(OH)Br 3

(5) Neutralization. M g ( O H ) B r + è H S 0 -> | M g B r + è M g S 0 2

4

2

4

+H 0 2

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

(6)

106

ADVANCES IN CHEMISTRY SERIES

WATER ACID

WATER

LI C H MgBr 6

LA

ETHER

ETHER

RES. WATER LAYER

AQUEOUS SOLUTION

5

REACTION

STEAM DISTILLATION

HYDROLYSIS a NEUTRALIZATION

AQUEOUS FILTRATE

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ETHER, METHANOL, ETC.

WATER SOUDS

PET. ETHERPRODUCT

-

PRECIPITATION OF SOLIDS

I

PET. ETHER SOLIDS FILTRATION

PET. ETHER SOLUTION OF BORINIC ACIO

β

PET. ETHER WASH Figure 2 .

Process flow diagram for preparation of benzeneboronic acid

F i g u r e 2 shows a s i m p l i f i e d process flow d i a g r a m o f t h e process i n v e s t i g a t e d . T h e process v a r i a b l e s i n v e s t i g a t e d were r e a c t i o n t e m p e r a t u r e , r a t e o f a d d i t i o n o f G r i g n a r d t o m e t h y l b o r a t e , m o d e o f a d d i t i o n a n d c o n c e n t r a t i o n o f r e a c t a n t s , effect of impurities i n the m e t h y l borate, a n d the procedure for isolating benzeneboronic acid. T h e p r o c e d u r e f o r i s o l a t i n g b e n z e n e b o r o n i c a c i d used t h r o u g h o u t t h i s i n v e s t i g a t i o n i n v o l v e d e x t r a c t i o n of t h e p r o d u c t f r o m the h y d r o l y z e d a n d n e u t r a l i z e d reaction m i x ­ ture w i t h ether, steam distillation of ether a n d volatile i m p u r i t i e s , c r y s t a l l i z a t i o n of t h e p r o d u c t , a n d p e t r o l e u m e t h e r w a s h o f t h e i s o l a t e d solids t o r e m o v e d i b e n z e n e b o r i n i c a c i d . T h i s i s a c o n s i d e r a b l y s i m p l e r process t h a n a n y p r e v i o u s l y r e p o r t e d . D e p e n d i n g o n t h e e x p e r i m e n t a l c o n d i t i o n s , v a r i o u s p r o d u c t s were f o r m e d d u r i n g t h e i n v e s t i g a t i o n o f t h e process s h o w n i n F i g u r e 2. F o r e x a m p l e , i n e x p e r i m e n t s a t 0 ° C , a b o u t 2 0 % o f t h e m e t h y l b o r a t e c o u l d b e a c c o u n t e d f o r i n t h e aqueous residue a n d v a r y i n g a m o u n t s o f t h e G r i g n a r d c o u l d b e a c c o u n t e d f o r as benzene. Qualita­ t i v e l y , a b o u t 2 t o 3 % p h e n o l a n d u p t o a b o u t 5 % b i p h e n y l were o b s e r v e d as p r o d u c t s of e x p e r i m e n t s c o n d u c t e d a t h i g h e r t e m p e r a t u r e s ( 1 5 ° t o 2 5 ° C ) . T h e benzene, p h e n o l , a n d b i p h e n y l were r e m o v e d d u r i n g t h e s t e a m d i s t i l l a t i o n s t e p . I n a l l e x p e r i ­ ments where the G r i g n a r d was added t o the m e t h y l borate, the v a r y i n g amounts of d i b e n z e n e b o r i n i c a c i d f o r m e d were s e p a r a t e d f r o m t h e b e n z e n e b o r o n i c a c i d b y w a s h i n g t h e f i l t e r e d solids w i t h p e t r o l e u m e t h e r . ADDITION OF P H E N Y L M A G N E S I U M B R O M I D E TO M E T H Y L BORATE.

E f f e c t of R a t e of

A d d i t i o n of Phenylmagnesium B r o m i d e . D u r i n g t h e early part of this work, t h e G r i g n a r d reagent w a s a d d e d t o t h e m e t h y l b o r a t e r e l a t i v e l y s l o w l y i n a c c o r d a n c e w i t h e s t a b l i s h e d l i t e r a t u r e p r o c e d u r e s (71) T h e a v e r a g e y i e l d ( T a b l e 1-1) f o r t h r e e s l o w runs a t 0 ° C . was 46.8%. A group of nine 3-mole runs was made a t 0 ° C . i n w h i c h t h e G r i g n a r d was added as r a p i d l y as possible w h i l e m a i n t a i n i n g a c o n s t a n t t e m p e r a t u r e ( T a b l e 1 - 2 ) ; t h e

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

WASHBURN, LEVENS, ALBRIGHT, BILLIG, AND CERNAK-BENZENEBORONIC ACID

Table

I.

A d d i t i o n of Phenylmagnesium

Bromide to M e t h y l

107

Borate 0 ° C .

(3.0-mole r e a c t i o n ) Addn. Time, Hr. 1.23 0.18 0.15 0.20 0.20

Solids

% Yield

A q . Filtrate

Run N o . Grams % B Grams le,6 154 8.68 885 2v 179 8.35 1086 3··/ 194 9.01 942 167 8.69 1011 4A 165 8.65 959 5» Methyl borate purified and stored before use. Slow reaction, average of 3 runs. Fast reaction, average of 9 runs. Average of 5 runs. * Methyl borate distilled directly into apparatus. f Fast reaction, average of 5 runs. ο Average of 4 runs. Fast reaction, 0.953 gram of methanol added to freshly distilled methyl » Fast reaction, 0.613 gram of boric acid added to freshly distilled methyl

Boromc acid 46.8 53.3 60.2 51.5 50.5

% B 0.48 0.35 0.34 0.30 0.28

Bonnie acid

—

18.2

Ο Ζ ο

70

Ο

m σο

Ζ

Μ m

Ζ

σο m

I

>

Ζ

το

m

Ό η

> ζ

Ρ

ρσ ι—

—I ν

Γ­ ΟΟ TO

>

Ζ

r— m < m

Ζ

C 3D

CO

> CO I

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

Figure 6.

uunu ι π

ira

mιυπυιιο Infrared spectrum of benzeneboronic anhydride nHYt

Downloaded by PURDUE UNIV on May 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch011

m CO

τα

CO m

-<

τα

m CO Ή

χ

Π

Ζ

m en

> Ζ η

D <

>

Downloaded by PURDUE UNIV on May 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch011

WASHBURN, LEVENS, ALBRIGHT, BILLIG, AND CERNAK-BENZENEBORONIC ACID

rigure

/.

οοιυοιιιτχ ο τ

DenzeneDoromc

aciu

METAL-ORGANIC COMPOUNDS Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

116

ADVANCES IN CHEMISTRY SERIES

> -J Ο CO

CO 2

< e> ο ο

IaJ

H

3

Downloaded by PURDUE UNIV on May 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch011

Ο CO

< 20

30

50

40

TEMPERATURE, ° C . Figure 8.

Solubility of benzeneboronic anhydride

T h e r e i s , h o w e v e r , a large a m o u n t of evidence t o i n d i c a t e t h a t a t e t r a c o o r d i n a t e b o r o n a t e a n i o n ( E q u a t i o n 8 ) a n d t h e e q u i l i b r i a i n w h i c h i t t a k e s p a r t are some of t h e m o s t i m p o r t a n t aspects o f t h e G r i g n a r d r e a c t i o n . K u i v i l a a n d c o w o r k e r s (41-Jfi) h a v e s h o w n t h a t a t e t r a c o o r d i n a t e b o r o n a t e a n i o n , I I , i s i n v o l v e d i n t h e r e a c t i o n of b e n z e n e b o r o n i c a c i d a n d i t s d e r i v a t i v e s w i t h b r o m i n e (43-40), i o d i n e (40), a n d h y d r o ­ gen p e r o x i d e (41, 4%, 4$)·

OH I

B—OH I

OH II M a n y e x a m p l e s o f b o r o n c o m p o u n d s a c t i n g as L e w i s acids are k n o w n . F o r e x a m p l e , s o d i u m h y d r i d e a n d s o d i u m m e t h o x i d e react w i t h m e t h y l b o r a t e t o g i v e s o d i u m t r i m e t h o x y - a n d s o d i u m t e t r a m e t h o x y b o r o h y d r i d e , r e s p e c t i v e l y (14)· T h e n , b y a n a l ­ ogy, i t m i g h t be e x p e c t e d t h a t p h e n y l m a g n e s i u m b r o m i d e w o u l d react w i t h m e t h y l borate according t o E q u a t i o n 8 to form a tetracoordinate complex.

OCH

3