Synthesis of Boron-Carbon Ring Compounds
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SHELDON L. CLARK, JAMES R. JONES, and HUGO STANGE
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Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch025
Energy Division, Olin Mathieson ChemicalCorp., Niagara Falls, Ν. Y.
A procedure for the preparation of simple mixed trialkylborane compounds has been devised. n-Butyldiethylborane and di-n-butylethylborane have been prepared. AI kyl group exchange has been shown to occur when a Grignard reagent is allowed to react with tri-n-butylborane or di-n-butylchloroborane. In one attempt to prepare α,ω-bis(di-n-butylboryl)alkanes, 1,4-bis(di-n-butylboryl)butane apparently was isolated. This reaction could not be repeated. Failure to isolate α,ω-bis(di-n-butylboryl)alkanes has been attributed to either disproportionation of this product or to exchange of alkyl groups during the reaction. As a result of this, two boron-carbon ring compounds, 1-n-butylboracyclopentane and 1-n-butylboracyclohexane, have been prepared.
A l l reactions were conducted under anhydrous conditions with the maintenance of a nitrogen atmosphere at all times. Boron analyses were determined by oxidation with hydrogen peroxide, accord ing to the method of Erickson (#). Molecular weights were determined cryoscopically b y measuring the freezing point depressions of benzene solutions. n-Butyldiethylborane from Di-n-butylchloroborane To a cold (0°C.) ethereal Grignard solution prepared from 12.0 grams (0.5 gramatom) of magnesium and 55.0 grams (0.5 mole) of ethyl bromide were added drop wise 40.0 grams (0.25 mole) of di-n-butylchloroborane (1), dissolved i n an equal volume of ether. The reaction mixture, after being heated to reflux for 2 hours, was hydrolyzed at 0°C. with a 10% solution of hydrochloric acid. T h e ethereal layer was washed with water, 5 % sodium bicarbonate solution, and again with water, and finally was dried over anhydrous magnesium sulfate. After the ether h a d been removed i n vacuo, distillation of the residue gave 8.8 grams (28%) of n-butyldiethylborane, boiling point, 45° to 46°C. (15 m m . ) . Analysis. Calculated for C H B : boron, 8.75; molecular weight, 126. F o u n d : boron, 7.73; molecular weight, 125, 126. I n addition, 5.5 grams of an impure liquid, boiling point 63° to 73° C . (15 mm.), and 9.5 grams (21%) of tri-n-butylborane were collected. 8
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D u r i n g the course of this work, i t became necessary to find a solvent for the Present address, Pennsylvania State University, University Park, P a . ' Present address, Food Machinery and Chemical Corp., Princeton, N . J . • Present address, New Haven, Conn. 22a 1
In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
CLARK ET AL.
Boron-Carbon Ring Compounds
229
butylchloroboranes. I t was discovered that ethers are stable to monochloroboranes ( > B — C I ) but not to dichloroboranes ( — B < ) . These results are i n agreement with those of Wiberg and Sutterlin, who studied the action of methoxychloroboranes on ethyl ether (4).
Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch025
n-Butyldiethylborane from Tri-n-butylborane A Grignard solution was prepared i n the usual manner from 12.0 grams (0.5 gram-atom) of magnesium and 55.0 grams (0.5 mole) of ethyl bromide. T o this were added dropwise at 0 ° C , 57.6 grams (0.3 mole) of tri-n-butylborane dissolved i n an equal volume of ethyl ether. The reaction mixture was heated to reflux for 1.5 hours, allowed to stand overnight, and finally was heated to reflux for an additional 4 hours. The mixture was hydrolyzed and the products were isolated as described above. A total of 8.2 grams (20.5%) of n-butyldiethylborane, boiling point 42° to 43°C. (15 m m . ) , was collected.
Di-n-butylethylborane from
Di-n-butylchloroborane
The procedure was the same as for the preparation of n-butyldiethylborane from di-n-butylchloroborane, except that the Grignard reagent was added to the d i n-butylchloroborane. F r o m 3.0 grams (0.125 gram-atom) of magnesium, 13.6 grams (0.125 mole) of ethyl bromide, and 20.0 grams (0.125 mole) of di-n-butylchloroborane were recovered 5.5 grams (27%) of di-n-butylethylborane, boiling point, 75° to 76°C. (15 m m . ) . Analysis. Calculated for C H B : boron, 7.15; molecular weight, 154. F o u n d : boron, 7.45; molecular weight, 146, 160. 1 0
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1 -n-Butylboracyclopentane The di-Grignard of 1,4-dibromobutane was prepared i n an 8 0 % yield from 48.0 grams (2.0 gram-atoms) of magnesium and 216.0 grams (1.0 mole) of 1,4-dibromobutane, i n the usual manner. The Grignard solution was added dropwise, with stirring, to 361.0 grams (2.2 moles) of cold di-n-butylchloroborane dissolved i n an equal volume of ether. The mixture was stirred for 1 hour at 0 ° C . and then filtered through glass wool. Distillation, after ether removal, gave 67.0 grams (67.5%) of 1-n-butylboracyclopentane, boiling point, 79° to 79.5°C. (75 m m . ) . Analysis. Calculated for C H B : boron, 8.73; molecular weight, 124. F o u n d : boron, 8.80, 8.80; molecular weight, 121. I n addition, 119.7 grams (82.5%) of tri-n-butylborane were isolated. The above experiment was repeated, except that the di-n-butylchloroborane was added to the di-Grignard solution. F r o m 12.0 grams (0.5 gram-atom) of magnesium, 54.0 grams (0.25 mole) of 1,4-dibromobutane, and 80.0 grams (0.50 mole) of di-n-butylchloroborane were isolated 21.5 grams (69.5%) of 1-n-butylboracyclopentane (boiling point, 37° to 39° at 10 mm.) and 34.8 grams (75.6%) of tri-n-butylborane (boiling point 87° to 98°C. at 12 m m . ) . 8
Isolation of
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l,4-Bis(di-n-butylboryl)borane
T o the cold di-Grignard solution prepared from 6.0 grams (0.25 gram-atom) of magnesium and 25.0 grams (0.13 mole) of 1,4-dibromobutane were added 32.0 grams (0.20 mole) of di-n-butylchloroborane dissolved i n an equal volume of ether. The addition required 1 hour, and the mixture was stirred for one more hour at 0 ° . I t was heated to reflux for 3 hours and then allowed to stand for 6 days. Distillation produced 10.7 grams (35%) of a material boiling at 80°C. (10 m m . ) . Molecular weight determinations gave values of 303 and 296 and a low value of 195 as compared to a calculated value of 306 for l,4-bis(di-n-butylboryl) butane. Redistillation 15 days later yielded only 1-n-butylboracyclopentane and tri-n-butylborane. In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
ADVANCES IN CHEMISTRY SERIES
230 1 -n-Butylboracyclohexane
This preparation is the same as for 1-n-butylboracyclopentane. The quantities used were 12.0 grams (0.50 gram-atom) of magnesium, 57.5 grams (0.25 mole) of 1,5-dibromopentane, and 34.0 grams (0.21 mole) of di-n-butylchloroborane. The yield of 1-n-butylboracyclohexane, boiling point, 60° to 62°C. (10 mm.), was 15.3 grams (100%) and that of tri-n-butylborane, boiling point, 100°C. (15 mm.), was 20.5 grams (100%). Analysis. Calculated for C H B : boron, 7.85; molecular weight, 138. F o u n d : boron, 7.65, 7.86; molecular weight 138, 138, 142, 144. Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch025
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Reaction of 1-n-Butylboracyclopentane with Hydrogen Chloride Anhydrous hydrogen chloride was bubbled through 28.0 grams (0.226 mole) of 1-n-butylboracyclopentane for 7.5 hours while the liquid was heated to an external temperature of 110°C. Subsequent distillation produced 28.0 grams (78%) of di-n-butylchloroborane, boiling point 60°C. (10 m m . ) . About 2.5 grams of an u n identified liquid also were collected. Discussion I n a proposed study of α,ω-bis (di-n-butylboryl)alkanes ( I ) , i t first had to be shown that simple mixed trialkylborane compounds ( C H ) B (CH ) B (C4H9) 2 I 4
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2
2
n
were capable of existence. A n attempt was made to prepare di-n-butylethylborane (II) b y the following equation. C H M g B r + (C H ) BC1 2
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4
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(C H ) B(C H ) + MgBrCl II
2
4
9
2
2
6
(1)
W i t h the use of a 100% excess of the Grignard reagent, the products isolated were n-butyldiethylborane (III) and tri-n-butylborane ( I V ) . These unexpected products can be accounted for if i t is assumed that an exchange of alkyl groups is possible between a Grignard reagent and a trialkylborane. C H MgBr + (C H ) BC H -» C H B(C H ) + C H MgBr III 2
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4
9
2
2
6
4
C H M g B r + (C H ) BC1 4
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4
9
2
9
2
6
2
4
9
(C H ) B + MgBrCl IV 4
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S
(2) (3)
I n view of the fact that two moles of the Grignard reagent actually were used, where one would have been sufficient, i t is not unreasonable to propose such a sequence of reactions. If a l k y l group exchange occurs, i t is to be expected that reaction between excess ethylmagnesium bromide and tri-n-butylborane should lead both to n-butyldiethyl borane and di-n-butylethylborane. C H M g B r + (ΟΆ)ζΒ 2
( C J S ^ B C A + C H MgBr
6
4
9
C H MgBr + (C H ) BC H -» C J ^ i C t H , ) , + C H MgBr 2
6
4
9
2
2
5
4
9
(4) (5)
When a 6 6 % excess of the Grignard reagent was used, based on Equation 4, only n-butyldiethylborane and unreacted tri-n-butylborane could be found. The failure to isolate di-n-butylethylborane can be explained by the fact that the system, as represented by Equations 4 and 5, is i n equilibrium. The relative amounts of products most likely depend on the relative rates of reaction. Material distilling between n-butyldiethylborane and tri-n-butylborane was obtained, but i t appeared to be a mixture and was not identified. Di-n-butylethylborane finally was prepared by add ing an equimolar quantity of ethylmagnesium bromide to di-n-butylchloroborane. N o other product was obtained. I t thus became apparent that, although exchange of In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
CLARK ET AL.
231
Boron-Carbon Ring Compounds
alkyl groups actually was occurring, any desired product could be achieved with the use of the proper ratio of reactants and the proper mode of addition. H a v i n g shown that the synthesis of mixed trialkylborane compounds presented no insurmountable difficulties, the preparation of a^-bis-(di-n-butylboryl)alkanes was attempted. The reactions of tetramethylenedimagnesium dibromide and pentamethylenedimagnesium dibromide with di-n-butylchloroborane were investigated i n an attempt to prepare l,4-bis(di-n-butylboryl)butane ( V ) and l,5-bis(di-n-butylboryl)pentane ( V I ) , respectively. (C4H ) B(CH ) B(C4H ) + 2 M g B r C l V
(6)
B r M g ( C H ) M g B r + 2(C H ) BC1 - ( C H ) B ( C H ) B ( C H ) + 2 M g B r C l VI
(7)
B r M g ( C H ) M g B r + 2(C H ) BC1 Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 12, 2014 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch025
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2
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2
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2
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4
9
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Instead of the desired products, Reaction 6 yielded 1-n-butylboracyclopentane ( V I I ) and Reaction 7, 1-n-butylboracyclohexane ( V I I I ) . Tri-n-butylborane was also iso lated i n both reactions. These cyclic structures represent a new class of boron-con taining compounds, none of which appear to have been reported previously. Subse quent to the writing of this article, the preparations of 1-phenylboracyclopentane and 1-phenylboracyclohexane have been reported (3). CH —CH 2
2
\
CH —CH 2
Β—C H 4
CH,—CH VII
CH
9
2
2
CH —CH VIII 2
2
/
Β—C H 4
9
2
These products can be accounted for most logically b y assuming a disproportiona tion of the l,4-bis(di-n-butylboryl)alkane. CH (C H ) B(CH ) B(C H ) 4
9
2
2
n
4
9
2
2
(CH,)^^
^B—C H 4
9
+ (C H ) B 4
9
3
(8)
One piece of evidence was obtained that supports this hypothesis of disproportiona tion. I n one attempt to prepare l,4-bis(di-n-butylboryl)butane, a liquid was iso lated which molecular weight determinations tentatively identified as the desired product. Redistillation of this liquid yielded tri-n-butylborane and 1-n-butylbora cyclopentane. The formation of di-n-butylchloroborane b y reaction of 1-n-butylboracyclopentane with hydrogen chloride is i n accord with this cyclic structure. Hydrogen chloride removes an alkyl group from trialkylboranes (1). Although two reaction paths are possible with the cyclic compound, only the product of ring cleavage was isolated. CH —CH 2
(C H ) BC1 4
2
CH —CH;
\ /
Β—C H 4
9
+ HC1 -
9
2
CH —CH 2
2
(9) 2
\ B—CI Έ
+ C H 4
1 0
CH,—CH References (1) (2) (3) (4)
Booth, R. E., Kraus, C. Α., J. Am. Chem. Soc. 74, 1415 (1952). Erickson, C., E., Ph.D. thesis, Cornell University, 1953. Torssell, K., Acta Chem. Scand. 8, 1779 (1954). Wiberg, Egon, Sütterlin, Walther, Z. anorg. u. allgem. Chem. 202, 1-21 (1931).
WORK supported by the Bureau of Aeronautics, Department of the Navy. In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
