3683
The Stereochemistry of Base-Cat a1yzed /?Elimination from 2-Brom0butane~'~ Richard A. Bartsch Contribution from the Department of Chemistry, Washington State University, Pullman, Washington 99163. Received September 22, 1970
Abstract: The stereochemistry of p elimination from d,I-erythro- and d,I-threo-3-deuterio-2-bromobutane (3 and respectively) induced by a variety of base-solvent combinations has been determined. Both cis- and trans-2butene are formed by exclusive anti elimination with EtOK-EtOH, sec-BuOK-sec-BuOH, tert-BuOK-tert-BuOH, tert-BuOK-DMSO, tert-BuOK-THF, and n-Bu4NF-DMF. Within experimental error, equal primary deuterium isotope effects were observed for the formation of trans-2-butene from 3 and cis-2-butene from 4 in a given base-solvent system. 4,
T
he role of syn- and anti-elimination transition states (1 and 2, respectively) in base-catalyzed /3 eliminations from simple acyclic compounds has recently reB
ondary alkyl halides report anti elimination in reactions of d,l-erythro- and d,l-threo-3-deuterio-2-bromobutane with potassium ethoxide and potassium hydroxide in ethanol. 1 3 , 1 9 Therefore, an investigation of elimination stereochemistry in reactions of d,l-erythroand d,l-threo-3-deuterio-2-bromobutane with a variety of base-solvent combinations was undertaken.
Results Photochemically initiated free-radical addition of deuterium bromide to trans- and cis-2-buteneZ0at - 90" ceived considerable attention 1 * 3 - 1 4 Stereochemical inproduced d,I-erythro- and d,l-threo-3-deuterio-2-bromovestigations of eliminations for secondary alkyl brobutane (3 and 4, respectively). Analysis by mass l 2 trimethylammonium mides, p-toluenesulfonates,6'8,11, of these diastereomeric compounds and salts,3-5,7,9-11,13,14 and dimethylsulfonium ~ a l t s indi~ ~ n ~ spectrometry ~ the butenes resulting from their reactions with pocate that syn-elimination stereochemistry is more comtassium tert-butoxide in dimethyl sulfoxide revealed a mon than previously imagined. Particularly intriguing lower degree of stereospecificity in the free-radical addiis the effect of the reaction solvent upon the relative tion than previously reported.20 The d,I-erythro-3amounts of syn and anti e l i m i n a t i ~ n . ~ - " ~ ~ - ~ * deuterio-2-bromobutane was contaminated with 2 f On the basis of the very high Irans-2-hexene:cis-21 of the threo isomer and 3 f 1 % of undeuterated hexene ratios observed in eliminations from 2-hexyl material. The d,l-threo-3-deuterio-2- bromobutane coniodide, bromide, and chloride induced by potassium tained 7 + 1 % of the erythro compound and 7 f 1 tert-butoxide in dimethyl sulfoxide, a syn-elimination of undeuterated 2-bromobutane. mechanism was proposed. 1 5 $ 1 6 The only stereochemProducts from Base-Catalyzed P-Elimination Reacical studies cf dehydrohalogenation of acyclic sections of d,l-erythro- and d,I,-threo-3-Deuterio-2-bromo( I ) For a preliminary communication of a portion of this work, butane. Reactions of 3 and 4 with a variety of basesee R. A. Bartsch, Tetrahedrou Lett., 297 (1970). solvent combinations were conducted in an apparatus (2) Presented at the 25th Northwest Regional Meeting of the American Chemical Society, Seattle, Wash., June 1970. designed to sweep the evolved butenes from the reaction (3) D. S. Bailey and W. H . Saunders, Jr., Chem. Commun., 1599 solution. 2 1 A methanolic solution of the trapped ( 19 68). butenes was subjected to analytical gas-liquid partition (4) D. S. Bailey, F. C. Montgomery, G. W. Chodak, and W. H. Saunders, Jr., J . Amer. Chem. Soc., 92, 6911 (1970). chromatography (glpc) to determine the relative ( 5 ) D. S. Bailey and W. H. Saunders, Jr., ibid., 92, 6904 (1970). amounts of the olefinic products and to preparative glpc (6) D. H. Froemsdorf, W. Dowd, W. A. Gifford, and S . Meyerson, to separate and collect the trans-2-butene, cis-2-butene, Chem. Commun., 449 (1968). (7) D. H. Froemsdorf, H. R . Pinnick, Jr., and S. Meyerson, ibid., and 1-butene. The relative olefinic proportions ob1600 (1968). served in reactions of d,l-erjjthro- and djl-threo-3-deu(8) D. H. Froemsdorf and M. D. Robbins, J . Amer. Chem. Soc., 89, 1737 (1967). terio-2-bromobutane and 2-bromobutane with a vari(9) M. Pankova, J. Sicher, and J. Zakada, Chem. Commun., 394 of base-solvent combinations are presented in ety (1967). Table I. The deuterium content of the three isomeric (IO) M. Prinkova, J. Zrivada, and J. Sicher, ibid., 1142 (1968). (11) J. Sicher, J. Zavada, and M. Pankova, ibid., 1147 (1968). olefins was determined by mass spectrometry at low (12) J. Zavada, M. Pankova, and J. Sicher, ibid., 1145 (1968). ionizing voltages. Tables I1 and I11 record the ob(13) J. Zavada and J. Sicher, Collect. Czech. Chem. Commun., 30, 1
2
z
z
438 (1965). (14) J. Zavada and J. Sicher, Proc. Chem. Soc., 96 (1963). (15) R. A. Bartsch and J. F. Bunnett, J . Amer. Chem. Soc., 91, 1382 (1969). (16) A syn-anti elimination dichotomy, i.e., formation of trans-olefin
by syn elimination and cis-olefin by anti elimination, has been postulated in eliminations from cycloalkyl bromides promoted by potassium terrbutoxide in fert-butyl alcohol and in benzene.17 (17) J. Zavada, J. KrupiCka, and J. Sicher, Collecr. Czech. Chem. Commun., 33, 1393 (1968).
(18) G. H . Helmkamp and N. Schiiautz, J . Org. Chem., 24, 529 (1959). (19) P. S. Skell, R . G. Allen, and G. H. Helmkamp, J . Amer. Chem. Soc., 82, 410 (1960). (20) P. S. Skell and R. G. Allen, ibid., 81, 5383 (1959). (21) Isomerization of olefinic products with strongly basic potassium
tert-butoxide in dimethyl sulfoxide is negligible under these conditions.15.*2 (22) R . A. Bartsch, J . Org. Chem., 35, 1334 (1970).
Bartsch 1 /3 Elimination from 2-Bromobutane
3684 Table 1. Relative Olefinic Proportions in Base-Catalyzed Elimination from 2-Bromobutane Base-Solvent
Temp, "C
EtOK-EtOH
70
___-______
Total butenes, tram-2-Butene cis-2- Butene
Isomer d,l-erythro
Obsd Corrb Obsd Corrb
d,l-threo sec-BuOK-sec-BuOH
terr-BuOK-tert-BuOH
70
Undeuterated d,l-erythro
70
d,l-threo Undeuterated d,l-erythro
Obsd Corrb Obsd Obsd Comb Obsd Corrb
d,l-threo tert-BuOK-DMSO
30
Undeuteratedc d,l-erytliro
Obsd Corrb Obsd Corrb
d,l-tlireo teit-BuOK-THF
ti-BudNF-DMF
25
Undeuteratedc d,l-erytliro
50
d,l-threo Undeuterated d,l-erythro
30.6 f O , l = 28.8 65.9 f 0 . 6 69.0 58.6 f 0 . 2 19.9 f 0 . 2 18.3 52.4 i 0 . 2 46.9 f 0 . 3 10.1 f 0.2 7.6 33.3 f 0 . 3 35.1 30.7 22.0 f 0 . 2 20.2 56.7 + 0 . 1 59.3 55.0 16.7 f 0 . 2 14.8 56.0 f 0 . 3 51.8 f 0 . 4 34.4 i 0 . 2 32.4 69.9 f 0 . 6 64.5 f 0 . 4
Obsd Corr* Obsd Obsd Corrb Obsd
d,l-threo Undeuteratedd
34.2 35.3 9.2 6.5 20.6 31.4 32.2 8.9 21.4 25.9 27.3 8.7 6.4 20.5 26.0 26.8 6.4 3.9 15.5 23.1 23.8 4.0 14.3 33.2 34.4 8.8 18.7
i 0.2 &
0.3
f 0.1 i 0.1 =t0 . 2
f 0.2 f 0.3 f 0.1 i 0.3
f 0.1 f 0.2 i 0.3 i 0.2 f 0.4 i 0.2 f 0.5
Standard deviation. * Corrected for butenes from diastereomeric contaminant and unlabeled impurity. J . Org. Chem., 35, 1023 (1970).
1-Butene 35.2 i 0 . 1 35.9 24.9 i 0 . 4 24.5 20.8 0.3 48.7 =t 0 . 1 49.5 38.6 f 0 . 2 31.7 i 0 . 2 64.0 i 0 . 2 65.1 58.0 f 0 . 3 58.5 48.8 52.0 i 0 . 3 53.0 36.9 zt 0 . 2 36.8 29.5 60.2 i 0 . 4 61.4 40.0 i 0.1 33.7 i 0 . 4 32.4 & 0 . 4 33.2 21.3 =t 0 . 7 16.7 f 0 . 1
*
R . A. Bartsch ,
Reference 22.
Table 11. Deuterium Content of Olefinic Products from Reactions of d,l-eryrliro-3-Deuterio-2-bromobutane with Several Base-Solvent Systems butenes $ y---..0 cis-2-Butene do 4
__---______-Total Temp, Base-Solvent
rrcirw-2-Butene do di
T
EtOK-EtOH
70
see-BuOK-sec-BuOH
70
tert-BuOK-tert-BuOH
70
tert-BuOK-DMSO
30
teit-BuOK-THF
25
u-Bu~NF-DMF
50
Obsd C a l c d )* Obsd Calcd'fc Obsd Calcd,d Obsd Calcd , e Obsd Calcd'J Obsd CalcdQ9
28.9 28.9 18.7 18.7 9.6 9.6 20.8 20.8 15.6 15.6 32.4 32.4
1.7 1.7 1.2 1.2 0.5 0.5 I .2 1.2 1.1 1.1 2.0 2.0
Calculated for contaminating 2.8% of 2-bromobutaneandthe threoisomer. isomer. e 2.0% threo isomer. f 2.0% threo isomer. 2.8% threo isomer.
b
0.6 0.8 0.6 0.8 0.3 0 7 0.5 0.5 0.6 0.5 0.8 0.8 2.5% threo isomer.
I-Butene
33.6 33.4 30.8 30.6 25.6 25.2 24.5 24.5 22.5 22.6 32.4 32.4 c
do
di
2.2 0.6 3.3 0.9 3.3 1.4 1.9 0.8 3.0 1 .o 1.4 0.5
33.0 34.6 45.4 47.8 60.7 62.6 50. I 51.2 57.2 59.2 31 .O 31.9
2.2% threo isomer.
1.5% threo
Table 111. Deuterium Content of Olefinic Products from Reactions of d,l-rlireo-3-Deuterio-2-bromobutane with Several Base-Solvent Systems
Base-Solvent
Temp, "C
EtOK-EtOH
70
tert-BuOK-tert-BuOH
70
tert-BuOK-DMSO
30
Calculated for contaminating 7.1 erythro isomer.
tram-2-Butene dQ di Obsd Calcd,& Obsd Calcdutc Obsd Ca1cda.d
4.0 6.5 2.4 3.2 2.5 5.0
54.2 51.7 30.7 30.2 51.0 53.5
of 2-bromobutane and the erythro isomer.
served deuterium content of the three butene isomers for from and 49 as as culated values which include contributions from conJournal of the American Chemical Society
Total butenes, %---------------cis- 2-Butene do dl 9.9 9.9 7.0 7.0 4.9 4.9
2.8 2.8 1.7 1.7 2.1 2.1
* 8.0% erythro isomer.
1-Butene dU
[I,
2.2 1.5 5.6 3.5 1.7 1.7
26.9 27.6 52.4 54.5 35.3 35.3
6.9% erythro isomer.
7.37
