March 20, 1964
DIISOPINOCAMPHEYLBORANE
[COSTRIBUTIOS FROM ? H E
RICHARD B
WITH
trUnS
AND
HINDERED OLEFINS
WETHERILL 1,ABORATORY OF PURDUE USIVERSITY,
1071
LAFAYETTE, IKD ]
Hydroboration. XIX. The Reaction of Diisopinocampheylborane with Representative frans and Hindered Olefins. Triisopinocampheyldiborane as a Reagent for the Configurational Assignment of Alcohols and Olefins oia Hydroboration BY
HERBERT c. BROWN,NAGARAJ R.A%YY.ANGAR,lA N D GEORGEZWEIFEL RECEIVED OCTOBER 3 , 1963
In contrast t o the fast, highly stereoselective reactions of diisopinocampheylborane with reactive cis-olefins, t h e corresponding reactions of this reagent with trans and hindered olefins are slow, proceeding only with t h e displacement of a-pinene from the reagent, and t h e oxidized products exhibit much lower optical activities13 t o 22(;;, instead of t h e 70 t o 9O(z realized with t h e cis-olefins. Moreover, t h e rotations of t h e products are t h e opposite t o those observed in t h e products obtained from t h e cis-olefins or predicted from t h e model previously proposed. The fact t h a t t h e reaction proceeds only with the displacement of a-pinene indicates t h a t the hydroborating agent is triisopinocampheyldiborane, formed by t h e concurrent dissociation of sym-tetraisopinocampheyldiborane (diisopinocampheylborane dimer). Triisopinocampheyldiborane was prepared and used t o hydroborate some representative hindered olefins. T h e reaction proved t o be relatively fast and yielded results similar t o those realized previously in the slow reaction with diiS~Jpi~l(lCamphe?.Iborane.I n spite of the modest rotations exhibited by the products, t h e observed configurations proved t o be consistent, suggesting t h a t t h e reaction should be useful a s a n empirical method for establishing the configuration of optically active alcohols and olefins realized via application of t h e hydroboration reaction t o ti*ans and hindered olefins. This conclusion was tested by converting 1-methylcyclopentene into optically active ( - )-trans-2-methylcyclopentanol. The predicted configuration of the- product, 1 R : 2 R , was confirmed b y transformation of t h e alcohol into ( +)-3methylcyclopentene of known configuration. Consequently, both diisopinocampheyiborane and especially triisopinocampheyldiborane should be valuable as a means of converting trans and hindered olefins into products of moderate optical activitl- (-13 t o 225;) and for the establishment of their absolute configurations.
T h e hydroboration of cis-olefins with optically active to 23 mmoles of trans-2-butene utilized, and 24.6 diisopinocampheylborane provides a convenient method mmoles of a-pinene a t the completion of the reaction for the synthesis of alcohols in high optical p ~ r i t y . ~ ,(24 ~ hr.), corresponding to 50 mmoles of trans-2Moreover, the reagent permits the partial resolution butene reacted. These results indicate t h a t 1 mole of of racemic olefin^.^,^ Finally, the configurations of the a-pinene is being displaced from the reagent by each optically active alcohols and olefins obtained can be 2 moles of trans-2-butene taken up. Hydrolysis of deduced on the basis of a steric model proposed for the reaction mixture resulted in the liberation of hythe reaction path.3.5 drogen, corresponding roughly to the presence of one Preliminary experiments indicated that the hydroboron-hydrogen bond for each mole of a-pinene disboration of trans and hindered olefins with diisopinoplaced from the reagent. campheylborane proceeds quite slowly, with a-pinene Similarly, the reaction of trans-4-methyl-2-pentene becoming displaced from the reagent as the reaction with diisopinocampheylborane proceeds quite slowly. proceeds. T h e alcohols realized on oxidation of the Oxidation of the reaction product after 24 hr. yielded hydroboration product exhibited only a modest optical a mixture of 4-methyl-2-pentano1, 68%, and 2-methylpurity (13 to 22%). Moreover, their absolute con3-pentanol, 32%. This is in marked contrast to the figurations did not conform to predictions based on the highly selective course of the corresponding reaction model which served so well for the czs-olefins.3.5 of the reagent with czs-4-methyl-2-pentene, where a In view of these observations, a detailed study was 96 to 4 distribution of 4-methyl-2-pentanol and 2undertaken on the reaction of diisopinocampheylmethyl-3-pentanol is realized. borane with representative trans and hindered olefins. Gas chromatographic examination of the reaction In the course of the investigation it became evident mixture revealed the presence here also of a-pinene in that the active hydroborating agent in these reactions an amount corresponding to 1 mole of a-pinene for must be triisopinocampheyldiborane and the study was each 2 moles of the olefin reacted. This suggested the extended to this reagent. possibility that each mole of diisopinocampheylborane might react with 2 moles of trans-4-methyl-2-pentene. Results Accordingly, reaction mixtures were prepared in this ratio, and the uptake of two equivalents of olefin per Reaction of trans-Olefins with Diisopinocampheylbomole of reagent was observed. A modest change in the rane.-The reaction of cis-2-butene with (+)-diisopinorelative quantities of the two isomeric alcohols produced campheylborane a t 0" is essentially complete within 2 in the latter experiments was observed. hr. Oxidation of the product produces (+)-%butanol T h e results of these experiments are summarized In contrast, the reof 86 to 8iyGoptical in Table I . action of trans-2-butene with the reagent under the Reaction of Hindered Olefins with Diisopinocamphesame conditions was only 367, complete in 1 3 hr., y1borane.---hlthough cyclohexene is a cis-olefin, its and was essentially complete only after 24 hr. Oxireaction with dialkylboranes is very slow. For exdation yielded (-)-2-butanol, [ a ] -~1.73', indicating ample, its rate of reaction with disiamylborane is only an optical purity of 13?GIG.'j 1yG that of cyclopentene.',5 Cyclohexene also reacts In the course of isolating the Zbutanol, the presence with diisopinocampheylborane primarily by displaceof a significant quantity of a-pinene was indicated. ment, with 1 mole of a-pinene appearing for each 2 Gas chromatographic analysis revealed the presence moles of cyclohexene reacting. of 11.6 mmoles of a-pinene after 13 hr., as compared The reaction of 2-methyl-%butene with the reagent (1) Postdoctorate research associate, 1961-1963, on grants (G-6273 and is even slower, being only 73Yc complete in 60 hr. a t G 19878) provided by t h e National Science Foundation. 0'. Moreover, in this case the cr-pinene displaced ( 2 ) H. C Brown and G. Zweifel, J . A m . Chem. Soc., 88, 486 ( 1 9 6 1 ) . 13) H . C Brown, S K . Ayyangar, and G . Z>veifel, ibid, 86, 397 (1964) amounts to nearly 1 mole per mole of 2-methyl-2(4) H . C. Brown, h-,I
