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A Simple Procedure for Hydroboration Using Tetrabutylammonium

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A Simple Procedure for Hydroboration Using Tetrabutylammonium Indian Journal of Chemistry Vol. 37B, November 1998, pp. 1189-1190 Note A simple procedure for hydroboration which showed a multiplet centered around -26.4 using tetrabutylammonium borohydride ppm corresponding to B2H7- (ref. 6). This prompted us to carry out hydroboration studies S Narasimharr", S Swarnalakshmi" & R Balakumar with this reagent in chloro solvents. As there are a number of boron reagents 'Centre for Agrochemical Research, SPIC Science Foundation, Madras 600 032 available for hydroboration, for any new reagent, 'Departrnent of Chemistry, Guru Nanak College, the important features to be considered in Madras 600 032 hydroboration reactions are stoichiometry and Received 10 August 1998; accepted 28 September 1998 regio selectivity. This study would ensure the utility of the reagent for selective hydroboration. Tetrabutylammonium borohydride hydroborates The stoichiometry was followed by performing the unsaturated systems in refluxing chloroform. With reaction at different olefin: borohydride ratio and eneynes, selectivity is observed for terminal unsaturation. studying the reaction mixture after subjecting to However, selectivity is not observed with dienes. oxidation with alkaline H202 using GC (Table I). The results indicated that tetrabutylammonium Hydroboration of alkenes and alkynes can borohydride hydroborates only two equivalents of generally be achieved by using electrophilic borane terminal olefins leading to the formation of dialkyl reagents like BMS I, Borane- THFI etc. which need boron species. Hence taking 2 equivalents of extra care during their handling. Metal alkene per borohydride a number of terminal borohydrides have been shown to exhibit apparent alkenes have been converted to alcohols in good electrophilicity in the presence of esters by yields (Table II). Similarly internal and cyclic Tandem reduction-hydroboration process. Thus alkenes were also converted to the corresponding lithium borohydride/ and calcium borohydride' alcohols. were reported to effect hydroboration in presence Terminal alkynes undergo dihydroboration with of esters. However, a solution of zinc borohydride tetrabutylammonium borohydride resulting in the in tetrahydrofuran was found to hydro borate formation of a dibora adduct. Oxidative hydrolysis unsaturated systems even in the absence of esters". of dibora adduct with alkahne H202 yielded 80% Herein we report hydroboration observed with of the corresponding terminal alcohol (Table II). tetrabutylammonium borohydride which is a solid The formation of dibora adduct could not be reagent and hence can be conveniently handled. It prevented even by using an excess of alkyne. can be prepared by the metathesis reaction of Internal alkynes generally form vinyl boranes tetrabutylammonium salt with sodium borohydride which on oxidation yield ketones as the major and stored without the loss of hydride under product. Thus, 3-hexyne yielded 3-hexanone. nitrogen atmosphere for a fairly long period at room temperature'. However, we found that CH3CH2C=CCH2CH3~CH3CH2CO(CH2)2CH3 tetrabutylammonium borohydride was found to However, diphenyl acetylene yielded 1,2- liberate diborane in chloro solvents like CH2Ch, diphenylethanol as the major product indicating CHCh, CCl4 etc. as evidenced through 11BNMR that dihydroboration had occurred. The dibora Table I-Stoichiometry of hydroboration of alkene Olefin" BH4- Olefin remaining Primary alcohol Secondary alcohol Olefin reacted! inmmole in mmole in mmole in mmole in mmole BH4- in mmole 5 1.25 2.25 . 2.25 0.5 2.2 5 2.5 0.75 3.7 0.55 1.7 7.5 2.5 2.1 4.8 0.6 2.1 *Dodecene and decene were the olefins used. 1190 INDIAN J CHEM. SEe. B, NOVEMBER 1998 Table II-Hydroboration studies with alkenes, alkynes and H2C=CHCH2C=C(CH2)nCH3~ eneynes HO(CH2)3C=C(CH2)nCH3 Substrate Product Yield where n = 2,3 (%)" But, in the case of l-hexadecyne-t-ene, only the Alkenes alkyne group has reacted indicating that the l-Decene l-Decanol 90 reagent has terminal selectivity. l-Dodecene l-Dodecanol 91 HC=C(CH2)8CH=CH(CH2)3CH3~ 5-Decene 5-Dodecanol 92 Ethylidene cyclohexene 2-cyclohexyl ethanol 85 HO(CH2)IOCH=CH(CH2)3CH3 a-Pinene 3-Pinanol 75 This result was utilized in the synthesis of Z-II- I3-Pinene Myrtanol 75 aiken-I-ol which are pheromone components of Cyclohexene Cyclohexanol 80 many insect species. Methyl-l C-unde- ll-Hydroxy methyl- cenoate undecenoate 90 Experimental Section Alkynes Typical procedure for hydroboration. To tetra- l-Octyne l-Octanol 87 butylammonium borohydride (lOH-) (2.5 mmole) 1-Hexadecyne l-Hexadecanol 90 in chloroform was added substrate (5 mmole) and 3-Hexyne 3-Hexanone 90 the mixture refluxed for hr. The excess hydride Diphenylacetylene I,2-Diphenylethanol 83 5 Methyl-l O-unde- l l-Hydroxy rnethyl- was quenched with dil. H2S04• The reaction cynoate undecanoate 75 mixture was made alkaline by adding NaOH and Ene-ynes the organoboron intermediate was oxidized with 30% H202 at room temp. The organic layer was l-Octene-s-yne 4-0ctyne-I-o I 85 l-Nonene-s-yne 4-Nonyne-l-ol 86 separated from the aq. layer, dried over anhyd. l-Hexadecyne-l l-ene l l-Hexadecene-I-ol 82 Na2S04 and the solvent removed at diminished pressure. Yields are given in the Table II. [Substrate] : [H] = 5 : 10, "lsolated yield per mmole of the substrate reacted. Acknowledgement The authors thank Prof T R Govindachari for his support and encouragement. SL thanks Department adduct formation may be due to the better planarity of Chemistry, GuruNanak College for their and increased electrophilicity experienced by the support. alkyne during the course of the reaction. A similar •t observation was made in the reaction with silica References gel supported zinc borohydride'. I Andrew Pelter, Keith Smith & Brown H C, Borane reagents (Academic Press, New York), 1988. C6H5C=CC6H5~C6H5CH(OH)CH2C6H5 2 Brown H C & Narasimhan S, Organornetallics, I, 1982, To study the regioselectivity of the reagent, 763. hydroboration of dienes and eneynes were 3 Brown H C, Somayaji V & Narasimhan S, J Org Chern, 49, 1984,4822. performed. There was no selectivity observed with 4 Narasimhan S, Madhavan S & Balakumar R, Unusual I,II-dienes which yielded the primary and the reactivity of zinc borhohydride-Facile hydroboration of secondary alcohol in the ratio of 1: 1.5. alkenes and alkynes, Unpublished results. However, with eneynes we observed a peculiar 5 Brandstorm S, Junggren V & Lanam B, Tetrahedron Leu, selectivity during hydroboration. Thus, l-octene-4- 1972,3173. 6 Hertz R K, Johnson H D & Shore S G, lnorg Chern, 8, yne and l-nonene-4-yne yielded terminal alcochol 1973, 12. as the major product showing the selectivity of the 7 Ranu Brindaban C, Sarkar Arunkanti, Saha Manika & reagent towards terminal double bond (Table II). Chakraborty Rupak, Tetrahedron, 50,1994,6579..
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