Balakumar, R

Narasimhan, S

Swarnalakshmi, S

NOPR

Indian Journal of ChemistryVol. 37B, November 1998, pp. 1189-1190A simple procedure for hydroborationusing tetrabutylammonium borohydrideS Narasimharr", S Swarnalakshmi"& R Balakumar'Centre for Agrochemical Research, SPIC Science Foundation,Madras 600 032'Departrnent of Chemistry, Guru Nanak College,Madras 600 032Received 10 August 1998; accepted 28 September 1998Tetrabutylammonium borohydride hydroboratesunsaturated systems in refluxing chloroform. Witheneynes, selectivity is observed for terminal unsaturation.However, selectivity is not observed with dienes.Hydroboration of alkenes and alkynes cangenerally be achieved by using electrophilic boranereagents like BMS I, Borane- THFI etc. which needextra care during their handling. Metalborohydrides have been shown to exhibit apparentelectrophilicity in the presence of esters byTandem reduction-hydroboration process. Thuslithium borohydride/ and calcium borohydride'were reported to effect hydroboration in presenceof esters. However, a solution of zinc borohydridein tetrahydrofuran was found to hydro borateunsaturated systems even in the absence of esters".Herein we report hydroboration observed withtetrabutylammonium borohydride which is a solidreagent and hence can be conveniently handled. Itcan be prepared by the metathesis reaction oftetrabutylammonium salt with sodium borohydrideand stored without the loss of hydride undernitrogen atmosphere for a fairly long period atroom temperature'. However, we found thattetrabutylammonium borohydride was found toliberate diborane in chloro solvents like CH2Ch,CHCh, CCl4 etc. as evidenced through 11BNMRNotewhich showed a multiplet centered around -26.4ppm corresponding to B2H7- (ref. 6). Thisprompted us to carry out hydroboration studieswith this reagent in chloro solvents.As there are a number of boron reagentsavailable for hydroboration, for any new reagent,the important features to be considered inhydroboration reactions are stoichiometry andregio selectivity. This study would ensure theutility of the reagent for selective hydroboration.The stoichiometry was followed by performing thereaction at different olefin: borohydride ratio andstudying the reaction mixture after subjecting tooxidation with alkaline H202 using GC (Table I).The results indicated that tetrabutylammoniumborohydride hydroborates only two equivalents ofterminal olefins leading to the formation of dialkylboron species. Hence taking 2 equivalents ofalkene per borohydride a number of terminalalkenes have been converted to alcohols in goodyields (Table II). Similarly internal and cyclicalkenes were also converted to the correspondingalcohols.Terminal alkynes undergo dihydroboration withtetrabutylammonium borohydride resulting in theformation of a dibora adduct. Oxidative hydrolysisof dibora adduct with alkahne H202 yielded 80%of the corresponding terminal alcohol (Table II).The formation of dibora adduct could not beprevented even by using an excess of alkyne.Internal alkynes generally form vinyl boraneswhich on oxidation yield ketones as the majorproduct. Thus, 3-hexyne yielded 3-hexanone.CH3CH2C=CCH2CH3~CH3CH2CO(CH2)2CH3However, diphenyl acetylene yielded 1,2-diphenylethanol as the major product indicatingthat dihydroboration had occurred. The diboraTable I-Stoichiometry of hydroboration of alkeneOlefin" BH4- Olefin remaining Primary alcohol Secondary alcohol Olefin reacted!inmmole in mmole in mmole in mmole in mmole BH4- in mmole5 1.25 2.25 . 2.25 0.5 2.25 2.5 0.75 3.7 0.55 1.77.5 2.5 2.1 4.8 0.6 2.1*Dodecene and decene were the olefins used.1190 INDIAN J CHEM. SEe. B, NOVEMBER 1998Table II-Hydroboration studies with alkenes, alkynes andeneynesSubstratel-Decenel-Dodecene5-DeceneEthylidene cyclohexenea-PineneI3-PineneCyclohexeneMethyl-l C-unde-cenoatel-Octyne1-Hexadecyne3-HexyneDiphenylacetyleneMethyl-l O-unde-cynoatel-Octene-s-ynel-Nonene-s-ynel-Hexadecyne-l l-eneProduct Yield(%)"Alkenesl-Decanol 90l-Dodecanol 915-Dodecanol 922-cyclohexyl ethanol 853-Pinanol 75Myrtanol 75Cyclohexanol 80l l-Hydroxy methyl-undecenoate 90Alkynesl-Octanol 87l-Hexadecanol 903-Hexanone 90I,2-Diphenylethanol 83l l-Hydroxy rnethyl-undecanoate 75Ene-ynes4-0ctyne-I-o I 854-Nonyne-l-ol 86l l-Hexadecene-I-ol 82[Substrate] : [H] = 5 : 10,"lsolated yield per mmole of the substrate reacted.adduct formation may be due to the better planarityand increased electrophilicity experienced by thealkyne during the course of the reaction. A similarobservation was made in the reaction with silicagel supported zinc borohydride'.C6H5C=CC6H5~C6H5CH(OH)CH2C6H5To study the regioselectivity of the reagent,hydroboration of dienes and eneynes wereperformed. There was no selectivity observed withI,II-dienes which yielded the primary and thesecondary alcohol in the ratio of 1: 1.5.However, with eneynes we observed a peculiarselectivity during hydroboration. Thus, l-octene-4-yne and l-nonene-4-yne yielded terminal alcocholas the major product showing the selectivity of thereagent towards terminal double bond (Table II).H2C=CHCH2C=C(CH2)nCH3~HO(CH2)3C=C(CH2)nCH3where n = 2,3But, in the case of l-hexadecyne-t-ene, only thealkyne group has reacted indicating that thereagent has terminal selectivity.HC=C(CH2)8CH=CH(CH2)3CH3~HO(CH2)IOCH=CH(CH2)3CH3This result was utilized in the synthesis of Z-II-aiken-I-ol which are pheromone components ofmany insect species.Experimental SectionTypical procedure for hydroboration. To tetra-butylammonium borohydride (lOH-) (2.5 mmole)in chloroform was added substrate (5 mmole) andthe mixture refluxed for 5 hr. The excess hydridewas quenched with dil. H2S04• The reactionmixture was made alkaline by adding NaOH andthe organoboron intermediate was oxidized with30% H202 at room temp. The organic layer wasseparated from the aq. layer, dried over anhyd.Na2S04 and the solvent removed at diminishedpressure. Yields are given in the Table II.AcknowledgementThe authors thank Prof T R Govindachari for hissupport and encouragement. SL thanks Departmentof Chemistry, GuruNanak College for theirsupport.•tReferencesI Andrew Pelter, Keith Smith & Brown H C, Boranereagents (Academic Press, New York), 1988.2 Brown H C & Narasimhan S, Organornetallics, I, 1982,763.3 Brown H C, Somayaji V & Narasimhan S, J Org Chern, 49,1984,4822.4 Narasimhan S, Madhavan S & Balakumar R, Unusualreactivity of zinc borhohydride-Facile hydroboration ofalkenes and alkynes, Unpublished results.5 Brandstorm S, Junggren V & Lanam B, Tetrahedron Leu,1972,3173.6 Hertz R K, Johnson H D & Shore S G, lnorg Chern, 8,1973, 12.7 Ranu Brindaban C, Sarkar Arunkanti, Saha Manika &Chakraborty Rupak, Tetrahedron, 50,1994,6579.

A simple procedure for hydroboration using tetrabutylammonium borohydride

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A simple procedure for hydroboration using tetrabutylammonium borohydride

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