An Operationally Simple Hydroboration-Oxidation Experiment George . W. Kabalka, Prakash P. Wadgaonkar, and Narayana Chatla University of Tennessee. Knoxville, TN 37996 T h e hvdrohoration-oxidation seauence provides an efficient ro&e to the a n t i - ~ a r k o v n i k &hydration of alkenes and is one of the most widelv used svnthetic reactions (1-4). T h e beauty of the hydrobbration ieaction as a synthetic method stems from the facts that i t is broadly applicable, generally quantitative, occurs at ambient temperature, and is hiehlv - . rerio- and stereoselective. The hydrohoration-oxidation experiment might thus be expected to enjoy a prominent place in the undergraduate lahoratory curriculum. However, i t is generally not considered to be suitable for the following reasons: (1)The key reagent, diborane, is somewhat difficult to prepare by traditional methods requiring the use of metal hydrides and strong mineral or Lewis acids (e.e.. boron trifluoride etherate) (5-7).(2) The most comi&iy used and commercially available hydroborating reaeents. borane-tetrah\drofuran (RH ,-THF) and borane-di&hyi sulfide (BMS); are difficult to manipulate requiring drv and oxvaen-free conditions. (3) The standard oxidation requires the use of a dangerous 30% hydrogen peroxide solution (68). We have developed a hydroboration-oxidation experiment that can he readily performed in an undergraduate laboratory in a 4-h laboratory period. The experiment utilizes commercially available, stable, inexpensive, safe, and easily handled reagents and standard lahoratory glassware. The reactions involve the use of in situ generated diborane (via reaction (9, 10) of sodium horohydride with iodine) as the hydroborating reagent and sodium perhorate (11,12) as the oxidizing agent. ~
~
~
6RCH=CH2 (RCH,CH,),B
-
~~
+ ZNaBH, + 1% 2(RCH2CH2)3B+ 2NaI + Hz t + 3NaB0, + NaOH + 9H20
-
3RCH,CH,OH
+ 4NaB(OH),
Experimental All reagents are available from Aldrich Chemical Company Inc. and were used without further purification. Apparatus A standard, single-necked 125-mLround-bottom flaskcontaining a magnetic stirring bar is fitted with a Claisen adapter carrying a separatory funnel and a reflux condenser fitted with a gas outlet constructed from a small piece of glass tubing inserted into a thermometer adapter that was connected to a paraffin bubbler (figure). Conversion of Cyclopentene to Cyclopentanol In a dry 125-mLsingle-necked round-bottomflask equipped with a magnetic stirring har, dropping funnel, and reflux condenser (figure) were placed tetrahydrofuran (50 mL) and cyclopentene (3.27 g, 48 mmol).' Finely powdered sodium borohydride (0.76 g, 20 mmol) was added and the flask cooled in an ice bath.z A solution of iodine (2.91 g, 11.46 mmol) in tetrahydrofuran (15 mL) was added dropwise, via the additian funnel to the reaction flask over a period of 0.5 h3 (caution: hydrogen evolved!). After the addition of iodine was complete, the ice bath was removed and the reaction mixture stirred for 1.5 h at room temperature. The mixture was cooled in an iee-water bath, and excess hydride was decomposed by the carefuladdition of water (25mL) (caution: hydrogen evolved!).The
Experimental setup forhydrobaratlon-oxldatlon experiment.
organoborane was oxidized by adding 3 N sodium hydroxide (5 mL), followed by the additian of sodium perborate4(9.24 g, 60 mmol) in one p ~ r t i o n .In ~ 10-15 min a mildly exothermic reaction commenced; the readion mixture was stirred for an additional 0.5 h to ensure completion of the oxidation rea~tion.~." Saturated sodium thiosulfatesolution (5 mL) was added to the reaction flask, followed hv the addition of enhvdrous ootassium carbonate to saturate the nqueow layer. The aqueous and organic phases acre carefully repamred.Theorganichyer wasdiluted ~ i t heranet40mL),driedover h ~~~~
~
~
'
It is not necessary to flush the reactlon fiask with an inert gas (nitrogen or argon) during the hydroboration reaction. The hydrogen gas evolved during the reaction helps maintain an inert atmosphere. An excess of sodium borohydride was utilized in the experiment to compensate for the hydride lost due to the traces of moisture in the experimental setup. Furthermore,the reaction of sodium borohydride with iodine is not quantitative (9). in the initial stages of the addition of Iodine to the suspension of sodium borohydride in tetrahydrofuran, the iodine coior disappears instantaneously. In the final stages the iodine coior persists for a slightly longer time but eventually disappears with time. Sodium perborate is moisture-sensitive and should be stored in a closed bonie in a cool place. It is necessary to cool the flask with ice water during the oxidation reaction to avoid the formation of side products. During the oxidation the iodine color (yellow) sometimes reappears, but it does not interfere with oxidation of the organoborane. 'The oxidation is complete when all the solid sodium perborate dissolves. Volume 67
Number 11
November 1990
975
magnesium sulfate, filtered, and the solvents removed by evaporation at room temoerature usine a rotarv evs~oratorto vield essent i a l l y pure ('.C N.MR) c y c l o p e n t a n c d (3.14 g, W ' o ) . The pnrdurt ran be d i s t i l l e d under reduced pressure ( w a t e r a s p i r a t o r ) t u u h r a i n pure colorless cyclopentand ( 2 . 8 9 g, 70%)'; bp 62 T I 2 8 mm Hg. ~
. .
~
The yield of cyclopentanol from cyclopenteneis good, and the procedure is applicable to a variety of alkenes as illustrated in the table. The advantages of this procedure are the commercial availability, stability, and ease of handling of the reagents utilized, all of which serve to make the sequence a valuable addition to the undergraduate laboratory curriculum. Literature Clted 1. Pelfer. A.; Smith, K.: Broan. H. C. Borarv Reagents. (Best Synthetic Methoda): Academic: Now York, 1988. 2. Brown, H. C.: Kramer, A. W.: Levy,A. B.: Midland, M. M. Orgonie Synfheais uio Bomnes; Wiley-Interadonce: New Yark, 1975. 3. Crsgg, G. M. L. Orgonobomneain Orgonie Synfheais: Dekker: New Yark. 1973.
OThe isolated yleld of cyclopentanol is l o w e r than GLC yield: this is amibuted to p h y s i c a l losses d u r i n g t h e isolation p r o c e d u r e .
976
Journal of Chemical Education
Conversion 01 A I k e n a into A l c o h o l s v l a Aikene
Hvdroboralion-Oxldatlon
Alcohol
Relative
Froducts
Amounts (%)'
Total Yield (%Ib
- .. .. Cyclopentene
(-)-&Pinme
Cyclopenlanol (-)-cisMyttanol
90 91
4. Brown, H. C. Boronea in Orgonie Chamlatry: Cornall University: Ithaea, NY, 1972. 5. Bmwn.H.C.;Murrsy,K. J.;Murrsy,L. J.:Snovor.J.A.:Zweifel,G. J.Am.Chrm.Soc. IQEO ill,d9ll . ...,-.
6. Zweifel, G.: Brown. H. C. Org. Rsoct. 1963.13,1, 7. Fallet, M. Chem.lnd. (London) 1986,123. 8. Brown, H.C.;Snyder.C.:SubbaRso, B.C.:Zweifel.G. Tetmhodmn 1986.42,5505. 9. Fre8euard.G. F.:Lone.L.H. Cham.Ind. ILondonl 1965.471.
