5. H. G o h Univers~tiMalaya Kuala Lumpur, Malays10
Dihalocarbene Insertion Experiment
One of t h e most fascinating reactions in organic chemist~ is the insertion reaction. the most familiar e x a m ~ l eof which is provided by the insertion of carbenes into carbon-hvdroeen . " bonds. T h e insertion of methvlene ( C H d into isopentane is fairly indiscriminate (relative reactivity of 3":2":1° C-H honds being 1.4:1.2:1) showing t h a t free methylene is one of the most reactive intermediates in organic chemistry. Although methylene may be extremely reactive other carhenes generally show a lowered reactivit y due t o the stabilization afforded by suhstituent groups. Thus the reduced reactivity of dichlorocarhene is reflected in t h e high selectivity of its insertion reaction^?.^ Only the more reactive C-H bonds, e.g. tertiary and a-C-H bonds of ethers, undergo insertion. Primary C-H honds of hydrocarbons do not react; secondary C-H bonds react with difficulty h u t do not react detectably if tertiary C-H honds are present. Intermolecular carhene insertions are usually of higher energy a s compared t o addition or dimerization reactions and hence are not eenerallv useful in organic synthesis. On the other hand intramolecular insertions mav he of use in the svnthesis of strained rinn svstems,3 eyg., syntheses of m&hyltricyclo (4.1.0.02,7)'heptanes. The insertion into the tertiary C-H bonds is favored 9:l over t h a t of the secondary C-H bond in this reaction.
agent the carbene when formed remains in the aqueous phase and reacts with hydroxide ions almost exclusively. T h e reaction can easily he adapted for undergraduate students t o illustrate dihalocarbene insertions into diisopmpyl ether. In fact this experiment provides t h e simplest demonstration of a carhene insertion reaction. Experimental
Reagents Chloroform and diisopropyl ether were purified by passing through 10 g of activated alumina before use. Other chemicals needed are sodium hydroxide, tri-tert-butylphenol as antioxidant, and a tetraalkylammonium salt (any of the readily available chloride or bromide salts is suitable, e.g. benzyltrimethylammonium, benzyltriethylammonium, cetyltrimethylammonium, and tetrabutylammanium). Procedure The experiment should be conducted in the fume hood as carbon monoxide is farmed as a side product. In a flat-bottom flask place sodium hydroxide pellets (40 g, 1 mole) and 40 ml water; swirl to dissolve while cooling with running water. Add diisopropyl ether (20.4 g, 0.2 male), chloroform (24 g, 0.2 mole), tetraalkylammonium salt (0.5 g), and tri-tert-butylphenol (0.1 g). Introduce a stirrer magnet, attach an efficient condenser, and reflux with stirring for 6 hr. Allow to cool, slowly add 40 ml water and 30 ml petroleum ether (bp 40-60'C) and then transfer to a separatory funnel. Separate off the lower aqueous layer, dry the organic layer with 5 g of sodium hydroxide pellets for 15 min, pass the crude mixture through a dry column of activated alumina (15 g), and elute with a further 30 ml of petroleum ether. Distil off the solvents over a water bath and finally distil the product under reduced pressure: 8-12 g (20-30% yield), bp 82-84"C/30 mm, nmr (CClr) 6 1.13 (6 H, d, J 6 Hz), 1.36 (6 H, s), 3.87 (1 H, septet, J 6 Hz), and5.49 (1 H, s). The insertion of dibromoearhene into diisopmpyl ether may be studied similarly using 25.3 g (0.1 male) of bromoform in the above procedure (10 hr reflua) to yield 3-5 g (11-18%) of product, hp 110-114"C/30 mm, nmr (CCM 6 1.15 (6 H, d, J 6 Hz), 1.44 (6 H, s), 3.86 (1 H, septet, J 6 Hz) and 5.52 (1 H, s). Cumene is also a suitable substrate, the insertion of dichlorocarbene into the benzylic C-H band gives 25% yield of product, bp 140-142'C/30 mm, nmr (CCU 6 1.54 (6 H, s), 5.84 (1 H, s), and 7.23 (5 H, s). Acknowledgment
Recently4 the use of phase transfer catalysts has enabled dihalocarhenes t o be conveniently generated and trapped by olefins in aqueous sodium hydroxide-organic solvent mixtures. By use of this method it is also possible to effect carbene insertion reactions into tertiary C-H bonds and a-C-H honds of ethers. Tetraalkylammonium salts used a s phase transfer catalysts can dissolve in both water and organic solvents. Trichloromethyl anion generated in this heteroeenwus medium is readilv taken into the organic phase and its subsequent decomposition to dicblorocarhene allows the carhene t o he trapped in the .same phase; whereas in the absence of a phase transfer
T h e author thanks T h e Chemical Society (London) for a grant of chemicals and Universiti Malaya for support. 'Kirmse, W., (Editor), "Carbene Chemistry," Academic Press, New York, ZndEd., 1971, Chap. 7. %eyferth, D., and Cheng, Y. M., J Amer. Chem. Sot., 95, 6763 1147RI \L".",.
3Moore, W. R. and King, B. J., J Org. Chem., 36, 1877, 1882 (1971); Paquette, L. A., Wilson, S. E., Hanzel, R. P., and Allen, G.R., J. Amer. Chem. Soc., 94,7761 (1912). 4Goh, S. H., J . CHEM. EDUC., 50, 678 (1973); Dehmlow, E. V.,Angew. Chem. Internat. Ed., 13,170(1974).
Volume 52, Number 6. June 1975 / 399
