Saturated and Unsaturated Acyclic Hydrocarbons

BY D. C. HORWELL

1 Saturated Hydrocarbons

Metallocarbenes have been implicated in the iridium-catalysed isomerization of branched hydrocarbons, such as that of 2-methylpentane (1) to 3-methylpentane (3). Studies with 13C-labelled (1) support a mechanism which proceeds via (2) as intermediate. Polymer-bound triphenylphosphine–lithium diorganocuprates may offer advantages in the Wurtz-type coupling of alkyl halides, in that work-up is easier and the product is not contaminated with residual tertiary phosphine. However, yields in general are comparable with those from the corresponding homogeneous reagents.

The air-stable, water soluble ruthenium(n) hydride, [(η6-C6Me6)Ru(µ- H)2 (µ-Cl)Ru(η6-C6Me6)]Cl, is extremely effective in the hydrogenation of double bonds and aromatic systems. Thus styrene is reduced to ethylcyclohexane in quantitative yield at 50°C under 50 atm pressure during 36 h. Cobalt(II) salts and sodium borohydride together appear to be a promising reagent for the selective reduction of olefins. For example, the reagent is able selectively to reduce the terminal double bond of limonene in 79% yield, with no reduction of the trisubstituted double bond.

Di-iododimethylsilane appears to be an effective reagent for the mild deoxygenation of α-arylalkanols to the corresponding hydrocarbon. Aliphatic alcohol methanesulphonates are selectively reduced in good yield by an electrochemical method. The reaction is performed in a divided cell with a lead cathode and a platinum anode in dry DMF containing tetraethylammonium toluene-p-sulphonate. Yields are in the range 57 — 87%, and groups such as esters, olefins, nitriles, and even epoxides are inert under these conditions.

Kabalka and Chandler now report improved yields (83 — 98%) in the deoxygenation reaction of aldehydes and ketones, on treatment of their corresponding tosylhydrazones with catecholborane in the presence of tetrabutylammonium acetate as the base.

2 Olefinic Hydrocarbons

More evidence has appeared showing that the olefin metathesis reaction can tolerate the presence of functional groups. The catalytic system Re2O7- Al2O3, promoted by a small amount of tetramethyltin, effects metathesis of olefins in fair yield (17 — 40%) in the presence of unsaturated ethers and ketones, alkenyl esters, and halogeno-alkenes. The reaction is performed in carbon tetrachloride as solvent at room temperature over 6 h. Electro-reduction of tungsten hexa-chloride with an aluminium anode in halogenated solvents appears to form a complex suitable for a clean metathesis, exemplified by the conversion of pent-2-ene into its equilibrium mixture with but-2-ene and hex-3-ene.

A direct combination of acetylenes and alkanes in a novel pericyclic reaction to generate olefins has been reported. However, yields are low (0.2 — 20%) and vigorous conditions of temperature (350 — 400°C) and pressure (350 — 500 bar) are required (Scheme 1).

Two new procedures which bring about the isomerization of double bonds have been described. The readily synthesized secondary allylic ethers of 2-hydroxy-benzothiazole (4) react with functionalized organocuprates regioselectively in high yield to give the olefins (6). An E : Z ratio of 98:2 may be achieved when the reaction is performed at –78°C; the reaction probably proceeds via an intermediate such as (5) and has been particularly useful in the synthesis of E-monoene alcohol systems found in insect sex attractants, as exemplified in Scheme 2. Allylic acetates and phenyl ethers are readily converted into the corresponding terminal olefin in high yield, on reaction with ammonium formate in the presence of palladium catalysts. Thus geranyl acetate (7) is converted into dihydromyrcene (8) and the corresponding 2-olefin in a ratio of 94:6 in almost quantitative yield. 12

The regio- and stereo-selective alkylation, alkenylation, and arylation of olefins via metallated species continues to attract attention. This methodology enables the direct cross-coupling of olefins to give a wide variety of derivatives. The monoalkyl-olefin (9) may be alkylated in one step to the corresponding 1,1-dialkyl-olefin (10), on reaction with a two-fold molar excess of trialkylaluminium mixed with bis(cyclopentadienyl)titanium dichloride in methylene chloride as solvent. The reaction appears to be sensitive to steric effects, as larger alkylaluminium reagents give low yields. Trialkylboranes may now be transformed into three moles of the corresponding Grignard reagent on treatment with pentane-1,5-bis(magnesium bromide) in toluene. This technique has been applied to the direct stereospecific alkylation of terminal olefins with the so formed Grignard reagents of vinyl halides, in the presence of palladium catalysts (Scheme 3). Murahashi and his co-workers have provided further illustrations of the superiority of palladium-catalysed cross-coupling of alkenyl halides with organo-lithium reagents over other metal catalysts, in terms of yields and stereoselectivity. However, the economic factors should not be overlooked in comparing the use of this expensive metal with the other techniques that are available. Grignard reagents may directly replace the alkoxy-group of enol ethers (11) to give the corresponding alkylated olefins (12), in the presence of bis(triphenylphosphine)nickel chloride. Yields are good and the reaction generally proceeds with retention of configuration.

Alkenylboranes are readily obtained by monohydroboration of acetylenes. Palladium catalysts in the presence of a base, such as sodium ethoxide, effect the coupling of the alkenylboranes with aryl halides to give arylated E-alkenes in good yield. The reaction proceeds with retention of configuration with respect to the alkenylborane. A general procedure for the preparation of the useful E-2-methyl-1-alkenyliodides has been described. These compounds are versatile precursors of trisubstituted olefins. The procedure involves the addition of trimethylaluminium to acetylenes in the presence of organo-zirconium reagents, followed by iodination. E-1-Chloro-1-alkenes and mixed 1,1-dihalogeno-1-alkenes are also readily prepared from 1-chloroacetylenes on reaction with lithium aluminium hydride followed by addition of the appropriate halogen.

Conditions have been described whereby both the alkenyl groups of a homocuprate reagent (13) can be utilized in the...