Saturated and Unsaturated Acyclic Hydrocarbons

BY J. C. SAUNDERS, B. P. SWANN AND D. E. TUPPER

1 Saturated Hydrocarbons

Synthesis. — The radical anion from di-t-butylbiphenyl is found to be superior to lithium naphthalene for the reductive removal of halogen from alkyl chlorides (Table la). Alcohols are reduced directly to the corresponding hydrocarbon by the addition of a silane R3SiH to the alcohol in methylene chloride followed by addition of boron trifluoride gas. The reduction proceeds rapidly (< 10 min) and is generally superior to the procedure which utilizes R3SiH in trifluoroacetic acid. The latter conditions often lead to both extensive decomposition of the silane and dehydration or rearrangement of the alcohol, whereas the R3SiH-BF3, combination enables even tertiary alcohols to be reduced without dehydration, although in some cases, such as octan-2-01, yields are low (ca. 50%); formation of non-volatile byproducts is possibly responsible for the lower yield since octane was the only product detected by g.1.c. An alternative procedure for primary and secondary alcohols involves prior conversion to the tosylate followed by treatment with sodium iodide and zinc powder in refluxing 1,2-dimethoxyethane. Yields are generally good but β-elimination can occur if the tosyl group site is hindered, leading to olefins (Scheme 1).

Aromatic aldehydes and ketones are reduced to the corresponding hydrocarbons in good yield by catalytic transfer reduction using cyclohexene or limonene as a donor, palladium-carbon as catalyst and a Lewis-acid promotor such as ferric chloride. The major competing reaction is decarbonylation, otherwise the reaction is straightforward and simply involves heating the catalyst, carbonyl compound, and donor under reflux for 3 — 5 h, furthermore the method is convenient and dispenses with elaborate equipment or potentially explosive hydrogen.

Alkyl halides react with superacids such as HF-TaF5, HCl-AlCI3, and HBr-AlBr3 initially to give the corresponding alkane via a hydride transfer. Naturally the reaction conditions detract from its synthetic utility. Readily available transition metal complexes such as Ni(acac)2 and Fe(acac)3 can be induced to react in an electrochemical system with alkyl halides to produce coupled hydrocarbon products. Low valency metal complexes are probably intermediates and these undergo reaction with alkyl halides to form ω-bonded alkyl transition metal intermediates, which then decompose by known pathways. The coupled products appear to arise from a free radical pathway, although the disproportionation products, alkane and alkene, may not be formed by this route. In particular organic halides having hydrogens on the carbon atom β- to the halide atom tend to yield alkanes and alkenes in addition to coupled products.

Reactions. — Alkanes can be oxidized by iodine tris(trifluoroacetate) to a mixture of mono and bis(trifluoroacetates). With certain alkanes, particularly those with tertiary carbon centres, the reaction can be of synthetic utility (Scheme 2). This is a further example of the parallel of the chemistry of iodine (III) and lead (IV) since lead tetra(trifluoroacetate) is reported to react similarly. Iodine tris(trifluoromethanesulphonate) is reported to be even more reactive. A similar type of functionalization can be carried out in superacid-S03ClF mixtures by addition of ozone. Mechanistic studies suggest that the active species is protonated ozone, 03H+ which undergoes electrophilic insertion into a σ-bond. Product analysis shows that oxygenation is followed by C -> O alkyl migration, analogous to the acid-catalysed cleavage and rearrangement reaction undergone by hydroperoxides. Direct amination of several acyclic and alicyclic alkanes can be effected with trichloramine-aluminium chloride. In general rearrangement and degradation of the hydrocarbon substrate occurs but isobutane and iso-octane give good yields of t-butylamine. Isoalkanes are brominated by dropwise addition of the isoalkane in sulphur dioxide at -80 °C to a solution of bromine in SbF5 or SbF5-FS03H in sulphur dioxide at -25 °C. Thus isopentane gives EtCBr(Me)CH2Br and isooctane gives a mixture of Me2CHCH2Br and Me2CBrCH2Br by a fragmentation–bromination sequence.

2 Olefinic Hydrocarbons

Synthesis. — Catecholborane offers a number of advantages over other boron hydrides for the reduction of αβ-unsaturated tosylhydrazones (1) to olefins. Only one equivalent of hydride is used and the conditions are mild. No alkane formation is observed. A study of the isomeric pulegone tosylhydrazones (2) and (3) and the role of stereochemistry upon the olefin formation has shown that the stereochemistry plays a critical role (Table lb). There would appear to be two decomposition routes, one of which is strongly stereochemically dependent, whereas the other has little, if any, dependence on tosylhydrazone stereochemistry. Suitable control quenching experiments show that the monoanions of the tosylhydrazones maintain their original stereochemistry. The exact mechanism has yet to be fully clarified. Treatment of ketone tosylhydrazones with a threefold excess of butyl-lithium provides a convenient route to vinyl-lithium reagents. The carbene (4), generated from the tosylhydrazone (5) fragments to give cis-1-allyl-2-ethynylcyclopropane, which readily rearranges to 1,2,5,7-octatetraene (6) in good yield (Scheme 3).

Several new reagents have been reported for dehydrating alcohols to olefins. Among these were the carbodimidium salt (7), methyltriphenoxyphosphonium iodine (8) in HMPT. and the use of dialkylcyanamides followed by a [3,3] sigmatropic rearrangement to ureas (9) and (10). Examples are shown in Scheme 4.

The use of vinyl and alkyl cuprates in organic synthesis shows no signs of abating. Amongst new examples which are useful in olefin synthesis are the cross-coupling of alkyl and arylcopper(1) reagents with (E)-2-iodo-l-alkenyl sulphones to give β-alkylated or β-arylated 2-alkyl-1-alkenyl sulphones, and the use of diphenylphosphate esters in the synthesis of olefins from ketones. The latter procedure involves conversion of the ketone to its enol phosphate via the regiospecifically generated anion, and displacement of the phosphate by a lithium dialkylcuprate, in parallel with work on the corresponding vinyl iodides. The final step involves three equivalents of lithium dialkylcuprate ; yields are...