Acetylenes, Alkanes, Allenes, and Olefins

BY D. W. DUNWELL, J. C. SAUNDERS, AND B. P. SWANN

1 Acetylenes

Synthesis of Acetylenes. — Full experimental details have been given for the preparation of alkynes and dialkynes by the reaction of mono- and di-halogenoalkanes with lithium acetylide-ethylenediamine complex. Dimethyl sulphoxide (DMSO) is the preferred solvent for this reaction. Alkylcopper compounds RCu react with copper acetylides to give disubstituted acetylenes, the products of unsymmetrical coupling.

Lithium ethynyl borates, which can be readily prepared from trialkylboranes and lithium acetylide-ethylenediamine complex, are decomposed by iodine to produce the terminal alkylacetylene in high yield. This represents an extension of the previously reported synthesis of internal acetylenes by an analogous procedure (Scheme 1). A similar type of transformation can also be carried out using methanesulphinyl chloride in place of iodine.

Only the strong base combination of sodamide and sodium t-butoxide was found to be capable of producing the required syn-elimination of hydrogen bromide from 1-bromocycloalkenes to give the corresponding cyclo-alkynes.

An ingenious new synthesis of acetylenes starts with bis(tri-n-butylstannyl)-ethylene. Treatment with one equivalent of butyl-lithium converts this compound into the corresponding monolithium derivative, which can then be alkylated and the tri-n-butylstannyl group cleaved with lead tetra-acetate in acetonitrile to give the acetylene (Scheme 2). An advantage in the use of the lithium intermediate is that addition of this compound to a copper acetylide gives an R2CuLi species, the tri-n-butylstannylethenyl group of which can be transferred to an αβ-unsaturated ketone. Oxidation of the resulting vinylstannane derivative with lead tetra-acetate gives the terminal acetylene, and the overall process constitutes a method for the preparation of ethynyl ketones [e.g. (1)] which are not accessible by conjugate addition using dialkynylcopper-lithium species.

Two convenient syntheses of t-butylacetylene from pinacolone have been reported, which involve the use of either phosphorus pentachloride-potassium t-butoxide in DMSO or trifluoromethanesulphonic anhydride in carbon tetrachloride containing pyridine.

Potassium fluoride and tetraethylammonium fluoride are known to be good agents for the promotion of dehydrohalogenation reactions. Good yields of acetylenes are obtained from vinyl halides when the anti configuration is present. The syn-compounds generally give good yields of the corresponding allenes. The reactions are catalysed by crown ethers.

A simple preparative procedure for acetylenic carboxylic acids by alkylation of the lithium salts of acetylenes by ω-bromoalkanoic acids in hexamethyl-phosphortriamide has been described.

The novel vinylcuprate reagent (2) has been found to be a useful three-carbon fragment. A survey of its reactivity with organic halides shows that the reagent is very specific for propargyl and allyl halides. Benzyl bromide is not attacked.

The reaction of 1-bromoalk-1-ynes with two equivalents of butyl-lithium gives 3-butylalk-1-ynes (3) after prolonged treatment. The intermediate is a dilithioalkyne, which can also be prepared directly from terminal alkynes, and which undergoes alkylation at the 3-position.

αβ-Unsaturated ketones react with chlorovinyltriphenylphosphorane to give the corresponding vinyl chloride. Treatment of these compounds with butyl-lithium gives an unsaturated carbene, which rearranges to give the corresponding alkenylacetylene. The reaction of dimethyl phosphonodiazomethane with aldehydes has been extended to include sugars and sugar aldehydes, which are converted into acetylenes containing one more carbon atom.

Trimethylsilylacetylenes are converted into αβ-acetylenic amides on treatment with dialkylcarbamoyl chlorides and aluminium chloride. Use of Castro–Stephens conditions for the coupling of m-iodophenylacetylene-copper(I) salt in pyridine gives a 4.6% yield of the cyclic hexamer.

Cyclo-octatetraene oxide can be converted into enynes by the sequence shown in Scheme 3.

3,3,6,6-Tetramethyl-1-thia-4-cycloheptyne (4), the first isolable cycloheptyne, has been synthesized by lead tetra-acetate oxidation of the corresponding dihydrazone. The large deformation of the C — C [equivalent to] C — C angles causes the 13C [equivalent to] C resonances to shift to lower field, and the reactivity of (4) in addition processes is enhanced. Thus (4) will react with phenyl azide, nitrones, carbon disulphide, and dichloroketen to give the products (5a), (5b), (6a), and (6b), respectively (Scheme 4).

The first synthesis of cyclononyn-5-one involves the fragmentation of the tosylhydrazone (7) with base.

Use of Acetylenes in Synthesis. — Interest in the use of organoborane derivatives of acetylenes as synthetic reagents has continued unabated. The alkynyl-trialkylborates (8), which are prepared by the addition of alkynyl-lithium salts to trialkylboranes, are particularly useful. Protonolysis by acetic acid gives olefins in good yield, and use of the thexylborane derivatives enables good stereospecificity to be maintained. The reaction of (8) with acetyl chloride followed by heating gives the 2-oxa-3-borolens (9), which can be oxidized by the Jones reagent to give αβ-unsaturated ketones. This method has proved to be particularly useful for the preparation of hindered ketones. Similarly, the reaction of chloroalkyl alkyl ethers with (8) gives a mixture of isomers (10) and (11), depending on the solvent and the structure of the borane. Addition of water to the mixture results in selective hydrolysis of (10) to (12). Hydrolysis of the residue with acetic acid converts (11) into (13), and the overall transformation constitutes a stereospecific synthesis of allyl alkyl ethers.

The reaction of (8) with oxirans gives the six-membered cyclic compounds (14). These intermediates can be cleaved with hydrogen peroxide to give γ-hydroxy-ketones (15); protonolysis by acetic acid gives trisubstituted alkenes of the homoallylic alcohol (16) stereospecifically; and the tetrasubstituted derivatives (17) can be obtained by use of a sodium hydroxide–iodine work-up procedure. These transformations are summarized in Scheme 5.

In a similar series of reactions, dialkylboranes can be converted into alcohols and into amines (Scheme 6).

Thexylborane reacts with two equivalents of 1-iodo-alk-1-ynes to give (18). Addition of two mole equivalents of sodium methoxide to (18; R = Bun) gives trans-1,4-di-n-butylbuta-1,2,3-triene. Hydroboration of the product with di-isoamylborane and protonolysis gives pure cis,trans-dodeca.-5,7-diene by addition across the central double bond.

Perhaps the most...