Saturated and Unsaturated Hydrocarbons

BY N. SIMPKINS

A number of cyclic and bicyclic hydrocarbons can be formed by cyclisation of suitable unsaturated alkyllithiums (Scheme 1). The in situ formation of the initial alkyllithium is carried out at -78°C using tBuLi. Quenching at low temperature provides simple non-cyclised products, whereas warming to room temperature (and in some cases addition of TMEDA) effects cyclisation. Benzylic alcohols are reduced to the corresponding hydrocarbons by means of the familiar Me3SiCl - NaI combination in CH3CN. The method gives very good yields, and tolerates other functionality. The combination of Mo(CO)6 and phenylsilane comprises a powerful reagent for conjugate reduction of Michael acceptors, including ketones, esters and amides, usually in near-quantitative yield.

Hydrogenation of organic compounds can be carried out effectively using soluble polyethylene-bound Wilkinson's catalyst, and using a new biphasic reduction system. Electrocatalytic hydrogenation, using specially prepared cathodes, is also an effective method for reducing carbon-carbon double bonds. Other functionality can also be reduced by the system, such as aromatic aldehydes and nitro compounds.

2 Olefinic Hydrocarbons

Deoxygenation of epoxides to the corresponding olef ins can be effected by treatment with SmI2. The reaction requires the use of HMPA and/or other additives for high yields in reasonable reaction times, especially for non-terminal epoxides. A report has detailed the use of titanium on graphite as a highly effective reagent for McMurry coupling of carbonyl compounds to give alkenes.

Methylenation of enolisable carbonyl substrates is a common problem in synthesis, due to the basicity of the reagents employed. The combination of CeCl3 with the Peterson reagent Me3SiCH2Li offers one solution to this problem (Scheme 2). The combined Li/Ce reagent proved superior to Li reagents, Mg reagents or combined Mg/Ce reagents in all cases.

An interesting study of the elimination of stabilised phosphorus ylide adducts brings into question the reversibility of the addition step as an explanation for (E)-olefin selectivity in such Wittig reactions. A variety of α-ydroxy ketones undergo accelerated Wittig reactions with stabilised phosphoranes to give trisubstituted alkenes with good (E)-selectivity, e.g. Scheme 3.

As indicated in Scheme 4, the boron-Wittig reaction of aromatic aldehydes can be effected to give either (E) or (Z) products. The initial erythro boron adduct with the aldehyde is thought to undergo selective syn (TFAA), or anti (HF) elimination, to account for the overall stereoselectivity.

Palladium catalysed cross-coupling reactions provide a powerful means of synthesising alkenes, as indicated by the examples in Scheme 5. Thus, the use of manganese compounds adds to the ranks of coupling partners which can be used for enol phosphonates or triflates. Trisubstituted systems, e.g. (2), are available by coupling of vinylalanes such as (1), which are themsel ves readily available from acetylenes. 14 The formation of the substituted vinyl silane (3) contrasts with previous palladium-catalysed couplings of CH2 CHSiMe3 with aryl iodides, in which aryl-desilylated products were obtained. The N-nitroso-N-arylacetamide acts as a source of ArPdOAc. More ambitious processes along the same lines allow sequential introduction of two groups. Thus, both 1,1- and 1,2-disubstituted ethenes can be prepared, as indicated in Scheme 6. Both methods are one-pot procedures and have the advantage of using readily available starting materials.

Piers has further extended the chemistry of bis-stannylesters such as (4). A series of metallation-alkylation reactions allowed these compounds to be efficiently converted into differential ly tetra substituted alkenes (Scheme 7; see also Scheme 54). The sequence is highly stereoselective, either isomer of (4) giving the same product in the first step. Surprisingly, direct metallation of (5), as a protected derivative, was not efficient, hence the need to convert to the corresponding iodide. What amounts to an intramolecular version of this chemistry has been used by Negishi et al. in another solution to the exocyclic alkene problem, e.g. Scheme 8.

The use of a new alkylidenation reagent derived from a 1,1-dibromoalkane, zinc and TiCl4 allows conversion of esters and lactones to the corresponding vinyl ethers(Scheme 9). Both chemical yields, and Z/E selectivity are, on the whole, very good, and the method looks operationally quite simple. Vinyl ethers derived from lactones suffered partial hydrolysis to hydroxyketones as indicated in the Scheme; some isomerisation of a cis double bond incorporated into one starting substrate was also observed. A study of the Heck arylation of vinyl ethers describes factors responsible for regiocontrol in the reaction. Vinylic chlorides are available from alkenes by reaction with PhSeCl3, followed by hydrolytic selenoxide elimination (Scheme 10). In many examples regiochemical problems arise, the most useful application of the method being the preparation of 2-chloro-1-alkenes (7) by oxidation of the selenide (6) to the dichloroselenide using SO2Cl2, followed by elimination. The reaction of 1,1-dichloro-l-alkenes with Grignard or organozinc reagents in the presence of [PdCl2 (dppb)] allows replacement of just one chlorine group with high stereoselectivity, e.g. Scheme 11. In each case a new group is introduced trans to the existing substituent, to give the vinyl chloride in good yield. By changing the catalyst to [PdCl2(PPh3)2] the second chlorine could also be substituted, resulting in a very elegant route to trisubstituted alkenes.

Vinyl sulphides and selenides have been prepared by free radical addition to suitable unsaturated starting materials(Scheme 12). The addition of PhSeH to allenes was found to require the presence of oxygen, and presumably takes place via attack of PhSe at the central carbon atom. The use of Et3B allows addition of thiols to acetylenes, although with very poor stereoselectivity. Analogous reactions of acetylenes with Ph3GeH, and with R3SnH, have been reported by the same group of workers.

A variety of vinyl sulphoximines can be prepared by a simple two-step sequence involving dehydration of β-hydroxyalkyl sulphoximines (Scheme 13). Alternative dehydration conditions could also be used to furnish the corresponding N-formyl or...