Saturated and Unsaturated Hydrocarbons

BY N. SIMPKINS

A catalyst comprising fused iron promoted by V2O5 is extremely efficient in the gas-phase hydrodeoxygenation of ketones and alcohols at relatively low pressures. Reductive d ecyanation of a variety of nitriles can be accomplished very cleanly using potassium metal in combination with a crown ether. The use of ultrasound allows for short reaction times in the reaction of gemdihalogenopropanes with various metals to form the usual carbenoid-derived products. The reduction of C-C multiple bonds has been found to take place in the presence of platinized TiO2 under illuminated conditions. A variety of unsaturated substrates react, although the reaction times are q uite long (ca. 26 h). A useful method for conjugate reduction of α,β-unsaturated ketones and aldehydes involves reaction with diphenylsilane catalysed by Pd0 in combination with ZnCl2. Excellent yields of reduced compounds were obtained using this method, which does not affect α,β-unsaturated nitriles or esters (Scheme 1).

The use of a trialkylaluminium-alkylidene iodide mixture to effect cyclopropanation has been re-examined. The reaction was found to work well when conducted in CH2Cl2, and shows contrasting regioselectivity to the Simmons-Smith reagent in reaction with geraniol (Scheme 2).

A new method which allows enantioselective cyclopropanation of α,β-unsaturated aldehydes employs acetals derived from tartrate esters. The method appears operationally straightforward and gives good yields and enantiomeric excesses (e.e.) (Scheme 3).

2 Olefins

Sodium borohydride can now be used for the reduction of acetylenes, by employing a NaBH4-PdCl2-polyethylene glycol-CH2Cl2 system. A variety of reduced products were obtained including cis-olefins and fully reduced materials.

The use of transition metal catalysts for dehydrogenation of alkanes has received more attention. The iridium complex [(Pri3P)2IrH5] exhibits unusual selectivity for this type of reaction in that methyl groups are attacked preferentially. Similarly, a photolytic dehydrogenation reaction was observed using [IrH2(CF3CO2)(PR3)2], even in the absence of the usual hydrogen acceptor t-butylethylene.

The reductive removal of allylic oxygenated functions can be carried out effectively using nickel boride. Allylic alcohols and their silyl ethers react , although they require much longer reaction times than the corresponding acetates (Scheme 4). Another new deoxygenation procedure constitutes the latest conversion of epoxides into the corresponding olefins, and utilizes arylseleno-carboxamides. The method is stereospecific (retention) although it requires the presence of a strong acid (CF3CO2H) and does not convert more sterically hindered epoxides such as norbornene oxide.

Luche has reported the reaction of carbonyl compounds with allylic halides in aqueous media. The reaction can be performed using either zinc or tin, and displays good chemoselectivity between aldehydes and ketones (Scheme 5).

Asymmetric coupling of aryl Grignards with allylic pivalates is possible in good e.e. by use of NiCl2[(S,S)-chiraphos] in only catalytic (1 mol%) amounts. Another allylic coupling reaction uses palladium to mediate displacement of an acetoxy-group from an allylic geminal diacetate by a sta bilized nucleophile, e.g. Scheme 6. Depending upon the substituents present on the reacting partners, the regioselectivity alters and a variety of products can be prepared.

A number of allylated and related products having quaternary carbon atoms may be prepared by radical chemistry starting from tertiary alcohols.

Allylstannanes have been prepared in a regioselective fashion by a selenoxide elimination route, and also via direct metallation of hydrocarbons. The latter procedure when combined with a protodestannylation step enables isomerization of various terpenes, e.g. Scheme 7.

A number of reports have focused interest on the synthesis of various allylic sulphur compounds. A one-pot procedure for the preparation of allylic sulphides from the corresponding alcohols involves initial rearrangement of the xanthate followed by extrusion of COS (Scheme 8). Allylic sulphides and sulphones are available from the corresponding nitro-compounds. Thus (1) on treatment with NaSPh in HMPA gave the sulphide (2), whereas the sulphone (3) was produced by reaction of (1) with PhSO2Na in DMF in the presence of [Pd(PPh3)4] (Scheme 9). Although the contrasting regioselectivity of the reactions is interesting, the products are perhaps more readily available by other methods [e.g. in the case of (3) by alkylation of the allylic sulphone anion]. Another research group has published similar chemistry starting from vinyl nitro-compounds. Warren et al. have published more chemistry leading to allylic (and also vinylic) sulphides, utilizing both β-hydroxy-sulphides and allylic phosphine oxides. Other applications of the phosphine oxide chemistry to the preparation of allylic products have also appeared. Vinyl sulphides have also been prepared by benzyne-induced fragmentation of 1,3-oxathiolanes and via hydroboration of 1-iodoalkynes (Scheme 10).

Vinyl alkyl selenid es can be prepared from the more readily obtainable vinyl methyl selenides by a d emethylation/alkylation sequence which retains the stereochemistry of the starting materia1. The chemistry of vinylic compounds containing silicon groups have received considerable attention. Acetylenes can be disilylated using a reagent derived from Me3SiLi, MeMgI, and MnCl2. Distannylation can also be achieved. Addition of (trimethylsilyl) trimethylstannane across the triple bond of alk-1-ynes gives products of type (4) in regio- and stereo-specific fashion.

Vinyl nitriles containing silicon groups have been obtained by palladium-catalysed ad dition of TMSCN to acetylenes, and by the addition of HCN to silylated acetylenes mediated by nicke1. The regioselectivity of the copper-catalysed silylzincation of terminal acetylenes described by Oshima can be very effectively controlled by the correct choice of reagent (Scheme 11).

Corey has now published additional details concerning the chemistry of the reagent derived by treatment of methylenetriphenyl-phosphorane with an additional equivalent of alkyl-lithium. The reagent formulated as (7) methylenates even very sterically hindered ketones, and also opens epoxides (Scheme 12). In contrast to this...