Substrates Bearing Polar Groups

Polar-substituted alkenes where the functionality is not attached to a strained ring are considerably more discriminating in their compatibility with metathesis catalysts and as a rule require relatively high catalyst charges. In the aliphatic series, unsaturated esters have received the most attention. Boelhouwer reported in 1972 the metathesis of the ester methyl oleate and its trans isomer, methyl elaidate, with a homogeneous catalyst based on a 1/1.4 molar combination of WCl6/(CH3)4Sn (23). At 70°C and an ester/W molar ratio of 33, near-thermodynamic equilibrium was attained, and 49 and 52% of the respective esters were converted to equal amounts of 9-octadecene and the dimethyl ester of 9-octadecene-1,18-dioic acid. 

Cross-metathesis of methyl oleate with 3-hexene under similar conditions led, in addition to the above products [Eq. (54)], to 3-dodecene and the methyl ester of 9-dodecenoic acid. 

Triglycerides of unsaturated fatty acids, which are an abundant commodity, were shown to disproportionate with the WCl6/(CH3)4Sn catalyst 

Intermolecular metathesis of linseed and soybean oils, which are triglycerides of linoleic and linolenic acids, offers an alternative method of upgrading these drying oils into stand oils by a net increase of their molecular weights. 

Boelhouwer s discovery (23) prompted a flurry of activity in this area. Baker applied the Boelhouwer catalyst to the metathesis of w-olefinic esters (88). At an ester/W molar ratio of 20/1 (68°C), symmetrical olefinic diesters were formed in 34-36% yields with concomitant elimination of ethylene. In addition, Baker identified products recovered in 3-8% yield corresponding to addition of HC1 across the terminal double bond. 

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SRN1 substitution with substrates bearing two leaving groups and bidentate or monodentate nucleophiles, followed by a polar or catalyzed reactions. 

As expected, substitution by azide ion occurs more readily when the alkyl substrate bears electron-withdrawing groups. For example phenacyl bromide and its derivatives give high yields of azides when treated with sodium azide in the cold °. Secondary alkyl substrates undergo Sj,2 reactions with azide ion ° ° but with less facility than primary alkyl substrates in accordance with the normal polar influences and primary steric eflfects in reactions . Selective substitution is therefore possible and this has been effectively applied in carbohydrate and steroid synthesis (sections II.B.5,6). These effects are also exemplified in the alicyclic series where it has been reported that menthyl halides and 2-methylcyclohexyl halides afford unsatisfactory yields of azide ° . The unsubstituted alicyclic azides, however, are obtained in good yields by the procedures outlined above(Table 1). 

The oxygenation of tertiary and benzylic C-H bonds of substrates bearing a number of common polar functional groups by CAN in aqueous solution is catalysed by 1,4,7-trimethyl-1,4,7-triaza cyclo-nonane-RuCl3 complex (62). A stepwise radical-rebound mechanism, based on chemoselectivity trends and kinetic isotope effect, is proposed. ... 

The metathesis of olefins bearing functional groups provides potential routes to many valuable compounds. Metathesis catalysts, most of which contain a Lewis acid, are unfortunately poisoned by polar and basic compounds. As yet, only a few catalytic systems have proved to be active in metathesizing functionalized alkenes. In the homogeneous disproportionation of these substrates, the catalytic combination WCl —Sn(CH3)4 is still the best known [63]. In addition to its well known application in the conversion of unsaturated oxygen containing olefins, such as fatty acid esters and unsaturated ethers, its effectiveness for the homogeneous metathesis of unsaturated amines has also been described [64]. 

Derivatives bearing a 3 -monopbosphoryl group were originally classified as totally resistant to venom exonuclease. As the quality of the enzyme preparation improved, these compounds were found susceptible but required 1000-fold more enzyme than was needed to hydrolyze 5 -monophosphate-bearing compounds. This unusual resistance led to another erroneous conclusion, that the polarity of exonuclease changes (20). The basis for this belief were the experiments in which a mixture of tri-, tetra-, and pentanucleotides of the type d-N pNPpN pN p were used as substrates. The early products were nucleosides and nucleotides, whereas 3, 5 -mononucleoside diphosphates appeared considerably later. It is clear now that the mixture was contaminated with a small amount of dephosphorylated chains which were rapidly hydrolyzed to completion. 

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