Allyl carbonates

Allylic carbonates are most reactive. Their carbonylation proceeds under mild conditions, namely at 50 C under 1-20 atm of CO. Facile exchange of CO2 with CO takes place[239]. The carbonylation of 2,7-octadienyl methyl carbonate (379) in MeOH affords the 3,8-nonadienoate 380 as expected, but carbonylation in AcOH produces the cyclized acid 381 and the bicyclic ketones 382 and 383 by the insertion of the internal alkene into Tr-allylpalladium before CO insertion[240] (see Section 2.11). The alkylidenesuccinate 385 is prepared in good yields by the carbonylation of the allylic carbonate 384 obtained by DABCO-mediated addition of aldehydes to acrylate. The E Z ratios are different depending on the substrates[241]. 

The term allylic refers to a C=C—C unit The singly bonded carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon Conversely doubly bonded carbons are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents 

I-Hexyne and allyl carbonate undergo dimerization-allylation to give (Z)-5-allyl-4-butyI-l,4-undecadien-6-yne (765) in 85% yield[475]. 

Since allylation with allylic carbonates proceeds under mild neutral conditions, neutral allylation has a wide application to alkylation of labile compounds which are sensitive to acids or bases. As a typical example, successful C-allylation of the rather sensitive molecule of ascorbic acid (225) to give 226 is possible only with allyl carbonate[l 37]. Similarly, Meldrum s acid is allylated smoothly[138]. Pd-catalyzed reaction of carbon nucleophiles with isopropyl 2-methylene-3,5-dioxahexylcarbomite (227)[I39] followed by hydrolysis is a good method for acetonylation of carbon nucleophiles. 

FIGURE 10 3 (a) The spin density (yellow) in allyl radical is equally divided between the two allylic carbons There is a much smaller spin density at C 2 hydrogen (b) The odd electron is in an orbital that is part of the allylic tt system 

Pyridone (333) is allylated with allylic carbonates on the nitrogen atom rather than on the oxygen atom, but 2-thiopyridone (334) is allylated on the sulfur atom[204]. 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

Phenols arc highly reactive 0-nucleophiles and allylated easily with allylic carbonates under neutral conditions. EWGs on phenols favor the reac-tion[213]. Allylic acetates are used for the allylation of phenol in the presence of KF-alumina as a base[214]. 

Only the heteroannular diene 523 is formed by treatment of both a- and, 3-allylic carbonates 522 and 524 in a hydrindan. system with a Pd catalyst. No homoannular diene is formed. 

On the other hand, the expected alkene 598 was regioselectively formed from the allylic carbonate 597[388]. In these reactions, the hydride from formate preferentially attacks the tertiary carbon rather than the secondary carbon. 

Some organosilicon compounds undergo transmetallation. The allylic cyanide 461 was prepared by the reaction of an allylic carbonate with trimethylsi-lyl cyanide[298]. The oriho esters and acetals of the o. d-unsaturated carbonyl compounds 462 undergo cyanation with trimefhylsilyl cyanide[95]. 

Hydride attacks regioselectively at the Si-substituted carbon in the hydro-genolysis of the silylated allylic carbonate 626 with formate, affording the allylic silane 627[I42]. 

Chemoselectivity in the cycloaddition of 2-methylenecycloheptenone (174) changes on addition of In(acac)3. The allylic carbonate 175 reacts with the ketone 174 in the presence of In(acac)3 to give the methylenetetrahydrofuran 176, and the methylenecyclopentane 177 is obtained in its absence[l 13], The cycloaddition of ynones to produce the methylenetetrahydrofuran proceeds smoothly only in the presence of In(acac)3 (10 mol%)[114]. 

Allylalion of the alkoxymalonitrile 231 followed by hydrolysis affords acyl cyanide, which is converted into the amide 232. Hence the reagent 231 can be used as an acyl anion equivalent[144]. Methoxy(phenylthio)acetonitrile is allylated with allylic carbonates or vinyloxiranes. After allylation. they are converted into esters or lactones. The intramolecular version using 233 has been applied to the synthesis of the macrolide 234[37]. The /i,7-unsaturated nitrile 235 is prepared by the reaction of allylic carbonate with trimethylsilyl cyanide[145]. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

The reaction can be applied to the synthesis of q, /3-unsaturated esters and lactones by treatment of the ketene silyl acetal 551 with an allyl carbonate in boiling MeCN[356]. The preparation of the q,, 3-unsaturated lactone 552 by this method has been used in the total synthesis of lauthisan[357]. 

---