Ethers allylic, coupling

The a-bromo-7-lactone 901 undergoes smooth coupling with the acetonyltin reagent 902 to afford the o-acetonyl-7-butyrolactone 903[763j. The o-chloro ether 904, which has no possibility of //-elimination after oxidative addition, reacts with vinylstannane to give the allyl ether 905, The o -bromo ether 906 is also used for the intramolecular alkyne insertion and transmetallation with allylstannane to give 907[764],... 

An interesting extension of this reaction is shown in the asymmetric kinetic resolution of cyclic allylic ether 44 under alkene coupling conditions. Use of (R)-12 as the catalyst gives (R)-45 in > 99% ee at 58% conversion. The ethylated product 46 is also formed in the reaction in 94% ee (Eq. 7) [25]. The reaction is effective for six- to eight-membered 3-oxacycloalkenes 47 as well as for a wide variety of alkoxycycloalkenes 48 [27], with some resolution dependency on the ring size of 47 (Fig. 2) [26]. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

When, furthermore, phenols (368) are coupled with 1 in the presence of a Pd° catalyst, the phenoxy-methyl-1,3-dienes 369 are produced [158]. As aryl allyl ethers, these can be made to undergo a Claisen rearrangement (205 °C, DMF) and the ensuing 2-(l,3-dienylmethyl)phenols 370 finally cydize in the presence of a trace of acid to a mixture of exo-methylene chromans 371 (major product) and dihydrobenzofur-ans 372 - a remarkable generation of functional and structural complexity from simple starting materials with 100% atom economy and underlining impressively the synthetic versatility of modern allene chemistry ... 

Transition metal-catalyzed allylic substitution with phenols and alcohols represents a fundamentally important cross-coupling reaction for the construction of allylic ethers, which are ubiquitous in a variety of biologically important molecules [44, 45]. While phenols have proven efficient nucleophiles for a variety of intermolecular allylic etherification reactions, alcohols have proven much more challenging nucleophiles, primarily due to their hard, more basic character. This is exemphfied with secondary and tertiary alcohols, and has undoubtedly limited the synthetic utihty of this transformation. 

Tab. 10.7 summarizes the results of the application of rhodium-catalyzed allylic etherification to a series of ortho-substituted phenols. The etherification tolerates alkyls, including branched alkanes (entries 1 and 2), aryl substituents (entry 3), heteroatoms (entries 4 and 5), and halogens (entry 6). These results prompted the examination of ortho-disubstituted phenols, which were expected to be more challenging substrates for this type of reaction. Remarkably, the ortho-disubstituted phenols furnished the secondary aryl allyl ethers with similar selectivity (entries 7-12). The ability to employ halogen-bearing ortho-disubstituted phenols should facilitate substitutions that would have proven extremely challenging with conventional cross-coupling protocols. 

A series of pyrido[2,3-rf pyrimidine-2,4-diones bearing substituents at C-5 and/or C-6 were synthesized using palladium-catalyzed coupling of uracil derivative 417 with vinyl substrates or allyl ethers to give the regioisomeric mixtures of 418/419 and 420/421, respectively. The ratio of the isomeric structures was dependent on the substituent R. In the case of the reaction with -butyl vinyl ether, only the product 419 was obtained. However, the reactions with acrylonitrile, ethyl acrylate, 2-trifluoromethylstyrene, and 3-nitrostyrene afforded only 418. Also, reaction with allyl phenyl ether gave only 420. The key intermediate 417 was prepared by the reaction of 6-amino-l-methyluracil with DMF-DMA (DMA = dimethylacetamide), followed by N-benzylation with benzyl chloride and vinyl iodination with iV-iodosuccinimide (NIS) (Scheme 15) <2001BML611>. 

Carbon-carbon coupling of radicals observed in the photo-Kolbe reaction could also be observed with other surface generated radicals. Kisch and coworkers have shown, for example, that cyclic allylic ethers undergo alpha deprotonation under photoelectrochemical activation, producing radicals that can be oxygenated, Eq. (29). On colloidal zinc sulfide, hydrogen evolution accompanies the photocatalytic... 

Benzylic silyl ethers couple with allylsilanes in the presence of trityl tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (TFPB) catalyst leading to carbon-carbon bond formation (equation 77). The corresponding Z11CI2-catalyzed reactions with an allyl silyl ether lead to a mixture of regioisomers145,146. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Bicyetic acetals.5 Cyclic allylic alcohols couple with ethyl vinyl ether when treated with Pd(OAc)2. Only a catalytic amount of Pd(II) is required if Cu(OAc)2 is present as a reoxidant. The absence of double-bond isomerization is a useful feature of this coupling. 

Interestingly, cyclic allylic ethers do not give analogous coupling products with zirconium-benzyne complexes.47 Reaction of 52 with 2,5-dihydrofuran... 

When propargyl allyl ethers are subjected to transition metal-catalyzed enyne cyclization reactions, 3-alkylidene-substituted tetrahydrofurans are usually formed. A useful variation of this scheme is the Pd(0)-catalyzed tandem enyne cyclization/Suzuki coupling reaction with various arylboronic acids (Equation 90) <2005JOC1712>. The stereoselectivity of this reaction is explained by invoking a chairlike transition state. 

Allylic ethers (165) were first observed as products about ten years ago while allylic ketoalcohols (164) had been isolated much earlier. Investigation of the reaction of biacetyl with a-trideuteriomethylstyrene showed that the simple radical coupling mechanism illustrated above is not correct124) since the deuterium scrambling expected from reaction of a symmetrical 2-phenylallyl radical was not observed in the... 

Propenyl Ethers and Unsaturated Cyclic Ethers Propenyl ethers (CH3—CH=CH—OR R = ethyl, isobutyl, etc. cis- and trans-isomers) and 3,4-dihydrofuran are linear and cyclic a,/3-unsaturated ethers, that can be regarded as / -substituted vinyl ether derivatives. For these monomers a few controlled/living cationic polymerizations have been reported. The HI/I2 system is generally effective for both linear and cyclic monomers [181,182,183], whereas a recent study by Nuyken indicates that the IBVE-HI adduct coupled with nBu4NC104 is suited for 3,4-dihydrofuran (see Section V.A.4) [184]. A variety of mono- and bifunctional propenyl ethers can readily be prepared by the ruthenium complex-catalyzed isomerization of corresponding allyl ethers [185]. 

On the basis of the desymmetrization concept, the kinetic optical resolution of a racemic substrate [66] can be recognized as an intermolecular version of desymmetrization. The kinetic resolution of a racemic allylic ether by the glyoxylate-ene reaction also provides efficient access to remote but relative [64] asymmetric induction. The reaction of allylic ethers catalyzed by the (f )-BINOL-derived complex (1) provides the 2R,5S)-syn products with > 99 % diastereoselectivity and > 95 % ee (Sch. 18). The high diastereoselectivity, coupled with the high ee, strongly suggests that the cata-lyst/glyoxylate complex efficiently discriminates between the two enantiomeric substrates to accomplish the effective kinetic resolution. In fact, the relative rates of the reactions of the enantiomers, calculated by use of the equation ... 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

Table 1-6. Catalyst effect on the Grignard (PhMgBr) coupling with allyl ether (data from [107])...

Allyl silanes react with epoxides, in the presence of Bp3 OEt2 to give 2-allyl alcohols.The reaction of a-bromo lactones and CH2=CHCH2Si(SiMe3)3 and AIBN leads to the a-allyl lactone.On the other hand, silyl epoxides have been prepared from epoxides via reaction with iec-butyllithium and chlorotri-methylsilane. ° a-Silyl-A-Boc-amines were prepared in a similar manner from the A-Boc-amine. " Arylsilanes were prepared by reaction of an aryl-lithium intermediate with TfOSi(OEt)3. In the presence of BEs etherate, allyl silane and a-methoxy A-Cbz amines were coupled. Benzyl silanes coupled with allyl silanes to give ArCHa—R derivatives in the presence of VO(OEt)Cl2 " and allyltin compounds couple with allyl silanes in the presence of SnCl4. Allyl silanes couple to the a-carbon of amines under photolysis conditions. 

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