Propargylic ylides

Alternatively, bromo trienyne 66, prepared by the Wittig reaction of TMS-capped propargyl ylide with , -5-bromo-2,4-pentadienal, could be coupled with dienyl zinc reagent 67, as illustrated in equation 3657. Subsequent desilylation followed by treatment with trimethyl aluminum in the presence of catalytic Cp2ZrCl2 afforded the alane of tetraenyne 68 which, on exposure to chloroformate, gave essentially all- polyene ester 69. 

Table 21 also includes reactions of allylic and propargylic ylides with relatively complex aldehydes (entries 83-109). These examples involve a wide range of experimental conditions, and the Z E ratios are more variable than for the simpler entries of Table 15. Several of the reactions are surprisingly Z selective (for example. Table 21, entries 93,95,102,104) by comparison with simpler analogues, and the results appear to be strongly influenced by lithium ion. Overall, the situation is reminiscent of the benzylide reactions of Table 14 where large variations in reported Z E ratios were noted because the results were obtained using a variety of experimental conditions.

Many different research groups have contributed to the collection of empirical data in Tables 6-21. In some cases where the results do not fit overall trends, there are differences between similar experiments due to troublesome experimental or structural variables. A few of the misfits are probably due to uncertain structural assignments, and many of the others correlate with experimental procedures in which the concentration of electrophilic byproducts or contaminants (lithium ion, hydroxyl groups, protic or Lewis acids) is not controlled. There are also some misfit cases among the reactions of unsymmetrical ketones, and in reactions where allylic, benzylic, or propargylic ylides are employed. However, the great majority of cases are well behaved and follow general patterns that are summarized in Table 22 and are discussed in the section on interpretation of stereoselectivity trends. 

Tnfluoromethyl-substUuted 1,3-dipoles of the propargyl-allenyl type and trifluoromethyl-substituted nitrilium betaines. Tnfluoromethyl- [164, 765] and bis(trifluoromethy])-substituted [166, 167] nitrile ylides have been generated by different methods and trapped with various dipolarophiles to yield [3+2] [768] and [3+1] cycloadducts [769], respectively... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

A more direct access to the unstable and non isolated sulfonium ylides 58a- c is the reaction of diisopropyl diazomethylphosphonate 57 with allylic sulfides, catalyzed by Cu(II), Rh(II) [39], or ruthenium porphyrins.[40] For example, the a-phosphorylated y,d-unsaturated sulfides 59-61 are obtained through the [2,3] -sigmatropic rearrangement of 58a-c. This method allows the use of a greater variety of starting allylic sulfide substrates, such as 2-vinyl tetrahydrothiophene, or propargylic sulfides (Scheme 15). 

Azomethine ylides of pyrrolo[l,2- ]pyrazine <1996JOC4655> and 3,4-dihydro pyrrolo[l,2-tf]pyrazine <1997T9341> undergo 1,3-dipolar cycloadditions with a number of dipolarophiles. For example, the ylide 178 reacts with propargylic ester 179 to give the tricyclic derivative 180 (Equation 43). 

Ylide-derived products may be formed as minor by-products from propargylic chlorides or ethers this contrasts with the inertness of allylic chloride or ethers... 

The Cu(I)-catalyzed cyclization for the formation of ethyl ( )-tetrahydro-4-methylene-2-phenyl-3-(phenylsulfonyl)furan-3-carboxylate 82 has been accomplished starting from propargyl alcohol and ethyl 2-phenylsulfonyl cinnamate. Upon treatment with Pd(0) and phenylvinyl zinc chloride as shown in the following scheme, the methylenetetrahydrofuran 82 can be converted to a 2,3,4-trisubstituted 2,5-dihydrofuran. In this manner, a number of substituents (aryl, vinyl and alkyl) can be introduced to C4 <00EJO1711>. Moderate yields of 2-(a-substituted N-tosyIaminomethyl)-2,5-dihydrofurans can be realized when N-tosylimines are treated with a 4-hydroxy-cis-butenyl arsonium salt or a sulfonium salt in the presence of KOH in acetonitrile. The mechanism is believed to involve a new ylide cyclization process <00T2967>. 

Propargylic silylated telluronium ylides react analogously. 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

Further study by Katsuki, McMillen, Hashimoto, and Wang improved the enantioselectivity up to a moderately high level.Wang and co-workers further extended the asymmetric catalysis to the [2,3]-sigmatropic rearrangement of propargyl sulfonium ylide to give allenic products with up to 81% ee (Equation (18))." ... 

Allenyl- and propargyl-phosphonium salts have also been used as precursors of heterocyclic compounds in the presence of various functionalized nucleophiles279, via the intermediacy of vinylphosphonio compounds however, an ylide extrusion has been sometimes observed815,816, depending on the nature of the functions in the nucleophile (reaction 250). Allenylphosphonium salts are able to add such weak CH acids as in ketones, even in absence of basic catalysts817,818, without any modification of the keto group (reaction 251). 

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm-1 that underwent a 66-cm I shift with I5N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative propargyl-like structure (9). This conclusion was supported by the spectra from the l3C- and 2H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitrilio benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

Benzylic, allylic, and propargylic alcohols can be oxidized by o-iodosylbenzoic acid (IBX) 7 in the presence of stabilized Wittig ylides to generate a>/3-unsaturated esters 30 in a one-pot procedure, Scheme 11. This is useful when the intermediate aldehydes are unstable and difficult to isolate [70]. 

Propargylic groups are also known to participate in [2,3]-sigmatropic rearrangements and lead to the isolation of allenes as the major products. For example, the nitrogen ylide 65 was formed by treating the corresponding ammonium salt with NaH in DMSO afforded allene 66 in 86%... 

Dilithio-1,3-dienes were found to react with aldehydes to form polysubstituted 2,5-dihydrofurans in a regio- and stereoselective manner <02OL2269>. Like other metal-carbene reactions, rhodium-catalyzed tandem carbonyl ylide formation - cycloaddition with propargyl bromide gave the 2,5-dihydrofuran in good yield <020L1809>. 

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