Propargyl compounds nucleophiles

Most of the synthetic routes to allenes utilize the reaction of propargylic compounds as electrophiles. In contrast, if the propargylic compounds serve as nucleophiles, a wide variety of substituted allenes, which are not easily accessible by the reaction of propargylic compounds with nucleophiles, are available. However, in order to synthesize enantioenriched allenes by this method, it is necessary to generate configurationally stable propargyl or allenylmetal reagents (cf. Chapter 9). 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g., HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compounds show a special reactivity by a special (Se2 or Se2v) mechanism. 

In allylic and propargylic compounds the 8, 2 reaction in addition to undergoing nucleophilic substitution takes place with a rearrangement of a double bond (Fig. 3). 

No cross-coupling of propargyl compound 94 with an excess of Et2Zn occurred. Instead, a nucleophilic propargyl species is generated by umpolung and reacts with benzaldehyde to give 95 [28],... 

In general, Pd-catalyzed reaction of propargyl compounds provides synthetically valuable allenyl compounds through addition, transmetallation, or oxypalladation of allenylpalla-dium intermediates. Exceptionally, soft carbon nucleophiles such as malonate and methyl acetoacetate attack the sp carbon of allenylpalladium intermediates to afford allylic compounds and furan derivatives. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

In a method for propargylating an alkyl halide without allylic rearrangement, the halide is treated with lithio-l-trimethylsilylpropyne (122) which is a lithium compound protected by an SiMca group.Attack by the ambident nucleophile... 

Propargylic substitution reaction is one of the most important routes to allenic compounds [1, 2], As shown in Scheme 3.1, replacement of a leaving group at the propargylic position with an incoming nucleophile via an SN2 pathway rearranges the C=C-C skeleton into a C=C=C moiety to give a propadienyl species. With certain... 

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