Nucleophilic substitution propargylic compounds

Starting material 8 is treated with the propargylic bromide 9 in the presence of potassium carbonate as base in a nucleophilic substitution, producing compound 10... 

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). 

The direct substitution of hydroxyl groups can also be extended towards propargylic alcohols. In the presence of FeCl3, a plethora of O-, N- or S-nudeophiles are able to react with substituted propargylic alcohols [17]. Amongst the variety of nucleophiles, allylsilanes occupy an important position since this example resembles a cross-coupling reaction between an organometallic compound and an alcohol (Scheme 7.12) [17]. 

The reaction of primary or secondary alcohols with thionyl chloride is a general method for preparing the corresponding chloro compounds. In the first step a chlorosulfne ROSOC) is formed from which S02 is eliminated in a relatively slow step. This decomposition is facilitated by a tertiary amine, e.g. pyridine. The ammonium salt RO-SON+.Cl— formed from the chlorosulftte is subsequently attacked on carbon (in R) by CF. Since nucleophilic substitutions on propargylic carbon proceed more easily than on carbon in saturated compounds, it may be expected that the conversion of propargylic chlorosulfites into the chlorides will take place under relatively mild conditions. 

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). 

The overall process, considering that the propargylic alcohols are generated from carbonyl compounds, constitutes a reductive nucleophilic substitution with additional introduction of the synthetically valuable sulfoxide functionality. 

The gold(III)-catalyzed direct nucleophilic substitution of allyl or propaigyl alcohols is possible with many nucleophiles, including allyl silanes, electron-rich aromatics, p-dicarbonyl compounds, alcohols, and sulfonamidesJ With thiols, the corresponding propargyl thioethers were obtained with moderate yield (Scheme 4-121). 

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. 

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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