Propargylic compounds rearrangement

A-Allyl compounds can be rearranged to A-propenyl derivatives. Similarly, A-propargyl compounds (969) give A-allenyl derivatives (970). 

Further mechanistic work with optically active and labeled compounds confirmed that the rate-determining step in such rearrangement was the silver coordination to the alkyne moiety and revealed that the silver-catalyzed allene epimerization was 2-40 times faster than the propargyl ester rearrangement.53... 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

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 equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

When X=OH, this conversion of acetylenic alcohols to unsaturated aldehydes or ketones is called the Meyer-Schuster rearrangement The propargyl rearrangement can also go the other way that is, 1-haloalkenes, treated with organocopper compounds, give alkynes. ... 

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... 

The Claisen rearrangement of propargyl vinyl ethers directly delivers the allene no equilibrium is observed. This reaction was also successful with complex substrates in order to show this, of numerous examples [375, 513-536], the compounds 159 [537] and 161 [538] are depicted (Scheme 1.71). 

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... 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

The configurational stability of chiral allenylmetal reagents depends to a large extent on the nature of the metal substituent. The mechanism of the racemization process has not been studied in detail, but two reasonable pathways can be proposed, based on known reactivity characteristics of these compounds. The first entails reversible intermolecular SE- rearrangement to the propargylic isomer. This process could proceed by a pure syn or anti pathway, in which case no racemization would take place. However, the occurrence of both pathways would result in racemization (Scheme 9.5). 

As appropriate model compounds for these reactions240 the bridgehead substituted di hydro-4-methyleneazulenes 474 were employed. Allyl-, crotyl- and propargyl-substituted dihydroazulenes 474 and 476 can be easily rearranged to the 4-substituted azulenes 475 and 477 (equations 179 and 180) whereas all attempts to obtain 4-benzylazulene 479 by rearrangement of precursor 478 gave only polymeric products (equation 181). Undoubtedly, this failure can be explained by the fact that the Cope rearrangement becomes very... 

Attempts to isolate 1,3-dilithiated propargylic ethers with two equivalents of BuLi at temperatures above -20 give unsatisfactory results, because the dilithio compounds are unstable. In the case of HCsCCH2Or-Bu, HCsCCH(-f-Bu)OH is found after aqueous hydrolysis, possibly as a result of a Wittig-rearrangement [2]. At temperatures below -20 C the dilithiation is too sluggish to be of practical interest. With the super-basic reagent BuLi.t-BuOK in a THF-hexane mixture dipotassiation can be effected in a relatively short time at low temperatures. 

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