PROPARGYL COMPOUND

There is evidence in some cases that allylation may precede decarboxylation see ref. 77. 

(a) Guibe, F. Tetrahedron 1998, 54, 2967 (b) Protective Groups in Organic Synthesis, Wuts, P. G. M. Greene, T. W. Wiley, Hoboken, 2007. 

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Reactions of Propargylic Compounds Catalyzed by Pd(Q) 5.2 Reactions with Alkenes and Terminal Alkynes... 

Synthesis of y-lactones using tungsten alkynyl and propargyl compounds 98PAC1111. 

Acylzirconocene chlorides 78, which are easily available through the hydrozirco-nation of alkenes or alkynes with Cp2Zr(H)Cl and subsequent CO insertion, can be used as acyl anion equivalents Cu(I)-catalyzed reactions with propargyl compounds 77 afford allenyl ketones 79 (Scheme 3.40) [86]. The use of an excess of 77 (2 equiv. to 78) is important for the selective preparation of 79, which prevents an undesirable side reaction of the allenic products 79 with 78. 

The rhodium(II)-catalyzed reaction of propargyl compounds 169 and diazo compounds 170 gave corresponding functionalized allenes 171 together with cydopro-penes 172 (Scheme 3.87) [126]. Rh2(pfb)4, where pfb represents perfluorobutyrate, was found to be an excellent catalyst for preparing the allenes 171. An analogous rhodium(II) complex, Rh2(OAc)4, afforded mainly 172 with only a trace amount of 171 (<5%). 

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). 

Only a few examples exist describing the products from the allenylic/propargylic carbanion resulting from the deprotonation of 18 and reaction with other electrophiles instead of protons which lead to products analogous to 19 [48]. Thus, treating the propargyl compound 21 with tetrabutylammonium fluoride (TBAF) in the presence of benzaldehyde furnishes the C,C-connected compound 22 [41]. 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

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