Propynoic acid amides

Scheme 4 Access to mixed N/O-substituted allenylidenes 6 starting from propynoic acid amides...

As shown in Scheme 4, alkynyl metallates derived from propynoic acid amides can also be used as source of the allenylidene C3 unit, their reactions with the tetrahydrofuran solvates [M(CO)5(THF)] affording the N/O-substimted allenylidene complexes 6 after selective O-alkylation with [R30][BF4] [29]. 

The same synthetic approach is also applicable for the preparation of analogous N/N- and N/C-substituted allenylidenes 7-10 (Fig. 4) by using C-ethynylimines, such as HC=CC(=NMe)Ph, 2-ethynylpyridines, 2-ethynylquinoline, or 2-ethynyl-pyrimidine, instead of propynoic acid amides [29, 30]. 

Fischer and coworkers have exploited thoroughly the synthetic route based on functionalized acetylides [4, 9]. Thus, by using deprotonated propynoic acid amides (alkynyl metallate) the reaction with M(CO)5(THF)] followedby treatment with [R3O] BF4 affords N/O-substituted allenylidene complexes (Equation 2.3). 

Analogous N/C substituted allenylidenes [M =C=C=CPh(NMeR) (CO)5] (M = Cr, W R= H, Me, Et) are obtained by using C-ethynylimines HC = CC(=NMe)Phinstead of propynoic acid amides. 0/C-, 0/0-, and N/S-substituted allenylidenes are also accessible from ethynyl ketones HC = CC(=0)R, propynoic acid esters HC=CC(=0)0R and propynethioic acid amides HC=CC(=S)NR2, respectively, after sequential deprotonation and the corresponding alkylation (representative examples are shown in Figure 2.2) [4a, 9]. 

Other possible relays are mostly alkynes, which, after carbopalladation, undergo a surprisingly clean reductive demetallation with formic acid salts (Scheme 2). At this point it is crucial that the catalyst cocktail has the appropriate composition, as yields under different conditions can vary substantially. The high degree of regioselectivity starting from propynoic acid amides has been attributed to coordination with the amide functionality. 

Similar studies have been reported for many other, previously unknown acetylenes, including aminoacetylene (11) as well as the difunctional alkynes X—C=C—Y (X,Y = 0H,NH2,0CH3)4 2 ionll+- is accessed by decarbonylation of the amide of propynoic acid (Scheme 12) ions X—C=C—Y" (12 -15 ) are available from appropriately substituted squaric acids (Scheme 12). All these ions produce structure characteristic CAD and spectra, demonstrating that both the acetylenic ions and the corresponding... 

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