Allenyl ketones, cycloisomerization/dimerizations

Hashmi et al. have found that terminal allenyl ketones give dimerization products when Pd(ll)-catalysts are employed furnishing 2,4-disubstituted furans (Equation 15) <1995AGE1581, 1997JOC7295>. Tetrakis(2,2,2-trifluoroethoxycarbonyOpalladacyclopentadiene proved to be the most effective catalyst to obtain predominantly dimerization instead of cycloisomerization. 

Disubstituted furans 62 are obtained from a gold-catalyzed cycloisomerization/dimerization pathway involving terminal allenyl ketones and a,(3-unsaturated ketones <00AG(E)2285>. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

The cycloisomerization of allenyl ketones was initially described as being catalyzed by rhodium(I) or silver(I) by Marshall et al.21 The activity of copper, silver, and gold for this reaction was first compared in two papers published later (Scheme 12.7).22 In the case of copper and silver, only a cycloisomerization was observed (Table 12.4, entries 1 and 2) with gold, a dimer is obtained as well (entry 3). 

TABLE 12.4. Cycloisomerization or Cycloisomerization/Dimerization of Allenyl Ketones (R1 = Me)... 

In the presence of 1 mol% AuCls, allenyl ketones undergo a rapid cycloisomerization and can be reacted with a Michael acceptor. In a cross-dimerization of terminal allenyl ketones and a,/3-unsaturated ketones, 2,5-disubstituted furans can be obtained (Equation 16) <2000AGE2285>. 

Allenyl ketones undergo cycloisomerization on exposure to (PhCN)2PdCl2. Dimeric products are produced if an a-substituent (at the aUenyl group) is absent from the substrates. ... 

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