Allenyl ketones dimerization

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. 

With certain substitution patterns such as in the p-methoxybenzyl allenyl ketone 103, the products from reactions with Ag(I), Au(III), Pd(II) and Hg(II) are entirely different 104, the dimers 105 or 106 or even 107 were formed (Scheme 15.28) [63, 64]. 

Hydroalkoxylation of Allenes In the year 2000, during their investigation of transition metal catalyzed reactions of allenyl ketones [29], Hashmi et al. discovered that gold(III) salts were able to lead the cydoisomerization and dimerization of these substrates (Equation 8.2) with a considerable improvement related to other assays with Ag (I) or Pd (II) catalysts [18]. 

As mentioned in the introduction, the reactivity of the allenes is high. Therefore the oxidative and cyclizative cross dimerization of two different allenic substrates just recently reported by Ma et al. can be considered to be a milestone in this field They discovered that one equivalent of 2,3-dienoic acids 69 and five equivalents of allenyl ketones 70 with the simple PdCl2(MeCN)2 catalyst can deliver up to 92% of the cross dimerization product 71... 

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)... 

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. 

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. ... 

We have demonstrated the first example of oxidative dimeric cyclization between two different classes of allenes, i.e., 2,3-allenoic acids and 1,2-allenyl ketones. The product is a dumb-bell-type bicyclic product 4-(3 -furanyl)-butenolide71 (Scheme 33) [24]. 

The dimerization of allenyl ketones 59, catalyzed by triphenylphosphine, affords the cyclodimers 60 formed by a [4+2] cycloaddition reaction in moderate yields". ... 

For example, terminal allenyl ketones undergo cycloisomeriza-tion/dimerization leading to 2,4-disubstituted furans. This process, apart from the C-0 bond formation, also involves C-C bond formation (eq 26). 

Scheme 17.12 Cyclization-dimerization of a-allenyl acids and ketones.

Scheme 17.13 Alternative mechanism for the cyclization—dimerization reaction of an a-allenyl acid and a ketone.

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