1.2-Allenyl ketones, 4- palladium

Table 11 Palladium-catalyzed silastannative cyclization of allenyl ketones...

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. 

A similar isomerization of an allenyl ketone, catalyzed by a Cr(CO)sL complex, is most probably the mechanistic key step of the palladium-catalyzed conversion of chromium carbene complexes and propargyl bromide to furans. In control experiments different aryl and alkyl allenyl ketones 96 isomerized to the furans 99 in the presence of 10 mol% of Cr(CO)5(NEt3) in good yields (Scheme 15.31) [70],... 

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

Allenyl ketones and aryl halides undergo coupling and cyclization in the presence of a palladium-triphenylphosphine catalyst and silver carbonate. The reaction leads to the formation of furane derivatives, the aryl group being introduced into the 3-position (3.72.) 90... 

The formation of 2 furane rings was achieved in one transformation by Ma and co-workers. Allenoic acids and allenyl ketons were reacted in the presence of a palladium catalyst to give the unsymmetrical bifuryl product, arising from the cyclization of both allene derivatives mediated by the same palladium centre followed by their coupling (3.73.) 91... 

Palladium-catalyzed isomerization of ynones to furans has been an active area of research over the last twenty years. Huang et al. described a Pd-catalyzed rearrangement of a,P-acctylcnic ketones to furans in moderate yield [175]. For example, Pdfdbajj promotes the isomerization of alkyne 214 to a putative allenyl ketone intermediate 215, which subsequently cyclizes to the corresponding furan 216. 

Allenyl-ketones pre-synthesised, or prodnced in situ from palladium(O)- or copper(I)-catalysed isomerisation of conjugated or non-conjugated ° alkynyl-ketones, can be cyclised to fnrans. The ring closure can be effected with palladium " or gold " catalysis. Acylation of silylallenes leads to a fnran directly. ... 

Allenyl ketones, presynthesised,or generated in situ by acylation of silyl-allenes, or by palladium(0)-catalysed isomerisation of conjugated or non-conjugated alkynyl ketones, can be cyclised to furans. The ring closure has been effected with silver or palladium catalysis acylation of silylallenes leads to the furan directly. In the presence of water, palladi-um(2)-catalysed closure of p,y-alkynyl ketones is believed to proceed via the enol, not the allene. l,2,3-Trienyl-4-ols have also been shown to cyclise to give furans. 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

Palladium(0)-catalyzed asymmetric arylation of tert-cyclobutanol 33 with bromobenzene, involving enantioselective C-C bond cleavage, furnished the chiral y-arylated ketone 34 (Scheme 3.13) [24]. Good enantioselectivity was observed when a chiral ferrocenyl P,N-ligand was used. y-Alkenylated and y-allenylated ketones were also obtained by asymmetric ring opening. 

Arylative or silylative cyclizations of allenyl aldehydes or ketones have been reported (Equations (101) and (102)).459,459a The intermolecular process, that is, three-component coupling reaction of aldehydes, allenes, and arylboronic acids, is catalyzed by palladium as well (Equation (103)).46O 46Oa These reactions are proposed to proceed through nucleophilic attack of the allylpalladium intermediates to the carbonyl groups. 

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

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