Pyranylidene complex

In the presence of a catalytic amount of triethylamine, a readily enolizable carbonyl compound like acetylacetone (25) can undergo a Michael-type addition onto the triple bond of 23 with C-C bond formation, and subsequent 1,2-addition of the hydroxy group with elimination of an alcohol (MeOH or EtOH) to eventually yield a pyranylidene complex 28 (mode E) [29]. The most versatile access to / -donor-substituted ethenylcarbene complexes 27 is by Michael-type additions of nucleophiles, including alcohols [30-32], primary... 

The dihydronaphthalene-annelated pyranylidene complex 178, prepared according to reaction route E in Scheme 4 from /J-tetralone and complex 35, upon treatment with the pyrrolidinocyclopentene 174 n-1) or -cyclohexene 174 (n=2) at room temperature gave the tetracyclic compounds 179 in excellent... 

Scheme 39 Synthesis of tetracyclic skeletons 179 from the dihydronaphthalene-annelated pyranylidene complex 178 and cyclic enamines 174 [29]...

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

A further useful reaction sequence, reported by Aumann [219], is based on the Diels-Alder reaction of 2-pyranylidene complexes with enamines (Figure 2.36). Retro-Diels-AIder reaction of the initially formed 3-oxabicycIo[2.2.2]octan-2-ylidene complex leads to elimination of metal hexacarbonyl and formation of a substituted cyclohexadiene. Although this sequence can also be performed with the corresponding carbonyl compounds (2//-2-pyranones), these normally... 

Fig. 2.36. Preparation of cyclohexadienes by Diels-Alder reaction of 2-pyranylidene complexes with enamines [219].

On Cr, ene-carbonyl-vinylidene complexes undergo conversion to 2-pyranylidene complexes (Scheme 1.19) [376]. 

Scheme 10. Pyranylidene complexes by metathesis of tertiary enaminones.

Pyranylidene complexes, which are easily obtained from (l-alkynyl)car-bene complexes, prove to be most valuable building blocks for the generation of open-chain l-metalla-l,3,5-trienes as well as of cyclohexadiene annulation products. 

Pyranylidene Complexes by C-Addition of Enolizable Carbonyl Compounds... 

Addition of carbon nucleophiles to the C=C bond of a compound la,b includes reactions of enolizable carbonyl compounds, enol ethers, and ena-mines, as well as lithium alkyls and zinc alkyls. Condensation of the enolizable ketone 68 with la,b (M = Cr, W)26 is induced, for example, by catalytic amounts of triethylamine in pentane and under these conditions affords a 90% yield of crystalline pyranylidene complex 57 directly from the reaction mixture.102 This reaction proceeds via the 2-ethoxy-l-metallatriene L, which, because of the presence of triethylamine, rapidly undergoes ring closure to the pyranylidene (pyrylium ylide) complex 69 by 1,6-elimination of ethanol (Scheme 22). Chromanylidene complexes 71 are obtained from condensation of a /3-tetraIone 70 (R = H, OMe) with compound 1a,b. 

Addition of enolizable carbonyl compounds to the C=C bond of (1-alkynyl)carbene complexes can proceed by C- or O-addition of the enol unit. The C/O ratio depends very much on the substrate as well as on the reaction conditions. Reaction of cyclic 1,3-diketones 72 in diethyl ether in the presence of catalytic amounts of Et3N results in the formation predominantly of (red) divinyl ethers 73 (by O-alkylation) together with smaller amounts of (blue) pyranylidene complexes 74 by (C-alkylation) (Scheme 23).26103 For related reactions, see Scheme 67. 

Aminolysis of pyranylidene complex 69b with primary amines RNH2 (R = n-Bu, t -Pr, CH2Ph, allyl) gives easy access to the formation of amino... 

Amino-l-metalla-l,3,5,7-octatetraenes via 6-Alkenyl Pyranylidene Complexes... 

The 6-methyl group of the pyranylidene complex 69b is even more acidic than the acetyl group and readily undergoes base-catalyzed H/D exchange reactions as well as condensation reactions with aldehydes. Thus, a route to styryl pyranylidene complexes 79 other than that shown in Scheme 25... 

Scheme 26. Condensation of pyranylidene complexes with aldehydes.

Enaminolysis of pyranylidene complexes affords cyclohexadiene annulation products.26 For example, steroid-like molecules 83 are obtained most straightforwardly by this route if the chromanylidene complex 81 is added to the enamine 82. The annulation is highly regioselective, exemplified by the formation of compound 86 from chromanylidene complex 85 and enamine 84, and also by the straightforward generation of the sterically... 

Scheme 3. Proposed mechanism for formation of pyranylidene complexes.

Two research groups independently extended the electrocyclization system into the stoichiometric formation of pyranylidene complexes (25 and 27), which also proceeds via vinylidene complexes as key intermediates (Scheme 21.10) [16,17], Further reactions of the pyranylidene complexes 25 with a variety of alkenes gave the corresponding naphthalenes 28 in good to high yields. In addition, Miki and others found the catalytic formation of cyclopropanes bearing a furan ring 30 from reactions of ene-yne ketones 29 with a variety of alkenes, where furyl carbene complexes 31 were proposed as key intermediates (Scheme 21.11) [18],... 

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