Pyranylidenes

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]...

Alkynylcarbene complexes react with /J-dicarbonyl compounds and catalytic amounts of a base to generate formal [3+3] pyranylidene derivatives [87]. The... 

In a reaction closely related to the latter, pyranylidene derivatives are obtained by the intermolecular radical coupling of alkynyl- or alkenylcarbene complexes and epoxides. Good diastereoselectivities are observed when cyclic epoxides are used. Moreover, the best results are reached by the generation of the alkyl radical using titanocene monochloride dimer [90] (Scheme 43). 

The reaction of ethyl 2,2-diethoxyacrylate with alkynylalkoxycarbene complexes affords 6-ethoxy-2H-2-pyranylidene metal complexes [92] (Scheme 48). The mechanism that explains this process is initiated by a [2+2] cycloaddition reaction (see Sect. 2.3), followed by a cyclobutene ring opening to generate a tetracarbonylcarbene complex. This complex can be isolated and on standing for one day at room temperature renders the final 6-ethoxy-2Ff-pyranylidene pentacarbonyl complex. This last transformation requires the formal transfer of one carbonyl group and one proton from the diethoxy methylene moiety to the metal and to the C3 2H-pyranylidene ring, respectively, with concomitant cyclisation. Further studies on this unusual transformation have been extensively performed by Moreto et al. [93]. 

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). 

Tetraphenyl-4,4 -bi-4H-pyranyliden wird in einfacher Weise durch Elektro-reduktion und nachfolgende Dehydrierung von 2,6-Diphenyl-pyrylium-perchlorat in Acetonitril/Tetrabutylammoniumperchlorat zu 48% d.Th. erhalten (als Nebenprodukt fallt 15% d.Th. 1,5-Dioxo-l,5-diphenyl-pentan an)8 ... 

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]. 

Some 4//-pyrans may also be prepared by addition reactions of appropriate 2- or 4-pyranylidene compounds. Thus butyl-, methyl-, or phenyl-lithium were found to add to monosubstituted 1,2-benzoxalenes 343 to give adducts 344, which were alkylated, acylated, or hydrolyzed to 4//-pyrans 345 or 346, respectively.329,330 (Scheme 16). 

Pyranylidene oxazolone 347 gives 95% of 2,6-diphenyl-4-formyl-4ff-pyran (348a) by successive hydrolysis with sodium hydroxide and hydrochloric acid, as shown in Eq. (17).331,332 The already mentioned, trans-... 

Reagent 528 and 2,6-diphenylthiopyrylium perchlorate afford 4,4 -bis-4//-pyranylidene (533a).2624 Analogous transformations of various... 

Bis-4,4 -pyranylidene derivative 164 (R = Ph) was prepared from dimeric 4//-pyran compounds 328 as follows 322... 

Dihydrobenzenes.2 Pyranylidene pentacarbonyl chromium complexes such as 1, prepared as shown, react with electron-rich alkenes, such as enol ethers, to form an adduct a, which extrudes Cr(CO)6 to provide dihydrobenzenes (2) in high yield. Attempted chromatography of 2 results in aromatization. 

Heterocycles such as furane, thiophene, and pyrrole have rather moderate donor ability. However, substituting them with appropriate electron-donating substituents might decrease their IP values and make them suitable components for organic metals. Pyranylidenes and their chalcogena analogs (12) have very low IP values (6.3 eV for R = Ph, X = S) and have been utilized for the preparation of conducting materials [23]. 

Scheme 6. Base-catalyzed cyclization of 1-metallatrienes to pyranylidene and dihydropyri-dinylidene complexes.

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. 

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