Cyclobutene metallation

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]. 

The interaction of butadiynediyl dimetal complexes [Fp -C -CsC-M, Fp =FeCp (CO)2, M= Fp, Rp, SiMea, Rp= RuCp(CO)2] with diiron nonacarbonyl, Fe2(CO)9, results in the formation of a mixture of products, as is also observed in the case of their interaction with organic acetylenes. Interesting polymetallic complexes, propargylidene-ketene compounds, zwitterionic cluster compounds, and pa-p -propargylidene-cyclobutene compoimds were isolated from the reaction mixtures and successfully characterized. The product distributions were found to be dependent on the metal fragment (M) at the other end of the C4 rod. The results of the reaction are described... 

As in the case of dimerizations, MCP derivatives are known to undergo metal-catalysed [2 + 2] codimerizations with other alkenes in a few cases [2]. The examples are limited to strained olefins, such as norbornadiene (572) (Scheme 79) [152] and cyclobutene (574) (Scheme 80) [153], and to alkyl acrylates (Table 46) [154] and always compete with the alternative [3 + 2] addition of TMM species. 

Cycloaddition of the carbene chromium complexes 97 with CO incorporation provides a versatile method for naphthol synthesis, in which the metallacy-clic intermediates 99 are involved [47]. An alternative entry to 101 is achieved by metal carbonyl-catalyzed rearrangement of the cyclopropenes 98 via the same metalla-cyclobutenes 99 and vinylketene complexes 100 [52], Mo(CO)6 shows a higher activity than Cr(CO)6 and W(CO)6. The vinylketene complex 103 is formed by the regioselective ring cleavage of 1,3,3-trimethylcyelopropene 102 with an excess of Fe2(CO)9 [53]. (Scheme 35 and 36)... 

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

Three syntheses of 1,2-dicyanocyclobutene (2) have been previously described. The first, involving dehydration of cyclobutene-1,2-dicarboxamide does not specify the yield.9 The second procedure involves a concomitant chlorination and catalytic dehydrochlorination of 1,2-dicyanocyclobutane in the gas phase, yielding 1,2-dicyanocyclobutene (2) in a mixture of several other products.10 The third method consists of dechlorination of 1,2-dichloro-1,2-dicyanocyclobutane using metals, such as zinc copper couple,11 Raney nickel,11 and, especially, Raney cobalt.2 In comparison with this last-mentioned synthesis, the overall yield of the present procedure is 5-10% higher. Furthermore, the reaction is performed in less time and utilizes considerably cheaper reagents. 

Ring expansion of vinylcyclopro-penes and cyclobutenes 8-34 Ring expansion of vinylcycloal-kanes cyclization of diynes 8-39 Metathesis of dienes 8-40 Metal-ion-catalyzed o-bond rearrangements... 

Much less information is available about [2 + 2]-cycloadditions. These allow the formation of cyclobutane derivatives in the reaction between two alkenes, or that of cyclobutenes from alkenes and alkynes. The reaction can be achieved thermally via biradical intermediates,543 by photoreaction,544 and there are also examples for transition-metal-catalyzed transformations. An excellent example is a ruthenium-catalyzed reaction between norbomenes and alkynes to form cyclobutenes with exo structure ... 

X-Ray crystal structure analyses of both squaric acid158 (3,4-dihydroxy-3-cyclobutene-l,2-dione) and the dianion159 are the usual benchmarks used in consideration of structural data of metal complexes. The relevant dimensions are shown in Table 9, from which it can be seen that there is substantial conjugation and C2 symmetry in the acid molecule while the dianion has DAh symmetry... 

Cycloisomerization or metathesis also occurs, which can be understood as the formation of cyclobutene 326 by reductive elimination of 321. The metathesis product 327 is formed by isomerization of 326. The metatheses involving metal-carbene complexes are discussed in Section 7.2.6. They are closely related, but somewhat different from the metathesis explained here. Balance between the ene and the metathesis reactions seems to be delicate. 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

Many metal-catalyzed reactions are accelerated by light irradiation [88], In the case of the Vollhardt reaction [89], which is performed with conveniently available Co catalysts, the irradiation with visible light is included in the standard conditions. In a triple [2 + 2 + 2] cycloaddition, the nonaalkyne derivative 86 is transformed into the [7]Phenylene 87 (Scheme 5.17) [90]. Compound 87, containing six benzo-cyclobutene moieties, is a partial structure of the archimedene C12o 88. In this way a variety of similar benzocyclobutene structures such as helical phenylenes [91] can be built up. 

Conversely, the photoinduced ring-opening of cyclobutenes allows the construction of medium-sized hetero- or carbocycles, starting from bicyclic frameworks. Alkenes can undergo thermal (metal-assisted) or photochemical [2 + 2]-cycloaddition to a variety of alkynes the subsequent two-carbon ring-enlargement can serve as a versatile method for the preparation of hydroazulenes and dioxacyclooctadiene derivatives (Scheme 9.44) [74, 75]. 

A transition metal mediates rearrangement of a substituted (noncyclo) C5 species. This includes ring expansion reactions of, for example, substituted 3-vinyl- 1-cyclopropene (cf. Scheme 22) or cyclobutene, both bonded to a metal to give supracyclopentadiene Rh and Ru complexes (92- 94). The rearrangement of vinylcyclopropenes to cyclopentadienes can also occur photolytically (66). 

The intrinsic reactivity of strained cycloalkenes such as norbornene and cyclobutene ensures that they react as desired, and simple homogeneous metal halide catalysts are often effective for this transformation. However for less strained cyclic substrates, manipulation of catalyst activity/selectivity by means of modifying ligands is required. This is where the well-defined alkylidene catalysts pioneered by Grubbs and Schrock have come to the fore. An interesting example illustrating the range of catalyst reactivity is provided by the... 

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