Cumulene analogs

The replacement of a benzene nucleus by a thiophene nucleus in compounds containing other chromophores causes more or less pronounced bathochromic shifts. These effects have been noticed in the thiophene analogs of malachite green (23), in cumulenes such as... 

Another [5]radialene ketone, the tetraallene 102a, has been prepared in 74% yield by treating the [5]cumulene 98a with stoichiometric amounts of Ni(CO)2(PPh3)267 the sterically very shielded ketone 102b is produced analogously in 32% yield from the corresponding terminally bridged [5]cumulene 98b (equation 11). In both cases the dimers of the [5]cumulenes, the peralkylated [4]radialenes are formed as well (see Section II.B, Scheme 11). 

Analogous -substitutions take place when cumulenic ethers anti Grignard compounds are allowed. to interact in the presence of catalytic quantities of cqpper(I) salts. Organocopper compounds are the presumed intermediates [163,164]. 

The homologous cumulene 44 prepared by an analogous vinylidene insertion reaction is spontaneously transformed to the dimer 45.46 By contrast the stable tert-butyl derivative 46 undergoes thermal cycloaddition at one of the inner double bonds giving 47. The structure of this dimer was confirmed by X-ray diffraction.47... 

If there is an analogy between alkenes/alkynes and alkylidenes/alkyli-dynes, then the metal-based analogue of an allene would be a vinylidene (alkenylidene), LnM=C=CR2, with cumulenated M=C and C=C double bonds. Such complexes are well established and generally arise directly... 

Unlike the other naturally occurring enediyne molecnles, neocarzinostatin chromophore (30) is activated into an enyne-butatriene. This unusual cumulene 57 undergoes a cyclization analogous to the Bergman cyclization, proposed concurrently by Myers and Saito. This cyclization, now referred to as Myers-Saito cyclization, produces the diradical 58 that can abstract hydrogen atoms from DNA. Thus, neocarzinostatin s bioactivity is remarkably similar to calichaemicin (Figure 4.14). 

Cumulenes are a varied class of compounds, including species such as ketenes, allenes, ketenimines, and isocyanates as well as analogs where carbon is replaced by silicon or germanium, oxygen is replaced by sulfur or selenium, and nitrogen by phosphorus or arsenic. 

T -Complexes contain a metal-carbon single bond (Figure 1.1). The organic group may be alkyl, vinyl, alkynyl, aryl or acyl. With the exception of the acyl complexes, there are analogous compounds of more familiar metals, such as magnesium and zinc. It is also possible to have complexes with metal-carbon double and triple bonds these are known as carbenes and carbines. Cumulenes are also known, such as in vinylidene complexes. 

Following the isolation of these complexes, all of the mechanistic studies on the carbonylation and reduction reactions of nitroarenes catalysed by Ru3(CO)i2, even in the presence of several promoters, have focused on the reactivity of these or related clusters [157-164]. Moreover, many studies have been also conducted on analogous osmium [165-172] and iron (see paragraph 6.6.) clusters, including insertion reactions of isocyanates, which yield potential intermediates in the carbonylation reaction (Insertion reaction of other cumulenes into the Ru-N bond will not be discussed here. However, see the paragraph of the synthesis of heterocycles later in this chapter). Although not all of the previously mentioned studies were intended to be a basis for a mechanistic understanding of the reactions here discussed, they still contain a lot of information on the possible transformations of amido or imido moieties on a trinuclear cluster. 

Slde-on attack between cumulenes (left) or polyacetylenes (right), where one of the reactants may be ionic, can lead to carbon-ring formation through cross-bonding. This corresponds to the Inverse of the mechanism proposed for the transformation of graphite at high temperatures (33) which is analogous to the transformation of benzene to acetylene. 

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