Other 1-Carbon Cumulenes

Louie J (2005) Transition metal-catalyzed reactions of carbon dioxide and other hetero-cumulenes. Curr Org Chem 9 605-623... 

By definition, cumulenes are compounds with double bonds adjacent to each other. The parent compound of carbon cumulenes is allene, CH2=C=CH2, in which the center as well... 

A variety of other cyclization reactions are also observed with many of the carbon cumulenes. Especially, allenes and ketenes undergo many of these reactions and gold catalysis has achieved a new dimension in selectivity. From bis-allenes, complex natural products, such as 18,19 norsteroids, are generated in one step. 

The reaction of hexaphenylcarbodiphosphorane with carbon dioxide, carbon disulfide or isothiocyanates gives switter ionic adducts, which undergo a Wittig reaction on heating to give Ph3P=C=C=X. These reactions are described by Matthews and Birum and by Bestmann. Several other phosphorus cumulenes, such as... 

The cumulenes discussed in this book are subdivided into carbon- and noncarbon cumulenes, and the 1-carbon cumulenes (sulfines, sulfenes, thiocarbonyl S -imides and thiocar-bonyl S -sulfides) are excellent dipolar species. The 2-carbon or the center-carbon cumulenes (carbon dioxide and carbon sulfides) are less reactive but their imides (isocyanates, isothiocyantes and carbodiimides) readily participate in many of the discussed reactions. The 1,2-dicarbon cumulenes (ketenes, thioketenes and ketenimenes) similarly participate in cycloaddition reactions, as well as the more exotic 1,2-dicarbon cumulenes (1-silaalene, 1-phosphaallene and other metal allenes). In contrast, 1,3-dicarbon cumulenes are only... 

A anmileue is a compound with three adjacent double bonds. Draw an orbital picture of a cumulene. What kind of hybridization do the two central carbon atoms have What is the geometric relationship of the substituents on one end to the substituents on the other end What kind of isomerism is possible Make a model to help see the answer. 

A preparation of the thiazine ring system which should be investigated for generality is that of Baranova et al.,2il who interacted the l-aroyl-2-thioureas 151 with carbon suboxide to give 152. Compounds of structure similar to 152 were also obtained starting with 1,1-disub-stituted thioureas.247 Treatment of other cumulenes 153, ketenes(X = O)... 

Cumulative double bonds are those present in a chain in which at least three contiguous carbon atoms are joined by double bonds non-cumulative double bonds comprise every other arrangement of two or more double bonds in a single structure. The generic name cumulene is given to compounds containing three or more cumulative double bonds. 

Carbynes are supposed to be formed of sp-hybridized carbon atoms bound linearly, where two n electrons have to be involved, giving two possibilities, i.e., an alternative repetition of single and triple bonds (polyyne) and a simple repetition of double bonds (cumulene) (Figure 2.1) [19]. The detailed structure of carbynes is not yet clarified, but some structural models have been proposed [19-22], A structural model is illustrated in Figure 2.11, where some numbers of sp-hybridized carbon atoms form chains that associate together by van der Waals interaction between jr-electron clouds to make layers, and then the layers are stacked. Foreign atoms are intercalated between the layers that are supposed to stabilize the carbyne structure. In the carbyne family, the variety of structures seems to be mainly due to the number of carbon atoms forming a linear chain, in other words, to the layer thickness, and to the density of chains in a layer. 

This simplest cumulene is pictured above. The carbons at the end of the cumulated double bonds are sp2-hybridized and form one 7t bond to the "interior" carbons. The interior carbons are -hybridized each carbon forms two n bonds - one to an "exterior" carbon and one to the other interior carbon. If you build a model of this cumulene, you can see that the substituents all lie in the same plane. This cumulene can thus exhibit cis-trans isomerism, just as simple alkenes can. 

As we have noted in a previous section of this chapter, cumulenic resonance forms present one extreme description for dimetal complexes linked by all carbon bridges. They are frequently encountered in some oxidation state (mostly the dioxidised one) and also contribute to intermediate ones between the purely cumulenic and oligoynediyl or the cumulenic- and the alkynyl-bridged dicarbyne forms (see Schemes 6.6-6.8). Purely cumulenic wires are encountered in ot,a)-di- or -tetraferrocenyl substituted cumulenes, but no evidence other than a splitting of the ferrocenyl-based redox waves has been presented to support the presence of electronic interactions between the fer-rocenyl end groups across the cumulenic ligands. Based on the results of ex-... 

A good approach for the synthesis of infinite polyyne or cumulene may be the polymerization of a monomeric compound. Hay [10] in 1969, Matsuda et al. [11] in 1984 and Kudryavtsev [12,13] have shown the possibility of synthesizing carbyne by oxidative dehydropolycondensation of acetylene. They found that carbyne powder was obtained by passing acetylene through an aqueous ammoniacal solution of a Cu(II) salt. The carbyne obtained by this method was nano-crystalline but it might be unstable under an oxygen atmosphere. Most recent re-investigation of this synthetic route by Cataldo and Capitani [14], who used solid state C-NMR, infrared and Raman spectroscopy and other analytical techniques, has revealed that the carbonaceous matter obtained is indeed rich in carbynoid structures but also consists of sp -and sp -carbon atoms. 

On the other hand, it seems that in higher cumulenes (226, 227) inner carbon atoms (C3, C4 ) tend to become more or less electronically equivalent showing an average C nmr position of 5c 120 ppm, irrespectively of whether the cumulene is of the planar or antiplanar type. Generally, the terminal carbon-13 atom resonances of all the cumulenes under consideration (including ethylene) are related to their total CNDO/S electron densities according to Equation 88 (r = 0.9842) (Fig. 21). 

Another class of hydrocarbons has multiple 7i-bonds the allenes. The parent compound is 20 and is named allene. Formally, 20 is a diene and the lUPAC name is 1,2-propadiene or propan-1,2-diene. Allene is an example of a cumulene—a molecule that has cumulative %-bonds. The term cumulative indicates that there are three or more adjacent sp or sp hybridized carbon atoms. Note that the central carbon has two Ji-bonds and is sp hybridized, whereas the two flanking carbons are sp hybridized. This arrangement requires that the two 7i-bonds are perpendicular—one to the other—as shown in 20B, which positions the methylene units (the -CH2- units) in different planes. 

Dialkylketenes react with carbon dioxide, mediated by Ph3P, to aflbrd the six-membered ring [2 + 2 + 2] cycloadducts 170 (Scheme 58) (1971JOC2205). The C=C bond in ketenes also contribute in cycloaddition reactions with other cumulenes such as carbon disulfide to provide cycloadduct 171 (1971JOC2205). 

In allenes, the double bonds share a central carbon atom. There are other cumulated molecules in which heteroatoms (noncarbon atoms such as oxygen or nitrogen) take the place of one or more carbons of an allene and/or in which there are more than two double bonds in a row. When one end carbon of allene is replaced with an oxygen, the compound is called ketene (Fig. 12.6). If both end atoms are oxygens, the molecule is carbon dioxide. Cumulated hydrocarbons with three double bonds attached in a row are called cumulenes, although they can also be named as derivatives of 1,2,3-butatrienes (Fig. 12.6). Cumulenes are very reactive and most difficult to handle. 

C=C units are separated from each other by one or more 5p -hybridized carbons, and to cumulated dienes, or cumulenes (also called allenes), in which two C=C units share a single carbon (C=C=C). 

The chemical reactivity of the cumulenes under discussion ranges from highly reactive species to almost inert compounds. While some cumulenes can only be generated in a matrix at low temperatures, others are indefinitely stable at room temperature. For example, sulfines and sulfenes are only generated in situ, but some cumulenes with bulky substituents are sometimes isolated at room temperature for example, C=C=S was detected in interstellar space by microwave spectroscopy, and its spectrum was later verified by matrix isolation spectroscopy. In contrast, some cumulenes, such as carbon dioxide and carbon disulfide, are often used as solvents in organic reactions or in the extraction of natural products. The reactivity of some center carbon heterocumulenes in nucleophilic reactions is as follows isocyanates > ketenes > carbodiimides > isothiocyanates. However these reactivities do not relate to the reactivities in cycloaddition reactions. Often reactive cumulenes are isolated as their cyclodimers. Aromatic diisocyanates are more reactive than aliphatic diisocyanates in nucleophilic as well as cycloaddition reactions. 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

The direct [2+2] reaction of carbon dioxide with other cumulenes or activated double bonds is rare and often these compounds are not stable. However [2+2+2] cycloaddition reactions are more often observed. 

Cyclopentadienylcobalt complexes are also good for co-cyclotrimerization of alkynes with other unsaturated compounds containing the carbon-heteroatom double bonds, especially when they are part of the cumulene system such as isocyanates, diimides, and carbon dioxide. The reaction conditions are essentially the same as in the previously mentioned processes. However, the biggest problem remains the selectivity for the formation of heterocycles, because of the strong competition for the formation of benzene derivatives. Whereas co-cyclotrimerization of diimides and isocyanates results in the formation of reasonable yields of the corresponding heterocycles 170 and 171 (Scheme 75), in the case of carbon dioxide the yields are generally low [108, 109]. Recently, it has been shown that the ruthenium complex 106 is capable of efficient catalysis of co-cyclotrimerization of diynes and isocyanates [110] and isothiocyanates [111] under mild reaction conditions. 

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