Carbon Cumulenes

The authors of this work proposed a somewhat different evolution of intermediate VI with the production of structure IX (see Scheme 26.3). This structure, geometrically unacceptable for a carbon cumulene, is allowed for sulfur. 

Huisgen s introduction of the dipolar [3+2] cycloaddition reaction has provided an enormous variety of synthetically usefiil click reactions. The example quoted by Kolb and Sharpless is only the tip of the iceberg . Over 1000 literature references on this reaction were reported in recent years. I had summarized the cycloaddition reactions of heterocumulenes in 1967 °, but in the meantime many new cumulenes have emerged and the cycloaddition reactions of carbon cumulenes, such as allenes, butatrienes and higher cumulenes, are also well investigated. 

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... 

The first book on the reactions of carbon cumulenes, treating the cycloaddition reactions of ketenes in depth, was written by Staudinger in 1912 Staudinger already realized that cycloaddition reactions of ketenes are common, and often ketenes were only isolated as cyclodimers. The cyclodimers of isocyanates became prominent in the development of polyurethanes in the IG Farben Laboratory in Leverkusen, Germany in the early 1930s and the cyclotrimerization of diisocyanates led to the development of polyisocyanurate foams, with thermal stability superior to rigid polyurethane foams in the 1960s. Today, polyisocyanurate foams are used in the insulation of the fuel tank of the space shuttle. Also, carbodiimide derived cellular plastics with improved thermal stability are of interest. In recent years, cumulene derived polymers became of interest as one-dimensional molecular wires. 

The cyclotrimerization of carbon cumulenes is usually initiated by heat or catalysis. Especially, the use of a catalyst assures that trimerization can be accomplished in quantitative yields. The base catalyzed cyclotrimerization reaction seems to be limited to ketenes, isocyanates, isothiocyanates and carbodiimides. In the trialkylphosphine catalyzed trimerization of methyl isocyanate an asymmetric trimer is obtained. 

In cycloaddition reactions of carbon cumulenes with suitable substrates, [2+1], [2+2], [3+2] and [4+2] cycloaddition reactions giving rise to the formation of cyclic compounds are observed. In general, [2+1] cycloaddition reactions afford three-membered ring compounds with an attached double bond, and sometimes the initially formed cycloadducts rearrange to form an isomeric three-membered ring cycloadduct. An example is the addition of diphenylcarbene to dialkylthioketenes where the initially formed cycloadduct 15 on photolysis produces the isomer 16, with bulky substituents on the three-membered ring. ... 

Carbon cumulenes undergo [2+2] cycloaddition reaction with numerous double or triple bonded substrates to give four-membered ring cycloadducts. Examples of cycloaddition to C=C, C=C, C=0, C=N, C=S, N=0, N=N. N=S. S=0, P=C, P=0, P=N and P=S bonds are known. When the two adjacent double bonds in the cumulenes are different, cycloaddition across either one of the double bonds occurs, and sometimes addition across both bonds is observed. However, more often the cycloaddition reactions follow only one pathway. As a general rule, in ketenes the non-catalyzed cycloaddition occurs preferentially across the C=C bond, whereas catalyzed cycloaddition reactions proceed across the C=0 bond. In thioketenes, isothiocyanates and sulfenes addition mainly occurs across the C=S bond. In isocyanates addition across the C=N bond is preferred. 

In [2+2] cycloaddition reactions of carbon cumulenes, often only one four-membered ring compound is obtained. This reaction is of considerable importance in the synthesis of 8-lactams from ketenes and C=N double bond containing substrates. The j8-lactam structure is present in a variety of antibiotics. Also, j8-thiolactams are obtained from thioketenes and imines. 

The 1,3-dipolar systems involved in the cycloaddition reaction with cumulenes include azides, nitrile oxides, nitrile imines, nitrones, azomethine imines and diazo compounds. However, some 1,3-dipolar systems are also generated in the reaction of precursors with catalysts. Examples include the reaction of alkylene oxides, alkylene sulfldes and alkylene carbonates with heterocumulenes. Carbon cumulenes also participate as 1,3-dipols in [3+2] cycloaddifion reactions. Examples include thiocarbonyl sulfides, R2C=S=S, and l-aza-2-azoniaallenes. 

The [4+2] cycloaddition reaction of dienes with dienophiles, which is generally known as the Diels-Alder reaction, is one of the most useful reactions in synthetic organic chemistry. Many examples of carbon cumulenes participating as dienes, dienophiles, or both, are known. Even aryl substituted cumulenes sometimes react as dienes in [4+2] cycloaddition reactions. 

I have also included in this book the insertion reactions of carbon cumulenes into polarized metal single bonds, which can be perceived as an initial [2+2] cycloaddition, which subsequently rearranges to give a linear adduct. The reactivity of the metal substituent appears to be NR2 > OR > SR. When the metal compound contains several reactive groups, stepwise insertion occurs. For example, Sn(OR)4 reacts with phenyl isocyanate to give the tetracarbamate Sn[N(Ph)COOR]4. Mixed insertion products are obtained using different isocyanates. In the insertion reactions of carbodiimides sometimes ionic cyclic amidinate complexes are formed. 

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. 

Proton magnetic resonance spectroscopy can also be used to identify carbon cumulenes. The protons attached to the same carbon atom to which the cumulene group is attached are deshielded by the cumulene group and the chemical shifts of these protons are sufficiently separated from that of ordinary alkyl protons to allow characterization and also quantitization. Of course, this method is only of value in the aliphatic series because in aryl substituted cumulenes only p protons are present and the deshielding effect is minimized. 

The higher carbon cumulenes are assumed to be present in interstellar regions. Thus far stable dimers of the higher carbon cumulenes have not been isolated. 

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