Bonding allene

Unlike triple-bond participation, no remote cumulative double-bond (allenic) participation in solvolysis has so far been observed or reported. A summary of triple-bond and allenic participation in solvolysis involving possible vinyl cations is given in Table VI. 

In recent years The Chemistry of Functional Groups series has included three volumes on composite functional groups in which a C=C double bond was attached to another group. The chemistry of enones (edited by S. Patai and Z. Rappoport) appeared in 1989 The chemistry of enols (edited by Z. Rappoport) appeared in 1990 and The chemistry of enamines (edited by Z. Rappoport) appeared in 1994. We believe that the time has arrived for a book dealing with the combination of C=C double bonds, namely dienes and polyenes. The two double bonds can be conjugated, and conjugated dienes have a chemistry of their own, but even non-conjugated dienes show certain reactions that involve both double bonds. Allenes and cumulenes, which represent a different combination of the double bonds were treated in The chemistry ofketenes, allenes and related compounds, edited by S. Patai in 1980. 

Khulbe and Mann [155] have obtained infrared spectra of allene adsorbed on silica-supported cobalt, nickel, palladium, platinum and rhodium. The spectra were similar for all the metals, although variations in band intensity from metal to metal were observed. Addition of hydrogen to the allene-precovered surface resulted in similar spectra to those found for chemisorbed and hydrogenated propene in which the surface species was thought to be an adsorbed prop-1-yl group. The authors concluded that the initial allene spectrum was consistent with the adsorbed species being a 1 2-di-o-bonded allene (structure K)... 

The hydrogenation of cumulative double bonds (allenes) takes place at a rate comparable to that of the corresponding acetylene but no isomerization to the acetylene is observed during the reaction. "5 Selective hydrogenation to the alkene occurs since the allene readily displaces the product olefin from the catalyst before the hydrogenation can take place. Selectivities for monoalkene formation of greater than 95% were commonly obtained in the hydrogenation of various substituted allenes." This reaction selectivity is probably facilitated by the fact that the n electron clouds of the two double bonds are perpendicular to... 

Addition to CC multiple bonds. Allenes are carboxylated at the central carbon by the Ni-catalyzed reaction with CO2, hydroxylation follows on subsequent exposure to oxygen7... 

Allene complexes containing o-bonded allenes as ligands are also known u9>. 

A number of hydrocarbons are known which are isomeric with members of the acetylene series, but do not contain a triple bond. They contain four less hydrogen atoms than the saturated hydrocarbon with the same number of carbon atoms. A study of the reactions of these hydrocarbons has shown that the property of unsaturation which they possess is produced as the result of the presence of two double bonds. Allene, which is isomeric with allylene, has the structure represented by the formula H2C = C = CH2. The hydrocarbons of this series are called diolefines on account of the fact that they contain two double bonds. Allene is sometimes called propadiene. The first two syllables indicate that the compound contains three carbon atoms (from propane CaHs) the termination diene indicates the presence of two double bonds in the compound. Other compounds are named in a similar way. The methods of preparation and reactions of the members of this series are analogous to those of the hydrocarbons of the ethylene series. 

The properties of dienes depend upon the arrangement of the double bonds and the other substituents. If the double bonds are widely separated(as in isolated dienes),the compound behaves like a simple alkene. If the compound is conjugated, there is potential delocalization of the tt electrons. Conjugated double bonds are, in general, more stable than isolated double bonds because of the resonance effect. Cumulated double bonds (allenes) are more unstable than isolated double bonds because of their geometry and electron density. 

Polyynes (polyacetylenes). Compounds with very diverse structures containing several C/C triple bonds are produced mainly by fungi (basidiomycete cultures) and plants of the families Asteraceae, Apiaceae, and Araliaceae. In addition to conjugated triple bonds the R often also contain C/C double bonds, allene units, thiophene and furan rings. On account of the close biosynthetic relationships between these compounds, the term R is used as a collective name even when only one C/C triple bond is present in the molecule. As result of the work of Bohimann, E. R. H. Jones, Sorensen, and others more than one thousand natural R are now known. The antibiotically active mycotnycin (C H, g02, Mr 198.22, mp. 75 °C) from basidiomycete cultures, dehydromatricaria ester (CiiHgO, Mr 172.18, mp. 105-106°C) from Asteraceae, and the thiarubrins may be mentioned as typical examples (see also ter-thienyls). 

It could be argued that much of Allen s work, especially the early work, responds to iconic masculinity in cinema, for example. Play It Again, Sam (1972), is directly about the character s relationship with Humphrey Bogart. However, the Bond/Allen relationship has not been explored in any depth in the scholarly literature. 

Alkyne Hydrocarbon that contains a carbon-carbon triple bond. Allene The compound H2C=C=CH2. 

Across C=S bonds Allenes undergo a thermal and a photochemical [2+2] cycloaddition reaction with thioketones to give four-membered ring heterocycles. For example, irradiation of tetramethylallene and thiobenzophenone in benzene affords the [2+2] cycloadduct 246 in 64 % yield. As the result of the thermal reaction the linear adduct 247 was formed in 36 % yield... 

Across other double bonds Allenes undergo [2+2] cycloaddition reactions with azo compounds. For example, reaction of difluoroallene with diethyl azodicarboxylate affords a low yield of the [2+2] cycloadduct 274 °. 

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