Cumulene, cyclic

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

Strained Cyclic Alkynes, and Strained Cyclic Cumulenes. 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Studies on Cg show that cyclic cumulenes are not well described by a one-determinant wavefunction. In a valence-CASSCF calculation on cyclic Cg, for example, the Hartree-Fock determinant has a weight of only 0.40. The problem is assumed to be aggravated for larger systems, as the HOMO-LUMO gap diminishes. 

Disilene 21 can react with the C=N bond in a cyclic imine 73 and cumulenes such as carbodiimide75 and keteneimine74 (Eqs. 18-21). The reactions with cumulenes are not straightforward, although the initial products are most likely [2 + 2] cycloadducts. 

Cyclononatriene, the smallest cyclic [3]cumulene isolated so far, polymerizes when its solutions are concentrated57. On the other hand, several radialenes have been isolated which represent cyclodimers of seven- and eight-membered 1,2,3-trienes (Scheme 9). 

The A state of 4b is predicted to be a transition state for interconversion of the enantiomers of a cyclic cumulene (7 in Fig. 12) that is an isomer of 3b. In 7 the roles of the lone pairs on nitrogen and carbon are reversed, so that the nitrogen is the central atom of a 2-aza-allene unit, formed by overlap of the... 

When the cyclic acetylene 260 is generated from a suitable precursor, it undergoes an isomerization reaction spontaneously generating naphthalene (263) and benzo-fulvene (264) as the finally isolable products. Very likely the process begins with a retro-Diels-Alder reaction to the [3]cumulene 261, which in a cascade reaction via the semicydic allene 262 rearranges to 263 and 264 [111]. 

Arynes, Strained Cyclic Alkynes, and Strained Cyclic Cumulenes. .. 42 147... 

Aside from fuUerene research, carbon clusters have always attracted much interest among scientists. " Linear cumulene-like Ce (Pooh) 132 is well known from computational studies as well as experimentally. " Cyclic Cg, which will be considered as the extreme case of a dehydrobenzene here, has a long and varied history. " In particular, two cyclic isomers possessing D h and Dg/, symmetry (133 and 134) have been discussed. " According to the most recent calculations, 133 ( Aj) is lowest in energy 134 ( Ai ) and 132 ( S )) being less stable by 8 and 10 kcal/mol, respec-tively. 42-... 

Cyclic allenes in general are less strained than their acetylenic isomers.345 The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air,34 ... 

Many examples of ylides possessing the P=C=P functionality have been synthesized and characterized. Symmetric, asymmetric, cyclic and acyclic examples are known36-43. The known crystal structures of these ylides are listed in Table 4. Whereas most small cumulenes are linear, cumulenes having more than two double bonds are frequently bent48 - 50. The P=C=P angle is the most characteristic feature of these types of ylides. 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

Two reactions which involve addition of sulfur reagents to alkenes have been reported for the first time. In the first, a mercapto[15]annulenone can be prepared from an annu-lenone by heating with sodium sulfide and elemental sulfur313 (equation 50). In the second, addition of sulfur to cyclic cumulenes gave rise to a series of interesting cyclizations, the... 

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