Allenes diallenes

It should be further noted that the factor between the 4-3IG and experimental azz values for acetylene and ethylene is 0.91 (for no other unsaturated hydrocarbons are experimental results available). Assuming that this value remains reasonably constant for other unsaturated hydrocarbons as well, one can define the combined factor/ = 1.57 = l/(0.7 X 0.9) to predict from STO-3G anticipated experimental azz values z (exp) = a (STO-3G) X 1.57. Bodart et employed this procedure to scale their STO-3G azz values both in the vinylacetylene series (proceeding to = 3) and for a larger number of other unsaturated hydrocarbons (details are available elsewhere ). In a subsequent paper Delhalle et also applied the same scaling procedure to the allene, diallene. 

Although this route to a,cu-diallenes from terminal bisolefmic precursors has often been used in allene hydrocarbon chemistry, the reaction is actually complex and yields product mixtures containing cyclic and bicyclic hydrocarbons when the spacer between the double bond and the allene group is short (between one and three methylene groups), see (a) Th. Miebach, H. Wiister,... 

Geminal dibromocyclopropanes have been reduced by chromium(lI) - (-i-)-tartrate complex and by butyllithium - (+)-sparteine complex to give cyclic allenes and 1-phenyl-l,2-butadiene with very low enantioselectivity (< 1 % op)122. In addition, cyclic diallenes of unknown optical purity have been reported123, l24, e.g., 3,4,9,10-cyclododecatetraene-l,7-dione is prepared using a stoichiometric amount of methyllithium and a sixfold excess of (-)-sparteine123,124. 

Cook and co-workers took advantage of the molybdenum-based methodology and reported one of the most graceful applications of the allenic PKR. They intended to use PKR to synthesize a theoretically interesting tetracyclic compound 55, which has 14 vr-electrons and is supposed to be aromatic in principle. To this end, they have applied PKR to various alkynyl allenes, and finally employed a diyne-diallene substrate 54 successfully (Equation (27)). Double PKR by employing molybdenum hexacarbonyl in excess and DMSO afforded the requisite intermediate in a high yield. 

Since the synthesis of allenes from dihalocyclopropanes has been reviewed,130"132 only representative examples highlighting the process, along with the more recent results, are presented in this section. The formation of diallenes (equation 48) illustrates the usefulness of this process.133,134 Two other interesting examples are given in equations (49) and (50).133-137... 

The work reported by Eaton and coworkers can be summarized as reactions where an allene system in conjugation with a further unsaturated functionality reacts with carbon monoxide in the presence of an iron-carbonyl complex such as Fe(CO)5 under photochemical and thermal conditions when Fe2(CO)9 is used. When diallenes are used (X=R2C=C, Scheme 9.24), five-membered carbocyclic products are obtained [51, 52], whereas when allenyl ketones (X = O) are applied, five-membered lactones are generated [53, 54]. The use of allenylimines (X = NR) leads to five-membered lactams under these conditions [55]. 

The reaction with unsymmetrical allene subunits result in the predominant formation of the E-configured exocyclic double bond, typically with a ratio of 80 20 for the lactones and 90 10 for the lactams. An identical trend is observed for the diallene system, where both exocyclic double bonds exhibit predominantly the E-configuration in the product. 

As mentioned in the introduction [lc], no selectivity was observed in early dimerization experiments of 1. But when other partners were offered, the corresponding crossdimerizations were quite selective. Probably methylene metallacyclopentenes 2 [4], which could be isolated, are intermediates that then react with the other partners. Generally, the related 1,3-dienes are less reactive than 1 with its reactive allenic double-bond and do not react in a similar manner [4a]. Rh-catalyzed [4+1] cycloadditions with CO as a second reaction partner led to alkylidene cyclopentenones 3 and 4 [4, 5], while in Pd-catalyzed reactions where 1 was generated in situ and a base was present, only 4 [6] was formed. When Pt(0) was used instead of Rh(I) in the carbonylation reaction, both in the presence of the (R,R)-DuPHOS-ligand, opposite enantiomers of 3 were obtained [5b], This observation still needs a precise explanation. [Fe(CO)5]-mediated reactions of diallenes form dialkylidene cyclopentenones 7 (Scheme 2, here 10 mol-% of catalyst are needed) [7],... 

Braverman, S., Duar, Y. Thermal rearrangements of allenes. Synthesis and mechanisms of cycloaromatization of it and heteroatom bridged diallenes. J. Am. Chem. Soc. 1990,112, 5830-5837. 

Not surprisingly, many of the methods which can be used in the synthesis of conjugated dienes can also be applied to dimerization of allenes. Thus, the bromoallenes (61) can be smoothly dimerized, for example, by treatment with 2 equiv. of copper(I) chloride in DMF at ambient temperature to give good yields of the diallenes (62). ... 

Some thermal cyclization reactions of allene derivatives in their own crystals are described. Intramolecular cyclization of diallene derivatives within their crystals proceeds stereoselectively according to the Woodward-Hoffmann rule. For example, heating of colorless crystals of meso- 19 and rac-diallene 23 at 150 °C for 1 h gave the in,out 22 and a 1 1 mixture of in,in 26 and out,out 27 cyclization products, respectively, in quantitative yields (Scheme 2.2.1) [14]. Since no liquid state was observed during the cyclization reactions, the reaction in the crystal is a real crystal-to-crystal reaction. An X-ray analysis showed that 19 and 23 have an s-trans configuration in the crystal [15], In order to cyclize to 22, and to 26 and 27, 19 and 23 should first isomerize to their s-cis-isomers 20 and 24, respectively, in the crystal. The conformational change, from the s-trans form to the s-cis form, requires... 

Treatment of the diselenocyclic allene shown with diphenyldiazomethane in refluxing benzene affords 2,2-diphenyl-1,3-selenane 46 in modest yield (Equation 6) <2001JOC7202>. Presumably, the reaction involves a complex series of diphenyldiazomethane additions, losses of nitrogen, and rearrangements. Irradiation of the diallene shown affords the acetylenic diselenane 47, again in modest yield (Equation 7) <2001JOC1787>. 

The (15Z)-isomer of the diallenic compound mimulaxanthin (202) has also been synthesized (Chapter 3 Part IV) and the (9Z,9 Z)- and (9Z,9 Z,15Z)-isomers were obtained by direct synthesis via an analogous route from the corresponding allenic (Zj-Cis-aldehyde 39 [37] (Scheme 15). 

Treatment of the a r/-tetrabromide (268) with methyl-lithium in the presence of (—)-sparteine gave two diastereomeric diallenes, (269) and (270). Their identity was shown by the fact that (269) is optically active whereas (270) is not, consistent with its we o-formulation. Both allenes were hydrolysed to their respective ketones, which were also optically active and inactive... 

Reaction with Allenes. A diallene, hexatetraene (eq 32), reacts with Fe(CO)5 with insertion of carbon monoxide to form 2,5-dimethylenecyclopent-3-enone. Allenyl ketones, on the other hand, form dinuclear complexes. ... 

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