Diallenic

The thermal solid-to-solid cyclization reaction of diallene derivatives also proceeds stereospecifically. Reaction of 1,6-diphenyl-1,6-di(p-tolyl)hexa-2,4-diyne-l,6-diol (113) with HBr gave meso- (114) and rac-3,4-dibromo-l,6-di-... 

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,... 

Nevertheless, new examples have also been reported of allenyl sulfoxides [101] and especially allenyl trichloromethyl sulfoxides [39, Y+-X = S(0)CC13] [102]. There have been many trials to use these sigmatropic rearrangements for the synthesis of diallenes (compare 40 — 42), e.g. the reaction of the diols 43 with trichloromethane-sulfenyl chloride 44 to the diallenes 45 (Scheme 7.7) [103]. 

We found that 43 (R1 = R2 = Me) and 44 lead to a mixture of the diallenes meso-and rac-45, which can be characterized by spectroscopy without doubt at low temperature [104], The isolation of 45 (R1 = R2 = Me) failed because this compound rearranged rapidly at room temperature most probably to the dienyne 46. Braverman et al. [105] analyzed spectroscopically the diallenes 48 resulting from diols 47 and the reagent 44. In one case, even isolation was possible. However, the cumulenes 48 cydize irreversibly to mixtures of the stereoisomers of the stable compounds 49 at room temperature, mostly at lower temperature. 

The heterocycles 205 are accessible by addition of the reagents NuH2, acting as double nucleophiles, to the diallenes 204 [104]. Compound 206 can also be prepared by this method and C,H-acidic compounds can serve as double nucleophiles as well. 

Diallenes such as 256 can lead to carbocydes by electrocyclic ring closure (Scheme 7.38) [80], Whereas 257 is formed on distillation of 256 at -160 °C, other diallenes cyclize at significantly lower temperatures to produce dimethylenecyclobu-tenes (see Scheme 7.7) [105]. 

Diallenes undergo rhodium-catalyzed cascade carbobicyclization similar to the SiCaB reaction (see Section 4.2) [21]. For example, the reaction of diallene 87 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at ambient temperature and pressure of CO gives ds-2-... 

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. 

Symmetrical incorporation of two units of 113 or 114 into one ring should yield either the monocyclic diallenes of D2 symmetry or of C-2h symmetry, depending upon the ways of combining the two enantiomers. Preparation of monocyclic diallenes in these two modifications was accomplished in Sondheimer s laboratory72a,b). 

Cyclodecatetraene, mp 36 °C, a 10-membered monocyclic diallene, was prepared 75,76) and was shown by means of X-ray analysis 77) to posses a meso-form. Force-field calculations were carried out in order to study possible conformations of this mesoform and the dl-form of this compound 78a,b). 

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... 

Keywords frarcs-diallene, [2+2]conrotatory cyclization, dimethylenecyclobutene... 

Ab initio and DFT calculations of the Diels-Alder reactions of vinylallene and diallene with dipolarophiles show that these reactions are concerted processes and that cumulated dienes are less reactive than non-cumulated dienes.153 Electron-rich... 

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. 

DFT studies on heteroatom diallenes have found, in line with previous experimental data, that biradical cyclization to a five-membered heteroaromatic ring is the preferred reaction pathway, although protonation of the heteroatom has been found to promote a competing cyclization to a six-membered, initially biradical, ring.59... 

Bowes, C. M. Montecalvo, D. F. Sondheimer, F. o-Dipropadienylbenzene and 2,3-dipropa-dienylnaphthalene. The oxidation of diallenes to cyclic peroxides with triplet oxygen. Tetrahedron Lett. 1973, 3181-3184. 

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],... 

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