1.3- Dienes cyclopropenes

Cycloadditiai.—To IJ-Dienes. Cyclopropenes will undergo [4 + 2] cycloaddition with dienes and the further transformation of the ring-fused cyclopropane products can provide useful synthetic routes to new structures. An example of this is shown in Scheme 51. 

From detailed HNMR spectroscopic analyses of 4//-azepine systems it has been concluded that 4//-azepines are energetically more favorable than their bicyclic 3-azabieyclo[4.1.0]hepLa-2,4-diene valence tautomers 85 however, the valence tautomer 10 of methyl 6-phenyl-4//-azepine-2-carboxylate (9) has been trapped as the [4 + 2] cycloadduct 11 with 3,3-dimethyl-cyclopropene.112... 

The reactive compound chlorosulfonyl isocyanate (ClSOaNCO, forms P-lactams even with unactivated alkenes, as well as with imines, allenes, conjugated dienes, and cyclopropenes. With microwave irradiation, alkyl isocyanates also... 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

Very recent work111 has shown that the predominant formation of the endo adduct in the reaction between cyclopropene and isotopically substituted butadiene could be attributed to an attractive interaction between a C—H bond of cyclopropene and the jt bond being formed in the diene moiety. 

According to recent quantum mechanical calculations, die importance of secondary orbital interactions, which have also been frequently used to explain die endo diastereoselectivity of Diels-Alder reactions, seems to be questionable and to be reserved for special cases like the addition of cyclopropene to various dienes. T. Karcher, W. Sicking, J. Sauer and R. Sustmann, Tetrahedron Lett., 33, 8027 (1992) R. Sustmann and W. Sicking, Tetrahedron, 48, 10293 (1992) Y. Apeloig and E. Matzner,./. Am. Chem. Soc., 117, 5375 (1995). 

The thermolysis of certain members of the carbomethoxy series (146.C and 145.c) results in a decarbonylation reaction that affords the exclusively E -diene complexes 150 or the E/Z-diene complex mixtures 151, respectively. These products may be formed by direct thermolysis of the precursor cyclopropene (144) in the presence of diiron nonacarbonyl, isolated in 80% overall yield and in a approximate ratio of 3 2 1 (150 Z-151 -151), a product distribution matching that obtained from the isolated vinylketene... 

In the addition of Me2CuLi reagents to electron-deficient acetylenes [85-88], DCD-type complexes have been identified by NMR [84, 89]. As shown below, an ynoate affords a vinylcopper intermediate, while an ynone instead affords an allenolate (Eq. 10.9). The origin of this diversity remains unclear. A related carbocupration mechanism has also been proposed for the reaction with allenylphosphme oxide [53]. Olefin carbocupration of dienes [90] and cyclopropenes [34, 36] is known, but these mechanisms also remain unclear. 

The introduction of 1,2-bromochlorocyclopropene (27) by Billups as a synthon for cycloproparenes constituted a major achievement and made a large number of previously unaccessiblecycloproparenes, lacking substituents at Cl, available. The cyclopropene 27 is labile and decomposes at room temperature, but nevertheless undergoes addition to a variety of dienes around -20 °C. In some cases reaction times of several days or even weeks are necessary in order to obtain complete conversion. The stereoselectivity for the addition of 27 to open-chain dienes is usually exo, as in the case of the tetrahalogenocyclopropenes. Bis-(hydro,halo-geno)elimination is carried out with r-BuOK in THF, as in the case of the adducts of tetrahalogenocyclopropenes. 

Metal-catalyzed cyclodimerization has been restricted to buta-1,3-dienes, norbornadienes (see Houben-Weyl, Vol. 4/4, pp 295-299) and more recently to strained alkenes such as cyclopropenes and methylenecyclopropanes (see Sections, 3., l.B.2.2.2. and 2.B.2.4.2.). 

Similarly, the complex 3 was formed from 3,3-dimethylcyclopropene and 2,2 -bipyridyl( 4-cy-cloocta-l,5-diene)nickel, which on treatment with maleic anhydride at 25 °C gave anti-3,3,6,6-tetramethyltricyclo[3.1.0.02,4]hexane (4) in >90% yield. Displacement of the hydrocarbon ligand from 3 with 3,3-dimethylcyclopropene proceeds at > 90"C. Since the complex 3 is regenerated in this step, 3 is the catalyst in the cyclodimerization of the cyclopropene.120... 

The reactive compound chlorosulfonyl isocyanate796 CIS02NC0 forms 3-lactams even with unactivated alkenes,797 as well as with allenes,798 conjugated dienes,799 and cyclopropenes.8 l, OS V, 673 65, 135, 140. 

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