Cyclopropenes special

Trimethylenemethane is a special type of alkene that does not exist as the free compound. Various synthetic equivalents to the synthon 43 shown below have been reported. Trost, in particular, has exploited these compounds in 1,3-dipolar cycloaddition reactions.138 139 A metal-bound, isolated trimethylenemethane species was recently reported by Ando (Scheme 6). It resulted from the complexation of an ero-methylenesila-cyclopropene with group 8 carbonyls (Fe, Ru).140,140a The structure was proved by X-ray crystal structure analysis.29Si NMR data were consistent with the -structure shown. 

Transition-metal catalyzed decomposition of alkyl diazoacetates in the presence of acetylenes offers direct access to cyclopropene carboxylates 224 in some cases, the bicyclobutane derivatives 225 were isolated as minor by-products. It seems justified to state that the traditional copper catalysts have been superseded meanwhile by Rh2(OAc)4, because of higher yields and milder reaction conditions217,218) (Table 17). [(n3-C3H5)PdCl]2 has been shown to promote cyclopropenation of 2-butyne with ethyl diazoacetate under very mild conditions, too 2l9), but obviously, this variant did not achieve general usage. Moreover, Rh2(OAc)4 proved to be the much more efficient catalyst in this special case (see Table 17). 

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 reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Of special interest is the first recording of a cyclopropene C(metal) l3C NMR signal which appeared at <5126.2 ppm (t, 7C P 23.0 Hz), considerably lower than the <5108.7 ppm observed for cyclopropene264. 

Metallic hydrides, such as lithium aluminum hydride and sodium borohydride, do not in general reduce carbon-carbon double bonds, although this can be done in special cases where the double bond is polar, as in 1,1-diarylethenes and in enamines." Lithium aluminum hydride reduces cyclopropenes with a pendant alcohol in the allylic position to the corresponding cyclopropane. ... 

These reactions are summarized in more detail in Section 1.2.1., as topologically they constitute syntheses from C, + Cj building blocks. Some special intra- and intermolecular cases in which cyclopropenes serve as alkenes are discussed in Section 1.1.6.3.1.2. 

P-H bonds in H-phosphonates and secondary phosphine oxides could also be added to special aUcene derivatives, cyclopropenes, with Pd catalysts [25]. Scheme 15 shows the proposed mechanism, including P-H oxidative addition, insertion into the Pd-H bond, then P-C reductive elimination. The observed... 

Very recently, it has been shown that on the basis of the energetic criterion of antiaromaticity and the proton affinity of 3-cyclopropenyl anion (13) this ion does not merit being differentiated from other aUylic anions and is therefore best thought of as non-aromatic. Cyclopropene is the smallest cycloafkene, and its conjugate base at C3 is considered to be a special anion that is destabihzed due to the presence of 4jt electrons in this fuUy conjugated monocycHc species. Its acidity, however, follows the same correlation as for cyclobutene, cyclopentene, cyclohexene, and propene. No additional parameter beyond the central C—C—C bond angle is needed to explain or account for the weak acidity of cyclopropene. 

Rhodium carboxylates Rh2(02CR )4 (specially R =CH3, C3H7, C(CH3)3) are so far the best catalysts for promoting, under mild conditions, the cycloaddition of carbalkoxycarbenes to mono- and di-substituted triple bonds to give cyclopropenes in good to excellent yields (90 O [69]. 

A highly specialized use of a ketone involved addition of1.290 to cyclopropen-one 1.289, which proceeds with ring opening to give methyl 3-amino-2-methyl-3-phenylprop-2-enoate (1.291) 1 The yield of amino acid is poor, and the scope of the process is limited to forming only a few amino acids due to the difficulties in preparing cyclopropenone derivatives. 

The cyclopropene ring system has special interest because of its highly strained double bond. The vinyl C-H bond has additional s-character that is in between that of a vinyl C-H and an acetylenic C-H. 1-Lithiocyclopropene (5) was calculated to be approximately 27 kcal mol" more stable than 3-lithiocyclopropene, but still about 26 and 35 kcal mol higher in energy than allenyllithium and propynylithium, respectively. ... 

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