Cyclopropenes cyclopropanation with

Addition to a carbon-carbon triple bond is even more facile than addition to a carbon-carbon double bond, and there are now several reports of intermolec-ular [71] and intramolecular reactions [72-74] that produce stable cyclopropene products with moderate to high enantioselectivities. One of the most revealing examples is that shown in Scheme 9 [72] where the allylic cyclopropanation product (30) is formed by the less reactive Rh2(MEPY)4 catalyst, but macrocy-clization is favored by the more reactive Rh2(TBSP)4 and Rh2(IBAZ)4 catalysts and, as expected, the highest enantioselectivities are derived from the use of chiral dirhodium(II) carboxamidate catalysts. 

Alkynes can be converted into cyclopropenes by inter- [587,1022,1052,1060-1062] or intramolecular [1070] cyclopropanation with electrophilic carbene complexes, Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result from rearrangement or other transformations of the cyclopropenes initially formed (cf. Section 4,1,3),... 

Cyclopropenation reactions are also effectively catalyzed by dirhodium(II) compounds, and high enantiocontrol has been achieved with the Rh2(MEPY)4 catalysts (Scheme 15.3) [47]. A striking example of the catalyst effect on selectivity is found in the behavior of substrate 25 toward Rh2(5S-MEPY)4 and the more reactive Rh2(4S-IBAZ)4 (Eq. 10) [48]. With the less reactive Rh2(5S-MEPY)4 it preferentially undergoes allyhc cyclopropanation with high chemoselectivity and enantiocontrol. With the more reactive Rh2(4S-IBAZ)4 addition to the carbon-carbon triple bond is favored even though this involves construction of a ten-membered ring. 

Cyclopropanation and Cyclopropenation. At this time, intermolecu-lar cyclopropanation with alkyl diazoacetates is best accomplished with cobalt cat-... 

The interesting reaction of cyclopropenes 20 with nitriles mediated by gallium(III) or indium(III) salts offers a new route to isomerically pure pyrroles 21 in low to moderate yields. A mechanistie rationale for this transformation was also provided <030BC4025>. Likewise, pyrroles 21 have been obtained via a cascade comprising [3 -i- 2] dipolar cycloaddition of a wide variety of nitriles to the donor-acceptor containing cyclopropanes 22, following dehydration, and isomerization <030L5099>. 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

The addition of phenyllithium to cyclopropene occurred with 99% stereoselectivity to give cw-l-lithio-2-phenylcyclopropane, albeit in 3 /o yield. With 3-methylcyclopropene, the reaction was more efficient and, after protonation of the lithiocyclopropane, 2-methyl-1-phenyl-cyclopropane was obtained in 44.5% yield. The product is about 94% in the trans form thus attack of the organolithium occurs on the opposite face to that bearing the larger substituent at C3. When the cyclopropene is 3,3-disubstituted, e.g. 17, both faces are sufficiently hindered to slow the addition process and metalation of a vinylic hydrogen predominates. ... 

Here, decomplexations of ligands in which the cyclopropane ring is preserved are described. In some cases the ring system can be modified during decomplexation. Thus, the cyclopropene complex 1 obtained via complexation of cyclopropane with bis(cyclopentadienyl)niobium(IV) dichloride under reductive conditions, upon treatment with hydrochloric acid, liberated cyclopropane in > 90% yield. 

This was not done in our previous paper [28] where the experimental data with respect to TMS for cyclopropene, cyclopropane and benzene were presented. 

Cyclopropenes and Methylenecyclopropanes as Multifunctional Reagents Table 10. Codimerizations of (diphenylmethylene)cyclopropane with alkenes... 

Besides strained cyclopentenes, cyclobutenes such as dewar benezene derivatives (see Table 5) also undergo cyclopropanation with respectable yields. In contrast, cyclopropenes react with diazomethane in a complex manner to form mixtures of monomeric... 

Reaction of cyclopropane with sodium or potassium amides in liquid ammonia results in the loss of a vinylic proton rather than a methylene proton [198,199] similarly methyl cyclopropene-1--carboxylate exchanges its vinylic but not its methylene proton in base [200]. The structure of the cyclopropenide anion is clearly shown by its H-n.m.r. spectrum. In the case of the sodium salt this is as follows 56.73 (t,lH), -0.45 (d,2H), J = 1.94 Hz [199]. These compounds dimerise on steuiding with alkyl halides they give mono- and di-alkyl cyclopropenes [199] ... 

The metal-carbenoid intermediate has been widely applied in organic synthesis for cycloaddition, cyclopropanation, and selective C-H bond insertion [245, 246]. The traditional methods to prepare metal carbenoids are from diazo compounds, and the recent reports have shown the feasibility to generate metal carbenes or carbenoids in situ from some precursors, such as alkynes [247] and cyclopropenes [248]. With great efforts, the metal-carbenoid chemistry was esteemed as one efiftcient redox-neutral C(sp )-H bond functionalization protocol (Scheme 2.42). Herein, we list several key reviews in this topic to readers for extending reading [249-254]. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The similarity between the reactions of alkenes and cyclopropanes is further demonstrated by the reactions of electrophilic cyclopropanes and cyclopropenes with enamines. Cyclopropylcyanoester74, when treated with the pyrrolidine enamine of cyclohexanone, undergoes what would be a 1,2 cycloaddition in the analogous alkene case, but is actually a 1,3 cycloaddition here, to form adduct 75 (90). A similar reaction between the... 

Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, em-dialkyl, r-butyl, and angular-methyl substituents (779), compounds often available only with difficulty otherwise (.77,5i,55,750,756), Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26). 

The relevant orbitals to consider are the molecular orbitals 5Aj and 6A, of cyclopropene (III.46) which correlate with the lowest acc orbital of cyclopropane (3A, III.56) and with one of the higher degenerate <7CC orbitals (3E, III.56). The cyclopropene orbitals are very similar to their cyclopropane counterparts. In particular the 6Aj orbital is a linear combination of all three crcc bond orbitals and extends over the entire cyclopropene molecule. Its amplitude, shown in Fig. 46, is fairly accurately represented by the... 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

More than twenty sterols with cyclopropanes or cyclopropenes have been isolated from marine organisms [53], The majority of these come from sponges, although significant numbers have been isolated from dinoflagellates and co-elenterates. There are no terrestrial organisms known to contain such sterols and thus their biosynthesis is purely a marine problem. 

A variety of functionalized [3]radialenes have been prepared starting from the appropriately substituted cyclopropanes or cyclopropenes. West and Zecher have pioneered the chemistry of [3]radialenes with quinoid substituents. The general strategy of this synthesis is outlined in Scheme 513. A tris(4-hydroxyphenyl)cyclopropenylium... 

Reaction of Me3GeCl with a substituted cyclopropene in the presence of lithium diisopropylamide (LDA) yields different products depending on the order of addition of the reagents.97 Addition of LDA to a mixture of the reactants gives the dimetallated cyclopropene (Equation (76)). Dilithiation of the cyclopropane followed by addition of Me3GeCl gives the allene (Equation (77)). 

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