Alkynes, -cycloaddition with cyclopropanes

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Trialkylsiloxyallylidene)cyclopropanes 58, prepared from the corresponding ketones 57, underwent facile cycloadditions with activated alkenes and alkynes to give 59 and 60, respectively (Table 6). The enol ethers were hydrolyzed directly upon aqueous workup. 

The [3-1-2] cycloaddition is mostly catalyzed by Ni or Pd catalysts. The MCPs can have substituents on the olefin or cyclopropane, and the two-atom partners can be electron neutral or deficient alkenes, alkynes, and carbon-heteroatom multiple bonds. Since there are different reaction courses of MCP in cycloaddition, introducing substituents on either MCP or the two-atom reaction partners complicates the reaction even more, considering the associated selectivity issues. Consequently, cycloaddition with multi-substituted substrates often gives mixtures though sometimes good selectivity can be achieved by adjusting reaction conditions and substituents. The intramolecular version of [3-1-2] cycloaddition, mainly developed by Motherwell [81], Nakamura [82], Lautens [83], and Mascarenas [87-91], may address the issues of selectivity to some extent, and leads to polycyclic structure meanwhile. AU these have been summarized by several excellent reviews [1, 84—86] and herein we will only update the [3-1-2] cycloaddition of MCP for synthesis of carbocycles with some recent representative examples. 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

In a mechanism for the [67i+27t]/[67i+27i]/[2o +27t] process, the first two steps involved in this overall transformation were presumed to parallel the pathway observed for the above described [67t+27t], [67t+27t] process. A third cycloaddition of the cyclopropane unit with an additional alkyne component occurred via a [2[Pg.113]

If the [3-h 2]-cycloaddition reaction of a methylenecyclopropane is performed with a remote alkene or alkyne moiety within the same molecule, products 2 of the bicyclo[3.n.0]-type (n < 3) or the bicyclo-[n.3.0]-type (n > 3) can, in principle, be obtained. The former of these product types can be considered as unhkely, as an alkene or alkyne separated by only one or two atoms from the methylenecyclopropane would give rise to a strained annulated ring, i.e. a cyclopropane or a cyclobutane. It is known from investigations of photochemically initiated intramolecular [2 3-2] cycloadditions of alkenes that the minimum size required for a spacer between 7t-systems is three atoms in a reaction of a non-crossed type. There are actually a few examples of photoinduced intramolecular cycloadditions of the crossed type involving substrates with short spacers (n =... 

Dihydro-2-isopropyl-3.6-dimethoxypyrazine, a bis(lactim)ether, is converted into the 5-di-azo compound 1 by lithiation and diazo group transfer. The intermediate diazo compound reacts at room temperature with olefins such as cyclohexene to produce the cyclopropane derivatives with excellent diastereoselectivity . The derivative from cyclohexene is hydrolyzed by acid treatment to give methyl 7-cv(cfo-aminobicyclo[4.1.0]hcptane-7-carboxylate. The bis(lac-tim)ether diazo compound 1 is also involved in an exceptional asymmetric [2 + 1J cycloaddition producing cnantiomerically pure cyclopropenc derivatives4. Thus, reactions of the diazo compound with monosubstituted alkynes afford the spiro compounds as one diastereomer. Hydrolytic removal of the auxiliary and protection of the amino group provides enantiomerically pure methyl l-amino-2-arylcyclopropene-l-carboxylates in moderate overall yield. 

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