Allenes cyclopropane ring

The process of transfer of one hydrogen atom from an allylic alkene (an ene) to an electrophilic alkene (an enophile) followed by formation of a new cr-bond and migration of allylic double bond is known as ene reaction or Alder ene reaction [1]. The ene may be an alkene, alkyne, allene, cyclopropane ring or arene, whereas an enophile may be an alkene, carbonyl, thiocarbonyl, imino or diazo compound having an electron withdrawing substituent. Oxygen may also serve as an enophile. The reaction usually takes place from left to right, since a new [Pg.161]

In 1998, Saigo et al. [150] discovered the Rh(I)-catalyzed allenic version of the vinylcyclopropane rearrangement. With a subsituent on the cyclopropane ring two different isomers can be formed depending on which of the different allylic C-C bonds in the three-membered ring is broken. Depending on R2, different regioselec-tivites were obtained with 250. Alkyl-substituted substrates lead to 251 and aryl-substituted substrates to 252 (Scheme 15.79) [150]. 

Carbonylative [5 + l]-cycloaddition of allenylcydopropanes was successfully achieved by use of an Ir(I) catalyst to yield (2-alkylidene)cyclohexenones in good yields (Scheme 16.43) [43]. No carbonylative [5 + l]-cycloaddition was observed in the case of an allenylcyclopropane lacking substituents at the allenic terminus. It can be deduced that the metal is too distant to open the cyclopropane ring, probably owing to the preferred -coordination at the allenic Jt-bond distal to the cyclopropyl group. 

In the case of Feist s acid 114 the possibility was admitted that the large value = 2.63 Hz was due to the electronic nature of the cyclopropane ring which imparts an allene type character to the molecule ... 

The inability to trap cyclopropanethione ifself has lead to a great deal of pessimism as to the catching of this thermodynamically unstable molecule. The best hope probably lies in the preparation of derivatives of cyclopropanethione in which the cyclopropane ring has been modified in such a way that there is a decrease in the stability of the allene episulfide form relative to the cyclopropanethione structure. It has been calculated that the energy difference at the MP-2 level between the optimized structures for 33 and its thiirane valence tautomer 34 is 2.4 kcal-moP, favoring structure 33. ... 

Other competing reactions include intramolecular alkylations of the intermediate meta-lated cyclopropanes when the leaving group is substituted at the cyclopropylmethyl carbon, e. g. 36 and 38. In this case no allene is produced and the major products are bicyclobutanes. However, when the leaving group is substituted at a site further removed from the cyclopropane ring, e. g. 41 and 43, allenes are produced in reasonable yield. 

Cyclopropylallene (55) and a number of alkyl substituted products have been investigated. In Fig. 16 we derive the h est occupied MO s of 58 from those of an allenic moiety and a cyclopropylmoiety. As anticipated there is essentiaily the same interaction between the cyclopropane ring and the adjacent double bond... 

This structure also accounts for all the observed chemistry for example the formation of cyclopropane rings with alkenes can be considered as (8+2]ir cyclo-addition reactions, with the allene actin as a conjugated tetraene whose termini share the same carbon atom. 

This section will be mainly devoted to intramolecular radical addition to acetylenic bonds. Some studies with radical addition to allenic systems and cyclopropane rings will also be discussed. Compared to the well-known inter-molecular free radical addition to ethylenic bonds, very little is known about free radical additions to acetylenic or allenic bonds, the same is true of intramolecular processes. 

The diazetidine (125) is slowly formed in the reaction of tm j -2,3-dimethyl-1-methylenecyclopropane with 4-phenyl-l,2,4-triazoline-3,5-dione. Retention of the cyclopropane ring in (125) contrasts with the products of the analogous reaction of the allene (126). A variety of ring systems, including... 

A novel iron-catalyzed, cationic cyclopropane ring opening was reported. Aryl-substituted cyclopropyl acetylenes gave ring-opened allene intermediates by a cationic rearrangement. In some cases, this gave functionalized naphthalene products. 

Gold(I)-catalysed allene-vinylcyclopropane cycloisomerization has been reported to give the tricyclic framework of the protoilludanes in a single step involving cyclopropane ring expansion and a Prins cyclization (Scheme 122). ... 

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