Mechanism, cyclopropane ring alkenes

The complexes are binuclear, contain a bridging carbonyl and are coordinated by /(-bonded, five-carbon flyover carbene ligands. Their formation has been explained as being the result of the fragmentation of the cyclopropane ring after alkene insertion, providing indirect evidence for the proposed Magnus mechanism. 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

Stereochemical investigations of the above mentioned reactions of cyclopentadienyliron complexes have been helpful in understanding the mechanism of [2+1] cyclopropanation of alkenes with cationic cyclopentadienyliron carbene complexes. Based on the results of these investigations, the cyclopropanation is believed to occur due to electrophilic attack by the carbene center of metallacarbenoid 5 at the less substituted position of the alkene to produce cationic complex 6 which undergoes back-side ring closure to alford 7 or front-side ring closure to afford 8 depending on cation stability, lifetime and rotation of the fS-y bond in 6. ... 

The cyclopropane ring represents an interesting intermediate between alkanes and alkenes. Theoreticians have had great fun with this molecule, for which no simple (i.e. non-wave mechanical) picture is satisfactory. There is certainly some electron density in the middle of the ring, and it can be hydrogenated to propane quite easily, although less easily than propene. Alkyl- and alkenylcyclopropanes react in interesting ways (Chapters 7 and 11). 

Simmons-Smith reaction Areactionin which a cyclopropane ring is produced from an alkene. It uses the Simmons-Smlth recent, which was originally diiodo-methane (CH2I2) with aZn/Cu couple. Usually, diethyl zinc is used rather than Zn/Cu. The mechanism involves the formation of H2C(I) (Znl) and carbene transfer from the zinc to the double bond of the alkene. 

Linear correlations in the stoicheoimetric cyclopropanation by W(CHPh)(C0)5 with alkenes in a comparison with the catalytic reactions of Rh2(0Ac) suggest a unifying mechanism. Cyclopropanation stereochemistry is determined by interactions in the TT-complex and at the ring forming stage. 

DFT calculations have been carried out to shed some light on the mechanism for cyclobutene formation. No direct pathway for the formation of cyclobutene 11-14 from flnf/-exo-cyclopropyl gold carbene 11-13 was found (Scheme 3.9). In contrast, iyw-exo-cyclopropyl gold carbene 11-13 forms cyclobutene 11-14 by a cyclopropane ring expansion. The formation of iyn-exo-cyclopropyl gold carbene 11-13 has been postulated to occur by a iyn-type attack of the alkene to the alkyne gold moiety in 11-15. However the anti attack is more favorable the syn attack could compete if the substitution at the alkene and/or the alkyne does not favor the skeletal rearrangement [Ref. 15 in Chap. 1]. 

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