Cyclopropanation and Cyclopropenation

The C - C hond formation reactions are among the most important reactions in organic synthesis 

The previous highly reactive, electrophilic carbenoid [Rh-Rh] =C intermediate, formed during these catalytic reactions, remained elusive until a recent seminal study by Berry and Davies et al. 

17 M-Rh2(02CCF3)2- [(d-BrCgHjlPld-BrCgH lPh]- [(C6H4)PPh2]-2CH3C02H 60 50 50 83 H Et [96] 

Davies et al. carried out a comparative study on the catalytic activities of Rh2(S-DOSP)4 (3), RhjlR-BNP)4 (4), and Rh2(S-PTAD)4 (5) toward asymmetric cyclopropanation reactions (Table 9.2, entries 3-9) [94]. In general, Rh2(S-DOSP)4 was found to be the most effective catalyst for asymmetric intermolecular cyclopropanation of methyl aryldiazoacetates with styrene. Rh2(S-PTAD)4 exhibited lower levels of enantioinduction with 4-substituted aryldiazoacetates but it proved to be superior with the 4-methoxy substituted aryldiazoacetate (Table 9.2, entry 4, 96% ee) [94, 98]. Rh2(i -BNP)4, however, functions as an extremely effective catalyst with all the aryldiazoacetates substituted with three methoxy groups (88-97% ee). The asymmetric inductions varied with the aryl groups on the aryl diazoacetate but the choice of olefin had no effect on the selectivity. 

A range of catalysts were surveyed by Fox et al. for the cyclopropanation of alkenes with a-alkyl-a-diazoesters [92]. Catalyst 1 (Table 9.2, entry 1) exhibited better catalytic activity and selectivity 

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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... 

A comprehensive review on NMR properties of cyclopropanes and cyclopropenes is available". Thus, only some fundamental facts are collected here. 

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

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

Gem-Dihalocydopropanes belong to the most readily available cyclopropane derivatives known today. They have been shown to be extremely valuable starting materials for the preparation of cyclopropanes and cyclopropenes, they may be converted to bicyclobutane derivatives and spiropentanes, can lead to allenes and the higher cumulenes, cyclopentenes and cyclopentadienes, and many other classes of compounds, both hydrocarbon systems and derivatives with valuable functional groups. The article summarizes the preparative developments in the area of gem-dihalocyclopropane chemistry during the last decade. 

To summarize, gwi-dihalocyclopropanes may serve as starting materials for the preparation of cyclopropane and cyclopropene derivatives, they can lead to compounds with bicyclobutane and spiropentane structures, provide allenes and... 

Carbenes are much more reactive toward carbon-carbon double bonds than toward single bonds. Without doubt the most useful feature of a elimination is that it provides a practical route to cyclopropanes and cyclopropenes by [2 + 1 ] cycloaddition of carbenes to double or triple bonds. These additions are stereospecific suprafacial additions if they involve singlet carbenes, but can give mixtures with triplet carbenes ... 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

P. Rademacher, Photoelectron spectra of cyclopropane and cyclopropene compounds. Chem. Rev. 103, 933-976 (2003)... 

This chapter is concerned with advances of three-membered diphospha-cyclopropanes and -cyclopropenes, and from a formal point of view following on from Koenig s contribution <1996CHEC-II(1)469>. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Nuclear magnetic resonance and infrared spectra of cyclopropanes and cyclopropenes... 

The aim of this review is to show something of the NMR and IR characteristics of cyclopropane and cyclopropene rings both alone, in the presence of substituents and when built into more complex ring systems. Whereas for cyclopropanes a division into five subheadings has been made, the smaller amount of subject matter in the later sections has not impelled such a subdivision. 

The nitrogen extrusion from 1-pyrazolines and 3H-pyrazoles giving cyclopropanes and cyclopropenes, respectively, has been extensively reviewed The cyclopropane synthesis from 1-pyrazolines can be executed thermally as well as photochemically, but the latter method generally gives substantially better results than the former. The major side reaction observed in the thermal process is the production of olefins, which arise in the migration of a substituent from the C(4) to C(3) position. A retro-1,3-dipolar addition producing a diazoalkane and an olefin has been observed in certain cases. The decomposition of 3-acyl- or 3-alkoxycarbonyl-1-pyrazolines is catalyzed by acids, such as perchloric acid and boron trifluoride and by Ce The stereochemical course... 

MINDO/3 calculations relative to cyclopropane and cyclopropene show that the two highest molecular orbitals for silacyclopropane and silacyclopropene are destabilized. As a result, silacyclopropane and silacyclopropene are predicted to be more reactive toward electrophiles than their carbon analogues <79JCS(P1)443>. 

Also shown in Figure 1.11(c), for purposes of comparison, are three neutral classically bonded hydrocarbons, propane, cyclopropane, and cyclopropene. For these systems, and for electron-precise systems in general, the number of electrons available for bonding (n) is equal to the number of AOs available (and so precisely the right number to fill the nil bonding MOs). 

Carbenoid and nitrenoid reactions. Decomposition of diazoacetic esters by the novel Cu complex (1) in the presence of alkenes and alkynes gives cyclopropanes and cyclopropenes, respectively. Similarly, a nitrenoid is generated from TsN=IPh, forming aziridines in similar reactions. 

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