Cyclopropanes small ring compounds

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

The electroreductive preparation of highly strained small-ring compounds is a useful general synthetic method [555-557]. The electroreduction of 1,3-dibromide (468), leading to the cyclopropane derivative (469) has been achieved via an organo Hg species, which involves sequential one-electron reductions with the intervention of organo Hg(I) radicals and dimeric Hg(I) species (Scheme 161) [558-560]. 

The authors, the editor and the publisher express the hope that this latest volume Small Ring Compounds in Organic Synthesis just like the previous five will help to stress the ever-increasing importance of cyclopropane chemistry and stir new interest in its multifold applications. 

The vibrational spectra of several small ring compounds have been studied in sufficient detail so as to provide force constants for the vibrational modes7 Cyclopropane has shorter CH bonds with larger stretching force constants than for cyclobutane. The CH bonds in cyclopropane resemble those in ethylene, corresponding to their increased i-orbital character. 

The structures of bicyclic and polycyclic small-ring compounds have attracted some interest. The relationship between the C-C-H bond angles at the bridgehead of bicyclo[1.1.0]butane (2) and the angle between its cyclopropane rings has been studied.31 Bicyclobutane and most other cyclopropane derivatives have bonds that are formed from orbitals that are bent in the same direction. However, with some mm -fused bicyclic compounds containing a cyclopropane... 

Comparisons have been made for some groups of small-ring compounds." Some examples are shown in Table 5.100 It can be seen that cyclopropane has larger C-H stretching force constants than propane, and that they are similar to those of ethene. This is in accord with the %s character in the C-H bonds. 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

Cyclopropanones act as three-carbon sources in [4 + 3] cycloadditions. However, these small-ring compounds are impractical for preparative scale experiments, because they are generally inaccessible and also must be handled with special precautions, in contrast to other oxyallyl species. One of the early descriptions of the synthesis of cyclopropanones appeared in 1932. NeverAeless, the development of the chemistry of cyclopropanones proceeded at quite a slow pace until the late 1960s when Turro reported their utilization in [4 -i- 3] cycloaddition reactions. 2,2-Dimethylcyclopropanone (14) adds to furan, cyclopentadiene, 6,6-dimethylfulvene and the relatively nucleophilic N-methylpyrrole to give the corresponding cycloproducts, but fails to react with anthracene and 1,3-butadiene. Parent cyclopropan-one cannot be used. 

Perhaps it is appropriate that this review be written some 100 years after the report of the first syntheses of cyclopropane derivatives by von Baeyer and Perkin and the formulation of the theory of ring strain by von Baeyer. The chemistry of small ring compounds has risen to prominence in the last 30 years. The popularity especially of the three-membered rings as intermediates in synthetic transformations has been due primarily to their latent energy content and to the almost endless number of chemical transformations in which these compounds and their derivatives can participate. New applications and novel permutations of the basic systems continue to appear at a fast pace. The fascinating chemistry... 

A review has discussed the photochemical reactivity of small ring compounds via charge transfer complexes. The oxidation of the cyclopropane derivatives (74) on illuminated ZnS or CdS has been studied. Irradiation of the cyclopropane (75) in pentane for 20 h yields the dimers (76, 35X) and (77, 15X). 

For a review see Salaiin, J. Small Ring Compounds in Organic Synthesis Synthesis and Synthetic Application of 1-Donor Substituted Cyclopropanes with Ethenyl, Vinyl and Carbonyl Groups, Top. Curr. Chem. de Meijere, A., Ed. Springer Berlin, 1988 Vol. 144, pi, and references cited therein. 

The cyclopropane ring, due to its ring strain, can be considered as a functional group comparable to the double bond with the synthetic potential to generate functionalized three carbon chains via ring opening. Besides thermal-, photochemical-, oxidative-, reductive-, radical-, nucleophile- and Lewis acid or electrophile-mediated activation, the conversion of cyclopropanes mediated by transition metals plays an important role in synthetic uses of small-ring compounds. In most of these cases, prior to conversion, a complexation of the cyclopropane system by the transition metal is necessary. 

Bonding in Cyclopropane and Other Small Ring Compounds... 

The hunt for strained molecules is maintained by the competitive ambition to find the most unusual structures and by the enormous synthetic potential of small-ring compounds. Following the general trend of recent years asymmetric syntheses are at the cutting edge of this research. [1] Despite all advances the synthesis of enantiomerically and diastereomerically pure cyclopropane derivatives remains a considerable challenge, especially when particular functional groups are required. The most... 

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