5-Membered rings relative reactivity

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

There are very few totally synthetic antibiotics presently on the market. One of these is the 1-oxacephem, moxalactam (96). One may speculate that the enhanced potency of moxa-1actam stems in part from the substitution of the smaller oxygen atom for the sulfur normally present in the six-membered ring of cephalosporins thereby enhancing the reactivity of the adjoining four-membered ring. It is also partly a measure of the present stage of development of chemical synthesis and of the relative economics of production of 7-aminocephalosporanic acid that such an involved synthesis apparently is economically competitive. 

Five-membered heteroaromatic ring compounds have been extensively studied in terms of their relative reactivity and specific physicochemical properties. In particular, it is generally accepted that the degree of aromaticity follows the order... 

Rates of detritiation of the 1 and 2 positions of biphenylene and the 5 position of benzo [bjbiphenylene by lithium cyclohexylamide at 50 °C relative to benzene have been determined as 490, 7.0, and 1,865, respectively and the enhanced reactivity of the position a to the strained 4-membered ring has been attributed to the enhanced electronegativity of the strained bridgehead carbon atom. The same... 

Some trends in relative reactivity for intramolecular ketene cycloadditions have been examined by internal competitions.172 For example, 12 gives exclusively 13, pointing to a preference for five-membered rings over six-membered ones. 

Diazaphospholes constitute the most widely investigated class of heterophospholes, the 67t-aromatic phosphorus heterocycles [1,2]. Diazaphospholes are unique in the manner that the five-membered ring incorporates one phosphorus atom. First diazaphosphole representative, i.e. 2//-[l,2,3 diazaphospholc was obtained as early as 1967 [3] and until 1980s the interest of organophosphorus chemists remained in the development of different synthetic routes and in investigating their varied reactivity due to the structural diversity within the class [4], On the basis of the relative positions of the three heteroatoms in the five-membered ring, six monocyclic diazaphosphole systems (A-F) are possible and all of them have been reported (Structure 1). 

The corresponding cyclohexenyl system 56 (Scheme 16) remains relatively unreactive, however, even when the reaction is performed under an ethylene atmosphere after 24 h (10mol% lb, 1 atm ethylene, CH2Cl2), only 10-20% of chromene 47 is obtained. This persistent lack of reactivity is presumably because (1) the relatively strain-free six-membered ring is less prone (relative to cyclopentenyl and cycloheptenyl structures) to react with LnRu=CH2 [21], and (2) in case ring rupture does occur with the proper regiocontrol to afford 57... 

Relative Reactivities of Cyclic Hydrocarbons with 4-7 Membered Rings" (78)... 

The structure of the amine component has a profound influence on the propensity to p-7i delocalization. As an example, enamines derived from cyclic five-membered ring amines, such as pyrrolidine, are more than thousand times more nucleophilic than those derived from six-membered ring amines. In a series of fundamental studies, the Mayr group has determined nucleophilicities of different enamines, ranging from highly reactive pyrrolidine-derived amines to relatively... 

As encouraging as these results were, the authors were concerned that the relatively poor reactivity of their catalyst left little scope for the introduction of additional steric constraints that could potentially improve the selectivity. These concerns appeared to be a real issue when initial structural modifications led to no major improvement in either selectivity or reactivity. Worse, replacing the 5-membered ring phosphine 1 by either the corresponding 6-membered ring phosphine 2 or the... 

The chemistry of the closest relative of hydrocarbon 1- methylenecyclopropane (2) - has recently been reviewed extensively [2]. The presence of the second three-membered ring in 1 strongly increases the total strain of this molecule, this enhances its chemical reactivity and leads to specific physical properties. The meanwhile large variety of experimental material obtained for compound 1, some of its derivatives and analogs prompted us to summarize the results in order to help set the stage for future developments. 

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