Formation of a Four-Membered Ring

In most transition metal-catalyzed reactions, one of the carbene substituents is a carbonyl group, which further enhances the electrophilicity of the intermediate. There are two general mechanisms that can be considered for cyclopropane formation. One involves formation of a four-membered ring intermediate that incorporates the metal. The alternative represents an electrophilic attack giving a polar species that undergoes 1,3-bond formation. 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

The (phosphino)(silyl)carbene 2a has been shown to exhibit this diagnostic reactivity. Under rather drastic conditions (300°C, 10 2 mm Hg), 2a has been converted into the azaphospholidine 37, in high yield, as a mixture of four diastereomers, which result from the presence of three asymmetric centers.4 The high regioselectivity of the carbene insertion (no formation of a four-membered ring was detected) is intriguing, especially since the same regioselectivity is observed upon thermolysis of bis(diisopropyl-amino)phosphinodiazomethane li.4 Yet, in contrast, four-membered heterocycles were obtained with other phosphinocarbenes (Sections V,C,1 and 3). 

Formation of a four-membered ring of carbon atoms can be achieved only with substantial distortion of the normal valence angles of carbon, regardless of whether the ring is planar or nonplanar. In cyclobutane, for example, if the valence bonds are assumed to lie along straight lines drawn between the carbon nuclei, each C-C-C bond angle will be 19.5° smaller than the 109.5° tetrahedral value ... 

The chelating bonding mode is less common for triazenido and amidino ligands than the bridging mode. A factor may be the steric bulk of the ligand, but another factor is clearly the strain involved in the formation of a four-membered ring, which is clear from the acute NYN angle of about 102-105° (Table 3). 

Stork then analyzed the transition state requirements for the formation of a four-membered ring (28) and realized that it seemed easier to attain col-linearity in this case than in that of the five-membered ring (22), arriving at the unexpected conclusion that the former process could prevail over the latter. 

Barnett and Sohn (12, 13, see also 14) have discovered that the iodolactoni-zation of b,r-unsaturated carboxylic acid salts 37 yield, under kinetically controlled conditions, the Y-iodo-B-lactones 39 in preference to the more stable B-iodo-Y-lactones l. Similar results were obtained in the course of the bromolactonization reaction. Thus, here again, the formation of a four-membered ring is more facile than that of a five-membered ring. This can be rationalized on the basis of Stork s analysis, i.e. the internal opening by the carboxylate anion of the three-membered ring iodonium ion (or bromonium) 38 39 is preferred over the other mode of opening 40 41 for stereoelectronic reason. 

We have already shown that the formation of a four-membered ring from cis- and trans-piperylene with a nickel-ligand, catalyst is not consistent with... 

Formation of a Four-Membered Ring From a Carbonyl-Conjugated Alkene... 

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