Subject four-membered rings

Bimetallic silylene-bridged complexes have been known for a long time and numerous articles related to this subject have appeared. Several compounds have been characterized, some of them also by x-ray structure analysis [165-171], For instance, the complex Mn2(CO)8(Si(C6H5)2)2 shows a distorted (MnSi)2 four-membered ring with a Mn-Mn bond [169], In the following section selected examples which have been described recently or are of particular interest in the present context will be discussed. 

Gas-phase decarboxylation of /i-ketocarboxylic acids XCOCH2COOH (X = H, OH, and CH3) has also been the subject of theoretical studies.42 Ah initio calculations reveal that decarboxylation via a six-membered (rather than four-membered) ring transition state is favoured. Activation barriers of 23.8, 23.3 and 28.5 kcal mol-1 have been calculated for decarboxylation of 3-oxopropanoic acid, acetoacetic acid, and malonic acid, respectively. Only marginal effects of solvent on the energy barriers and on the geometries of the reactants and transition structures are predicted. The activation energy predicted for reaction of malonic acid agrees well with the experimental value and rate constants have been predicted for decarboxylation of 3-oxopropanoic acid and acetoacetic acid in the gas phase. 

The photochemistry of a-diketones has been a subject of interest for about a century. Since the appearance of comprehensive review articles u in 1969 and 1971, activity in this area has continued with investigation of a number of new systems, particularly unsaturated diketones and diketones incorporated in a four-membered ring. New types of chemistry of synthetic and mechanistic interest have been revealed. The purpose of this review is to summarize these newer developments with briefer reference to some significant developments in the older reactions. 

Cyclophosph(V)azanes with five-coordinate phosphorus are mostly dimeric, four-membered ring derivatives, [X3PNR]2 (6). Several reviews cover the subject. A simple preparation of these compounds involves the reaction of PCI5 with primary amines (or their hydrochlorides), the so-called Kirsanov reaction. Weakly basic amines form monomeric Cl3P=NR, but with more basic amines (e.g. with R = alkyl), the products are the cyclic dimers. Various anilines give either a monomer or a cychc dimer, depending upon their basicity. ... 

The synthesis and properties of ylidic four-membered rings with four 71-electrons provides the subject for a short review by Weber. The reactions of l,l,3,3-tetrakis(dimethylamino)-lX,, 3X -diphosphete (38) with Ph2PCl and Mel form the basis of a study by Rosche et al. These reactions are summarised in (scheme 6). 

This subject has recently been reviewed6 and we shall concentrate on the different approaches to grandisol outlined in that review. Many of these approaches can clearly be used for the other compounds. An early approach was a chiral pool strategy from the simpler and commercially available terpene (R)-( (-linalool 38. Though this did not lead to a useful synthesis, it pointed the way to effective methods, particularly the Cu(I)-catalysed photochemical closure of the four-membered ring. 

The intrinsic reactivity of small rings is ascribable to ring strain relief in nature, and in squaric acid chemistry it is accomplished by conversion of rather stable cyclobutene-1,2-dione to the more reactive 4-hydroxy-2-cyclobutenone [55,56] as described in the previous section. At the same time, this conversion step fulfills the regiospecific introduction of substituents required for the targeted heterocychc structure. Thereby, the set-up four-membered ring is now subjected to directed synthesis through variable ring transformation reactions. 

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