Formation of four-membered rings

Codimerisation occurs not only between different fluoroalkenes but also between fluoroalkenes and other unsaturated hydrocarbons. Moreover, some of these codimerisa-tions proceed more readily than the reactions involving only fluorinated alkenes. Examples of addition reactions of fluoroalkenes are shown in Table 7.13. Rate constants have been measured for [2tt + 2tt] and [2tt + 4tt] cycloadditions involving fluorinated alkenes, employing gas-phase NMR techniques [227]. 

A particular driving force appears to be the presence of Cp2= in the alkene 1,2-difluoroaUcenes, —CF=CF—, are much less reactive in this context but examples have been recorded [235]. 

With few exceptions, products are formed which result from a combination of alkenes in a head-to-head maimer, or a correspondingly regiospecific manner in codimerisations (see Table 7.13). Furthermore, the reactions are not stereospecific. 

In considering the mechanism of these reactions it is important to stress that they are [2tt - - 2tt] additions, which are formally forbidden as thermally induced [2tts + 2tts] processes according to the well-established Woodward-Hoffman rules for pericyclic reactions. Consequently, it is much more likely that these reactions proceed via a pathway that involves radical intermediates, although concerted processes have been claimed [238]. 

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The formation of four-membered-ring sulfones and a-sulfonyl amides has also been applied to the reaction of methanesulfonyl chloride with ketene aminals and acetals (470-473). 

The lithium as well as the sodium derivative of 1,1,3, 3-tetra-tert-butyl- l-fluoro-3-hydroxydisiloxane should be predestined for the formation of four-membered rings by intramolecular salt elimination. The results of X-ray structural analysis seem to confirm this assumption. Each molecule of the lithiated derivative forms a six-membered ring showing intramolecular LiF-contact three molecules form a trimer, containing a central Li-O six-membered ring with three-coordinated lithium [8]. The sodium analogue of this compound reveals a surprisingly similar structure. 

A review which includes the formation of four-membered rings among a range of other heterocycles has appeared <99MI637>. 

Examples of the formation of four-membered rings by intramolecular C-H insertion of electrophilic carbene complexes are listed in Table 4.4. 

As exemplified in Scheme 30, the formation of four-membered ring systems via inter- or intramolecular [2-1-2] cycloadditions between the Co,=Cp moiety and C=N or C=C double bonds have also been described [89, 129, 189]. 

Configurational match/mismatch governs these reactions so that different products may be produced in high yield and selectivity when enantiomeric catalysts are apphed to the same substrate [61, 62], as illustrated by the reaction processes in Scheme 15.8 [61]. Additional examples in the steroidal field have also been reported [66]. In these cases the formation of four-membered ring y9-lactones is common. 

Formation of Four-membered Rings by Making Two Bonds from [2+2] Atom Fragments 471... 

Michael addition has been employed as a key step in the formation of four-membered ring compounds.6 As outlined, indirect generation of carbanions from 3,3-dimethyl-4-tosyloxybut-... 

The influence of various buffers on the solvolysis of 5-(trifluoromethanesulfonyloxy)pent-2-yne in anhydrous 2,2,2-trifluoroethanol at 25 C for 24 hours has been studied.24 Sodium and calcium carbonate, 2,6-dimethylpyridine, pyridine and quinoline all favored the formation of four-membered rings, whereas potassium carbonate, triethylamine and sodium 2,2,2-trifluo-roethoxide suppressed formation of the rearranged products. One of the solvolysis products is 2,2,2-trifluoroethyl-2-methylcyclobutenyl ether (see Section 8.A.2.I.). 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

Reactions of cyclopropanones with nucleophiles frequently lead to ring enlargement reactions since the formation of four-membered rings from the reactive intermediates is accompanied by a considerable reduction in strain energy. Thus, 2 reacts with diazomethane to form cyclobutanone96>, with hydrazoic acid to form (3-lactam 76,89) and, under special conditions, with amines and hydroxyl amine derivatives to form N-sub-stituted (3-lactams 87> (Scheme 24). 

Before Interrante s studies, synthesis of poly(silylenemethylenes) was limited by the availability of the monomer. It was found that ethoxy substituents on silicon lead to improved yields for the formation of four-membered rings <1996JOM(521)l> (Scheme 19), then ethoxy functionality can be converted into chloride. Therefore disilacyclo-butane monomers became easier to approach. 

In this chapter, the theoretical methods, crystal structures, characteristic nuclear magnetic resonances, reactivities, and formation of four-membered rings with two heteroatoms including selenium or tellurium that have appeared in the last decade are described. 

The generation of the carbodiimide groups in the presence of excess isocyanate groups leads to the formation of four membered ring cycloadducts 3, which can react further with isocyanate or carbodiimide to give the thermally stable six membered ring cycloadducts 4 or 5, respectively. The formation of 4 or 5 accounts for the observed... 

The two groups of reactions with low frequency factors, namely, the decompositions of the acetals and the decompositions of the chloroformate esters, would both have to go through the formation of four-membered-ring transition complexes, although the acetals can also decompose via a six-membered ring. While it might be expected that the formation of a... 

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