Formation of three-membered rings

Nucleophilic additions of titanium nucleophiles to nitriles or isonitriles are relatively rare. Eisch et al. recently reported the formation of three-membered ring titanacycles of type 84 and their reactions with benzonitrile and carbon dioxide, respectively (Scheme 34).119,120... 

These cycloadditions lead to the formation of three membered rings commonly seen in the carbene addition to double bonds. 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

Formation of three-membered-ring systems can also be the result of a nucleophilic addition leading to a zwitterionic intermediate, followed by loss of nitrogen. In these cases, the reactions are related to the addition of sulfur or phosphorus ylides to Michael acceptors. ... 

The formation of three-membered rings is reversible cf. ref. [159c]. With allenes as alkenes, < >Heck-Stille... 

In this section, we have seen the formation of three-membered ring intermediates when a neighbouring group participates in a reaction. We will now look at some other reactions involving three-membered rings, but this time ones that exist in either the starting material or the product, rather than just as an intermediate. 

Although the formation of three-membered rings by cyclopropanation of olefins with metal carbenoids is commonplace, the construction of such systems via intramolecular C-H insertion is quite rare. This is because 1,2 migration of any hydride atoms a to the carbenoid center is typically very facile, rendering it inactive toward further transformations [56], It was found, however, that [i-tosyl a-diazo carbonyl compounds 37 are suitable substrates for intramolecular 1,3 C-H insertion reactions catalyzed by achiral rhodium carboxylates 25 (Scheme 6) [57],... 

Further Wittig analysis of (33) gives aldehyde (35). Optically active ( )-perillaldehyde (36) is available (it is a natural product) so disconnection (35a) becomes very attractive. Formation of three-membered rings by displacement at a tertiary centre is known (Chapter 30). 

Formation of three-membered rings. Among chiral ligands for the Simmons-... 

The most common application of carbenes in synthesis is in the formation of three-membered rings by addition to multiple bonds. This is a typical reaction of all carbenes that do not undergo intramolecular insertion. Generation of the carbene in the presence of an alkene gives a cyclopropane product. Addition of halocarbenes to alkenes is a stereospecific cis reaction, but this is not necessarily the case with all carbenes. Hence Z-2-butene 109 gives the cyclopropane 110, in which the two methyl groups remain cis to one another (4.86). The stereospecificity... 

Now Sn2 opening of the protonated lactone with the soft nucleophile (bromide ion) gives the y-bromoketone that cyclizes through its enolate. The formation of three-membered rings is favoured kinetically. 

The formation of three-membered rings is reversible Compare Ref. [38c]. 

F. Formation of Three-Membered Rings, including Cyclopropanations... 

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