Rings 3-membered, substitution

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

The comparatively unreactive complex Mn(CO)3(C5H5BMe) (14) with MeCOCl/AlCl3 produces the 2-acetyl derivative 84 and small amounts of [Mn(CO)3(PhMe)]+ (27). The product ratio is rather insensitive to reaction conditions. It is reasonable to assume a common intermediate 85 (of unspecified stereochemistry at C-6) which under kinetic control may either irreversibly deprotonate to 84 or undergo a rearrangement ultimately leading to the ring-member substitution product (27). 

Because of the high ring strain of the four-membered ring, even substituted oxetanes polymerize readily, ia contrast to substituted tetrahydrofurans, which have tittle tendency to undergo ring-opening homopolymerization (5). 

A simple example of a ring-opening substitution reaction is the acid-catalysed hydrolysis of epoxides. In the example shown, protonation of the epoxide oxygen improves the leaving group, and an Sn2 reaction may then proceed using water as the nucleophile. Three-membered rings must of necessity... 

Fig. 17 Variation of reaction free energy (kcal/mol) with temperature (K) for alkyl C—H hydrogen atom loss in (a) five-membered ring methyl-substituted heteroaromatic rings and (b) six-membered ring methyl-substituted heteroaromatic rings.

The phosphorus ring member has phosphenium character. It adds tetrachloro or/Ao-quinone and 2,3-butanedione and it also reacts with dimethylaniline and anisole in a pura-substitution... 

The thermal isomerization of dihalocyclopropanes is facilitated by the presence of aryl, alkyl and alkoxy substituents in the three-membered ring. Phenyl substituted systems may give rise to indenes, and in the alkyl series the dehydro-halogenation of 2,3-dihalocyclopropane derivatives may lead to dienes. 

Reactions of Nondonor Chelate Ring Member in the Intact Ring. In some cases, chelate ring members will react without impairment of the stability of the complex. The most studied example involves substitution at the central carbon atom in / -diketone complexes (16-19, 21, 45, 64). This is illustrated with the bromination reaction in Equation 26. Acylation, formylation, and nitration reactions have also been carried out. 

Electron donor 2-substituents orient substitution in furan, thiophene and selenophene into the 5-position. In pyrrole, although the ratio of a to (3 reactivity is much smaller than in the other five-membered rings, 5-substituted 2-alkylpyrroles still appear to be the major products. 

Investigation of the geometry of 1, related three-membered rings and substituted 1 has been a major research goal of structural chemists for decades5,26 2 32,33-71- 83 94->25 since these... 

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