Chelate ring, discovery

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

This reaction process is depicted in general terms in equation (34), and includes the category generalized in equation (35). The earliest example of this type of reaction appears to be the ready transesterification of uncomplexed ester groups shown in Scheme 39,133,134 which intramolecular participation forms a new chelate ring in the transition state. Since this discovery, numerous studies have been made on the intramolecular catalysis of ester hydrolysis by metal-complexed hydroxide... 

Kspecially interesting is the fact that the epoxide ring is activated by intramolecular chelation" <37). Pnor to this discovery, 18 standard reagents had tailed in opening the epoxide.7 The reducing agent attacks activated epoxide 37 selectively from the sterically more accessible side and produces a tertiary alcohol Selective protection of the secondary alcohol leads to compound 16... 

In fact, the discovery of 4.69 (and indeed the majority of supramolecular chemistry), was preceded by the observation in 1967 that the much more flexible bidentate Lewis acid F2BCH2CH2BF2 (4.70) is capable of chelating methoxide ion to give five-membered rings as shown in Scheme 4.5. 

Attempts to model the intrastrand d(GpG) crosslink with nucleobases have met with only moderate success. Usually the 06 atoms of the two guanosine rings are on opposite sides of the platinum coordination plane ( head-to-tail isomer). Only for cw-[Pt(NH3)2(9-EtG)2] was the correct isomer obtained. Nucleobase complexes of the ds-diammineplatinum(II) moiety have been valuable for testing the controversial proposal of N7,06 chelate formation, which to date has not been observed. Several interesting discoveries of metal-nucleobase chemistry are that metal binding can stabilize rare tautomers, for example, the... 

The significant progress of the stereoselective aldol reaction is the discovery of a six-membered transition state to realize the acyclic stereocontrol (Scheme 8.2). The cation is chelated with an enolate oxygen and an aldehyde oxygen in the transition state, which constructs the six-membered ring to be more stable. Acyclic stereocontrol reduced labors and shortened the steps taken in the synthesis. Therefore, the speed of synthesis has been accelerated and numerous natural polyketides have been synthesized. 

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