Chelated adduct

Some of the available data on the neutral donor adduct formation of M(TTA)4 chelates are compared against the neutral donor basicities in Fig. 6. The parallel relationships observed between different metal ions suggest that the adducts involved are, most probably, isostructural. A similar relationship showing the diluent effect is given in Fig. 7 and a comparison between the stabilities of the Th(TTA)4 adducts and Th(IPT)4 adducts is shown in Fig. 8. Such correlations are of much practical value in estimating the stabilities of the neutral donor-metal chelate adducts from the data available for similar systems. 

In the previous section it was noted that in solution tin-derived Lewis acids, like titanium, prefer to form 1 2 acid carbonyl or 1 1 chelated adducts. Consistent with these findings, the crystal structure of... 

The mechanism of the HWE olefination is not fully understood. In the Still-Gennari modified HWE olefination the phosphorous has two electron-withdrawing trifluoroalkoxy groups. In this case the rearrangement from the chelated adduct to form the oxaphosphetane is favored and the elimination step is faster than the initial addition which essentially becomes irreversible (unlike in the case of the regular HWE olefination). As a result the formation of the (Z)-stereoisomer is predominant. 

Reactions of the a-methyl- -benzyloxy aldehyde 97 with allyltri-n-butylstannane 98 are summarized in Table 11-7 [83]. While little stereocontrol is observed in the BF3 OEt2-promoted allylation reaction, the chelate-controlled reaction catalyzed by either TiC or SnCl4 is much more selective, favoring formation of the Cram-chelate adduct 100 with up to 98 2 selectivity. The chelate transition state 101, where C-C bond formation occurs anti to the aldehyde a-methyl group, rationalizes the observed stereoselective formation of 100. Although the BF3-OEt2-cata-... 

The mesomeric diazo compound lOOa/lOOb, derived from diazo group transfer to the appropriate pyrazol-3-one, was irradiated for 10 h in tetrahydrofuran to give the hydrazone chelated adduct 101 as the only product (78H199) (Scheme 29). Mesomeric lOOa/lOOb reacts also with triphenylphosphine to give intermediate ( /Z)-phosphazine 102 which is extremely sensitive to water and hydrolyzes to EjZ)-hydrazone 103. 3-Hydroxy-5,5-dimethyl-cyclohex-2-enone 104 and 3-hydroxy-inden-1-one 106 readily couple with lOOa/lOOb in the presence of sodium acetate to give hydrazono derivatives 105 and 107, respectively. 

Table II. Effects of Uranyl Chelate Adducts on t in ScF COaat 50 C...

The chelate adducts formed between the lanthanide shift reagent [Eu(fod)a] and [Co(iS-diketonates)3] have relatively long lifetimes for dissociation (ca. 10- s). ... 

The generation of the cis-enolate 176 was accomplished through the utilization of Sn(OTf)2 (Scheme 4.18) [90]. Importantly, substrate 170 and Sn(II) form a chelated adduct 175 in which the ethyl group orients itself in a manner that avoids a double gauche pentane interaction while at the same time... 

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