Bidentate Leaving Groups

Bidentate Leaving Groups. A comparison of the rates of the acid-catalysed removal of pentane-2,4-dione (acac) from [Cr(hedta)(acac)] ion and [Cr(edda)-(acac)] has been made. The pendant carboxylate arm in the former complex accelerates the rate of acac loss by a factor of 10 compared with the usually observed rate, and this complex is more reactive than the protonated derivative, [Cr(hedtaH)(acac)]. For [Cr(edda)(acac)J, a normal rate is observed at 25 °C, 10 k=2.99 s- (7= 1.0 M). 

Values of AH, AS, and AV for the acid-catalysed aquation of cis-[Cr(LL)2(OH2)2]- ions (LL=ox or mal) are collected in Table 14 (see Introduction, ref. 11). The small positive AV values for the aquation reactions are consistent with a mechanism in which proton attack at the co-ordinated uniden- 

Reiter, V. V. Udovenko, and I. Berna, Russ. J. Inorg. Chem. Engl. Transl.), 1979, 24,27. 

Some of the steps in the reaction of [Cr(ox)2(gly)] ion with oxalic acid (Haox) in acidic nitrate media are as follows  

All three steps in the successive H+-catalysed loss of salicylaldehyde (L) from [CrLj] have been examined. For each step, / obs = / o + i[H+], with the H+-independent pathways catalysed by anions such as NOg and SO4 , except for the first step i.e. when co-ordinated H2O is not present). Anation of [Cr(H20)s] + by sulphate ion was also investigated and proceeds via an h mechanism.  

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We certainly do not intend these rules to restrict future research, but only to encompass a large amount of past experience with platinum(II) complexes. Obviously exceptions will, and have already, occurred. For example, the high activity of bidentate leaving groups such as oxalate and malo-nate (see structures of Fig. 4) first synthesized by M. Cleare and J. Hoe-schele in this laboratory are not encompassed nor is the effect of cyclic amines, developed by Tobe, which decrease the solubility of the complexes, but markedly enhance the antitumor activity. Here, studies of the rela-... 

More recently, antitumor activity has been observed with compounds containing bidentate leaving groups, such as malonate, sulfate or nitrate (for instance 35, 55-57). 

L, L = mono- or bidentate ligand S = solvent or vacant LG = leaving group Nu = nucleophile... 

Zr L = a bidentate hydroxypyranonate or hydroxypyridinonate ligand and X is a halide or alkoxide leaving group (264,265). Replacement of chloride by thiocyanate, pyrazine, or water (in acetonitrile solution) in 3-hydroxy-4-pyranonate or 3-hydroxy-4-pyr-idinonate tin(IV) complexes in all cases follows a two-term rate law of the type... 

In this example, significant chiral induction can only be realized if silver salts are added to trap the iodide ions, and thus a chelating coordination mode of the chiral bidentate ligand is facilitated during coordination of the olefin (Scheme 2, cationic pathway). Even better results are obtained from substrates with non-coordinating leaving groups such as vinyl triflates. 

SRN1 substitution with substrates bearing two leaving groups and bidentate or monodentate nucleophiles, followed by a polar or catalyzed reactions. 

Another approach to ring closure is the reaction of a substrate having two leaving groups in o-position and a bidentate nucleophile. For instance, in the photostimulated reaction of 0-bromochlorobenzene with 3,4-toluenedithiolate ions (332), 3-methylthianthrene (333) is formed (55%). When the substrate, such as 0-diiodobenzene, has a better leaving group, an increase in the yield of 333 to 64% is observed (equation 197)343. 

Studies other than preparative-type transformations have so far been limited to B-pyrazolylpyrazaboles. For example, the latter compounds have been found to react with R BX (where X is a readily leaving group) to form polynuclear boron spiro-cations (see Sect. V.A). It has also been shown that the pyrazolylpyrazaboles offer themselves as a class of neutral bidentate chelating ligands For example, the complexes H2B(pz)jB(pz)2ZnCl2 and Cl2Zn(pz)2B(pz)2B(pz)jZnCl2 were readily accessible by combination of the two reactants in an appropriate solvent. Obviously, a more detailed study of this feature is mandated. 

The bidentate ligands (RR C(pz)2) react with R"2BX compounds, where X is a leaving group, to form boronium cations [(RR C(pz)2)BR"2]+ isosteric with pyrazaboles and isolable as PF6 salts.202... 

The 4,5-dihydro-3//-pyrazole nucleus, and hence cyclopropane derivatives, can be constructed through a number of reactions in which hydrazine (or a hydrazine derivative) acting as a bidentate nucleophile undergoes addition to a three-carbon fragment such as A, a 1,3-dihalide, B, an o ,/ -unsaturated carbonyl compound, C, a carbonyl compound with a leaving group on the 8-carbon or D, an a,/S-epoxy carbonyl system. 

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