Chelation structure

Usually, iodides and bromides are used for the carbonylation, and chlorides are inert. I lowever, oxidative addition of aryl chlorides can be facilitated by use of bidcntatc phosphine, which forms a six-membered chelate structure and increa.scs (he electron density of Pd. For example, benzoate is prepared by the carbonylation of chlorobenzene using bis(diisopropylphosphino)propane (dippp) (456) as a ligand at 150 [308]. The use of tricyclohexylphosphine for the carbonylation of neat aryl chlorides in aqueous KOH under biphasic conditions is also recommended[309,310]. 

Hydrogen bonding plays a major role in pyrazolone tautomerism, and the formation of a chelate structure can shift the equilibrium towards the chelated form. Structures (135) and (136) are two representative examples of such stabilized tautomers. Structure (137) is a hypothetical example of stabilization of the NH tautomer. 

Acyl-, 4-alkoxycarbonyl- and 4-phenylazo-pyrazolin-5-ones present the possibility of a fourth tautomer with an exocyclic double bond and a chelated structure. The molecular structure of (138) has been determined by X-ray crystallography (Table 5). It was shown that the hydroxy group participates in an intramolecular hydrogen bond with the carbonyl oxygen atom of the ethoxycarbonyl group at position 4 (8OCSCII21). On the other hand, the fourth isomer is the most stable in 4-phenylazopyrazolones (139), a chelated phenyl-hydrazone structure. 

The crystal structure of the adduct of titanium tetrachloride and the ester formed from ethyl 2-hydroxypropanoate (ethyl lactate) and acrylic acid has been solved. It is a chelated structure with the oxygen donor atoms being incorporated into the titanium coordination sphere along with the four chloride anions. 

Stabilization of the methylene tautomer may be attained also through the formation of a conjugated chelated structure, as is the case of 265 [76AHC(S1), p. 493]. 

Pyridyl)benzothiazole with [Re(CO)5X] (X = Cl, Br) forms the chelate structures 18 (X = C1, Br) (01ICA(314)91). With silver triflate, dehalogenation occurs followed by formation of 18 (X = OTf). The latter refluxed in acetonitrile gives the cationic complex of stoichiometry [Re(CO)3L(AN)](OT0-... 

The Group VI organometallic chemistry is mainly characterized by the occurrence of N- and C-coordination and carbene complex-formation, as well as by some unique cases of Se- (Te-) coordination, ring opening and deselenation. The Group VII organometallic chemistry is known for the carbene and chelate structures of the derivatized thiazoles. 

Structural Formula Orgotein is a complex protein with a molecular weight of about 33,000. It Is a divalent metal (Mg, Cu, Zn) chelated structure. 

Barluenga et al. have described novel vinylcarbene complexes containing a cyclic BF2 chelated structure which temporarily fixes the s-cis conformation of the exocyclic C=C and Cr=C double bonds. These boroxycarbene complexes behave as dienophiles with 2-amino-l,3-butadienes in a remarkably regio- and exo-selective way. Moreover, high degrees of enantioselectivity are reached by the use of chiral 2-aminodienes derived from (S)-methoxymethylpyrrolidine [101] (Scheme 54). 

Figure 7.9 Proposed chelate structures in the synthesis of high ortho novolac oligomers.

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

The bishgand chelate structure which is formed differs little from that of the parent eugenol dimer (Structure II in Figure 9.2a). The molecule is an electrically neutral chelate where two eugenolate molecules are attached to a central zinc atom in square planar or tetrahedral configuration (Figure 9.2b). 

Figure 9.26 The bisligand chelate structure of zinc eugenolate, showing bridging water molecules. After Wilson Mesley (1974).

In contrast to the early theoretical work of Rank and coworkers , C-NMR investigations had revealed that the metallated carbon atom in the a-sulphinyl carbanion is nearly planar . A four-centre chelate structure 315 has been proposed for a-lithiosulphoxides, and it is believed to be responsible for the planar configuration of the anionic carbon atom and for the greater stability of o(-sulphinyl carbanions in comparison with a-sulphenyl carbanions This chelation favours one of the two diastereoisomeric carbanions and for this reason a-sulphinyl carbanions react with electrophiles in a highly stereoselective manner (see below). 

The reactants are usually /V-acyl derivatives. The lithium enolates form chelate structures with Z-stereochemistry at the double bond. The ring substituents then govern the preferred direction of approach. 

The stereoselectivity of aldol addition is also affected by chelation.81 a- and P-Alkoxy aldehydes can react through chelated structures with Li+ and other Lewis acids that can accommodate two donor groups. 

These reactions have been applied to a-benzyloxy and a-(r-butyldimethylsiloxy)-thioacetate esters.164 The benzyloxy derivatives are anti selective, whereas the siloxy derivatives are syn selective. These differences are attributed to a chelated structure in the case of the benzyloxy derivative and an open TS for the siloxy system. 

A-methoxy-A-methyl amides.87 LiAlH4 and DiBAlH have both been used as the hydride donor. The partial reduction is again the result of the stability of the initial reduction product. The A-methoxy substituent leads to a chelated structure that is stable until acid hydrolysis occurs during workup. 

Prediction and analysis of diastereoselectivity are based on steric, stereoelectronic, and complexing interactions in the TS.82 In the case of the lactic acid auxiliary, a chelated structure promotes facial selectivity. In the TiCl4 complex of 0-acryloyl ethyl lactate,... 

This complex is formed with more than 1.0 equivalents of (C2H5)2A1C1 with concomitant formation of Li2AICI2. The open and chelated structures have been characterized by NMR.90 The chelated structure is substantially more reactive than the open complex, which accounts for the increase in enantioselectivity with more than 1.0 equivalents of catalyst. 

Examples of catalyst control of stereoselectivity have been encountered in the course of the use of the ene reaction to elaborate a side chain on the steroid nucleus. The steroid 4 gave stereoisomeric products, depending on the catalysts and specific aldehyde that were used.33 This is attributed to the presence of a chelated structure in the case of the SnCl4 catalyst. 

Figure 1.17 The texaphyrin-gadolinium chelate structure used as a photosensitizer and MRI contrast agent in the detection and treatment of cancer.

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