Leaving group complexes

The question as to whether the reactive intermediate is the phenol-metal/leaving group complex 21/22 or the free phenoxonium ion 17 has been studied in the particular case of hypervalent iodine. Pelter and co-workers presented permissive evidence in support of a mechanism involving the free oxonium species 17 (Scheme 7) Phl(OAc) is an extremely good nucleofuge, no transfer of chirality is observed when homochiral hypervalent iodine compounds are used, and calculations made on the cation species correctly predict the re-gioselectivity of the substitution reaction [32, 33]. 

These exemples show the kinetic effect of complexation of the substrate itself. A similar study carried out on nitro-substituted phenylacetates revealed that leaving group complexation is less efficient than substrate complexation. It should be borne in mind that the interpretation of complexation effect is complicated by solvent competition. For instance 1,3-dinitrobenzene accelerates the butylaminolysis of 4 -nitro-phenyl-3,5-dinitrobenzoate (A obs/ o>l) in 1,2-dichloroethane and acetonitrile. For 4 -nitrophenyl, 4-NN dimethyl aminobenzoate there is acceleration only in the former solvent (koiyjko =. 55). 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

Allylic amine is a less reactive leaving group[7], but the allylic ammonium salts 214 (quaternary ammonium salts) can be used for allylalion(l30,131]. Allylic sulfonium salts are also used for the allylation[130]. The allylic nitrile in the cyclic aminonitrile 215 can be displaced probably via x-allylic complex formation. The possibility of the formation of the dihydropyridinium salts 216 and subsequent conjugate addition are less likelyfl 32],... 

In aqueous dioxane, the endo-anti isomer gave a product mixture consistent of alcohol N and the corresponding ester (derived from capture of the leaving group p-nitrobenzoate). The other isomers gave much more complex product mixtures which were not completely characterized. Explain the trend in rates and discuss the structural reason for the stereochemical course of the reaction in the case of the endo-anti isomer. 

Other, closely related, nicotinic acid derivatives and the unsubstituted system itself have also been studied and undergo similar reactions. Moreover, the approach may be extended to 2,2 -bipyridyls. Newkome and his collaborators have used the 2,2 -bipyridyl unit 19) as an electrophile in which ortho-hr ommes served as leaving groups. They have also used halomethyl systems and formed the macrocycles from these systems . A compound derived from the latter starting material 20) is reported to form a cobalt complex, in which both nitrogens and only one of the oxygen atoms participate in the binding . The two precursor units are shown below as 79 and 20, respectively. 

Parker has suggested that the presence of a lone-pair on the entering group in the intermediate complex (234) increases the rate by assisting in breaking the bond to the leaving group (Le) as indicated. 

In many cases, the effects of the nucleophile observed with carbo-aromatics can be expected to carry over to heteroaromatics, e.g., changing the relative reactivity of leaving groups (Section II, D, 2, b) deceleration by charge-transfer complexing with the substrate... 

The transfer of simple alkyl groups (R in the table—mostly -Bu or Me), from tin to palladium complex 6 is a very slow process, and the substituent R (see table) is transferred selectively. The leaving group X on the coupling component... 

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