Attacking reagent, definition

The general subject of asymmetric synthesis has been reviewed extensively (1-5). The term asymmetric synthesis has been defined in more than one way (1,4) however, a useful definition is the one given by Morrison and Mosher (1) a process which converts a prochiral unit [refs. 6 and 7] into a chiral unit so that unequal amounts of stereoisomeric products result. The stereoisomeric products may be enantiomeric or they may be diastereomeric. The substrate molecule must contain either enantiotopic or diastereotopic groups or faces (8,9), since the attack of a reagent at equivalent groups or faces cannot lead to isomeric products. 

Although the final products of the amination of 5-bromo-4-R-pyrimidines are the 6-amino-4-R-pyrimidines,106 107 two mechanisms, by which these same products are formed, have been recognized. The substrate is converted in part to the product via C-6 adduct 41, whereas the remaining part follows an ANRORC mechanism requiring attack of the reagent at C-2.105 Evidence for the existence of C-2 adducts as intermediates in nucleophilic substitutions is hard to obtain. However, the formation of an adduct of this kind (34) from 4,6-diphenyIpyrimidine was definitely proved by H-NMR measurements (Table XI).98 C-6 adducts such as 42-44 have also been described (see also Table XI) and shown to be the sole intermediates in the amination of 5-chIoro-4-te/T-butyIpyrimidine and 5-X-2,4-di-tert-butyIpyrimidines (X = Cl, Br), the ANRORC mechanism remaining excluded with the latter substrates.98... 

Allyltin reagents were suggested to add to ir-allylpalladium complexes directly on the allyl ligand,137138 although a definitive stereochemical experiment could not be successfully carried out. The fact that these reactions proceed with allyl inversion of the stannane (equation 168) would require an unlikely, sterically unfavorable allylpalladium-R intermediate if metal attack was followed. 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which dififers appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Zvalue . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function c.t. for charge transfer complex is written as a linear combination of a wave function describing the unperturbed ground state of the molecule under attack, and a function 0 which dififers from in fhe replacement of one of its orbitals by the valence orbital of the reagent. Thus the wave function... 

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