Nucleofugality , leaving

The reaction of a prostereogenic, nucleophilic substrate (carbanion) with a reagent R1—X, containing an achiral electrophilic part R1 and a chiral nucleofugal leaving group X (PAC/ NEN-reaction) leads within an SN2 reaction to the stereoselective formation of C -C bonds 2 ... 

A / -elimination reaction also occurs with linear and cyclic glycol mono-alkyl ether radicals. In this case the nucleofugal leaving group is the alkoxy function, RO, as shown in Scheme 7 for base catalysis and a cyclic system in which alkoxide elimination leads to ring-opening [63] ... 

The reciprocal [67] mechanism for ring-opening is H+-catalysis, in which the oxygen at is protonated, resulting in enhancement in the nucleofugal leaving-group properties of the alkoxy function, e.g. 

As a matter of fact, Cl is such an excellent nucleofugal leaving group (from Cp) that activation at the adjacent carbon (i.e., at Cc) by an electron-donating substituent (such as OH) is not necessary for the reaction to occur rapidly and efficiently. For instance, the j8-chloroalkyl radical formed by H-abstraction from l-chloro-2-methylpropane, Me2C CH2Cl (this is the same radical as that formed in Scheme 1, Section 9.2.1), undergoes C-Cl heterolysis in aqueous solution with khet = 3.4 X 10 s- [75]. 

The overwhelming number of reactions presented in this account proceed via polar intermediates (or at least via transition states with large charge separation). This fact enables a simple classification of these reactions (equations 2-12) attack of nucleophiles N" or electrophiles on cyclopropanes is usually supported by electron acceptor A and electron donor D substituents, respectively, or by nucleofugal leaving groups LG. Comparison of equation 1 and equations 2 4 clearly demonstrates the close relationship... 

The sulfur species is thus activated by S-halogenation into a nucleofugic leaving group, which is substituted by fluoride. The fluorodesulfuration of thiocarbonyl compounds is supposed to follow a similar principal pathway. 

Silylation. Transsilylation from a 4-pentenyl silyl ether to an alcohol involves iodination of the double bond, rendering the pentenyloxy group nucleofugal (leaving as 2-iodomethyltetrahydrofuran). 

Alcohols with a nucleofuge leaving group in the r-position can be cyclized to give oxetanes. Thus the cyclodehydrohalogenation of halo alcohols occurs in an analogous way to the oxirane synthesis from y -halogenated alcohols (see p 20). Oxetanes can be prepared from 1,3-diols via monoarene sulfonates ... 

Nucleophilic substitution occurs faster if there is a nucleofuge leaving group, e.g. ... 

Ligands (NFI3, CN, pyridine) Nucleophiles (attacking L. bases) Nucleofuges (leaving L bases) Spectator anions Electron-rich pi systems H+ abstractors... 

The Siv2-type displacement mode applies also to borate reorganizations involving nucleofugal leaving groups other than chloride. Carbamates can fulfill this purpose quite effectively, as recent work demonstrates (Scheme 1-30). 

---