Lone pair assisted

Kanematsu and co-workers devised a simple pathway to isobenzofurans and dihydroisobenzofurans starting with substituted furans of type 62. Treatment with strong base results in an alkyne-allene isomerization. Subsequent intramolecular cycloaddition, ring opening (probably oxygen lone-pair assisted), and acidic workup give 63. 

Ad 2 (Addition, Electrophilic, Bimolecular), A -i- An and Hetero Ad Z, p.t. + AdN El (Elimination, Unimolecular), Dn + Dg and Lone-Pair-Assisted El, Ep + p.t. Se2Ar Electrophilic Aromatic Substitution, Ag + Dg ElcB (Elimination, Unimolecular, Conjugate Base), p.t.-i-Ep AdN2 (Addition, Nucleophilic, Bimolecular), AdN + P f ... 

This reaction is the reverse of the hetero Ade2 reaction. The lone-pair-assisted El uses a properly aligned lone pair to expel the leaving group, path Ep. The resultant cation is then deprotonated, path p.t. 

Likewise, a lone pair on a heteroatom can aid the loss of the leaving group in the El process. The following example shows first the protonation of the leaving group followed by the lone-pair-assisted El, path Ep followed by proton transfer. 

Lone pair assisted E+ attack from the fle-side... 

Steric-iepulsion-contiolled, lone pair assisted E attack from the S/-side... 

Pyrazolines substituted at position 4 or 5 with hydroxy or amino groups readily eliminate a molecule of water or amine yielding pyrazoles. The 4-substituted derivatives are relatively more stable than the 5-substituted ones, because for the last group the lone pair at N-1 assists the elimination (407) -> (408) -> (409). The sulfonyl group at position 1 is also easily eliminated and this property is taken advantage of in Dorn s elegant synthesis of 3-aminopyrazole (Section 4.04.3.3.1). 

The reason for the slow hydrolysis compared to that of structurally similar compounds like nitrones or 0,lV-acetals might be the following (b-67MI50800) in the protonated species (77) assistance of the lone pair of electrons at nitrogen is sterically hindered due to the large angle of its orbital to the plane of the ring. 

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. 

This has been mentioned at various points in this paper and may involve either a direct acid-base reaction of nitrene and nucleophile or, in some instances, reaction of the nitrene precursor with the nucleophile (or 1,3-dipolarophile) followed by loss of nitrogen. For example, the reaction of benzenesulphonyl azide with pyridine to give 31 (Ar=Ph) 69> could either involve a free nitrene or a concerted process in which the lone pair on the pyridine nitrogen atom assists the elimination of molecular nitrogen. That some free nitrene can be involved in these reactions is clear from the isolation of some 3-benzenesulphonamido-2,6-lutidine... 

The mechanistic implications of these facile formal cluster oxidations arachno nido closo by nett loss of Hp at moderate temperatures are considerable. The processes are accompanied by, and presumably assisted by a flexibility of coordination geometry about the Ir atom and also by its ready oxidation. The metal atom can be seen as a potential source of electrons for cluster bonding either by involving its lone pairs of electrons or by switching between Ir-H-B bridging and Ir-H terminal bonding... 

It would be reasonable to assume that, in a solvent of high dielectric constant, such as nitromethane, the nitrile (84) is formed by direct attack of cyanide ion on an ion-pair (86) in which the bromide ion has the a-D orientation. Departure, assisted by metal ions, of the halide ion from 83 or 86, with possible assistance by the lone pair of the ring-oxygen atom, would lead to an oxonium ion (87) that could... 

The direct interaction of the reaction center of a molecular entity with a lone pair of electrons of an atom within that same molecular entity that is not associated with the reaction center or, interaction of the reaction center with the reaction center nor conjugated with the reaction center. Rate acceleration by such a process is referred to as anchimeric assistance. See Intramolecular Catalysis Synartetic Acceleration. 

Examples of Beckmann degradation reactions with assistance by mesomeric donation of electron lone pairs (example Y = N, O, S) or n electrons (example R = Ph or aromatic group) are well documented in the literature ... 

Anchimeric assistance by the aUcoxyl oxygen lone pair promotes heterolytic S v 1 and reactions at nitrogen as well as homolysis of the N—Cl bond. A-Haloamides are also amidyl radical sources" but their heterolytic reactivity is known to involve positive, rather than negative, halogen ... 

If the ligands are more electronegative than M, then the bond pairs are drawn farther from M and away from each other, a situation that assists the lone pair in making the L-M-L angle smaller as it operates against this weaker BP-BP repulsion. For example, compare NH t (I06 45 ) wjth NF , (I02 9), an(j H,() (104 IT) Wlfh OF2 (I0I 30 ). 

In Mechanisms 1 and 2 the reactions are considered to proceed via an equilibrium concentration of the free base. Mechanism 3, Scheme (4), which was first proposed by Wenkert and Liu [10], starts with protonation at Nb, the most basic site and so the most likely to be protonated first. After protonation, the C-3 - Nb bond is cleaved due to participation of the indole nitrogen lone pair, giving a carbocation intermediate. Ring reclosure is assisted by the Nb lone pair to effect the inversion at C-3. 

Ring opening of this intermediate is assisted by one of the lone pairs of oxygen and restores the aromaticity of the ring. 

The various explanations that were provided to rationalize the observed re-gioselectivity involved (a) electronic repulsions between the lone pairs of the heteroatoms and the negatively charged oxygen of the perepoxide, (b) anchimeric assistance by the allylic substituent leading to regioselective opening of the possible perepoxide intermediate [Eq. (13)],... 

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