Mechanisms for Structure Formation

Suggest a structure for the product of nucleophilic substitution obtained on solvolysis of tert-butyl bromide in methanol and outline a reason able mechanism for its formation... 

Despite its apparent simplicity, the PK pyrrole synthesis has retained its mystique since being discovered. Several investigations into the PK mechanism have been reported, including a gas phase study. Current evidence (intermediate isolation, kinetics, isotope effects) suggests the following (abbreviated) mechanism for the formation of pyrrole 13. However, the specific PK mechanism is often dependent on pH, solvent, and amine and dicarbonyl structure, especially with regard to the ringclosing step. 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

This chapter is primarily concerned with the chemical microstructure of the products of radical homopolymerization. Variations on the general structure (CHr CXY) are described and the mechanisms for their formation and the associated Tate parameters are examined. With this background established, aspects of the kinetics and thermodynamics of propagation are also considered (Section 4.5). 

Only one example in this category has been described in recent years (as of 2003). Treatment of o-phenylenediamine (396) with 3,4,5,6-tetrachloropyridazine (397) in A-methylpyrrolidine at 115°C for 17 h gave a separable mixture of products, one of which was 2,3-bis(benzimidazol-2-yl)quinoxaline (398) (unstated yield). The structure (398) was confirmed by X-ray analysis,and a mechanism for its formation was suggested. ... 

As indicated, many of the more highly fimctionalized building blocks did not result in 2-pyridones. However, a thorough structure elucidation of by-products and intermediates was used to propose a mechanism for the formation of the 2-pyridone core based on a Michael addition followed by a Dimroth-type rearrangement (Fig. 3). 

Reaction of pyridinium thiocyanatoacetamides (106) with a strong base (e.g potassium t-butoxide) in ethanol gave mesoionic Af-[2-(l,3,4-thiadiazolo[3,2-a]pyridino)]acetamidates (107) or (108) whose structures were confirmed by the X-ray analysis of (107 R = Me). Possible mechanisms for the formation of the mesoionic derivatives were discussed <96BCJ1769>. 

In addition the structure of the 1,2-azathiabenzene 78 was also confirmed by chemical evidence as shown in Scheme 10. Protonation of 78a (R1 = R2 = Me) with 70% perchloric acid yielded the corresponding cyclic amino sulfonium salt 82a in 87% yield, but not the starting sulfonium compound 76a, suggesting predominance of sulfilimine structure 78a rather than cyclic sulfonium ylide stmcture 80a. Thus, compound 78 could be recognized as the first example of a 1,2-azathiabenzene having sulfur at a bridgehead position. A proposed mechanism for the formation of 78 and 79 is shown in Scheme 9. The most acidic proton adjacent to sulfur in 76 is deprotonated with... 

I.3.4.2.2. Nonaromatic Unsaturated Heterocycles Reactions of aromatic nitrile oxides with 1-azirines are followed by the ring opening of the latter to give 4-benzamidoisoxazoles 145 (314). The structure of 145 (R = 4-C1C6H4, Ar = Ar7 = Ph) was established by single-crystal X-ray analysis. A mechanism for the formation of 145 has been proposed, (see Scheme 1.29). 

A series of q3-allyltitanium compounds has been prepared, and their jr-structures have been confirmed by spectroscopic methods and X-ray analysis [9]. In these complexes, the alkyl substituents at C-l and C-3 preferably occupy the syn position with respect to the H (or R) at C-2 due to steric reasons. The proposed mechanism for their formation is outlined in Scheme 13.4 [8]. 

The possible mechanism for the formation of C—C coupled (1) and decoupled (2) bimetallic complexes from Cp2M has been investigated [1 lb,c]. We followed a stepwise procedure to arrive at a model that was practical and at the same time realistic. In the first stage, the substituted cyclopentadienyls were replaced by Cp and the substituents on acetylides and butadiynes were replaced by H. The relative energies showed that, the C—C coupled structure 1 for M = Ti when L = Cp and R = H is more stable than 2 by 3 kcal/mol, while 2 is calculated to be 14.8 kcal/mol... 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

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