Formation of Cyclics

Conducting ADMET in bulk monomer or in concentrated solution if the polymer or monomer is a sohd minimizes both of these processes. Providing the 

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Although some dibasic acids, e.g, succinic acid and phthalic acid, readily lose water on heating with the formation of cyclic anhydrides, most monobasic... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Formation of Cyclic Carbonates. In the absence of water, chlorohydrins such as 2-chloroethanol and l-chloro-2-propanol react with an alkah carbonate or bicarbonate to produce cycHc carbonates such as ethylene carbonate [96-49-1] and propylene carbonate [108-32-7] ia yields of up to 80%... 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, diphenylmethylenedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

Rearrangements of vinylogous urethanes to vinylogous carbonic acids and decarboxylation are other interesting enamine rearrangements which may be synthetically useful in the formation of cyclic enamines (623,624). 

One of the most powerful tools for the formation of cyclic molecules is the Diels-Alder reaction (/). The reaction generally involves the combination of a diene with a dienophile according to the diagram. There are surprisingly few limitations on the... 

Scheme 5.1-60 The use of NaCI/AICl3 (X(AICl3) = 0.68) in the formation of cyclic ketones.

Acidic hydrolysis of the amide group at pH 4.5 is a very slow reaction. Strong acidic conditions leads to a progressive insolubilization of the reaction product because of formation of cyclic imide structures ... 

Fig 2 Methods of Formation of Cyclic Polynitramines by Selective Cleavage of Hexamine... 

This interesting solvent effect was explained by Baro et al. on the basis of the formation of cyclic dimers (13.7) as suggested earlier by Hunter (1937) and by Le Fevre and Vine (1937). It was later confirmed in an X-ray investigation by Omel chenko and Kondrashev (1967). The X-ray analysis of 1,3-diphenyltriazene copper(i) (Brown... 

Formation of cyclic sulfoxide (R)-55 by t reatment of bromoarene (i )-54 with tributylstan-nane apparently proves that intramolecular homolytic substitution at the sulfur atom of the sulfoxide group proceeds with strict inversion of configuration (equation 50)103. 

The reactions of Fischer carbene complexes with 1,3-dienes (carbodienes or heterodienes) lead to the formation of cyclic products with different ring sizes depending upon both the nature of the reaction partners and the reaction conditions. Between these synthetically useful transformations are found [2c+2s], [3C+2S], [4S+1C], [3S+3C], [4S+2C], [4S+3C] and [2S+1C+1C0] cycloaddition reactions which will be summarised further on, in addition to the [2S+1C] cycloaddition processes here described. 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

The data indicate that the formation of cyclic intermediates creates a stabilization of the cationic chain ends (AH° < 0 and AH s < 0), also expressed by a decrease of both the acceptor strength (Ae(LUMO) > 0) and the donor strength (Ae(HOMO) < 0) of the cations. The positive charge of the cationic centre is distinctly decreased (Aqc+ < 0) as a consequence of the interaction of this centre with the oxygen of the methoxy group. A partially covalent C + —O-bond is formed (pt Q(f) > 0.6 rc+ 0if) 146 pm). 

Other evidence cited for SET mechanisms has been detection of radical or radical ion intermediates by ESR or CIDNP the finding that such reactions can take place at 1 -norbomyl bridgeheads and the formation of cyclic side products when the substrate has a double bond in the 5,6 position (such substrates are called radical probes). 

Formation of cyclic acetals reaction between diketones and acids... 

Polyfunctionality of the reactants is not sufficient in itself to assure formation of polymer the reaction may proceed intramolecularly with the formation of cyclic products. For example, hydroxy acids when heated yield either lactone or linear polymer (or both),... 

The principles set forth above account reasonably well for the course of bifunctional condensations under ordinary conditions and for the relative difficulty of ring formation with units of less than five or more than seven members. They do not explain the formation of cyclic monomers from five-atom units to the total exclusion of linear polymers. Thus 7-hydroxy acids condense exclusively to lactones such as I, 7-amino acids give the lactams II, succinic acid yields the cyclic anhydride III, and ethylene carbonate and ethylene formal occur only in the cyclic forms IV and V. 

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