Reverse additives

The mechanism of these reactions involves the rapid and reversible addition of a proton to the aromatic ring, followed by 1,2-intramolecular methyl shifts (10) ... 

The reversible addition of sodium bisulfite to carbonyl groups is used ia the purification of aldehydes. Sodium bisulfite also is employed ia polymer and synthetic fiber manufacture ia several ways. In free-radical polymerization of vinyl and diene monomers, sodium bisulfite or metabisulfite is frequentiy used as the reduciag component of a so-called redox initiator (see Initiators). Sodium bisulfite is also used as a color preventative and is added as such during the coagulation of crepe mbber. 

The consequences of these interactions with nucleophiles are obviously readily explained by a rapid but reversible addition of the nucleophile to the 3,4-double bond which accompanies the normal irreversible displacement of the chlorine atoms. 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

The same structural factors come into play in determining the position of equilibria in reversible additions to carbonyl compoimds. The best studied of such equilibrium processes is probably addition of cyanide to give cyanohydrins. 

Henbest and Jackson have rationalized these remote directive effects on the basis of the well-supported mechanism of alkaline epoxidations. The initial step in the reaction is the reversible addition of the hydroperoxide ion... 

The effect of a substituent may be substantially modified by fast, concurrent, reversible addition of the nucleophile to an electrophilic center in the substituent. Ortho- and para-CS.0 and pam-CN groups have been found by Miller and co-workers to have a much reduced activating effect on the displacement of halogen in 2-nitrohaloben-zenes with methoxide ion [reversible formation of hemiacetal (143) and imido ester anions (144)] than with azide ion (less interaction) or thiocyanate (little, if any, interaction). Formation of 0-acyl derivatives of 0x0 derivatives or of A-oxides, hydrogen bonding to these moieties, and ionization of substituents are other examples of reversible and often relatively complete modifications under reaction conditions. If the interaction is irreversible, such as hydrolysis of a... 

Ring-chain tautomerism via intramolecular reversible addition reactions to the C=0 group 95AHC(64)251. 

Although the previous protocol suggests it is not necessary to deprotonate the sulfonamide prior to exposure to the zinc carbenoid, a experimentally simpler procedure can be envisioned wherein the alcohol and promoter are deprotonated in a single flask (Fig. 3.15). In protocol IV, the alcohol and promoter are combined in flask A and are treated with diethylzinc, thus forming the zinc alkoxide and zinc sulfonamide. In sub-protocol IVa, this solution is transferred to flask C which contains the zinc carbenoid. Sub-protocol IVb represents the reversed addition order. Sub-protocol IVa is not only found to be the superior protocol in this sub-set, it is found to out-perform all of the previous protocols Despite the persistence of the induction period, a large rate enhancement over the uncatalyzed process is observed. This considerable rate enhancement also translates to a reduction in the overall reaction time when compared to sub-protocols la and Ilia. Selectivity rises... 

Reversible additions of sodium methoxide to certain 2,4,6-triarylsele-nopyrylium salts were reported to lead to unstable 2-methoxy-2//-selenopyran intermediates, which could be reconverted into the starting... 

Although lithium aldolates generally display a rather moderate preference for the u/f/z-isomer4, considerable degrees of diastereoselectivity have been observed in the reversible addition of doubly deprotonated carboxylic acids to aldehydes20. For example, the syn- and uw/z-alkox-ides, which form in a ratio of 1.9 1 in the kinctically controlled aldol addition, equilibrate in tetrahydrofuran at 25 C after several hours to a 1 49 mixture in favor of the anti-product20. 

Ring-opening provides a thiyl radical propagating species. Although the polymers have a double bond on the backbone there is little or no crosslinking (Scheme 4.34, Scheme 4.35). There is, however, evidence of reversible addition... 

This equation can be solved numerically to give values of Clr and Ctr.404 For reversible addition-fragmentation chain transfer (RAFT) (Scheme 6.5), the rate constant for the reverse reaction is defined as shown in eq. 22 ... 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

Although the term RAFT (an acronym for Reversible Addition-Fragmentation chain Transfer)38" is sometimes used in a more general sense, it was coined to describe, and is most closely associated with, the reaction when it involves thiocarbonylthio compounds. RAFT polymerization, involving the use of xanthates, is also sometimes called MADIX (Macromolccular Design by Interchange of Xambate) 96 The process has been reviewed by Rizzardo et [Pg.502]

Organic chemists have been aware of reversible addition-fragmentation involving xanthate esters in organic chemistry for some time. It is the basis of the Barton-McCombie process for deoxygenation of alcohols (Scheme 9.37).402 404... 

In 1988 a paper by Zard and coworkers4(, reported that xanlhates were a convenient source of alkyl radicals by reversible addition-fragmentation and used the chemistry for the synthesis of a monoadduct to monomer (a maleimide). Many applications of the chemistry in organic synthesis have now been described in papers and reviews by the Zard group.406 407... 

RAFT reversible addition-fragmentation chain transfer... 

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