Elimination functional groups

Amorphous metal oxides with basic and amphoteric surfaces are active catalysts for a variety of important reactions including olefin isomerization, hydrogenation, and functional group elimination.2... 

Functional groups, elimination (s. a. Replacement... by hydrogen) 19, 109 Functionalization... 

Some approaches to the synthesis of trifluoromethyl-l,2,3-triazoles by the azidation of compounds containing not a triple but a double C=C bond were described. In this case apparently a functional group elimination occurs in situ to provide the formation of an additional carbon-carbon bond. For instance, Miethchen et al. described a synthesis of 4-trifluoromethyl-1,2,3-triazole 53 linked to the C -atom of D-galactose and D-altrose. 1,3-Dipolar cycloaddition using the monosaccharide azides and the perfluoroalkyl- substituted phenylvi-nylsulfones was performed [55, 56],... 

Application of class reactions. The apphcation of selected reactions that indicate the presence or absence of certain functional groups, with due regard to the indications provided by tests 1, 2 and 3, will locate the class (or classes) to which the compound belongs or wUl, at least, serve to eliminate all but a few classes to which the compound can be assigned. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Sulfonate esters are subject to the same limitations as alkyl halides Competition from elimination needs to be considered when planning a functional group transforma tion that requires an anionic nucleophile because tosylates undergo elimination reactions just as alkyl halides do... 

The polymer repeat unit arises from reacting together two different functional groups which usually originate on different monomers. In this case the repeat unit is different from either of the monomers. In addition, small molecules are often eliminated during the condensation reaction. Note the words usual and often in the previous statements exceptions to both statements are easily found. 

Temperatures should not exceed 60°C for the Type I resins, and 40°C for Type II and acryflc resins. Thermal degradation and the loss of functional groups occur when these temperatures are exceeded. Elimination of siUca from the resin bed is further improved by preheating the bed with warm water before injecting the NaOH solution. 

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

Both conjugated and isolated dienes are usually accessible by extension of the methods suitable for mono-olefins. Allylic functions for ehmination may be produced by double bond introduction a to a functional group or by allylic substitution of an olefin. Both reduction of allylic systems to mono-olefins and elimination to give dienes, may involve rearrangement. 

The amino functional group is not commonly encountered in steroid synthesis except perhaps in steroidal alkaloids. However, certain elimination reactions have been shown to have theoretical and limited preparative importance, largely due to the efforts of McKenna and co-workers. The Hofmann rule for 2 elimination predicts that alkaline elimination of quaternary ammonium salts will occur towards the carbon carrying the most hydrogen atoms cf. the converse Saytzeff orientation, above). In cyclohexyl systems, the requirement for diaxial elimination appears to be important, as in other 2 eliminations, and the Hofmann rule frequently is not obeyed [e.g., (116) (117)]. 

Functional group transfonrrations that rely on substitution by the SnI mechanism are not as generally applicable as those of the Sn2 type. Hindered substrates are prone to elimination, and reanangement is possible when carbocation intennediates are involved. Only in cases in which elimination is impossible are SnI reactions used for functional group transfonnations. 

Advantages of the Stille reaction include neutral conditions under which the reaction takes place, often with full retention of stereochemistry, and compatibility with nearly all functional groups thus eliminating additional steps required for protection and deprotection. Conversely, a highly undesirable drawback is the use of toxic tin compounds and the ensuing difficult removal of these from the reaction mixture. 

The addition of living poly(styrene) to AIBN leads finally, especially for high coupling efficiencies, to the elimination of one nitrile group [72]. More recently, Ren et al. [73] have used bis(2-chloroethyl)2,2 -azodiisobu-tyrate (see scheme 19) to terminate anionically initiated poly(butadiene) chains. Since the azo transfer agent possesses two functional groups (Cl) that are able to termi-... 

Although no small molecule gets eliminated, the reaction can be considered a condensation polymerization. Monomers suitable for polymerization by ring opening condensation normally possess two different functional groups within the ring. Examples of suitable monomers are lactams (such as caprolactam), which produce polyamides, and lactons, which produce polyesters. 

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