Side reactions 7-bonds

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Not stereospecific racemization ac companies inversion when leaving group IS located at a chirality cen ter (Section 8 10) Stereospecific 100% inversion of configuration at reaction site Nu cleophile attacks carbon from side opposite bond to leaving group (Section 8 4)... 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

RM can be a traditional Grignard reagent or an organolithium, 2inc, aluminum, or mercury compound. The Grignard route is employed commercially for production of tertiary phosphines, even though these reactions are subject to side reactions. Yields are often low, eg, 40—50% for (C4H )2P prepared via a Grignard reaction (18). A phosphoms—carbon bond can form from the metathetical reaction of a phosphoms haUde and a pseudohaUde salt. 

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

Cyclopentadienyltitanium Compounds with Other Carbon Titanium Links. Cyclopentadienyltitanium trichloride and, particularly, CpgTiClg react with RLi or with RAl compounds to form one or more R—Ti bonds. As noted, the Cp groups stabilize the Ti—R bond considerably against thermal decomposition, although the sensitivity to air and moisture remains. Depending on the temperature, mole ratio, and stmcture of R, reduction of Ti(IV) may be a serious side reaction, which often has preparative value for Cp Ti(Ill) compounds (268,274,275). 

There are ample precedents for reductions of double bonds in conjugated enones with lithium in deuterioammonia (see section V-C). Examples of the reduction of saturated ketones in deuterated media appear only as side reactions (over reductions) during the above mentioned conversions. For experimental details, therefore, one should consult the literature for the analogous reductions in protic medium (see also chapter 1). The use of deuterioammonia is essential for labeling purposes since by using liquid ammonia and methanol-OD the resulting alcohol contains no deuterium. For the preparation of deuterioammonia see section IX-D. 

Displacement of an allylic halide is complicated by side reactions involving migration of the double bond. A good example is the reaction of 7a-bromo-3 -acetoxy-A -steroids (201) which gives, besides the expected... 

Generally, isolated olefinic bonds will not escape attack by these reagents. However, in certain cases where the rate of hydroxyl oxidation is relatively fast, as with allylic alcohols, an isolated double bond will survive. Thepresence of other nucleophilic centers in the molecule, such as primary and secondary amines, sulfides, enol ethers and activated aromatic systems, will generate undesirable side reactions, but aldehydes, esters, ethers, ketals and acetals are generally stable under neutral or basic conditions. Halogenation of the product ketone can become but is not always a problem when base is not included in the reaction mixture. The generated acid can promote formation of an enol which in turn may compete favorably with the alcohol for the oxidant. 

In general side reactions are rare. In a few cases an isomerization by shift of the double bond favored by formation of a conjugated system can be observed ... 

Various competitive reactions can reduce the yield of the desired Michael-addition product. An important side-reaction is the 1,2-addition of the enolate to the C=0 double bond (see aldol reaction, Knoevenagel reaction), especially with a ,/3-unsaturated aldehydes, the 1,2-addition product may be formed preferentially, rather than the 1,4-addition product. Generally the 1,2-addition is a kinetically favored and reversible process. At higher temperatures, the thermodynamically favored 1,4-addition products are obtained. 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

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