Reaction aliphatic substitution

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

The attack by a reagent of a molecule might be hampered by the presence of other atoms near the reaction site. The larger these atoms and the more are there, the higher is the geometric restriction, the steric hindrance, on reactivity. Figure 3-6e illustrates this for the attack of a nucleophile on the substrate in a nucleophilic aliphatic substitution reaction. 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

Compound 40 has not yet been synthesized. However, there is a large body of synthetic data for nucleophilic substitution reactions with derivatives of 41 [synthesized from aliphatic and aromatic aldehydes, pyridine, and trimethylsilyl triflate (92S577)]. All of these experimental results reveal that the exclusive preference of pathway b is the most important feature of 41 (and also presumably of 40). 

The opposite case—reaction of an arenediazonium species with an aliphatic substrate —is possible if a sufficiently acidic C—H bond is present e.g. with /3-keto esters and malonic esters. The reaction mechanism is likely to be of the Sel-type an electrophilic substitution at aliphatic carbon ... 

The Pictet-Spengler reaction has mainly been investigated as a potential source of polycyclic heterocycles for combinatorial apphcations or in natural product synthesis [149]. Tryptophan or differently substituted tryptamines are the preferred substrates in a cyclocondensation that involves also aldehydes or activated ketones in the presence of an acid catalyst. Several versions of microwave-assisted Pictet-Spengler reactions have been reported in the hter-ature. Microwave irradiation allowed the use of mild Lewis acid catalysts such as Sc(OTf)3 in the reaction of tryptophan methyl esters 234 with different substituted aldehydes (aliphatic or aromatic) [150]. Under these conditions the reaction was carried out in a one-pot process without initial formation of the imine (Scheme 86). 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Several distinct mechanisms are possible for aliphatic nucleophilic substitution reactions, depending on the substrate, nucleophile, leaving group, and reaction conditions. In all of them, however, the attacking reagent carries the electron pair with it, so that the similarities are greater than the differences. Mechanisms that occur at a saturated carbon atom are considered first. By far the most common are the SnI and Sn2 mechanisms. 

Salts of aliphatic or aromatic carboxylic acids can be converted to the corresponding nitriles by heating with BrCN or CICN. Despite appearances, this is not a substitution reaction. When R COO was used, the label appeared in the nitrile, not in the C02, and optical activity in R was retained. The acyl isocyanate... 

The chemical reactions of benzene and all aromatic compounds, with few exceptions, are unlike those of unsaturated aliphatic compounds (olefins) that is, addition reactions do not occur. Instead, the hydrogens on the ring are replaced by other atoms or groups of atoms. The aromatic ring remains unchanged by these substitution reactions. All six of the hydrogens in benzene can be replaced by other atoms. 

Because the Sn2 nucleophilic substitution of uncharged amines with uncharged aliphatic organic halides involves a transition state that is more polar than that of the starting materials, such substitution reactions... 

In a faster, selective and cleaner applications of the microwave-accelerated reactions, Stone-Elander et al. have synthesized a variety of radiolabeled (with 3H, 11C, and 19F) organic compounds via the nucleophilic aromatic and aliphatic substitution reactions, esterifications, condensations, hydrolysis and complexation reactions using monomodal MW cavities on microscale [121]. A substantially reduced level of radioactive waste is generated in these procedures that are discussed, at length, in Chapt. 13 [122]. 

Many theories have been put forward to explain the mechanism of inversion. According to the accepted Hugles, Ingold theory aliphatic nucleophilic substitution reactions occur eigher by SN2 or SN1 mechanism. In the SN2 mechanism the backside attack reduces electrostatic repulsion in the transition state to a minimum when the leaving meleophile leaves the asymmetric carbon, naturally an inversion of configuration occurs at the central carbon atom. 

The polymers used in this study were prepared by a nucleophilic activated aromatic substitution reaction of a bisphenate and dihalo diphenyl sulfone ( ). The reaction was carried out in an aprotic dipolar solvent (NMP) at 170°C in the presence of potassium carbonate (Scheme 1) (5,6). The polymers were purified by repeated precipitation into methanol/water, followed by drying to constant weight. The bisphenols used were bisphenol-A (Bis-A), hydroquinone (Hq) and biphenol (Bp). Thus, the aliphatic character of Bis-A could be removed while retaining a similar aromatic content and structure. The use of biphenol allows an investigation of the possible effect of extended conjugation on the radiation degradation. 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

Another group of bicyclic aliphatic phosphines has been introduced by Sasol [15], Their ligands are based on addition of PH3 to limonene (the R-enantiomer). A mixture of two diastereomeric compounds is obtained due to the two configurations of the methyl group at the C-4 position (Figure 7.9). The Lim-H compounds obtained can be functionalised at the phosphorus atom with the usual radical reactions with alkenes or substitution reactions of their conjugate bases formed after treatment with BuLi with electrophiles. 

Scheme 19 Allylic substitution reactions between aliphatic alcohols and allylic carbonates...

Compounds in the two groups differ in a number of ways. The two differ chemically in that the aliphatic undergo free-radical substitution reactions and the aromatic undergo ionic substitution reactions. In this chapter you examine the basics of both ciromatic and heterocyclic ciromatic compounds, concentrating on benzene and related compounds. 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

The nature of the aldehydes shows some effect on the reaction. Aliphatic, aromatic, and moderately activated aldehydes such as chloro and methyl gave high yields of products when compared with deactivated aldehydes such as nitro-substituted ones. 

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