With leaving groups

Section 8 14 Nucleophilic substitution can occur with leaving groups other than halide Alkyl p toluenesulfonates (tosylates) which are prepared from alcohols by reaction with p toulenesulfonyl chloride are often used... 

Synthesis. The first hiUy alkyl/aryl-substituted polymers were reported in 1980 via a condensation—polymeri2ation route. The method involves, first, the synthesis of organophosphine-containing alkyl or aryl substituents, followed by the ready oxidation of the phosphine to a phosphorane with leaving groups suitable for a 1,2-elimination reaction. This phosphorane is then thermally condensed to polymers in which all phosphoms atoms bear alkyl or aryl substituents. This condensation synthesis is depicted in Eigure 2 (5—7,64). 

Section 8.14 Nucleophilic substitution can occur with leaving groups other than halide. 

In highly concentrated acids, however, with leaving groups that facilitate bond cleavage, the A1 route may prevail, the intermediate being the acylium ion ... 

In the previous chapter, we saw that a substitution reaction can occur when a compound possesses a leaving group. In this chapter, we will explore another type of reaction, called elimination, which can also occur for compounds with leaving groups. In fact, substitution and elimination reactions frequently compete with each other, giving a mixture of products. At the end of this chapter, we will learn how to predict the products of these competing reactions. For now, let s consider the different outcomes for substitution and elimination reactions ... 

Salaun and de Meijere s groups have applied successfully the intramolecular PKR on 1-cyclopropylidene-1,6-enynes 421-426 (Table 34) [108], accessible via the Pd(0) catalysed alkylation of stabilized carbanions with 1-vinylcyclo-propanes 1-substituted with leaving groups [3a, 109]. 

O R Substitution of lactam N-atom Basicity of leaving group amine, electron-with-drawing substituents in amino moiety Increases with basicity Monobactams are ca. 100 times less reactive than penicillins with leaving groups of similar basicity [76] [90]... 

Another important site of structural variation in cephalosporins is C(3) (Table 5.4.J). Electron-withdrawing substituents at C(3) such as a Cl-atom or a MeO group increase base-catalyzed hydrolysis of cephalosporins by both resonance and inductive effects [92], For cephalosporins carrying 3-methylene-linked substituents with leaving group ability (e.g., acetate, thiol, or pyridine), it has been postulated that a concerted expulsion of the substituent facilitates the nucleophilic attack on the /3-lactam carbonyl group [104][105]. However, there are also arguments for a stepwise process in which the ex-... 

The acidity of the At-protons in amides is considerably higher (pK typically in the range 20-26) than those of amines, and (37b) has been successfully used as in situ formed EGB for At-carboxylation and subsequent cyclization of amides with leaving groups in the a-position according to Scheme 25 [115]. 

Extrapolation of the above considerations to acetate esters less reactive than POAc, i.e., with leaving groups more basic than PO , leads to the prediction of lower catalytic efficiencies because the rate of acetylation of the catalyst is expected to decrease more rapidly than the rate of background methanolysis. In contrast, with esters of phenols more acidic than pNPOH, the catalytic reaction is predicted to be obscured by increased rates of background methanolysis. Thus, for one reason or... 

The reaction can be carried out with leaving groups other than amines. 

Koltzenburg G, Behrens G, Schulte-Frohlinde D (1982) Fast hydrolysis of alkyl radicals with leaving groups in the (5 position. J Am Chem Soc 104 7311-7312 Koltzenburg G, Behrens G, Schulte-Frohlinde D (1983) 1,1-Dialkoxytrimethylene radical cations. An-gewChem Int Ed Engl 22 500-501... 

Carboxylic amides or related substrates, substituted with leaving groups at the /3-position, are suitable substrates for the synthesis of azetidin-2-ones. Relatively stable or labile, in situ generated, leaving groups can be applied. Selective activation of 3-hydroxy-2-hydroxymethyl-2-methylpropanamide 388 with P(NMe2)3-KPF6 and subsequent... 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

The alkoxide formed by addition of a Grignard reagent to an aldehyde or ketone is stable. Tetrahedral intermediates are similarly formed by addition of a nucleophile to a carbonyl group, so why are they unstable . The answer is to do with leaving group ability. 

We can also rationalize selectivity for elimination versus substitution, or attack of H versus attack on C in terms of hard and soft electrophiles (pp. 237-238). in an Sn2 substitution, the carbon centre is a soft electrophile—it is essentially uncharged, and with leaving groups such as halide the C-X a is a relatively low-energy LUMO. Substitution is therefore favoured by nucleophiles whose... 

---