Subject chain substitution

Benzene rings in both the skeleton structure and on the side groups can be subjected to substitution reactions. Such reactions do not normally cause great changes in the fundamental nature of the polymer, for example they seldom lead to chain scission or cross-linking. 

The mechanism composed of Eqs. (2), (3), (5), and (7) is observed typically for nuclear substitution in aromatic compounds, whereas side-chain substitutions, for instance, typically proceed according to Eqs. (2), (4), (6), and (8). (For a more thorough discussion of the reactions between radical cations and nucleophiles, the reader is referred to the rich literature on this subject [1,2].) Essentially the same mechanistic pattern is found in oxidative substitution driven by high valent inorganic ions [3-6]. 

The response functions of short conjugated chains (substituted or unsubstituted) have been the subject of a number of studies Cao and Lin78 (alkenes), Park and Cho79 (triazine), Choytun et a/711 (donor/acceptor substituted azines), Rosseto et al.81 (substituted alkynes). 

The allylic position of olefins is subject to attack by free radicals with the consequent formation of stable allylic free radicals. This fact is utilized in many substitution reactions at the allylic position (cf. Chapter 6, Section III). The procedure given here employs f-butyl perbenzoate, which reacts with cuprous ion to liberate /-butoxy radical, the chain reaction initiator. The outcome of the reaction, which has general applicability, is the introduction of a benzoyloxy group in the allylic position. 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

The dangerous reactions of alcohols, apart from the ones that involve the carbon chain, are linked either to the exothermicity of the reactions whose consequences are often aggravated by poor temperature monitoring, or the instability of the intermediate or final compounds formed. The first case often happens with oxidation reactions, the second especially with substitutions of active hydrogen or hydroxyl. Nitric acid will be the subject of special consideration since it can have both characters, without knowing which one played a role during accidents that have involved this compound. 

Entry 5 is an example of the use of fra-(trimethylsilyl)silane as the chain carrier. Entries 6 to 11 show additions of radicals from organomercury reagents to substituted alkenes. In general, the stereochemistry of these reactions is determined by reactant conformation and steric approach control. In Entry 9, for example, addition is from the exo face of the norbornyl ring. Entry 12 is an example of addition of an acyl radical from a selenide. These reactions are subject to competition from decarbonylation, but the relatively slow decarbonylation of aroyl radicals (see Part A, Table 11.3) favors addition in this case. 

The synthesis in Scheme 13.21 starts with a lactone that is available in enantiomer-ically pure form. It was first subjected to an enolate alkylation that was stereocontrolled by the convex shape of the cis ring junction (Step A). A stereospecific Pd-mediated allylic substitution followed by LiAlH4 reduction generated the first key intermediate (Step B). This compound was oxidized with NaI04, converted to the methyl ester, and subjected to a base-catalyzed conjugation. After oxidation of the primary alcohol to an aldehyde, a Wittig-Horner olefination completed the side chain. 

The reactions of the six-membered chlorocyclophosphazene were studied with a number of aliphatic diamines (169 175), aromatic diamines (176), aliphatic diols (177-179), aromatic diols (180,181) and compounds containing amino and hydroxyl functional groups (169,170,182). This subject has been reviewed (11,16,20). There are at least five different reaction products that are possible (Fig. 19). Replacement of two chlorine atoms from the same phosphorus atom produces a spirocyclic product. Replacement of two chlorine atoms from two different phosphorus atoms in the same molecule produces an ansa product. Reaction of only one end of the difunctional reagent, resulting in the substitution of only one chlorine atom, leads to an open-chain compound. Intermolecular bridged compounds are formed when the difunc-... 

Products of substitution of inosine and guanosine 5 -monophosphate for chloride or for water on ternary aminocarboxylate complexes such as [Pd(mida)(D20)], where mida = IV-methyliminodiacetate, or [Pd2(hdta)Cl2]2-, where hdta = 1,6-hexanediamine-A(7V,./V,./V,-tetraace-tate, is subject to mechanistic controls in terms of number of coordinated donor atoms and pendant groups and of the length of the chain joining the functional groups in the bis-iminodiacetate ligands. These factors determine the nature and stereochemistry of intermediates and the relative amounts of mono- and bi-nuclear products (253). 

Modification of the basic side-chain of metoclopramide has been the subject of numerous investigations. Earlier work led to the synthesis of YM 09151-2 (15a) [8], clebopride (15b) [9], dazopride (15c) [10] and cisapride (15d) [11]. Modification of the basic side-chain and aromatic ring substitution led to the synthesis of alizapride (4a) [5], sulpiride (16a) [ 12] and cinitapride (16b) [13]. Although a number of analogues have found clinical use for various indications,... 

---