Nucleophiles iodide ion

Utilization of the free compounds A and B instead of TPO indeed led to the same promoting effect. These observations suggest that the effect is primarily due not to direct interaction between the active metal center and the oxide, but to the generation of a high concentration of strongly nucleophilic iodide ions ... 

When HI adds to a double bond, the proton acts as an electrophile, giving an intermediate carbocation that then reacts with the nucleophilic iodide ion to give the product. In the reaction of HI with 1-methylcyclohexene, there is only one product, 1-iodo-l-methylcyclohexane no l-iodo-2-methyl-cyclo-hexane is formed. 

It should be noted that the reaction of [54] with either hydroxide ion on alumina or piperidine yields [56] (Ballester et al., 1967). This is excellent additional evidence in favour of those carbanions being reaction intermediates in dealkylation. As a nucleophile, iodide ion also causes dealkylation. 

Chapter 10 discusses the reaction of a nucleophile with a carbocation in the second step of a two-step mechanism, where the carbocation is generated by the reaction of a mineral acid with an alkene (Section 10.2). In Chapter 6 (Section 6.7), a nucleophile is defined as a species that donates two electrons to carbon, to form a new covalent a-bond to carbon. A carbocation has a positively charged carbon atom, and a nucleophile such as chloride ion, bromide ion, or iodide ion donates two electrons to that electron-deficient atom. The Lewis base analogy for a nucleophile is appropriate in these reactions, in which a carbocation 1 reacts with a nucleophile, iodide ion, to form a new C-I bond in 2. 

The reaction of a nucleophile with an electrophilic carbon atom is not limited to carboca-tions. A nucleophile can also donate electrons to a polarized carbon atom (C ) such as the one in 3, where the presence of an electronegative atom X (such as Cl or Br) generates an induced dipole at carbon. The nucleophilic iodide ion donates two electrons to the positive carbon in 3 to form a new C-I a-bond in 4. However, if a new bond is formed to a carbon that has four covalent bonds, one of those bonds must break. The relatively weak C-X bond breaks as the C-I bond is formed, and the products are alkyl iodide 4 and the X ion. The conversion of 3 to 4 constitutes a new type of reaction, a substitution at an sp hybridized carbon. 

The conversion of 64 to 74 is a two-step process (two chemical reactions), where the product of the first reaction is the intermediate 66 via ionization of 64. This transient product (the intermediate) reacts with the nucleophile (iodide ion) in a second reaction via collision to give the iodide product, 74. In this transformation, it is reasonable to ask if ionization or nucleophilic collision is the faster process. The attraction between a negative anion (iodide) and a positive cation (66) is great, so this reaction is expected to be very fast. 

Introduction of a strong nucleophile (iodide ion) into the reaction mixture in cmder to suppress elimination of the heterocyclic base from phosphonate intemnediates51 5257c or 57e entirely changed the reaction course (Scheme 11). Thus, chlorobutyne 14e and tri-ethylphosphite (110 °C, 2 h) in the presence of BU4NI furnished -3, 5 -diphosphonate 60e in 58% yield. This is an example of a simultaneous introduction of two phosphonyl residues into an acyclic (unsaturated) nucleoside analogue. A similar reaction of compound... 

It resembles tetracyanoethylene in that it adds reagents such as hydrogen (31), sulfurous acid (31), and tetrahydrofuran (32) to the ends of the conjugated system of carbon atoms suffers displacement of one or two cyano groups by nucleophilic reagents such as amines (33) or sodiomalononittile (34) forms TT-complexes with aromatic compounds (35) and takes an electron from iodide ion, copper, or tertiary amines to form an anion radical (35,36). The anion radical has been isolated as salts of the formula (TCNQ) where is a metal or ammonium cation, and n = 1, 1.5, or 2. Some of these salts have... 

Iodide and thiocyanate ion are effective catalysts for inducing a related rearrangement (62AG(E)S28). This reaction can be envisioned as proceeding by nucleophilic attack on the lesser substituted aziridinyl carbon atom by iodide ion to give an iodoethyl intermediate such as (132) which is subsequently converted to the final product. 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Nucleophilic catalysis is also observed with iodide ions. Fluoride ion does not form nitrosyl fluoride under diazotization conditions, as is to be expected from Pearson s hard and soft acids and bases principle which was discussed briefly in Section 3.2. More recently, nucleophilic catalysis has also been shown to occur with thiocyanate ion (SCN ), thiosulfate ion (HS2Of), dimethyl sulfide, and thiourea (H2NCSNH2) or its alkyl derivatives (see below). 

A fivefold variation in added iodide ion concentration produced no detectable trend in the rate coefficients, which had the mean value 99+10 at 20 °C, and therefore nucleophilic attack of iodide ion on tin appears kinetically non-significant. 

Chloride ion is a weaker nucleophile than bromide and iodide ions => chloride does not react with 1° or 2° alcohols unless zinc chloride or some Lewis acid is added to the reaction. 

Two examples of aquation/anation studies of chloro-platinum(II) complexes of possible medical relevance appeared in subsection 1 above 202,207). Aquation of cisplatin is slower in the presence of DNA but not in the presence of phosphate 220). DNA also inhibits substitution in [Pt(terpy)(py)]2+ and related complexes. For reaction of these charged complexes with iodide ion inhibition is attributable to electrostatic interactions - the complex is concentrated on the double helix and thus separated from the iodide, which distances itself from the helix. Intercalation of these complexes within the helix also serves to make nucleophilic approach by neutral reagents such as thiourea more difficult 221). 

In summary, our studies have revealed that Sml2 is not a suitable reagent for the reductive opening of epoxides. The high Lewis acidity of this metal combined with the high nucleophilicity of the iodide ions leads to the for-... 

The reduction of (2,3)-q - and (2,3)-jS-methylenepenam j6-sulfoxides to the corresponding sulfides using potassium iodide and trifluoroacetic anhydride (TFAA) is found to be much faster than for bicyclic jS-lactam jS-sulfoxides.- In the proposed mechanism, initial attack of the sulfoxide oxygen on TFAA is followed by rate-limiting, nucleophilic displacement of trifluoroacetate by iodide ion nucleophilic attack of iodide on the iodine atom then yields the sulfide and iodine. The rate enhancement is accounted for by the stabilization of the transition state in the rate-limiting step by interaction of the p-like orbital of sulfur and the cyclopropane a orbital. 

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