SN1 type reaction

The second explanation for the formation of the stereoirregular polymer is that the propagation of 2 is essentially an SN1 type reaction, and that attack d (broken arrow) in 14 is depressed by the interaction of the outer-ring oxygen atom with the positively charged carbon atom of the oxycarbenium ion from the upper side of the... 

The energy required to effect such a process decreases as t rises the process is also facilitated by increasing solvation, and consequent stabilisation, of the developing ion pair compared with the starting material. That such effects, particularly solvation, are of prime importance is borne out by the fact that SN1 type reactions are extremely uncommon in the gas phase. 

Figure 1.12 CF3 effect on Sn1 type reactions oftosylates and on protonation of fluoroolefins.

Notice that for SN2 substitution, the alkyl halide came from the less sterically hindered group. For SN1 type reactions, the alkyl halide forms from the fragment of the original molecule that forms the more stable cation. Thus, the reaction of t-butyl ethyl ether with HI gives t-butyl iodide and ethyl alcohol. The following mechanism occurs ... 

Organocatalytic Sn1-Type Reactions with Br0nsted Acids... 

Organocatalytic SN1-Type Reactions with Br0nsted Adds and Metals... 

Combination of Enamine Catalysis and Lewis Acids in SN1-Type Reactions... 

Simple kinetic measurements can, however, be an inadequate guide to which of the above two mechanisms, SN1 or SN2, is actually operating in, for example, the hydrolysis of a halide. Thus, as we have seen (p. 45), where the solvent can act as a nucleophile (solvolysis), e.g. H20, we would expect for an S 2 type reaction,... 

In general, the more stabilized the carbon cation derived from an alkyl halide, the more reactive the compound will be in SNl-type reactions. This is especially apparent in the reactivities of compounds with phenyl groups on the reacting carbon. As the number of phenyl groups increases from zero to three, the SN1 reactivity of the chlorides increases by more than 107 because of increasing stabilization of the carbon cation by the phenyl groups ... 

The protected compound is a much weaker acid than the alkyne, and the displacement reaction can be carried out with the alkynide salt without difficulty. To obtain the final product, the protecting group must be removed, and this can be done in dilute aqueous acid solution by an SN1 type of substitution (Sections 8-7D and 8-7E) ... 

When a solution of a diazonium salt is heated, nitrogen is evolved and the diazo group is replaced by a hydroxyl group in an SN1 type of displacement reaction. 

Initial explanations in terms of an associative SN2-type reaction proved untenable, and the reaction is now thought to involve deprotonation of a co-ordinated amine (Fig. 2-17). This is the SN1 cb or Deb mechanism. The key step is the formation of the amide intermediate, [Co(NFl3)4(NH2)Cl]+, which undergoes halide loss to generate the reactive five-co-ordinate intermediate [Co(NH3)4(NH2)]2+ (2.2) (Fig. 2-18). 

At the higher temperature, the reaction becomes reversible and is under thermodynamic control. This means there is enough energy available for either product to reform the allylic carbocation by an SN1 type ionization and then form the other product. As a result the products are in equilibrium. At equilibrium, the relative amount of the products is controlled only by the difference in energy between them (AG). In this case, the 1,4-addition product (called the thermodynamic product) is more stable than the 1,2-addition product, so more of it is present in the equilibrium mixture. This same equilibrium mixture of products (15% of the 1,2-addition product and 85% of the 1,4-addition product) is produced when the low-temperature reaction product mixture (80% of the 1,2-addition product and 20% of the 1,4-addition product) is heated to 45°C. 

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

As expected, heterocyclic enols and potential enols (i.e., compounds existing mainly in the CH form) behave toward diazomethane similarly to the open chain and isocyclic enols, i.e. they form enol methyl ethers by reactions of the SN1 type (cf. footnote 29). Examples of this behavior are barbituric acid,84 picrolonic acid,125 dehydroacetic acid (64), 125 3-methyl-l-phenylpyrazolin-5-one,36 1-phenylpyrazoli-dine-3,5-dione,61 1,2-diphenylpyrazolidine-3,5-dione,144 3-hydroxy-... 

When a carbocation is formed by loss of a nucleophile, as in the SN1 reaction, elimination of a proton will always compete with substitution, provided that there is a (3-C-H bond in the cation. These eliminations are described as unimolecular eliminations (El)because, as with the competing SN1 reaction, the rate-determining step is the unimolecular reaction of the original halide to give the carbocation. For example, /-butyl chloride in ethanol gives the /-butyl cation, which can either react with a solvent molecule to give the ethyl ether 36 (SN1, solvolysis, reaction 5.25) or a proton can be removed by a solvent molecule to give 2-methylpropene 37 (El, reaction 5.26). If more than one type of J3-C-H... 

Unsymmetrical ir-allyl-Pd complexes usually suffer attack of the hydride nucleophile at the less substituted position in an SN2-type reaction. However, the site selectivity of the process is controlled by steric and/or electronic effects. The reaction is strongly dependent on the structural features of the substrate and the reaction conditions. Opposite site selectivity is observed when the reduction occurs at the sterically more hindered position via a cationic intermediate (SN1-type). Very potent nucleophilic hydride sources, such as LiBHEt3 or LiAlH4, may rapidly attack intermediate it-ally 1 complexes at the less hindered terminal position to give the more substituted alkene, while less effective hydride-transfer reagents (NaBH3CN, NaBH4) attack the it-allyl systems at the site best able... 

The possibility of rearrangement must be borne in mind particularly when considering reactions of allylic compounds. Substitution of SN1 type, which dominates in polar solvents, always leads to mixtures of isomeric rearrangement products bimolecular substitution occurs without rearrangement. Thus choice of strongly nucleophilic reactants and, particularly, repression of SN1 reaction by selection of an apolar solvent such as acetone permit reactions of allylic compounds to be undertaken without rearrangement.13... 

The reaction of an alkyl halide or los3 late with a nucleophiJe/base results eithe in substitution or in diminution. Nucleophilic substitutions are of two types S 2 reactions and SN1 reactions, in the SN2 reaction, the entering nucleophih approaches the halide from a direction 180° away from the leaving group, result ing in an umbrella-like inversion of configuration at the carbon atom. The reaction is kinetically second-order and is strongly inhibited by increasing stork bulk of the reactants. Thus, S 2 reactions are favored for primary and secondary substrates. 

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