Lewis bases substitution

Several Lewis-base-substitution products of Ru3(CO)i2 have been prepared (752). PFj replaces up to six CO groups (277), whereas organic phosphines or arsines replace two or three (58, 65, 66, 230, 327). The mechanism of these substitution reactions has been elucidated (319-321) and can be summarized by equation (2) ... 

In the cases of [Ru3(CO)i2] and [Ru4H4(CO)i2], which have very unstable anion radicals, Lewis base substitution reactions are stoichiometric rather than catalytic in the reducing agent (167) dppe and the triruthenium cluster give [Ru3(CO)io(dppe)J, [Ru3(CO)n(dppe)], or [ Ru3(CO)n 2(/u.-dppe)], depending on the relative stoichiometries of the reactants (168). Complexes such as [Ru3(CO)uL] and [Ru4H4(CO)uL] [L = PPh2CH2CH2Si(OEt)3], precursors to surface-anchored species, have also been prepared by reductively induced substitution reactions (169). 

The Lewis acid Lewis base idea also includes certain substitution reactions m which one atom or group replaces another... 

When we discussed elimination reactions in Chapter 5 we learned that a Lewis base can react with an alkyl halide to form an alkene In the present chapter you will find that the same kinds of reactants can also undergo a different reaction one m which the Lewis base acts as a nucleophile to substitute for the halo gen substituent on carbon... 

The Lewis base that acts as the nucleophile often is but need not always be an anion Neutral Lewis bases can also serve as nucleophiles Common examples of substitutions involving neutral nucleophiles include solvolysis reactions Solvolysis reactions are substitutions m which the nucleophile is the solvent m which the reaction is carried out 8olvolysis m water (hydrolysis) converts an alkyl halide to an alcohol... 

As we have seen the nucleophile attacks the substrate m the rate determining step of the Sn2 mechanism it therefore follows that the rate of substitution may vary from nucleophile to nucleophile Just as some alkyl halides are more reactive than others some nucleophiles are more reactive than others Nucleophilic strength or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate By measuring the rate at which various Lewis bases react with methyl iodide m methanol a list of then nucleophihcities relative to methanol as the standard nucleophile has been compiled It is presented m Table 8 4... 

Substitution can take place by the S l or the 8 2 mechanism elimination by El or E2 How can we predict whether substitution or elimination will be the principal reac tion observed with a particular combination of reactants The two most important fac tors are the structure of the alkyl halide and the basicity of the anion It is useful to approach the question from the premise that the characteristic reaction of alkyl halides with Lewis bases is elimination and that substitution predominates only under certain special circumstances In a typical reaction a typical secondary alkyl halide such as iso propyl bromide reacts with a typical Lewis base such as sodium ethoxide mainly by elimination... 

As crowding at the carbon that bears the leaving group decreases the rate of nude ophilic attack by the Lewis base increases A low level of steric hindrance to approach of the nucleophile is one of the special circumstances that permit substitution to pre dominate and primary alkyl halides react with alkoxide bases by an 8 2 mechanism m preference to E2... 

FIGURE 8 11 When a Lewis base reacts with an alkyl halide either substitution or elimination can occur Sub stitution (Sn2) occurs when the Lewis base acts as a nu cleophile and attacks carbon to displace bromide Elimi nation (E2) occurs when the Lewis base abstracts a pro ton from the p carbon The alkyl halide shown is iso propyl bromide and elimi nation (E2) predominates over substitution with alkox ide bases... 

Tertiary alkyl halides are so sterically hindered to nucleophilic attack that the pres ence of any anionic Lewis base favors elimination Usually substitution predominates over elimination m tertiary alkyl halides only when anionic Lewis bases are absent In the solvolysis of the tertiary bromide 2 bromo 2 methylbutane for example the ratio of substitution to elimination is 64 36 m pure ethanol but falls to 1 99 m the presence of 2 M sodium ethoxide... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Nucleophilicity (Section 8 7) A measure of the reactivity of a Lewis base in a nucleophilic substitution reaction... 

Another group of isoprene polymerization catalysts is based on alanes and TiCl. In place of alkyl aluminum, derivatives of AlH (alanes) are used and react with TiCl to produce an active catalyst for the polymerization of isoprene. These systems are unique because no organometaHic compound is involved in producing the active species from TiCl. The substituted alanes are generally complexed with donor molecules of the Lewis base type, and they are Hquids or soHds that are soluble in aromatic solvents. The performance of catalysts prepared from AlHCl20(C2H )2 with TiCl has been reported (101). 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

The term nucleophilicity refers to the effect of a Lewis base on the rate of a nucleophilic substitution reaction and may be contrasted with basicity, which is defined in terms of the position of an equilibrium reaction with a proton or some other acid. Nucleophilicity is used to describe trends in the kinetic aspects of substitution reactions. The relative nucleophilicity of a given species may be different toward various reactants, and it has not been possible to devise an absolute scale of nucleophilicity. We need to gain some impression of the structural features that govern nucleophilicity and to understand the relationship between nucleophilicity and basicity. ... 

These mechanisms ascribe in jortance to the Lewis acid-Lewis base interaction between the allyl halide and the organolithium reagent. When substitution is complete, the halide ion is incorporated into the lifliium cluster in place of one of the carbon ligands. 

We have seen that an alkyl halide and a Lewis base can react together in either a substitution or an elimination reaction. 

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Other salts, especially fluoride salts, (e.g., KF) can be used to perform nucleophilic substitution. As is well known, halides, and particularly the fluoride anions, are rather powerful Lewis bases and can exert a catalytic effect on aromatic nucleophilic substitutions in dipolar aprotic solvents. Phenols can be alkylated in the presence of KF (or CsF) absorbed on Celite64,65 or Et4NF.66 Taking advantage of this reaction, halophenols and dihalides with bisphenols have been successfully polymerized in sulfolane at 220-280°C by using KF as the base. 

The bromination of benzene illustrates the difference between addition to alkenes and substitution of arenes. First, to achieve the bromination of benzene it is necessary to use a catalyst, such as iron(III) bromide. The catalyst acts as a Lewis acid, binding to the bromine molecule (a Lewis base) and ensuring that the outer bromine atom has a pronounced partial positive charge ... 

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