7.5 FLUJOS DE SALIDA
7.5.2. Pérdidas de agua potable facturada y no facturada Índice de Agua No
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Chapter 4
Nucleophilic Reactivities of Pyrroles
This chapter will be published as soon as possible by M. Westermaier, T. A. Nigst and H. Mayr in Eur. J. Org. Chem.
4.1 Introduction
Pyrrole and its derivatives are biochemically important compounds and can be found as substructures in many natural products, e. g. heme, chlorophyll and the pyrrole alkaloids.[1-4] Electrophilic substitutions of pyrroles, which incorporate electron-rich
π
-systems, have been investigated intensively.[5-12] In 1957, Treibs and Fritz derived a qualitative reactivity scale for alkyl substituted pyrroles from their reactions with diazonium salts of variable electrophilicity.[13] The most comprehensive quantitative comparison of the reactivities of arenes and heteroarenes has been based on the σ+arene constants, which are defined by theHammett-Brown relationship (4.1)[14-20] + ⋅ = arene k k/ 0 ρ σ lg (4.1)
The σ+arene constants (equivalent to σ+p or σm in the case of monosubstituted benzenes) are a
measure for the relative reactivities of one position of an arene in relation to one position of benzene. They were typically derived by competition experiments, where an electrophile was allowed to select between a pair of arenes, or from the rates of SN1 reactions of the side chain
of the corresponding arenes.
However, σ+arene parameters have only been determined for few pyrroles, e. g. the parent
compound and N-methylpyrrole. As knowledge of the nucleophilic reactivities of pyrroles is crucial for their well directed use in synthesis, particularly as nucleophiles in organocatalysis cycles (iminium catalysis),[21-30] we decided to obtain quantitative information on the nucleophilicities of alkyl substituted pyrroles (Scheme 4.1).
N H Me Me N H N Me N Si(i Pr)3 N Me Me Me Me NH Me N H Me Me Et 1 2 3 4 5 6 7
Scheme 4.1. Alkyl substituted pyrroles.
We have previously reported that the reactions of carbocations with π-systems follow Equation (4.2), ) ( ) 20 ( lgk °C =s N+E (4.2)
where electrophiles are characterized by one parameter (electrophilicity E) and nucleophiles are characterized by two parameters (nucleophilicity N and slope s).[31-35] Recently, it has been demonstrated that Equation (4.2) can also be employed for SN2 reactions, if an additional,
electrophile-specific parameter sE is considered.[36]
N and s parameters of N-methylpyrrole (2)[33] and N-(triisopropylsilyl)pyrrole (7)[37] have already been determined in dichloromethane and the N parameter of the parent compound (1) has been estimated.[37] We have now studied the reactions of four alkyl substituted pyrroles 3–6 with a series of benzhydrylium ions 8 (for structures see Table 4.1) in acetonitrile (Scheme 4.2) and used the kinetic data to determine the N and s parameters of these pyrroles.
H N Alkyl + Ar Ar CH3CN, 20 °C HN Alkyl Ar Ar 3−6 8 BF4 9−12 - HBF4
Table 4.1. List of carbocations 8 used in this study as reference electrophiles.
reference electrophile[a] E[b]
N N 8a –10.04 N N Me Me 8b –8.76 N N 8c –7.69 N N Me Me Me Me 8d –7.02 N N Ph Ph Me Me 8e –5.89 N N O O 8f –5.53 N N Ph Ph Ph Ph 8g –4.72 N N Me Me F3C CF3 8h –3.85 N N Ph Ph F3C CF3 8i –3.14
[a] All benzhydrylium ions were used as tetrafluoroborate salts. [b] Electrophilicity parameters E taken from ref. [33].