R 2 complexes

Given that many electrophiles form r-complexes with aromatic hydrocarbons, and that such complexes must be present in solutions in which electrophilic substitutions are occurring, the question arises... 

Tschitschibabin amination of pyridine, the mechanism of which has been established as involving an intermediate charge distribution of the thiazole molecule as well as of the thiazolium ion. 

Substituted tetrazoles readily exchange the 5-hydrogen for deuterium in aqueous solution. A major rate-enhancing effect is observed with copper(II) or zinc ions due to complexation with the heterocycle. The rate of base-induced proton-deuterium exchange of 1-methyltetrazole is 10 times faster than 2-methyltetrazole (77AHC(2l)323). 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

Iewin92] Lewin, R., Complexity Life at the Edge of Chaos, Macmillan Publishing Company (1992). 

These early results have since been confirmed and extended by a vast and still growing body of research. All of the contemporary spectroscopic techniques (ir, uv, visible, nmr, esr) have been brought to bear on the problem, and further confirmation has come from cryoscopic and conductometric studies. The early confusion that resulted from the coexistence of both donor-acceptor or non-covalently-bonded complexes) has been clarified. This research has been extensively reviewed10,13-15 and will not be detailed here. 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

Scheme 3 Representation of equilibria between Ti04 framework species and complexes inside TS-1 channels upon dosage of H2O2/H2O (left) and between r] and r] complexes upon dehydration (right). Adapted from [50] with permission. Copyright (2004) by WUey-VCH...

It seems likely that benzene forms a n complex (12) with, for example, Br2 (cf. p. 131), and that the Lewis acid then interacts with this. The catalyst probably polarises Br—Br, assists in the formation of a a bond between the bromine molecule s now electrophilic end and a ring carbon atom, and finally helps to remove the incipient bromide ion so as to form a [Pg.138]

Willie et al. [508] used Unear sweep voltammetry for the determination of molybdenum. The molybdenum was adsorbed as the Eriochrome Blue Black R complex on a static mercury drop electrode. The method was reported to have a limit of detection of 0.50 xg/l and the results agreed well with certified values for two reference seawater samples. 

In the methyl groups is particularly favorable because it allows both methyl groups to interact effectively with the positive charge. In the yr-complex the location of the methyl groups makes less difference. 

Weng, G., Bhalla, U. S. and Iyengar, R. Complexity in biological signaling systems. Science 284 92-96,1999. 

The primary defect in familial hypercholesterolemia is the inability to bind LDL to the LDL receptor (LDL-R) or, rarely, a defect of internalizing the LDL-R complex into the cell after normal binding. This leads to lack of LDL degradation by cells and unregulated biosynthesis of cholesterol, with total cholesterol and LDL cholesterol (LDL-C) being inversely proportional to the deficit in LDL-Rs. 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

An electrosynthetic method was used to generate the 25 different (P)Rh(R) complexes listed in Table 1(14,16). Many of the complexes in this Table had not been previously reported, especially (P)Rh(RX). In all cases, bulk electrolysis of... 

Table I. The Redox Potentials and UV-vls Spectral Data for the Electrogenerated (TPP)Rh(R) Complexes in THE Containing 0.2 M TBAP a...

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