Dipolar metals

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

Examples Involving Dipolar Metals. The dipolar metals are grouped in Table I along with a rough indication of their ease of detection. 

There is an enormous literature concerning the dipolar metals, and earlier reviews (8,10,15) constitute an important source of reference information. Recent contributions include trends in Pt (16) and Sn (17) chemical shifts, and in Sn-X coupling constants... 

Examples Involving Quadrupolar Metals. The quadrupolar metals are grouped in Table II, again with an indication of their ease of detection. The classification is considerably more arbitrary than for the dipolar metals since the ease of detection will depend markedly on the degree of symmetry around the metal and the magnitude of the quadrupole coupling constant. Where a choice exists, the most likely preferred isotope is indicated. 

As is the case for the dipolar metals, the number of applications involving quadrupolar metals is far too great to cover in a short review such as this. Dechter s coverage (10) of the literature from 1978 through September, 1983 is particularly timely in updating earlier reviews, and selected references which are thought to be currently representative will simply be mentioned here. 

From the X-ray structure we know that the arrangement of the cysteines bound to the metal ions is highly distorted, in particular one cysteine is in a completely distorted position with respect to the others and one P-CH2 proton is very near to both iron ions. Using these structural data we can calculate the proton T values reported in Table 2 by assuming a purely point-dipolar metal-centered model. For reasons we are going to discuss the experimental values cannot be reproduced. 

Reactions of sulphur dioxide with [Fe( -allylX -QH6XCO)2] are known to afford S-bonded sulphinate complexes. It is generally accepted that these reactions proceed via dipolar metal-alkene intermediates, and low-temperature n.m.r. studies have now provided evidence for this reaction intermediate (8). The rates of re-... 

Once a metal is unmersed in a solvent, a second dipolar layer will fomi at the metal surface due to the... 

Adsorption of a neutral (n ) onto a metal surface leads to a heat of adsorption of Q, as the electrons and nuclei of the neutral and metal attract or repel each other. Partial positive and negative charges are induced on each with the formation of a dipolar field (Figure 7.4). 

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Nucleophilic Substitution Route. Commercial synthesis of poly(arylethersulfone)s is accompHshed almost exclusively via the nucleophilic substitution polycondensation route. This synthesis route, discovered at Union Carbide in the early 1960s (3,4), involves reaction of the bisphenol of choice with 4,4 -dichlorodiphenylsulfone in a dipolar aprotic solvent in the presence of an alkaUbase. Examples of dipolar aprotic solvents include A/-methyl-2-pyrrohdinone (NMP), dimethyl acetamide (DMAc), sulfolane, and dimethyl sulfoxide (DMSO). Examples of suitable bases are sodium hydroxide, potassium hydroxide, and potassium carbonate. In the case of polysulfone (PSE) synthesis, the reaction is a two-step process in which the dialkah metal salt of bisphenol A (1) is first formed in situ from bisphenol A [80-05-7] by reaction with the base (eg, two molar equivalents of NaOH),... 

Gothelf presents in Chapter 6 a comprehensive review of metal-catalyzed 1,3-di-polar cycloaddition reactions, with the focus on the properties of different chiral Lewis-acid complexes. The general properties of a chiral aqua complex are presented in the next chapter by Kanamasa, who focuses on 1,3-dipolar cycloaddition reactions of nitrones, nitronates, and diazo compounds. The use of this complex as a highly efficient catalyst for carbo-Diels-Alder reactions and conjugate additions is also described. 

Asymmetric Metal-catalyzed 1,3-Dipolar Cycloaddition Reactions... 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

Basic Aspects of Metal-catalyzed 1,3-Dipolar Cycloaddition Reactions 215 The normal electron-demand 1,3-dlpolar cycloaddition reaction... 

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