The literature on the chemistry of DMSO prior to 1962 has been reviewed,' and a comprehensive review of the solvent characteristics of dipolar aprotic solvents, including DMSO, has recently appeared.2 In the present discussion, the conclusions of some of the recent investigations which have extended our knowledge of DMSO chemistry and the reasons for the particular effects i t shows as a reaction solvent are reviewed. DMSO physical constant data selected from the literature are gathered in TABLE l.3-10 The DMSO molecule is a pyramidal structure with sulfur, oxygen and carbon atoms a t the corners.\" The sulfur-oxygen bond is quite polar so tha t the liquid has a high dielectric constant. As Lindberg pointed out,6 this polarity and geometry lead to considerable organization in the liquid state in which DMSO molecules are associated into chains by dipole attraction. The presence of a liquid structure and association forces is reflected in a rather high freezing point, a low entropy of fusion, a rather high boiling point and a high entropy of vaporization. The dielectric constant is one-third greater than that of methanol and is high enough to permit easy charge separation and hence good solubility for a variety of ionic, polar, and polarizable molecules. Sears et al. examined the conductivity of 19 salts in DMSO and found that all were completely dissociated in concentrations below l o p 3 molar. However, a t the higher concentrations used in most chemical reactions, some association of charged materials into ion pairs or aggregates occurs. Weaver & H u t c h i ~ o n ' ~ calculated the decrease in reaction rates attributable to this effect. The effect is less in DMSO than in most organic solvents because of its high dielectric constant. Since the sulfur atom is the positive atom of the dipole, one might expect the positive charge to activate the hydrogen atoms of the methyl groups of DMSO making them more acidic. However, this effect is very weak. The acidity constant for DMSO is about which makes it about .001 as acidic as diphenylmethane. Protons thus are removed from DMSO only by strongly basic reagents such as sodium hydride. The hydrogen atoms of DMSO are also quite resistant to removal in free-radical reactions, and consequently DMSO is useful as a solvent for olefin polymerizations. Because of the enhanced electron density a t the oxygen atom and the steric availability of the oxygen, DMSO solvates cations in general. A large number of stable solvates of metallic salts have been examined in which 1 to 8 molecules of DMSO were bound to the cation.' The DMSO ligand is almost

The chemical and physical properties of DMSO.

Publisher Summary This chapter discusses the preparation of pyrroles from ketoximes and acetylenes. The chemistry of pyrroles also includes complex pyrrole systems, porphirin, and phthalocyanine structures such as haemin, chlorophill, hemoglobin, bile pigments, cytochromes, and vitamin B 12 . Ketoximes react with acetylenes in a strong base/DMS0 system to form NH- and N-vinylpyrroles. The reaction of ketoximes with acetylene has become popular as a reliable preparative tool for wide application in the chemistry of pyrroles. The reaction of heterocyclization with acetylene to form NH- and Nvinylpyrroles involves all ketones having at least one methylene or methyl group in the a-position to the oxime function and no substituents susceptible to the action of bases. Extention of the reaction of ketoximes with acetylene to oximes of hydroxyalkyl ketones can lead to the synthesis of inaccessible, functionally substituted pyrroles and N-vinyl derivatives. The ketoximes containing only tertiary or aromatic radicals in any combination do not form pyrroles when reacting with acetylene in the presence of superbases. The chapter discusses influence of the acetylene structure and the use of acetylene equivalents. Systematic research on the synthesis of pyrroles and especially of N-vinylpyrroles from ketoximes and acetylene is in progress. This is expected to lead to not only further extension of the preparative possibilities of the reaction, but also to discovery of new versions and analogs. The increased access to N-vinylpyrroles stimulates more and more synthetic and theoretical investigations in this field as well as work dealing with polymerization and practical application of these compounds.

Preparation of Pyrroles from Ketoximes and Acetylenes

Strongly basic systems—V

Reactions of triads Se8KOHDMSO, Se8KOHDMSO, TeKOHHMPA with acetylenes

The Hammett acidity function concept has been used to determine simultaneously the pKa values of 24 substituted anilines and diphenylamines and the H− values of various solvent systems. The following solvent systems were examined (the most basic H− value obtained for each is shown in parentheses): water containing benzyltrimethylammonium hydroxide (16.20); pyridine–water containing tetramethylammonium hydroxide (18.91); sulpholane–water containing tetramethylammonium hydroxide (19.18); dimethyl sulphoxide–water containing tetramethylammonium hydroxide (18.61); water containing lithium hydroxide (14.31).In addition to these results adjustments have been made to previously published scales for hydrazine–water (15.08) and ethylenediamine–water (15.48).

Strongly basic systems: iii. the h− function for various solvent systems

Rate-equilibrium correlation for dissociation of a carbon acid

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Rate-equilibrium correlation for dissociation of a carbon acid