Strong Closed-Shell Interactions in Inorganic Chemistry.

One of the most exciting developments in the areas of catalysis, organometallic chemistry, and polymer science in recent years has been the intense exploration and commercialization of new polymerization technologies based on single-site and metallocene coordination olefin polymerization catalysts.1 The vast number of specifically designed/synthesized transition metal complexes (catalyst precursors) and main-group organometallic compounds (cocatalysts) allows unprecedented control over polymer microstructure, the generation of new polymer architectures, and the development of new polymerization reactions. Commercialization of new generations of single-site and metallocene catalyst-based technologies has provided the multibillion pound per year polyolefins industry with the ability to deliver a wide range of new and innovative olefin-based polymers having improved properties.2-4 The intense industrial activity in the field and the challenges to our basic understanding that have come to light have in turn 1391 Chem. Rev. 2000, 100, 1391−1434

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Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure−Activity Relationships

A plethora of methods have been developed over the years so that carbon dioxide can be used as a reactant in organic synthesis. Given the abundance of this compound, its utilization in synthetic chemistry, particularly on an industrial scale, is still at a rather low level. In the last 35 years, considerable research has been performed to find catalytic routes to transform CO(2) into carboxylic acids, esters, lactones, and polymers in an economic way. This Review presents an overview of the available homogeneous catalytic routes that use carbon dioxide as a C(1) carbon source for the synthesis of industrial products as well as fine chemicals.

Transformation of Carbon Dioxide with Homogeneous Transition-Metal Catalysts: A Molecular Solution to a Global Challenge?

Publisher Summary This is one of two articles in this volume that is concerned with the borane-carborane structural pattern. In the other, Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon 7∼ complexes, and various neutral or charged hydrocarbons. Boranes and carboranes may be regarded as cluster compounds in the sense defined by Cotton; they contain a finite group or skeleton of atoms held together entirely, mainly, or at least to a significant extent by bonding directly between those atoms, even though some other atoms may be associated intimately with the cluster. Examples of their structural pattern, however, can be found far beyond the confines of what is normally regarded as cluster chemistry, so this survey includes many systems not commonly referred to as clusters.

Structural and Bonding Patterns in Cluster Chemistry

Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization

Reaction du trimethyl(phenylethynyl) silane avec le catalyseur de Ziegler Cp 2 TiCl 2 +CH 3 AlCl 2 dans le chloroforme

Organometallic compounds of Group III. Part 41. Direct observation of the initial insertion of an unsaturated hydrocarbon into the titanium-carbon bond of the soluble Ziegler polymerization catalyst Cp2TiCl2-MeAlCl2

Synthesis of pentaphenylborole, a potentially antiaromatic system

Pentasubstituted boroles or boracyclopentadienes have been prepared by two routes: (1) the interaction of an (E&)-( 1,2,3,4-tetraaryl-1,3-butadien-l,4-ylidene)dilithium in ether solution with an aryl(diha1o)borane to form the borole etherate; and (2) the exchange reaction between a 1,1-dialky1-2,3,4,5-tetraarylstannole in nondonor media to produce the unsolvated borole. The boroles are unexpectedly strong Lewis acids, complexing with amines and even ethers and nitriles, and very prone to oxidation, solvolytic cleavage, and Diels-Alder reactions. The foregoing chemical behavior, taken together with their unusual nuclear magnetic and electronic spectral properties, can be straightforwardly interpreted in terms of the Hiickel antiaromatic character of the 4-*-electron boracyclopentadiene nucleus. By treating such nuclei as perturbed cyclopentadienyl 7r-systems, a qualitative understanding of both the spectral and chemical properties can be attained. From such considerations, it is evident that the conjugation between the tricoordinate boron's 2p,-orbital and the four-carbon butadienylidene array is destabilizing and is the source of the high reactivity of boroles. An $-hybridized boron atom might be expected to form cyclic conjugated systems with arrays of sp2-hybridized carbons, because both the covalent radius (0.80 A) and the Allred-Rochow electronegativity (2.01) of boron are similar to those of carbon (0.77 A and 2.51).4 In such conjugated rings the boron center might exert a stabilizing influence if the total number of x electrons would conform to the Hiickel 4n + 2 criterion for a r o m a t i ~ i t y . ~ ~ In fact, heterocyclic organoboranes bearing the isoelectronic J3=N unit in place of one or more C=C linkages of a benzenoid ring have been shown to possess chemical and physical properties similar to classic aromatic compounds.5b Boracyclopolyenes not possessing the appropriate number of ?r electrons might be destabilized by conjugation and accordingly display unusual reactivity or antiaromaticityS6 An example of the destabilization of a carbocyclic ring by such antiaromaticity is the instability of the pentaphenylcyclopentadienyl cation (1). This deep-blue ion is highly reactive and has a thermally populated triplet (compound 2). This behavior indicates that the gain in conjugated stabilization attained by placing the four A electrons as pairs in bonding molecular orbitals is not large.

Bora-aromatic systems. Part 8. The physical and chemical consequences of cyclic conjugation in boracyclopolyenes. The antiaromatic character of pentaarylboroles.

Organometallic compounds of Group III. XIX. Regiospecificity and stereochemistry in the hydralumination of unsymmetrical acetylenes. Controlled cis or trans reduction of 1-alkynyl derivatives

Rearrangements of organometallic compounds. XIII. Boraaromatic systems. IV. Synthesis of heptaphenylborepin via the thermal rearrangement of heptaphenyl-7-borabicyclo[2.2.1]heptadiene

John J. Eisch

Papers

Organometallic compounds of Group III. Part 41. Direct observation of the initial insertion of an unsaturated hydrocarbon into the titanium-carbon bond of the soluble Ziegler polymerization catalyst Cp2TiCl2-MeAlCl2

Synthesis of pentaphenylborole, a potentially antiaromatic system

Bora-aromatic systems. Part 8. The physical and chemical consequences of cyclic conjugation in boracyclopolyenes. The antiaromatic character of pentaarylboroles.

Organometallic compounds of Group III. XIX. Regiospecificity and stereochemistry in the hydralumination of unsymmetrical acetylenes. Controlled cis or trans reduction of 1-alkynyl derivatives

Rearrangements of organometallic compounds. XIII. Boraaromatic systems. IV. Synthesis of heptaphenylborepin via the thermal rearrangement of heptaphenyl-7-borabicyclo[2.2.1]heptadiene