Transition arylations

One of the most obvious approaches to defeating the putative optical nonlinearity-optical transparency trade-off follows from the theory of Marder and Perry and is illustrated in Fig. 1. When an amine donor group is connected to a thiophene bridging group, replacement of an alkyl amine with an aryl amine leads to an increase in bond length alternation and an accompanying blue shifting (increase in optical gap) of the interband electronic transition. Aryl to alkyl amine substitution leads, as expected from the theory of Marder and Perry, to an increase in molecular first hyperpolarizability (see Fig. 1), which is, as already noted, accompanied by an increase in optical transparency. This is in contrast to the more... 

Similar stability and reactivity have also been observed for bridged-Cp systems. The catalyticaHy active (CH2)2Si(C (CH2)4)2ThR2, where R = Cl [89597-06-8] alkyl, CH2CgH [89597-10A] aryls, or H [89597-11-5], Similar to Group 4 transition-metal Zeigler-Natta catalysts, stable cationic Th(IV) species, eg, [Cp 2ThCH2] [108834-69-17, have been isolated with a host of noncoordinatiag/nonreactive anions. MetaHacycle formation has also been... 

This can be circumvented by choosing alkyl groups with no P H, eg, methyl, neopentyl, trimethylsilylmethyl, phenyl and other aryl groups, and benzyl. The linear transition state for -elimination can also be made stericaHy impossible. The most successful technique for stabilization combines both principles. The pentahaptocyclopentadienyl ring anion (Cp) has six TT-electrons available to share with titanium. Biscyclopentadienyltitanium dichloride... 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

This rearrangement can be considered to occur via a transition state in which C(2)—C(4) bridging is accompanied by a 4 3 aryl migration ... 

Trifluoromethyl Derivatives of the Transition Metal Flements 1984 Synthesis and Properties of Aryl... 

The reactions of enamines as 1,3-dipolarophiles provide the most extensive examples of applications to heterocyclic syntheses. Thus the addition of aryl azides to a large number of cyclic (596-598) and acyclic (599-602) enamines has led to aminotriazolines which could be converted to triazoles with acid. Particular attention has been given to the direction of azide addition (601,603). While the observed products suggest a transition state in which the development of charges gives greater directional control than steric factors, kinetic data and solvent effects (604-606) speak against zwitterionic intermediates and support the usual 1,3-dipolar addition mechanism. 

Stabilization of /j -alkyl and -aryl derivatives of transition metals can be enhanced by the judicious inclusion of various other stabilizing ligands in the complex, even though such ligands are known not to be an essential prerequisite. Particularly efficacious are potential IT acceptors (see below) such as AsPh3, PPh3,... 

Alteration of the relative reactivity of the ring-positions of quinoline is expected and observed when cyclic transition states can intervene. Quinoline plus phenylmagnesium bromide (Et20,150°, 3 hr) produces the 2-phenyl derivative (66% yield) phenyllithium gives predominantly the same product along with a little of the 4-phenylation product. Reaction of butyllithium (Et 0, —35°, 15 min) forms 2-butylquinoline directly in 94% yield. 2-Aryl- or 6-methoxy-quinolines give addition at the 2-position with aryllithium re-agents, and reaction there is so favored that appreciable substitution (35%) takes place at the 2-position even in the 4-chloroquinoline 414. Hydride reduction at the 2-position of quinoline predominates. Reaction of amide ion at the 2-position via a cyclic... 

Transition metal-catalyzed synthesis of hetarylamines and hetaryl ethers from triflates and aryl/hetaryl halides or heterocyclic amines 98AG(E)2046. 

Among organic materials, poly(aryl ethers) and poly (aryl sulfides) have been known, as a class of engineering thermoplastics. The electron withdrawing sulfone and ketone groups usually activate the dihalo or dinitro compounds to facilitate the nucleophilic displacement through the transition state called Meisenheimer-Iike complex, and, thus, poly(aryl ether or sulfide) sulfones... 

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