1.3- dipole formation

In presence of one carbon-nitrogen triple bond —C—C=N In compounds with tendency to dipole formation, e.g., C=C—C=0 In aromatic compounds... 

Scheme 2.184. Synthesis of diverse five-membered heterocycles via domino 1,3-dipole formation/cycloaddition.

The rhodium( 11)-catalyzed formation of 1,3-dipoles has played a major role in facilitating the use of the dipolar cycloaddition reaction in modern organic synthesis. This is apparent from the increasing number of applications of this chemistry for the construction of heterocyclic and natural product ring systems. This chapter initially focuses on those aspects of rhodium(II) catalysis that control dipole formation and reactivity, and concludes with a sampling of the myriad examples that exist in the Hterature today. 

The First Examples of Transition Metal-Mediated 1,3-Dipole Formation... 

The cycloaddition reaction of dipoles has been known since the late eighteenth century however, before Huisgeris introduction of the concept of a 1,3-dipole, these reactions were considered to proceed via a diradical mechanism [16]. One of the earhest examples of metal-catalyzed 1,3-dipole formation involved the controlled decomposition of an a-dia-... 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

Radius of negentropically re-ordering vacuum energy at time t in seconds, after dipole formation... 

Tsuge. Azomethine Ylide Cycloadditions via 1,2- Prototropy and Metallo-Dipole Formation from Imines, R. Grigg and V. Sridharan. Index. ... 

In an apolar medium, fluorophore in the excited state will induce a dipole formation within the environment. The formation of a new dipole needs less energy than the reorientation of an already existing dipole. Thus, emission from an apolar environment yields an emission spectrum with a maximum located in the blue compared to emission occurring in a polar environment. 

Ibata was the first to show that the masked carbonyl ylide embedded within the isomiinchnone framework would readily undergo 1,3-dipolar cycloaddition with various dipolarophiles [34], The isolable isomiinchnone 4 was observed to react with dimethyl fumarate to produce cycloadduct 7 which possesses the 7-oxa-2-azabicyclo[2.2.1]heptane skeleton. When the reaction of 1 was carried out using catalytic amounts of Cu(acac)2 in the presence of various dipolarophiles, smooth dipolar cycloaddition was observed to occur giving mixtures of endo and exo isomers. In most cases, the exo isomers were favored. All of Ibata s isomiinchnone cycloadditions contain aromatic substituent groups, presumably selected to facilitate dipole formation. The synthetic utility of the cycloaddition reaction is diminished, however, because of the low reactivity of the aromatic substituents toward further manipulation. 

Scheme 24 Dipole formation from reactions of azines and substituted alkynes...

We should clarify here that the above cited studies are largely exploratory and the role of each parameter in reaction specificity is currently unclear. They show, however, the need for a fundamental understanding of molecular and electronic surface interactions that determine electrocatalytic as well as catalytic specificity. Thus, adsorption isotherms, surface states, molecular configurations, electronic distributions, dipole formation, and bond hybridization should be explored for well-characterized catalysts and model reactions in the presence and in the absence of an electric field. 

The tandem cyclization-cycloaddition reaction of 1-diazoacetyl-7,7-dimethylbicyclo[2.2.1]heptan-2-one (1) with dimethyl butynedioate catalyzed by rhodium(II) acetate dimer in benzene at 25 °C, afforded a formal [3 + 2] cycloadduct 2 in 85% yield with complete diastereofacial selectivity48. This reaction is interpreted to proceed via rhodium carbenoids and subsequent transannular cyclization of the electrophilic carbon onto the adjacent oxo group to generate a cyclic carbonyl ylide. followed by 1,3-dipolar cycloaddition. Similar reactions are observed with other dipolar-ophiles, such as propargylic esters and A -phenylmaleimide. Studies dealing with the geometric requirements of dipole formation were undertaken. 

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