No abstract

No abstract

Nitrogen-centered radicals, generated by the action of UV light, are capable of reacting with alkynyl Fischer carbene complexes in two ways, 1,2-and 1,4-addition. Our results constitute the first reported example of a photochemically driven reaction of this kind. ' INTRODUCTIONRadicals have shown impressive potential in synthesis. 1 We recently described 2 the use of acyloximes in the photochemical synthesis of isoquinoline derivatives in a two-step, one-pot reaction sequence. First, the iminyl radical is easily generated by a nitrogenÀoxygen bond cleavage. The addition of this radical to unsaturated moieties then gives the heterocyclic compounds with ease in good yields. 2 This reaction mechanism has been further explored by means of theoretical calculations 3 and EPR studies. 4 Several groups are able to react in the intramolecular version, such as aryl, heteroaryl, alkenyl, or alkynyl, 2 while the intermolecular reaction works particularly well with alkynes to give isoquinolines (Scheme 1). 2a This fact, together with our experience in the photochemistry of imine Fischer carbene complexes, 5 prompted us to study the reactivity of iminyl radicals with alkynyl Fischer carbene complexes.A survey of the literature showed that metal carbene complexes have been extensively used as synthons in organic and organometallic synthesis. 6 However, as far as we know, only four papers have been published on additions of radicals to unsaturated Fischer carbene complexes. All of these reactions were carried out under thermal conditions and none under photochemical ones. The strategy to add alkyl radicals, generated from epoxides and [Cp 2 TiCl] 2 , to R,β-unsaturated carbenes was developed by Merlic 7 and was also used by D€ otz to prepare carbohydrate-modified fused pyranosylidene complexes, 8 while Sierra used Et 3 B in the presence of traces of oxygen to produce an ethyl radical that adds to the carbene complex. 9 The lack of examples is not surprising considering that carbenes often react with several types of reagents for radical generation or are incompatible with the reaction conditions to create such species. 10 Sierra also attempted to promote this kind of reaction by forming the radical through irradiation (tungsten lamp or sunlight) of Barton esters, 11 but this process resulted in recovery of unreacted or oxidized metalÀcarbene complex. 9 We report here the first successful results for the photochemically induced addition of an iminyl radical to unsaturated metalÀcarbene complexes. ' RESULTS AND DISCUSSIONSIn our first attempt we reproduced the previous reaction conditions that successfully yielded isoquinolines. Irradiation of a solution of benzophenone O-acetyloxime (1a) and pentacarbonyl(1-ethoxy-3-phenyl-2-propynylidene)tungsten (2a) was performed, at room temperature, with a 400 W medium-pressure mercury lamp through Pyrex under an Ar atmosphere. However, we found that the same reaction conditions were not appropriate in this case since only decomposition of the carbene complex occurred.In our previous experim...

No abstract

A series of diaryltrisphaeridines have been synthesized by iodination of trisphaeridine and subsequent Suzuki coupling reactions. The photophysical and electrochemical properties of these compounds have been investigated and it has been found that they are dependent on the aryl substituents. In particular, the presence of dimethylamino groups induces a bright‐green emission, thus making this compound a promising candidate for fluorescence devices and molecular sensors. Moreover, the green emission disappears on addition of HCl and this material can therefore also be used as a sensor for protons. The results have been rationalized by DFT and TD‐DFT theoretical studies.