Heteroatomic halides

The reaction type (a) in Scheme 1 for the allylic amination reaction uses substrates which have an allylic C-X (X = heteroatom, halide) bond and is mainly nucleophilic amination of functionalized alkenes, whereas reaction type (b) is a direct allylic amination of an alkene, based on electrophilic amination of nonfunctionalized alkenes and involves a cleavage of a C-H bond. 

Nucleophilic amination of alkenes functionalized by an allylic C-X (x = heteroatoms, halides) as outlined in Eq. (1) is a simple and direct procedure for the synthesis of allyl amines, since very efficient methods for the selective allylic functionalization of alkenes are available. 

In eontrast, dialkylhalonium salts sueh as dimethylbromonium and dimethyliodonium fluoroantimonate, whieh we prepared from excess alkyl halides with antimony pentafluoride or fluoroantimonie acid and isolated as stable salts (the less-stable chloronium salts were obtained only in solution), are very effective alkylating agents for heteroatom eompounds (Nu = R2O, R2S, R3N, R3P, ete.) and for C-alkylation (arenes, alkenes). 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

Benzylic, allylic and propargylic positions enhance the cathodic cleavage rate of C— heteroatom bonds as, for example, in the reduction of benzylic and allylic halides or alcohols56. Similar activated sulphones, due to their acidity, are in a class apart. Figure 8 shows the similitude between the cathodic behaviour of an allylic sulphone and its isomer, i.e., the corresponding vinylic sulphone when the electrolyses are run in an aprotic solvent. However, in the presence of an excess of proton donor, discrepancies appear. 

The (EDT-TTF-I)2Br salt described above [36] and the 1 1 (TTFI4)I salt reported by Gompper [51] were the only structurally characterized salts with simple halide anions until Imakubo recently described an extensive series of Cl" and Br" salts from several ortho-diiodo tetrathiafulvalene, tetraselena-fulvalene and dithiadiselenafulvalene derivatives (Scheme 8) [62], The X-ray crystal structure analysis of the nine salts described there show a variety of halogen bonded motifs, demonstrating the adaptability of the supramolecu-lar interactions to other structural requirements imposed by the nature of the heteroatoms (O, S, Se) in the TTF frame. Indeed, in (EDT-TTF-l2)2X-(H20)2 (X = Cl, Br), a bimolecular motif (Fig. 6) associates two partially oxidized EDT-TTF-I2 molecules with one Br" anion and one water molecule. 

For the synthetic studies on the bicyclic 6-6 systems 1-3, with one ring junction nitrogen and five extra heteroatoms, refer to CHEC-II(1996) <1996CHEC-II(8)743>. In the studies of the newly available compounds, a one-pot procedure for the synthesis of [l,2,4]triazino-[4,3- ][l,2,4,5]tetrazine 4 derivatives is discussed below. Refluxing of hydrazonoyl halides 10 in chloroform (or ethanol) in the presence of triethylamine for 6h with either 4-amino-2,3-dihydro-6-substituted-3-thioxo-[l,2,4]triazin-5(4//)-ones 8 or 4-amino-3-methylthio-6-substituted-[l,2,4]-triazin-5(4//)-ones 9 yielded compounds 4 (Equation 1) <2000JPR342>. 

Halide displacement from the carbene ligands of Ru, Os, and Ir halocarbene complexes by N-, O-, and S-based nucleophiles frequently leads to the formation of new heteroatom-substituted carbene complexes. This important class of reactivity will be discussed in more detail in Section V,D, but it is appropriate here to illustrate the scope of this method with several examples ... 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacydopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacydopentadienes. 

The second salient feature of heterocycles is the marked activation at positions a- and y- to the heteroatom. For N-containing heterocycles, the presence of the N-atom polarizes the aromatic ring, thereby activating the a and y positions, making them more prone to nucleophilic attack. The order of SNAr displacement of heteroaryl halides with EtO is [14] ... 

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

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