Pseudohalides alkenes

Currently the most popular of the pseudohalides are N-haloamides. These reagents are used in conjunction with the solvent or an added nucleophilic reagent to give overall 1,2-addition of a halogen and a nucleophile to the alkenic substrate. The amides of choice seem to be N-halosuccinimides. The low nu-cleophilicity of the succinimdyl anion allows for a variety of nucleophiles to attack the initially formed halonium ion. 

General Characteristics. In accordance with the high tendency for formation of Co(CO)4 (2 ) and the acidity of (3), the former componnd behaves in many respects as a psendohalide see Pseudohalide). Relative rates of electrophilic snbstitntion with Mel and MeOTs classify (2 ) as a hard and relatively weak nncleophile see Hard Soft Acids and Bases). Compounds of the type RCo(CO)4 are prepared by two different methods reaction of an electrophihc halide, snlfate, or tosylate (RX) with (2 ), or reaction of HCo(CO)4 with alkenes. Simple alkyl derivatives have low stability nevertheless, many studies have been conducted in which species have been identified as RCo(CO)4 intermediates. Apart from alkyl and aryl derivatives, for example MeCo(CO)4 and PhCo(CO)4, (2 ) forms derivatives with main group residues R3E and R2E and with the main group metals Zn, Cd, and Hg as well as with transition metals, as shown by examples in Scheme 4. Co E bonding in these compounds appears to be predominantly covalent. The bond distances can be long, and consequently, the coordination number of Co can be high. 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

Alkenes react with many selenenyl electrophiles to form 1,2-adducts thus, selenenyl chlorides, bromides and pseudohalides afford a-chloro-, a-bromo-, or other a-functionalized selenides, respectively. When the addition of a selenenyl halide to an alkene is carried out in the presence of a nucleophile, in some cases the nucleophile is incorporated at the a position instead of the halide atom. 

Between 1953 and 1955, Ziegler and his coworkers developed a simple and direct synthetic route to aluminum trialkyls from aluminum, hydrogen, and the corresponding alkene [213,231]. Aluminum trialkyls, which are Lewis acids, show a strong tendency to react with alkali metal and tetraorganoammonium halides and pseudohalides to... 

R is potentially a very stable carbonium ion, the intermediate breaks down to generate the carbonium ion, and the products are those to be expected from a carbonium ion. When X is halide or pseudohalide anions ( CN, SCN, NJ, etc.) the preferred pathway is to the alkyl halide or pseudohalide by ligand transfer. If the R group is not capable of sustaining formation of the carbonium ion and no easily transferred anion is present, the organocopper intermediate is converted primarily to alkene by elimination of a proton. 

In particular, sulfonylhydrazones are valuable coupling partners in Pd-catalyzed cross-coupling reactions with aryl halides and pseudohalides giving rise to alkenes. As an example, the reaction of a chloroarene with the tosylhydrazone derived from 4-tert-butylcyclohexanone leads to an aryl substituted cyclohexenone (Experimental Procedure below). " " Moreover, the reaction can be conducted directly from the carbonyl... 

Abstract During the last decades a powerful set of protocols featuring C(sp )-N bond formation have emerged as convenient alternatives for the assembly of enamine and enamides. Those methods consist of mostly palladium-catalyzed oxidative amidations of alkenes and both palladium- and copper-catalyzed cross-couplings between generally vinyl halides or pseudohalides and amines or amides. In this review recent advances in both types of processes will be disclosed. Additionally, the synthetic value of the title processes will be illustrated by describing relevant total syntheses of natural products involving vinylation process as the key step. 

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