Addition of Other Electrophilic Reagents

Reaction of alcohol solutions of alkenes with potassium selenocyanate and cupric chloride gives j8-selenocyanato ethers  

This reaction proceeds by the generation of an electrophilic species, probably N=C-Se-Cl, which is formed by oxidation by Cu(II). 

This reagent then gives a cyclic selenonium ion intermediate which captures the solvent. Thiocyanate salts behave similarly under these reaction conditions.  

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

Entries 7 to 9 involve other reagents that react by similar mechanisms. In the case of thiocyanogen chloride and thiocyanogen, the formal electrophile is [NCS]+. The presumed intermediate is a cyanothiairanium ion. The thiocyanate anion is an 

For nitrosyl chloride (Entry 8) and nitrosyl formate (Entry 9), the electrophile is the nitrosonium ion NO+. The initially formed nitroso compounds can dimerize or isomerize to the more stable oximes. 

Related reactions that have been studied include the chlorination and bromina-tion of butadiene. Both reactions give mixtures containing slightly more of the 1,2-addition product than the 1,4-addition product. 

Reagent Control of regioselectivity Stereospecificity in aliphatic systems Product Ref. 

Addition via bridged intermediates is characterized by anti stereochemistry, since the nucleophile opens the ring by backside attack  

Electronic control of ring-opening— nucleophilic attack at more positive carbon 

Steric control of ring-opening—nucleophilic attack at less substituted carbon 

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Scheme 4.2. Addition Reactions of Other Electrophilic Reagents...

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85 , or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of 7r-complex 86 in a rapid equilibrium with the reagents . 

The addition of protons is certainly the most common nucleophilic reaction of reduced hydrocarbon species due to the almost ubiquitous availability of proton donors, which may or not be wanted. However, under strictly aprotic conditions it is also possible to add other electrophilic reagents such as alkyl halides, acyl halides, CO2, and SO2. 

However, it is possible to change the stereochemistry of cleavage by varying the nature of the electrophile by the addition of other reagents such as AICI3, PPh3, P(OEt)3. ... 

The stereospecific anti addition of phenylsulfenyl chloride to norbomene is a particularly interesting example of the stability of the intermediate. Neither rearrangement nor syn addition products, which are observed with many of the other electrophilic reagents, are formed.63 This result indicates that the intermediate must be quite stable and reacts only by nucleophilic attack.64... 

This is true for the addition of hydrogen halide (H-X), halogens (X-X) and other electrophilic reagents (NO —Cl, I — Nj, etc.) to cyclohexenes. The oxymercuration reaction (Hg(OAc>2) in the presence of several nucleophiles (H20, AcOH, ROH) is another well known example. 

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

Reactions of Enamines with Other Electrophilic Reagents Addition of aldehydes to enamines, followed by hydrolysis, leads to monoalkylidene and monoarylidene ketones (81).272 An example in which this reaction occurs intramolecularly is provided by the alkaloid ajmaline.273... 

It can be observed that in several cases the addition reaction proceeds with excellent diastereomeric excesses. Although the reaction temperature, the solvent and the nature of the counter anion are different from case to case, nevertheless the data reported in the table can be used to have preliminary indications about the ability of the various electrophilic reagents to transfer the chirality to the newly generated stereocenters. The information gained from these experiments, however, must be used with caution since the stereoselectivity is also a function of the alkene employed and examples are known in which a reagent which gives unsatisfactory results with styrene can, in contrast, be efficient with other alkenes. This is clearly evident from the data reported in Table 2 in which the diastereomeric excesses measured for the reactions of the electrophilic reagents derived from the diselenides 24 and 26 with various alkenes are reported. In both cases the results obtained with other alkenes are much better than those observed with styrene. 

Electrophilic additions of other reagents e.g. HOBr, INCO) to these olefins are quite normal in giving the diaxial products in high yield. The order in which the components of these molecules attack the steroid (1+ or Br+ first, followed by the anion) provides a most useful synthetic route to epoxides or ethyleneimines of the -configuration (p. 117), for the preferential formation of a halonium ion of a-configuration directs the 0- or iV species to the / -face of the molecule. It has already been noted that direct epoxidation of a steroid olefin usually gives the a-epoxide. 

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