Nucleophilic substitution sulfur/selenium nucleophiles

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

The SRN1 process has proven to be a versatile mechanism for replacing a suitable leaving group by a nucleophile at the ipso position. This reaction affords substitution in nonactivated aromatic (ArX) compounds, with an extensive variety of nucleophiles ( u ) derived from carbon, nitrogen, and oxygen to form new C—C bonds, and from tin, phosphorus, arsenic, antimony, sulfur, selenium, and tellurium to afford new C-heteroatom bonds. 

Nucleophilic substitution at sulfur, selenium and tellurium by organomagnesium compounds ... 

Two acids with mixed three-membered chains are known, namely selenopentathionic and telluropentathionic acid. They are derived from pentathionic acid by substitution of selenium or tellurium for the middle sulfur atom of the latter, and behave in reactions with nucleophilic reagents as thiosulfates of divalent selenium and tellurium. The first salts of scleno-pentathionic (84) and telluropentathionic (85) acid were isolated in 1949, although the formation reactions in aqueous solutions had been known earlier. Wood (229) has by paper electrophoresis shown that the preparative methods lead to pure products. 

Selenophene compounds react faster than the corresponding thiophene derivatives in both electrophilic and nucleophilic substitutions. This may be due to the capacity of selenophene to delocalize both positive and negative charges, since the selenium atom is larger and more polarizable than the sulfur atom and consequently selenophene can release its p-electrons and accept electrons into its free -orbitals more readily than thiophene. 

As mentioned in the introduction, partially fluorinated compounds are highly useful, however methods for their synthesis are strictly limited in many cases. For example, nucleophilic substitution occurs with difficulty at the position a to a trifluoromethyl group due to its strong electron-withdrawing effect, although sulfur and selenium nucleophiles undergo such a substitution rather efficiently (Scheme 6.1). 

Thus, the kinetic results show that selenophene undergoes both electrophilic and nucleophilic substitution reactions somewhat more readily than does thiophene. Hence, the reactivity of the heterocycle increases when sulfur is replaced by selenium a possible explanation might be that the selenium atom is larger and more polarizable, and therefore more willing both to release its p electrons and to accept electrons into its free d orbitals. 

Sulfur and selenium nucleophiles react with arsenic halides, amides and alkoxides to give the corresponding substituted products as shown in equations 154 155 156 , ... 

The preparative electrochemical oxidation of a-heteroatom-substituted tetraorganosilanes gives rise to cleavage of the C-Si bond and the introduction of a nucleophile such as methanol on the carbon [Eq. (27)]. Various reactions of this type have been reported for compounds containing oxygen [117-119], nitrogen [121], sulfur [116,119,120,122,123], and selenium [123]. 

Avoidance by choice of oxygenated starting materials Oxidation through Lithiation and Ort/ro-Lithiation Hydroxylation of Pyridines by ortho-Lithiation Synthesis of Atpenin B Introducing OH by Nucleophilic Substitution Part II - Oxidation of Enols and Enolates Direct Oxidation without Formation of a Specific Enol Selenium dioxide Nitrosation with nitrites Nitrosation with stable nitroso compounds Indirect Oxidation with Formation of a Specific Enol Enone Formation Pd(II) oxidation ofsilyl enol ethers Bromination of enols in enone formation Sulfur and selenium compounds in enone formation Asymmetric Synthesis of Cannabispirenones... 

In order to obtain isosteric sulfur and selenium analogs of retinyl ethers, the acetoxy group in retinyl acetate (9) was subjected to a nucleophilic substitution reaction with thiophenolate and selenophenolate to give (577) and (578), respectively (Welch and Gruber, 1979). The products were obtained in good yields, and the all-trans configuration was retained. 

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