Electrophilic alkylating reagents

Iodonium salts 49 and 50 are efficient electrophilic alkylating reagents towards a variety of organic nucleophiles, including silyl enol ethers. The reaction with silyl enol ether 51 proceeds under mild conditions and selectively affords the appropriate product of alkylation 52 along with iodobenzene as the by-product (Scheme 24) [41]. 

The crystal structures of [Ir(dppe)2Se2]Cl (dppe = Ph2PCH2CH2PPh2) and Os(Se2)-(CO)2(PPh3)2 have shown the presence of side-on-bonded Sc2 rj -Sc2)- The Se—Se bonds in these complexes have been found to be reactive to electrophilic alkylating reagents with retention of the Se—Se bond whereas low-valent transition metal species undergo oxidative insertion into the Se—Se bond. The reactions of these types of complexes, which involve oxidative addition and transition metal displacement, have recently been examined in detail. ... 

The principal electrophiles to attack ring sulfur are either oxidants or alkylating reagents. Thiophene sulfoxide and sulfone formation is discussed in Section 3.02.2.6. Alkylating agents capable of forming thiophenium salts include trimethyloxonium tetrafluoroborate (MeaO BF ) and alkyl fluorosulfonates (ROSO2F). The salts e.g. 87) are conveniently isolated as hexafluorophosphates (88). 

Alkynes react with electrophilic selenium reagents such as phenylselenenyl tosylate.155 The reaction occurs with anti stereoselectivity. Aryl-substituted alkynes are regioselective, but alkyl-substituted alkynes are not. 

Metal alkyl reagents react with the acidic OH groups of silica, probably by the electrophilic cleavage of the metal-carbon bond. For example, the electrophilic cleavage of the metal-carbon bond occurs when organometallic reagents react with the electrophilic OH groups of the silica surface (Scheme... 

The diastereomers 251/ewf-251 and 252/ent-252 could be separated and were decom-plexed separately. From the fraction of 251/ewt-251,253 was obtained with 85% ee (e.r. = 92.5 7.5), and the fraction of 252/ent-252 yielded ewt-253 with 88% ee (e.r. = 6 94). A similar situation results from the reaction with tributyltin chloride or alkylation reagents, but the diastereomeric ratio is strongly dependent on the electrophile. The following conclusion is drawn from these and further experiments The enantiomeric ratio is determined by a selection of the chiral base between the diastereotopic methylene groups, since the benzylic carbanionic centres are labile, whereas the diastereomeric ratio results from the relative rate of the electrophile approach syn or anti with respect to the A-methyl group. One question remains—why are opposite d.r. values formed in the alkylation by methyl iodide and ethyl iodide ... 

The previous sections have dealt primarily with reactions in which the new carbon-carbon bond is formed by an SN2 reaction between the nucleophilic carbanions and the alkylating reagent. Another important method for alkylation of carbon involves the addition of a nucleophilic carbon species to an electrophilic multiple bond. The electrophilic reaction partner is typically an a,/i-unsaturated ketone, aldehyde, or ester, but other electron-withdrawing substituents such as nitro, cyano, or sulfonyl also activate carbon-carbon double and triple bonds to nucleophilic attack. The reaction is called conjugate addition or the Michael reaction. Other kinds of nucleophiles such as amines, alkoxides, and sulfide anions also react similarly, but we will focus on the carbon-carbon bondforming reactions. 

The ruthenium atom is the target for electrophilic attack in reactions <90JOM(395)27> of (333) with a number of alkylating reagents, such as trimethylgermyl chloride (Scheme 33) or ethylene dibromide. 

As has been mentioned above, thermal decarboxylation of heterocycle 36 can be coupled with electrophilic trapping with either bromine or nitro groups to yield the substituted analogues. The same heterocyclic system 59 can be thiolated by treatment either with elemental sulfur (followed by alkylation with an alkyl halide) to afford 60, or the electrophilic thiolating reagent 58 to generate 57 (Scheme 5) <2005HCA1208>. 

The composition of the products from the alkylation of lithiated 3,6-dialkoxy-2,5-dihydropy-razines with bis-alkylating reagents depends on the equivalents of electrophile employed. For example, with three equivalents of bis-alkylating electrophiles, (25)-2,5-dihydro-2-isopropyi-3,6-dimethoxy-5-methylpyrazine (I) gives high yields of the 1 1 adducts 2 or 5 with > 97% de2. These products can be further converted to the bicyclic systems (3 and 6. respectively) which, upon hydrolysis, give optically pure cyclic amino acids (4 and 7). Methyloxirane is used as an acid scavenger in order to release the free amino acid zwitterion. 

The cyclopropanation of Af-substituted alkenes has been reported in a very few cases. Problems arising from Af-alkylation by the electrophilic zinc reagents were sometimes... 

The bridgehead-amine-derived electrophilic fluorinating reagents l-alkyl-4-fluoro-l,4-di-azoniabicyclo[2.2.2]octane ditriflate 5 a or bis(tetrafluoroborates) 5b and 6 can be prepared from l-alkyl-4-aza-l-azoniabicyclo[2.2.2]octane salts and elemental fluorine. For example, treatment of l-methyl-4-aza-l-azoniabicyclo[2.2.2]octane triflate (4a) and lithium triflate in acetonitrile at — 35CC with neat fluorine over three hours gives l-fluoro-4-methyl-l,4-dia-zoniabicyclo[2.2.2]octane ditriflate (5a).81 The reagents 5b, 6, 7, and 8 can be synthesized in a similar manner.76 83... 

Substituted cyclobutenes 2-substituted 1,3-dienes. The reagent reacts with various electrophiles (alkyl halides, carbonyl compounds, epoxides) to give the corresponding selenides, jS-hydroxy- and y-hydroxyselenides, respectively. Three methods can be used to convert these adducts to cyclobutenes, as shown in equations... 

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