Alkyl derivatives trapping reagents

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

Miyabe et al. developed a tandem addition/cycUzation reaction featuring an unprecedented addition of alkoxycarbonyl-stabihzed radicals on oxime ethers [117], and leading to the diastereoselective formation of /1-amino-y-lactone derivatives [118,119]. The reaction proceeds smoothly in the absence of toxic tin hydride and heavy metals via a route involving a triethylborane-mediated iodine atom-transfer process (Scheme 37). Decisive points for the success of this reaction are (1) the differentiation of the two electrophilic radical acceptors (the acrylate and the aldoxime ether moieties) towards the nucleophilic alkyl radical and (2) the high reactivity of triethylborane as a trapping reagent toward a key intermediate aminyl radical 125. The presence of the bulky substituent R proved to be important not only for the... 

The addition of different trapping reagents diverts the formation of ketone to the appropriate monosubstituted alkyl derivatives CBrCl3 leads to alkyl bromides, diphenyl diselenide produces (phenylseleno)alkanes, etc. 38-39(8 details below). Thus, the most important features of Gif-chemistry are given below. 

As mentioned above, the addition of different trapping reagents diverts the formation of ketone and alcohol giving other alkyl derivatives in similar yields to that of the oxygenation process -. This data proves that the reaction pathway includes an intermediate A, which is a precursor of intermediate B Al. 

Why then, is intermediate A not a free alkyl radical In order to examine the reaction pathway of genuine free alkyl radicals under Gif-conditions, they were generated at room temperature by the photolysis of N-hydroxy-2(7//)-thiopyridone derivatives of alkane carboxylic acids and were allowed to react with dioxygen or added trapping reagents (Scheme 2) ... 

Another triflate ester that recently has found growing application in organic synthesis is commercially available trimethylsilylmethyl trifluoromethanesul fonate. This powerful alkylating reagent can be used for the synthesis of various methylides by an alkylation-desilylation sequence A representative example is the generation and subsequent trapping by 1,3-dipolar cycloaddition of indolium methanides from the corresponding indole derivatives and trimethylsilylmethyl trifluoromethanesulfonate [108] (equation 54)... 

By enantiotopos-differentiating deprotonation the lithiated complex is formed in a reagent-controlled reaction with excellent selectivity. The lithiated center of the complex is assumed to have the S configuration, as follows from the carboxylation, to give an (7 )-lactic acid derivative based on the reasonable assumption of metalloretentive electrophilic attack. Trapping with chlorotrimethylstannane gave the corresponding chiral (.S -SjS-dimethyl-l-trimethylstannyl-alkyl-l-oxa-4-azaspiro[4.5]decane-4-carboxylates. Enantioselectivity of the overall transformation is excellent. 

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