Selenides, alkenyl

Generation of a selenoaldehyde, a selenoketone, and telluroaldehydes by [3,3] sig-matropic rearrangement of allyl alkenyl selenides and tellurides [139]... 

Selenals, selones, and tellurals generated in situ via [3,3] sigmatropic rearrangements of allyl alkenyl selenides and tellurides 167 can be trapped by 2,3-dimethyl-l,3-butadiene affording the expected cycloadducts (Equation 68) <1995CL135>. Higher yields were noted in less-hindered cases where the selenium and tellurium aldehyde... 

In the presence of base, phenylselenenyl bromide reacts with alkenyldihydroxyboranes to give alkenyl phenyl selenides with retention of configuration at die double bond (equation 73). 

Ley reported that selenium promoted carbocyclization reactions can also be effected by the enolic olefinic bonds of -dicarbonyl compounds [111]. These reactions occur with N-PSP in the presence of zinc iodide, tin tetrachloride or aluminium trichloride. An example is reported in Scheme 33. In the intermediate 219, derived from the -ketoester 218, cyclization through the oxygen atom to afford 220 is kinetically favoured. This reaction, however, is reversible, and upon prolonged reaction times and in the presence of strong acids the carbocyclization product 221 is formed. This procedure has been recently employed by Ley to effect the conversion of the alkenyl p-keto lactone 222 into the tricyclic selenide 223 (Scheme 33) which is a key intermediate in the preparation of model compounds with antifeedant activity [112]. 

An efficient way of introducing selenium in a radical precursor is the use of selenium containing building blocks. The selenides are particularly appropriate when the reaction sequence involves reaction steps that are incompatible with halides or when the corresponding halides are not stable. In Eq. (7), preparation of the selenenylated alkenyl sulfoxide 35 by alkylation of malonate 33 with the bromide 34 is described [18]. This procedure is not feasible with the corresponding halide due to cyclopropane formation via intramolecular alkylation. Radical cyclization of 35 in a 5-exo mode affords, after elimination of the phenylsulfinyl radical, the methylenecyclopentane 36 in good yield and excellent enantioselec-tivity. 

The 6-exo-trig cyclization of a 6-heptenylmetal to a (cyclohexylmethyl)metal is much slower than the analogous 5-exo-trig carbocyclization. Indeed, as shown in Scheme 7-64, the 6-exo rearrangement of 6-heptenyllithium requires the presence of TMEDA to facilitate the process [44]. However, in the case of 6-alkenyl benzyl selenides, the formation of the six-membered rings is possible by carbocyclization [70] as well as by the use of Ziegler-Natta conditions [71],... 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

Increasing the substitution of the alkyl group on the 3-carbon of 1-alkenyl phenyl selenides again favors metaliation at the vinylic site (Scheme 48). > KDA in THF has proved to be, without contest, the most successful reagent for this purpose. The reaction takes place at low temperature (-78 C) and the 1-phenylselenoalkenyl metal, produced in almost quantitative yield, retains its stereochemistry (Scheme 48, b-d). 

Related results have been described from pyridyl alkenyl selenides which, contrary to odier aryl alkenyl selenides (see below)," are easily metallated at the vinyl site with LDA (Scheme 48, compare i and j with f and h). The ready deprotonation and stereoselective methylation of these selertides have been ascribed to the presence of the nitrogen atom which can reduce the electron density of the double bond and also chelate with the lithium counterion." "... 

C—SeMe and the C—Cl bonds and often faster than that of the C— Br bond The reduction is highly chemoselective and leads to alcohols usually in almost quantitative yield (Scheme 161, a Scheme 164, a Scheme 168, a and b). In rare cases, however, such as when a ca n-carbon double or triple bond is present in a suitable position, the formation of a five- or six-membered ring takes place by trapping of the radical intermediate (Scheme 118). ° Tin hydride reduction has been advantageously extend (g P-hydroxy-y-alkenyl and -hydroxy-a-alkenyl selenides displayed in Scheme IM (a) and Scheme 168 (a and b) and derived from a-selenoalkyllithiums and enenones, and from 1-seleno-l-alkenyl metals and carbonyl compounds, respectively. 

The case of p-hydroxy-y-alkenyl selenides merits further comments. The rearrangement efficiently takes place using the thallium(I) ethoxide method and the presence of an additional double bond in the reactant does not introduce a serious problem associated with unwanted reaction with the dichlorocarbene intermediate. This is not the case when silver tetrafluoroborate is used. 

Alternatively, 1-lithio-l-alkenyl phenyl and methyl selenides have been obtained on reduction of the C—Se bond of l,l-di(seleno)alkenes with alkyllithiums this reaction is the only one which allows the synthesis of the methylseleno derivatives (Scheme 41). ... 

Alkyl and allyl sulfides and selenides can be transformed readily into the corresponding alkyl and alkenyl halides respectively. This reaction takes advantage of the easy formation of the corresponding sul-fonium and selenonium salts on reaction with alkyl halides, alkyl bromoacetates or bromine - (Scheme S3). A related process involves the intermediary formation of a selenoxide and its further reaction with hydrochloric or hydrobromic acids. ... 

J13.2 Alkenyl Sulfides, Sulfones, Selenides and Tellurides... 

Coupling with alkenyl selenides. Stereoselective synthesis of aiylalkenes (6... 

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