Selenides, allyl metallation

Allylic metals, in propargylic alcohol alkylation, 11, 129 ir-Allylic palladium complexes, and carbocyclization, 11, 426 Allylic position, alkenes, dienes, polyenes, metallation, 9, 6 Allylic selenides, [2,3]sigmatropic rearrangement, 9, 481 Allylic substitution reactions for C-N bonds via amination... 

Competing reactions to addition to a-hetero-substituted alkenes are metallation of vinylic or allylic protons (see 5.5.2.3.2) and cleavage of the carbon-hetero-element bond (see 5.5.2.2.1.). a-Metallations occur with vinylic chlorides, fluorides and ethers no addition of RLi occurs with vinyl fluorides, chlorides or ethers. For vinylic sulfides and selenides, whether metallation or addition (or even carbon-heterobond cleavage) occurs depends on the conditions and reagents. Addition of RLi (R = Et or n-Bu, not Ph) occurs to aryl vinyl sulfides ... 

LDA in THF or in THF-HMPT, LiTMP in THF and KDA in THF have all been successfully used for the metallation of phenyl or m-(trifluoromethyl)phenyl vinyl selenide (Scheme 44). The deprotonation was shown to be reversible with LDA in THF and irreversible with LiTMP when performed in the same solvent. Extension of this reaction to homologous derivatives proved difficult since metallation at the allylic sites often competes. Allylic metallation is particularly favorable with aryl 1-propenyl selenides (Scheme 46) - and, whatever the conditions used (LiTMP or KDA ), with aryl 1-(2-methyl-1-propenyl) selenides (Scheme 47). In the latter case both the (Z)- and the ( )-methyl groups have been metidlated leading to the corresponding a-metalloallyl selenides (Scheme 47). - ... 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The a-deprotonation of vinylic aryl selenides is strongly dependent on the substituents [4]. For phenyl vinyl selenide, the reaction was successfully carried out with LDA or KDA at -78°C in THF (Scheme 50, reaction 1). When a alkyl group is present, the use of LDA in THF leads to the abstraction of both vinyl and allyl protons. The a-deprotonation was improved using KDA in THF at -78°C (Scheme 50, reaction 2). With two )9,)9 -alkyl groups, the metalation occurred only at an allylic position (Scheme 50, reaction 3). 

Propargylic sulfides " and selenides have been metallated with n-butyllithium and lithium amide respectively, and allylated or alkylated selectively at the propargylic position (Scheme 34) with primary and secondary alkyl halides. - ... 

Allyldiethylamine behaves similarly, but the yields are low since neither the starting amine nor the products are stable to the reaction conditions. For the efficiency of the cyclopropanation of the allylic systems under discussion, a comparison can be made between the triplet-sensitized photochemical reaction and the process carried out in the presence of copper or rhodium catalysts whereas with allyl halides and allyl ethers, the transition metal catalyzed reaction often produces higher yields (especially if tetraacetatodirhodium is used), the photochemical variant is the method of choice for allyl sulfides. The catalysts react with allyl sulfides (and with allyl selenides and allylamines, for that matter) exclusively via the ylide pathway (see Section 1.2.1.2.4.2.6.3.3. and Houben-Weyl, Vol. E19b, pll30). It should also be noted that the purely thermal decomposition of dimethyl diazomalonate in allyl sulfides produces no cyclopropane, but only the ylide-derived product in high yield.Very few cyclopropanes have been synthesized by photolysis of other diazocarbonyl compounds than a-diazo esters and a-diazo ketones, although this should not be impossible in several cases (e.g. a-diazo aldehydes, a-diazocarboxamides). Irradiation of a-diazo-a-(4-nitrophenyl)acetic acid in a mixture of 2-methylbut-2-ene and methanol gave mainly l-(4-nitrophenyl)-2,2,3-trimethylcyclo-propane-1-carboxylic acid (19, 71%) in addition to some O-H insertion product (10%). ... 

Whereas free singlet carbenes are rather unselective with respect to formation of cyclopropane 22 or ylide 23 and the cyclopropane is favored under conditions that populate the triplet state of a carbene (see Section I.2.I.2.4.2.6.2.), the metal carbenes generated with copper or rhodium catalysts display a selectivity for functional groups which are more nucleophilic than a double bond. Thus, no cyclopropanes are obtained from dialkylallylamines allyl sulfides -allyl dithioacetals , and allyl selenides under carbenoid conditions (copper or rhodium catalysts). 

Allylic sulfoxides and sulfones are completely metallated in liquid ammonia by all alkali amides [1]. For metallation in organic solvents, LDA and BuLi are usually applied. The lithiation of allyltrimethylsilane can be conveniently achieved with BuLi TMEDA (or HMPT) or f-BuLi TMEDA (or HMPT) in THF-alkane mixtures [3], As in the case of the sulfur compounds, extension of the unsaturated system leads to an increased acidity. Thus H2C=CHCH=CHCH2SiMe3 can be metallated with the less strongly basic LDA in THF [4], For the metallation of allylic selenides, LDA seems to be the reagent of choice. Butyllithium will presumably attack on selenium. 

Lithiated allylic sulfoxides may be a-alkylated and the resulting products subjected to [2,3]-sigmatropic rearrangement induced by a thiophile to give allylic alcohols (eq 43). In contrast, alkenyl aryl sulfoxides produce a-lithiated species which are alkylated with Mel or PhCHO in good yields (eq 44). LDA has also been used to metalate allylic and propargylic selenides as weU as aryl vinyl selenides. ... 

In the presence of samarium metal-trimethylsilyl chloride, (Z)-allyl selenides 370 were stereoselectively afforded by a one-pot reaction of diselenides with MBH adduct under mild eonditions. Presumably, the diselenides are cleaved by the Sm-TMSCI system to form selenide anions, which then undergo S 2 substitution of MBH adduets to produce the corresponding (Z)-allyl selenides 370 in high yields (Scheme 3.165). ... 

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