Chemoselectivity elimination

At the cathode, a chemoselective elimination depending on the degree of alkylation of the v/c-dihalide is possible [73]. Chemically these reductions are more limited in scope they can be conducted with 1 in DMF, with Zn, Mg, or Cr " [74]. The cleavage of 1,2-diols can be inexpensively achieved at the nickel hydroxide electrode [13], while chemically the more expensive reagents Pb(OAc)4 or IO4 must be used. 

An investigation of the substrate dependence on the rate of samarium-mediated reductive elimination of jS-acyloxysulfones has provided insights into the mechanism of this transformation and allowed for the development of a chemoselective elimination process." Electron transfer to the sulfonyl and/or benzoyl group is probably reversible and can occur into both acceptor groups. A difference in carbon radical stabilities... 

Ballini and coworkers have developed a new strategy for alkenylation of carbonyl compounds based on the Michael addition followed by elimination of HN02 (see Section 7.3). A variety of 2-alkylidene 1,4-dioles have been conveniently prepared, in two steps, by the Michael addition of a nitroalkane to the appropriate enedione derivatives under basic conditions, followed by chemoselective reduction with LiAlH4 (Eq. 4.123).170... 

The synthesis of 2,3,5-trialkylpyrroles can be easily achieved by conjugate addition of nitroalkanes to 2-alken-l,4-dione (prepared by oxidative cleavage of 2,5-dialkylfuran) with DBU in acetonitrile, followed by chemoselective hydrogenation (10% Pd/C as catalyst) of the C-C- double bond of the enones obtained by elimination of HN02 from the Michael adduct. The Paal-Knorr reaction (Chapter 10) gives 2,3,5-trialkylpyrroles (Eq. 4.124).171... 

Alternatively, Ballini devised a new strategy to synthesize tri-alkylated pyrroles from 2,5-dialkylfurans and nitroalkanes <00SL391>. This method involves initial oxidation of 2,5-dimethylfuran with magnesium monoperoxyphthalate to cA-3-hexen-2,5-dione (6). Conjugate addition of the nitronate anion derived from the nitro compound 7 to 6 followed by chemoselective hydrogenation of the C-C double bond of the resulting enones 8 (obtained by elimination of nitrous acid from the Michael adduct) completes the conversion to the alkylated y-diketones 9. Final cyclization to pyrroles 10 featured improved Paal-Knorr reaction conditions involving reaction of the diketones with primary amines in a bed of basic alumina in the absence of solvent. 

It was apparent from the beginning (Scheme 16.7) that there were four potentially independent aspects of reactivity 1) the rate of bimolecular transfer of the diazo ester to the rhodium-complex [10a, 22] 2) the ratio [21] of C-H insertion to /9-H elimination [(34-1-35 -h 36 -h 37)/33] 3) the chemoselectivity [(34-i-35)/(36-i-37)] [4] and 4) the diastereoselectivity [9] of the insertion (34/35 or 36/37). As a prelude to the development of an effectively chiral catalyst, we felt that it was important to experimentally explore these aspects of reactivity. 

When artemisinin 9a was treated with 2-lithiothiazole followed by in situ O-acetylation, the thiazole carbonyl adduct 161 was formed chemoselectively in good yield (Scheme 22) the same reaction with PhLi produced a mixture of uncharacterized products. When this acetyl adduct 161 was exposed to TMSOTf, the corresponding elimination product 162 was formed, which was converted in three steps without purification of intermediates into aldehyde 163. This was reacted with high chemoselectivity in reactions with organometallics ( -BuLi, PhMgBr) and with phosphonium ylides in a Wittig procedure <1999T3625>. 

Carbenoid transformations involving competition between intramolecular cyclopropa-nation and /8-hydride elimination have been investigated149. The chemoselectivity of these catalytic transformations can be effectively controlled by the choice of catalyst. Rhodium(II) trifluoroacetate catalysed decomposition of diazoketone 111 proceeds cleanly to give only enone 112. However, rhodium(II) acetate or bis-(iV-t-butylsalicyladiminato) copper(II) cu(TBs)2 provides exclusively cyclopropanation product 113 (equation 102)149. 

Anomeric thioglycoslydes reacted with tripeptides containing dehydroalanine, derived from selenocysteine by oxidative elimination, thus affording S-linked neoglycopeptides through chemoselective Michael addition (Fig. 28e).116... 

The catalytic cycle of the Ni-catalysed dimerization of ethylene to give 1-butene (65) is explained by the insertion of ethylene to the nickel hydride 62 twice to form the ethyl complex 63 and the butyl complex 64. The elimination of /1-hydrogen gives 1-butene (65), and regenerates the Ni—H species 62. The reaction is chemoselective. Curiously, no further insertion of ethylene to 64 occurs. 

With higher alkenes, alkenyl acetates, allylic acetates and dioxygenated products are obtained [58], The reaction of propylene gives two propenyl acetates, 66 and 67, and allyl acetate (68) by the acetoxypalladation to form two intermediates, followed by elimination of /1-hydrogens. The chemoselective formation of 68 by a gas-phase... 

AcOH, followed by hydrolysis, gives the (/ )- , 3-dienecarboxylic acid 152. Then the acetoxylactone 153 can be prepared by the Pd-catalysed intramolecular trans-1,4-functionalization of the 1,3-cyclohexadiene 152. On the other hand, the acetate 155 is obtained by the Pd-catalysed chemoselective displacement of the allylic carbonate moiety in 154 with malonate under neutral conditions. The (5)-1,3-dienecarboxy 1ic acid 156 is obtained by Pd-catalysed 1,4-elimination of 155. The Pd-catalysed 1,4-functionalization of the 1,3-cyclohexadiene 156 and acetylation, afford 157, which is an enantiomer of 153 [101]. 

Specific dehydrogenation at the terminal positions of alkanes is a reaction that would be of high utility. The 1-alkenes obtained by such a reaction are the basis of a variety of additional products. Felkin and co-workers discovered that metal complexes are able to mediate the transfer of hydrogen from alkanes 13 to olefins 14 (Scheme 4) [17]. The specific advantages of a transition metal catalyst can be applied to the benefit of the chemoselectivity of this reaction. In a kinetically controlled process, it is predominantly primary C-H bonds that add to the metal complex. A subsequent /Miydride elimination affords the terminal alkenes... 

In principle, the bases Y are also nucleophiles, and, hence, they can react with the same alkyl halides and sulfonates via the SN2 mechanism. The point of reaction is the C atom that bears the leaving group. In order to carry out E2 eliminations chemoselectively, competing Sn2 reactions must be excluded. To understand the outcome of the competition (E2 elimination vs. Sn2 reaction), it is analyzed kinetically with Equations 4.1-4.3. 

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