Reductive cleavage metals

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Na or Li and ammonia, excellent yields. " A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. ... 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

Reductive cleavages of carbon-chlorine bonds by active metals and with photochemical activation figure in recent studies aimed at HFCs and HCFCs Sodium amalgam [3J] (equation 25), zinc powder [34] (equation 26), and alumi-mun/tin chloride [35] (equation 26) are all used in conjunction with protic solvents in reactions giving high yields and conversions... 

Protection of primary aryl amines as the triazene is accomplished by diazotiza-tion of the amine followed by reaction with pyrrolidine in aq. KOH. This group is stable to metalation of the aromatic ring by metal halogen exchange. The amine is recovered by reductive cleavage with Ni-Al alloy (aq. KOH, rt, 37-68% yield). ... 

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

The synthesis of the trisubstituted cyclohexane sector 160 commences with the preparation of optically active (/ )-2-cyclohexen-l-ol (199) (see Scheme 49). To accomplish this objective, the decision was made to utilize the powerful catalytic asymmetric reduction process developed by Corey and his colleagues at Harvard.83 Treatment of 2-bromocyclohexenone (196) with BH3 SMe2 in the presence of 5 mol % of oxazaborolidine 197 provides enantiomeri-cally enriched allylic alcohol 198 (99% yield, 96% ee). Reductive cleavage of the C-Br bond in 198 with lithium metal in terf-butyl alcohol and THF then provides optically active (/ )-2-cyclo-hexen-l-ol (199). When the latter substance is treated with wCPBA, a hydroxyl-directed Henbest epoxidation84 takes place to give an epoxy alcohol which can subsequently be protected in the form of a benzyl ether (see 175) under standard conditions. 

The reductive cleavage of allyl ethers 28 30 or sulfides is preferred over the metalation of allyl... 

The dissolving metal reduction is not suitable, because it effects a cleavage ol the benzylic C-N bond. b The cleavage of the N-N bond by hydrogenolysis is affected by some racemization. 

Subsequently, Julia, Uguen and Callipolitis104 105 used both lithium metal in ethyl-amine and sodium amalgam in ethanol to effect reductive cleavages of j8-hydroxysulphones or of allylic sulphones. The latter reaction is part of a synthetic sequence for the construction of alkenes that has been used with some considerable success... 

Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

Reductive cleavage of the Si-Si bond of bis(siloles) with alkali metals resulted in the formation of silole monoanions (Scheme 2.49). Silole monoanions were found to be aromatic on the basis of NMR spectral data and calculations. 

Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

M[pz(A4)] A = S2ML2. The octakis(.V-R)porphyra/,ines reported by Schramm and Hoffman (2), M[pz(S-R)8 (M = Ni, Cu), (60), can be converted to the octathiolate M[pz(S )g] (Scheme 11) via reductive cleavage of the sulfur-carbon bond when R = benzyl (Bn), and this tetra-bis(dithiolate) can then be peripherally capped with metal-ligand systems to yield peripherally tetrametalated star porphyrazines. The benzyl dinitrile 57 can be macrocyclized around magnesium butoxide to form [Mg[pz(S-Bn)8] (58) (35-40%), which can then be demetalated with trifluoroacetic acid to form 59 (90%), which is subsequently remetalated with nickel or copper acetate to form 60a (95%) and 60b (70%) (Scheme 11) (3, 23, 24). Deprotection of 60a or 60b with sodium in ammonia yields the Ni or Cu tetra-enedithiolates, 61a or 61b to which addition of di-ferf-butyl or n-butyl tin dinitrate produces the peripherally metalated star porphyrazines 62a (37%), 62b (80%), and 62c (41%). 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Reductive cleavage of carbon-carbon bonds upon alkali metal reduction of... 

Perhaloalkanes have been found to scramble halogen atoms via consecutive halophilic reactions following carbanion generation by halophilic attack by base. S l reaction of an allylsilane has been applied in a stereocontrolled synthesis of ( )-dihydronepetalactone, and functionalized aryl and arylmethyl carbanions have been generated by reductive cleavage of aryl and arylmethyl alkyl ethers by electron transfer from alkali metals. ... 

Polymetalated systems of this type without phenyl substitution at the lithiated carbon centre are only accessible when solutions of LiCioHg (144a) or LiDBB (145) in THF instead of a suspension of metallic lithium in THF are reacted with bis(phenylthiomethyl)silanes of type 155. In our group, variously substituted bis(lithiomethyl)silanes 117a, 156b-e and 101 were synthesized by reductive cleavage of the carbon-sulphur bond with LiCioHg... 

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