Metal alkyls protonation

Metalatlon of 2-alkynyl and 1,2-alkadienyl tetrahydropyranyl ethers furane synthesis. /-Butyllithium metalates the lithium alkoxide 1 to afford the allenyllithium compound a quantitatively. This anion reacts with alkyl halides or CH3OH to afford 2. Another metalation-alkylation protonation sequence proceeds via b to afford 3. Hydrolysis of the latter intermediate affords furanes directly. The overall sequence can be performed in one pot from a propargyl tetrahydropyranyl ether, r-butyl-lithium, and an aldehyde. ... 

The NH protons can also be deprotonated in reactions with metal alkyls to generate compounds with a very high metal content (252) (Eqs. 10,11). 

Hill et al. (127) reported the first proton magnetic resonance work on a series of cobalamins. This work was carried out at 60 MHz and the spectra are, therefore, of quite low resolution. Subsequently, this work was extended to a wider variety of molecules and also spectra were recorded at 100 MHz (128). Five low field resonances and those of the metal alkyl groups were assigned. Some representative chemical shifts for the low field resonances are shown in Table 1. 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

Rhenium-acyl complexes, such as 1, are isoelectronic with the iron-acyl complexes discussed above and many reactivity patterns are common to the two groups of compounds. Treatment of complex 1 with strong bases, such as butyllithium or lithium diisopropylamide, results in abstraction of a cyclopentadienyl proton which is followed by rapid migration of the acyl ligand to the cyclopentadienyl ring to produce the metal-centered anion 384. Alkylation of 3generates a metal-alkyl species, such as 4. 

When arylhydrazones of aldehydes or ketones are treated with a catalyst, elimination of ammonia takes place and an indole is formed, in the Fischer indole synthesis.515 Zinc chloride is the catalyst most frequently employed, but dozens of others, including other metal halides, proton and Lewis acids, and certain transition-metals have also been used. Arylhydrazones are easily prepared by the treatment of aldehydes or ketones with phenylhydrazine (6-2) or by aliphatic diazonium coupling (2-7). However, it is not necessary to isolate the arylhy-drazone. The aldehyde or ketone can be treated with a mixture of phenylhydrazine and the catalyst this is now common practice. In order to obtain an indole, the aldehyde or ketone must be of the form RCOCH2R (R = alkyl, aryl, or hydrogen). 

The protolysis of a basic metal-alkyl by the acidic proton in ketimines can lead to methyleneamido complexes (equation 95). 

The above results are consistent with a steric specific syn 1,2-addition-elimination of metal hydride intermediate which is formed fast in a pre-equilibrium [MH] [MD] and adds to the olefinic substrate to form the metal alkyl intermediate (equation 261). The /1-hydride elimination of the most stable rotamer (equation 262) is the RDS in the rearrangement, leading to a metal hydride-product complex, which starts a new cycle faster than uncoordinated metal hydride. The protonated catalyst, 434, produces a precursor... 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

The stereochemistry of the reduction of substituted cyclopentanones has not been studied in great detail and it is not possible to make detailed generalizations regarding their course. Based on limited experimental data, it appears that metal-alcohol and metal-NHs-proton donor reductions of alkyl-substituted cyclopentanones will usually afford a greater than equilibrium ratio of the thermodynamically more stable alcohol. In the absence of an added proton donor, pinacol formation may be a problem and different metals may give different ratios of epimeric alcohols. 

The highly electronegative metal ion affords the appropriate empty orbitals for the stabilization of the complex anion in the transition state. Both late and early transition metal alkyls are prone to this reaction, but its occurrence had to be particularly, invoked in the case of the early transition metals. Many similar reactions, such as the reaction of metal alkyls with other FIX compounds, could be described as if they followed this pathway, but the use of the term a-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. Two reactions have been depicted in Fig. 4.34. There are, of course, borderline cases when the reacting hydrocarbon is acidic, as in the case of 1-alkynes, a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also... 

III. Structural Limitations - Few structural features have been reported which prevent dianion formation. Even benzoic acid can be metalated by proton abstraction17 ( eq l) or by metal-halogen exchange18 (eq 2). All three toluic acids form dianions, and dimethyl benzoic acids (eq 3) can be selectively metalated and alkylated.19 Ortho, meta and para methyl groups of dimethylbenzoic acids react in the order o>p>m. 

This equation assumes each initiator and alcohol molecule to be a potential polymer chain. Alcohol or other protonic substances can thus be used to control polymer molecular weight. The molecular weight limitation due to exchange reactions, as represented by Eq. (10.11), does not, however, apply to polymerizations initiated by alkoxides and hydroxides in aptotic polar solvents, nor does it apply to polymerizations initiated by other initiators such as metal alkyls and aryls and the various coordination initiators, since the latter initiators are dissolved in aprotic solvents such as benzene or tetrahydrofuran (Odian, 1991). 

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