Meerwein

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

Meerwein-Ponndorf-Veriey Reduction opposite of Oppenauer oxidation Synthesis 1994, 1007 Organic Reactions 1944, 2, 178... 

No mechanistic aspects of organic chemistry (or, for this reason, any reaction intermediates) were ever mentioned by Zemplen in his lectures or writings, nor did he consider or accept their existence. I never heard him mention the names of Meerwein, Ingold, Robinson, or any other pioneers of the mechanistic electronic theory of organic chemistry. The possible role of organic ions was similarly never mentioned. He was. 

One of the most original and significant ideas in organic chemistry was the suggestion by Hans Meerwein that carbocations (as we now call all the positive ions of carbon compounds) might be intermediates in the course of reactions that start from nonionic reactants and lead to nonionic covalent products. 

Studying alkylations, we developed a series of selective ionic alkylating agents. Although Meerwein s trialkyloxonium and dialkoxycar-benium salts are widely used as transfer alkylating agents, they lack selectivity and generally are incapable of C-alkylation. 

In Cleveland office, 1976. The pictures on the wall are of Meerwein, Ingold, Winstein, Brown, and Whitmore. 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

The reactant corresponding to retrosynthetic path b in Scheme 2.2 can be obtained by Meerwein arylation of vinyl acetate with o-nitrophcnyldiazonium ions[9], Retrosynthetic path c involves oxidation of a 2-(o-nitrophenyl)ethanol. This transformation has also been realized for 2-(o-aminophenyl)ethanols. For the latter reaction the best catalyst is Ru(PPhj)2Cl2. The reaction proceeds with evolution of hydrogen and has been shown to be applicable to a variety of ring-substituted 2-(o-aminophenyl)ethanols[10]. 

Reduction with aluminium isopropoxide by the Meerwein-Pondorf procedure yields the alcohol (56, 57, 112). 

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

Metal alkoxides cataly2e the Tishchenko condensation of aldehydes (62), the transesterification of carboxyhc esters, the Meerwein-Poimdorf reaction (63), and other enolization and condensation reactions. 

Aluminum alkoxides are easily soluble in hydrocarbons and in chlorinated hydrocarbons, but sparingly soluble in alcohols. They are sensitive to moisture and dry storage is essential. Aluminum alkoxides are used extensively as intermediates, for example, in the Meerwein-Poimdorf reaction (94). 

Catalytic amounts of mercuric chloride are usually employed in this preparation. Aluminum isopropoxide is a useful Meerwein-Potmdorf-Verley reducing agent in certain ester-exchange reactions and is a precursor for aluminum glycinate, a buffering agent (see Alkoxides, metal). 

N-Alkylations, especially of oxo-di- and tetra-hydro derivatives, e.g. (28)->(29), have been carried out readily using a variety of reagents such as (usual) alkyl halide/alkali, alkyl sulfate/alkali, alkyl halide, tosylate or sulfate/NaH, trialkyloxonium fluoroborate and other Meerwein-type reagents, alcohols/DCCI, diazoalkanes, alkyl carbonates, oxalates or malon-ates, oxosulfonium ylides, DMF dimethyl acetal, and triethyl orthoformate/AcjO. Also used have been alkyl halide/lithium diisopropylamide and in one case benzyl chloride on the thallium derivative. In neutral conditions 8-alkylation is observed and preparation of some 8-nucleosides has also been reported (78JOC828, 77JOC997, 72JOC3975, 72JOC3980). 

Reaction of 2-ethoxycarbonyl-l-phenylpyrazolidine with Meerwein s salt (boron trifluoride etherate) yields the quaternary ammonium salt (426) which is not stable and undergoes ring opening to the hydrazine (427) with sodium ethoxide (76JOC1244). 

Oxabicyclo[2.2.1 ]heptyl 2-cation Wagner-Meerwein rearrangement, 1, 418... 

MEERWEIN - PONNOORF VERLEY Reduction Reduction ot carbonyl groups to aicohots by means ot AI(iPiO)3 and isopropanol... 

WESTPHALEN - LETREE Rearrangement Cartxx ation rearrangement of steroidal tert alcofxris (saeWagner-Meerwein). 

---