Zwitterion dehydrogenation

Similarly, hydroboration of p-methoxystyrene with HBcat using zwitterionic [(P-P)Rh(//6-catBcat)] ((4), P-P = dppe, dippe) or [(dippe)Rh(r/3-2-IVIc-a 11 y 1)] (dippe= l,2-bis(diisopropylphosphine)ethane) gave 99% of the internal hydroboration product. Complications arose in studying reactions of hindered styrenes due to the appearance of not only the hydroboration products (A and B), but also the dehydrogenative borylation products C and D (Scheme 5).32 The ratio of products was found to be highly catalyst dependent (Table 2).33... 

Pentadienyl anion-like [61] photocyclizations occur from molecules having a heteroatom connected to two C=C bonds (olefmic and/or aromatic). This results in a reactive zwitterionic intermediate 128 which, upon oxidative dehydrogenation, produces derivatives of pyrroles, furans, thiophenes, and selenophenes 129 (Scheme 8.37) [62],... 

Mechanistically, in these reactions the lone pair of electrons on the heteroatom contributes to the 6n electron system, which undergoes conrotatory electrocycliza-tions with a conservation of orbital symmetry. The dehydrogenation of zwitterionic... 

Some unusual zwitterionic pyrazines have been prepared by dehydrogenation of 1,4-disubstituted piperazine-2,6-diones. Honzl et al. (476a) prepared theanhydro-2,6-dihydroxy-l, 4-diphenyl-3,5-bis(phenylthio)pyrazinium dihydroxide [s/c] (92) (which yields adducts by dipolar cycloaddition of maleic anhydride or formaldehyde) by the reaction of 1,4-diphenylpiperazine-2,6-dione with benzenesulfonyl chloride in pyridine. Tanaka et al. (476b), from I,4-diphenylpiperazine-2,6-dione with benzoyl chloride, and tosyl chloride in pyridine at reflux, obtained the anhydro - 3 - benzoyl - 2,6 -dihydroxy -1,4 -diphenyl - 5 - (p - toly lthio)pyrazinium dihydroxide [src] (93), together with some of the S-p-tolyl analogue of (92). 

A family of neutral cationic and zwitterionic Rh(I) and Ir(I) complexes with P,N-, P,0-, P(S),0-, and P(S),N-substituted indene ligands have been tested in the hydrosilylation and the dehydrogenative silylation of st3rrene. Cationic rhodium complexes, such as [Rh(cod)2]+BF4, have recently appeared as regio- and stereoselective catalysts for hydrosilylation of alkynes and exclusive formation of vinylsilanes instead of alkylsilanes in the hydrosilylation of alkenes (73). 

The alternative formation of quinoxalines 205 and benzodiazepines 206 can be understood with the help of the proposed reaction mechanism (Scheme 2.14). 1,2-DABs 155a, b, f, m react with ketone 204 to form an imino-intennediate, which by A-protonation and C-deprotonation may form a zwitterion. In the case of the 2-substituted pyridine in MeOH solution, it is proposed that this intermediate cyclizes and is dehydrogenated to form quinoxaline derivadves 205. The formation of quinoxaline derivatives is limited to those reactions in which the aiyl group is 2-pyridyl and there still remains a question over the oxidation to the final product. When the zwitterionic intermediate reacts at carbon with another equivalent of the ketone, a new intermediate could be formed which could undergo further cyclisa-tion to give benzodiazepine derivatives 206. 

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