Rhodium 2-alkenyl pyridines

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

Ellman and co-workers developed a rhodium-catalyzed C-H alkenylation followed by 67t-electrocyclization to give di-, tri-, tetra-, and penta-substituted pyridines. The dihyropyridine intermediates 285 can be directly aromatized via hydrogenolysis to pyridine derivatives 286 in moderate to good yields. ... 

Cheng and co-workers reported a one-pot synthesis of substituted pyridines through a rhodium-catalyzed C-H alkenylation of a,(3-unsaturated ketoximes 287 with symmetrical alkyne substrates 288. 67t-Electro-cyclization of the azatriene intermediates 289 and subsequent loss of water afforded the desired pyridines 290 in moderate to good yields. ... 

A rhodium-catalyzed one-pot synthesis of substituted pyridine derivatives from a,(3-unsaturated ketoximes and alkynes was developed in 2008 by Cheng and coworkers [99], Good yields of the desired pyri-dines can be obtained (Scheme 3.48). The reaction was proposed to proceed via rhodium-catalyzed chelation-assisted activation of the (3—C—H bond of a,(3-unsaturated ketoximes and subsequent reaction with alkynes followed by reductive elimination, intramolecular electro-cyclization, and aromatization to give highly substituted pyridine derivatives finally [100]. Later on, in their further studies, substituted isoquinolines and tetrahydroquinoline derivatives can be prepared by this catalyst system as well [101]. Their reaction mechanism was supported by isolation of the ort/jo-alkenylation products. Here, only asymmetric internal alkynes can be applied. 

Ellman, Bergman, and coworker reported a rhodium-catalyzed procedure for the synthesis of pyridines from alkynes and a,/ -unsaturated N-benzyl aldimines and ketimines in 2008 [107]. The reaction proceeded via C-H alkenylation/electrocyclization/aromatization sequence through dihydropyridine intermediates. The C-H activated complex was isolated and determination by X-ray analysis. Good yields of highly substituted pyridines were produced in one-pot manner (Scheme 3.50). 

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