Lewis adds carbenes

The high reactivity of carbene complex 37 was successfully employed by K. B. Wagner for polymerizations of acyclic dienes, Eq. (35) [50]. Vinyl addition reactions could be avoided under these Lewis add free reaction conditions. 

For a long period, chiral Lewis adds were the most studied catalysts. Only recently have chiral bases, Br0nsted acids, and N-heterocycUc carbenes been utilized as effective organocatalysts for the oxa-hetero-Diels-Alder reaction. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Adds and electrophiles are electron-deficient species. According to the Lewis concept, all electrophiles (e.g., cations, carbenes, metal ions) are acids by definition. However, from long usage the term acid is frequently used to refer to a proton donor, whereas the term Lewis acid usually refers to charged electrophiles in general. 

Aziridines are commonly prepared from imine precursors. Carbene addition to the C=N bond is illustrated by the formation of aziridine 127 from imine 126. ° Difluorocarbene, generated from HFPO (1), also adds to imines such as 128 to give the highly fluorinated aziridine 129. In the presence of Lewis acids, diazo compounds react with imines to produce aziridines. Ethyl diazoacetate and imine 130 gave aziridine 131 in 93% yield, with a cis/trans ratio of 95 5. Chiral diazo compound 132 reacted with the aldimine precursor 133 to afford aziridine 134 in 81 % yield.The reaction displayed both high cis selectivity (>95 5) and excellent diasteroselectivity (94% de). Reductive removal of the chiral auxiliary gave the optically active hydroxymethylaziridine 135. 

It has been shown by Raubenheimer [11] that carbondisulfide adds to mono- and dianionic Fischer carbene complexes. We have used these anions as nucleophiles they react. with cationic Organometallic Lewis Acids [12] and with cationic ethylene and allyl complexes to give a series of novel ligand bridged dimetallic compoimds ... 

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