Cyclopropanes Lewis acid catalysis

The first examples of chiral Lewis acid catalysis in the opening of diactivated cyclopropane derivatives (224) with nitrones (225) has been demonstrated, giving rise to the tetrahydro-l,2-oxazines (226). High enantioselectivities (71-96% ee) were attained with Ni(C104)2 and Ph-DBFOX ligand (227).262... 

Lewis acid catalysis of the vinylcyclopropane rearrangement has been reported for vinylcy-clopropane 3, a key intermediate in the synthesis of the plant hormone antheridogen-An. Most recently, a report has appeared describing a highly stereoselective, diethylaluminum chloride promoted rearrangement of vinylcyclopropanes 5 to cyclopentenes 6. The cyclopen tenes appear to be formed directly, in some cases as a consequence of the rhodium-promoted cyclopropanation of enol ethers (see Section 2.4.3.1.3.). ... 

The cyclization step requires Lewis acid catalysis lithium chloride, formed during an attempt at silylation, is sufficient. The stereochemical course under these conditions (retention at the carbanionic center, inversion at C-3) was rigorously proven by stereospecific deuteration, and an X-ray structure analysis of amino ketone 171, as well [57]. Surprisingly, the use of the stronger Lewis acids f-BuMe2SiOTf or Bp3-OEt2 caused the formation of the opposite enantiomer ent-169 [57] the reasons are still unknown. The cyclopropanes 169 are easily depro-tonated by sec-butyllithium/TMEDA and substituted by many electrophiles with complete retention [57]. The reactions are quite general diastereomers are formed from 2-monosubstituted dicarbamates [108]. 

Unfortunately, NBD does not absorb in the visible and UV-A (400-320 nm) range of the solar spectrum. This problem may be overcome by the use of a wide variety of photosensitizers, which includes transition metal complexes and organic sensitizers like acridones and aromatic ketones . In this section, however, new trends in the exothermic back-isomerization Q NBD are of considerable interest because in this two cyclopropane ring moieties are involved. This ring-opening reaction may be achieved by transition metal complexes " or by Lewis acid catalysis the latter, however, leads to side reactions which include cationic oligomerization ". ... 

In 2009, Lambert reported a mild, efficient, and stereoselective synthesis of fused cyclopentenes via Mgl2-promoted isomerization of activated VCPs. The VCPs were prepared by Pd-catalyzed cyclopropanation of 1,3-dienyl p-ketoesters in moderate to high yields and good diastereoselectivities. Typical standard reaction conditions for the VCP-CP rearrangement (i.e., pyrolysis, transition metal catalysis, standard Lewis acid catalysis) were found to be ineffective for these substrates. However, the use of 1.5 equiv. of Mgl2 led to conplete conversion, providing bicyclic cyclopentenes in high yields fScheme 11.15L... 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

Alkyltin compounds carrying a leaving group at the y-position undergo 1,3-elimination to produce cyclopropanes. y,(5-Epoxyalkyltin derivatives are converted to cyclopropyl-methyl alcohols under catalysis of a Lewis acid (equation 5) . y-Hydroxyalkyl selenides... 

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

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