Lewis basicity carbonyl

The mono-alkoxy product from (+)-ethyl lactate and diisopropylsilane (le) is shown to be more reactive than la and If (Scheme 31). We can only speculate at this time that there may be some intramolecular interactions between the Lewis basic carbonyl oxygen of the (+)-ethyl lactate and the silicon atom, which may either, activate the silane towards oxidative addition to the metal catalyst or increase the rate at which the silane-metal complex is attacked by the nucleophilic alcohol. 

The Lewis basic carbonyl group forms a complex with the empty p orbital of the Lewis acidic borane. Hydride transfer is then possible from anionic boron to electrophilic carbon. The resulting tetrahedral intermediate collapses to an iminium ion that is reduced again by the borane. 

Lewis-basic carbonyl group P to double bond... 

A Lewis basic carbonyl group can be activated by coordination with a metal-centered Lewis acid, with profound reactivity and stereochemical consequences. In the context of asymmetric synthesis many Lewis acid-mediated reactions are known to proceed with better stereoselectivity than their non-catalyzed counterparts—very recently a variety of chiral Lewis acids have been shown to be remarkably efficient... 

The 13C NMR study clearly reveals that acetophenone largely shifts to the downfield when in association with BF3, while no shift is observed in trifluoroacetophenone under the same conditions. This fact suggests that no association of the markedly weak Lewis basic carbonyl group of 27 with Lewis acid is observed under the NMR analysis conditions. The nonfluorinated carbonyl group associates strongly with Lewis acid to enhance the electrophilic reactivity, while the corresponding fluorinated one does not (Scheme 1.19) [ 8]. 

Radical reductions typically involve a hydrogen atom transfer from a suitable donor (organotin or silicon hydride) to an acceptor, commonly a radical intermediate generated a to a earbonyl. In eases where enantiomerieally pure products are desired, a chiral Lewis acid is employed in order to coordinate to the Lewis basic carbonyl oxygen(s) and provide faeial bias for the ensuing hydrogen atom transfer. 

Although there are several locations in this molecule that are reasonable sites for protonation, one in particular, the Lewis basic carbonyl oxygen, is unusual Upon protonation, a resonance-stabilized cation is formed, and one of the resonance forms can be recognized as a substituted cyclopropenyl cation, an aromatic species (Problem 59). In fact, with the additional phenyl substitution, this species is remarkably stable. 

Lewis add-base interactions are very common in chemistry and are often rather subtle. You are about to meet, in the next chapter, an important way of making C-C bonds by adding organometallics to carbonyl compounds, and in many of these reactions there is an interaction at some point between a Lewis acidic metal cation and a Lewis basic carbonyl group. 

The observation that the isolated yields of macrocyclization products in RCM reactions can be influenced by the proximity of a Lewis-basic carbonyl to the site of metathesis has been made by other groups as well. Grubbs [29] and Fiirstner [30] have each reported similar problematic macrocyclizations due to the position of the olefin in a metathesis precursor. In each case, the formation of cyclic chelates between the Lewis-acidic Ru atom in the catalyst and a carbonyl moiety in the substrate was responsible for the low yields. 

In a search for a potential solution, Fiirstner and coworkers [31] repeated the macrocyclization of 53 to form 14-membered macrolactones, this time using li(Oi-Pr)4 as an additive (Scheme 12.16). The Ti-based additive was meant to competitively act as a Lewis acid to preferentially bind the Lewis-basic carbonyl, thereby inhibiting chelate formation between the Ru atom and the carbonyl oxygen. The choice of Ti(0/-Pr)4 as an additive was made because it did not promote any degradation of the catalyst, and its coordination to the polar carbonyl functionality was reversible. When 2 equiv of Ti(Oi-Pr)4 was added to the macrocyclization reaction of 53 using catalyst 12 (2.5 mol% in CH2CI2), the... 

While most metathesis additives have been used to coordinate to Lewis-basic carbonyls, additives - such as Ti(Oi-Pr)4 - can also be used to bind Lewis-basic nitrogen substituents. For example, Yang and coworkers studied the formation of nitrogen-containing heterocycUcs by RCM the starting dienes are known to be... 

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