Polarized multiple bonds reacting with

On the other hand these are compounds with marked 1,4-dipolar character, having electron lone pairs at the terminal heteroatom and an electrophilic center at C-4 Consequently, they can react with polarized multiple bond systems, even when these are extremely electron-poor [225]... 

The conversion of a nitrile R —CN into a ketone R —CO—R demonstrates that polarized multiple bonds other than C=0 also react with Grignard reagents, and that such reactions are synthetically useful. Esters 22 and acid chlorides can react subsequently with two equivalents of RMgX the initially formed tetravalent product from the first addition reaction can decompose to a ketone that is still reactive, and reacts with a second RMgX. The final product 23 then contains two substituents R, coming from the Grignard reagent ... 

Organometallics react with this sink by addition to the multiple bond (path Ad r). The more covalent, less reactive organometallics, like R2Cd, react very slowly with almost all of these sinks, whereas organomagnesiums, RMgX, and organolithiums react quickly. Complexation of the metal ion to the Y heteroatom catalyzes this reaction. Organometallics react much faster as nucleophiles with polarized multiple bonds than as bases with the adjacent C-H bonds, (carbon-acid, carbon-base proton transfer is slow). C=Y example ... 

Under equilibrium conditions (thermodynamic control), the allylic source adds to the polarized multiple bond (path AdN). However, the allylic source can also serve as a base and may deprotonate the sink, creating a mixture of sources and sinks and thus a messy statistical mixture of products. Clean products result if the source is just the deprotonated sink or if the sink has no acidic protons. With ketones, the equilibrium of the attack step favors the starting materials, and therefore the reaction goes to completion only if driven by a following elimination. In the next Adisj2 example, the source is the deprotonated sink. The product is an aldehyde-alcohol, or aldol, a name now used for the general process of an enol (acidic media) or enolate (basic) reacting with an aldehyde or ketone. 

Simple pi bonds are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron sink, then addition can occur via the lone-pair-stabilized carbocation as the sink. Figure 8.7 shows a mechanistic example from a short synthesis of the human hormone estrone. 

Aromatic rings are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron... 

Schrock carbene complexes undergo reaction with multiple bonds via four-center metallacyclic intermediates (51). Chapter 11 will consider what occurs when alkylidenes react with alkenes, a reaction known as alkene metathesis. Below are examples of Schrock carbenes reacting with polar multiple bonds such as C-N and C=0. 

The reactions of bis(trifluoromethyl)-subsatuted hetero 1,3 dienes are predomi nantly LLJMO controlled processes [238] With polar or highly polarizable dieno philes, the tendency to undergo stepwise cycloaddition reactions is considerable Notably these hetero-1,3-dienes react with a,(l unsaturated hetero multiple bond systems across the hetero multiple bond exclusively [243, 246 248] (equation 53)... 

Many silenes react cleanly in a [2 + 2] manner with carbon-carbon multiple bonds, and several examples of such behavior have been given above. Some additional examples are listed in Scheme 16, there now being too many examples known to allow a full listing. For the cases listed,33,65 185189 each of which involves a polarized carbon-carbon double bond, the products were isolated and well characterized and the main products were those of a [2 + 2] reaction. Wherever regioisomerism was possible, only a single regioisomer has been observed this includes cases in which unsymmetric alkynes were involved. 

For couplings of metallated cyclopropenes with carbonyl compounds liquid ammonia seems to be generally a less suitable solvent since enolizable carbonyl compounds may easily undergo deprotonation under strongly polar conditions, while aldehydes such as benzaldehyde may react with the solvent. For the lithiation of 1-methylcyclopropene both BuLi THF and LDA THF seem suitable systems. We prefer to use, however, the latter base because of its decreased tendency to add across multiple bonds. The thermodynamic basicity of LDA will be sufficient though in the less polar THF the rate of deprotonation might be lower than that with potassium amide in liquid ammonia. 

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