Organometallic reagents, reactions

The acidity of the hydroxyl group can be seen in the rapid proton-deuteron exchange that can take place when alcohols are dissolved in D O (Reaction I), alkali metals (Reaction II) and organometallic reagents (Reaction III and IV). 

The mechanistic data on the N-arylations are separated into data on reactions of hypervalent organometallic reagents, reactions of low-valent organometallic reagents, reactions of low-valent organometallic reagents with a co-oxidant, that are catalytic in copper, and reactions of aryl halides (I, Br, Cl). 

The dimer (q -Cp)2Fe2(CO)4 is commercially available and is a valuable starting material in organometallic chemistry. Reactions with Na or halogens cleave the Fe—Fe bond (equations 23.99 and 23.100) giving useful organometallic reagents, reactions of which are exemplifled in equations 23.101—23.104. 

Another important reaction via transmetallation is carbon-metal bond formation by reaction with bimetallic reagents. This is a useful synthetic method for various main group organometallic reagents. 

Acyl halides react with organometallic reagents without catalysts, but sometimes the Pd-catalyzed reactions give higher yields and selectivity than the Lincatalyzed reactions. Acyl halides react with Pd(0) to form the acylpalladium complexes 846, which undergo facile transmetallation. 

Application of 7r-allylpalladium chemistry to organic synthesis has made remarkable progress[l]. As deseribed in Chapter 3, Seetion 3, Tt-allylpalladium complexes react with soft carbon nucleophiles such as maionates, /3-keto esters, and enamines in DMSO to form earbon-carbon bonds[2, 3], The characteristie feature of this reaction is that whereas organometallic reagents are eonsidered to be nucleophilic and react with electrophiles, typieally earbonyl eompounds, Tt-allylpalladium complexes are electrophilie and reaet with nucleophiles such as active methylene compounds, and Pd(0) is formed after the reaction. 

Ketones can be prepared by trapping (transmetallation) the acyl palladium intermediate 402 with organometallic reagents. The allylic chloride 400 is car-bonylated to give the mixed diallylic ketone 403 in the presence of allyltri-butylstannane (401) in moderate yields[256]. Alkenyl- and arylstannanes are also used for ketone synthesis from allylic chlorides[257,258]. Total syntheses of dendrolasin (404)f258] and manoalide[259] have been carried out employing this reaction. Similarly, formation of the ketone 406 takes place with the alkylzinc reagent 405[260],... 

Before we describe the applications of organometallic reagents to organic synthesis let us examine their preparation Organolithium compounds and other Group I organometal he compounds are prepared by the reaction of an alkyl halide with the appropriate metal... 

When planning the synthesis of a compound using an organometallic reagent or indeed any synthesis the best approach is to reason backward from the product This method is called retrosynthetic analysis Retro synthetic analysis of 1 methylcyclohexanol suggests it can be prepared by the reaction of methylmagnesmm bromide and cyclohexanone... 

Carbon-Carbon Bond-Forming Reactions of Organometallic Reagents... 

A general method of forming a phosphoms—carbon bond is by reaction of an organometallic reagent and a phosphoms—halogen link. 

Pyran-4-one, 2,2-dimethyltetrahydro-reactions, with organometallic reagents, 3, 726 Pyran-4-one, 2,6-diphenyltetrahydro- C NMR, 3, 589... 

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

In most cases, the product ratio can be controlled by choice of reaction conditions. Ketones are isolated under conditions where the tetrahedral intermediate is stable until hydrolyzed, whereas tertiary alcohols are formed when the/Tetrahedral intermediate decomposes while unreacted organometallic reagent remains. Bxamples of synthetic application of these reactions will be discussed in Chapter 7 of Bart B. 

---