Reagents migratory insertion

According to the general migratory-insertion mechanism proposed by Cossee,58 chain propagation (4.105) is a two-step process in which the precursor metal reagent (I) forms an intermediate alkene complex (II) that subsequently rearranges to the insertion product (III),... 

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

In the process of carbonyl insertion the 1,1 migratory insertion of the coordinated CO ligand into the metal-carbon bond results in the formation of a metal-acyl complex (Figure 1-7). This process, as nearly all elementary steps discussed so far, is reversible, but even when using atmospheric CO pressure the equilibrium is mostly shifted towards insertion. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Scheme 23 Reagents and conditions (i) CH2Cl2 (ii) CH2X2 (X = Br, I), —80 °C (iii)reductive coupling (iv) oxidative addition (v) SN2 (vi) halide abstraction (vii) migratory insertion (viii) CD2Cl2, r.t.

Small unsaturated molecules often insert into the M-G bonds and cause expansion of metallacycles. The mechanism of the process is often described as migratory insertion when the reagent is first coordinated by the metal center and then enters further intramolecular reactions <1994JA12117, 1995RTC73, 1996CCR(155)209>. 

Acylzirconocene chlorides are easily accessible in a one-pot procedure through the hydrozirconation see Hydrozirconation) of alkene or alkyne derivatives with the Schwartz s reagent and subsequent migratory insertion see Migratory Insertion) of carbon monoxide into the alkyl- or alkenyl zirconium bond. The stability of the acylzirconocene chlorides is remarkable at room temperature, and consequently allows many applications in organic synthesis. 

In hydroboration, a boron hydride (R2BH) adds across an alkene (R CH=CH2) to give R CH2CH2BR2. The 16-electron, d° Zr(IV) complex Cp2Zr(H)Cl, popularly known as Schwartz reagent, undergoes a closely related reaction. The mechanism involves coordination of an alkene to the electrophilic Zr center followed by migratory insertion of the alkene into the Zr-H bond. The reaction proceeds for alkynes also, by exactly the same mechanism. 

RBr - RCHO. The reagent reacts with primary bromides in the presence of triphenylphosphine (25°) to give the corresponding aldehyde in high yield (75-85%, isolated) after protonation with acetic acid. The reaction is considered to involve oxidative addition, migratory insertion, and reductive elimination. 

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