From Hard Allylic Organometallics

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

Stereochemistry of Pd-catalyzed allylation of nucleophiles has been studied extensively. In the first step, formation of 7r-allylpalladium complexes 37 by the attack of Pd(0) on allylic compounds 36 proceeds with inversion of configuration (anti attack). Subsequent reaction of 37 with nucleophiles occurs in different stereochemistry depending on the nature of the nucleophiles. The soft (stabilized) nucleophiles which are derived from conjugate acids with < 25, such as active methylene compounds, attack 37 from the backside of the Pd atom to give 38 with inversion of stereochemistry. Thus overall retention is observed. On the other hand, hard nucleophiles (pK > 25), typically organometallic compounds of main group metals (Mg, Zn, B, Sn and others) generate 39 by transmetallation, and subsequent reductive elimination affords 40. Both the transmetallation and... 

Nickel catalysts have been shown to promote allylic substitutions with hard organometallic nucleophiles and allylic ethers or acetals as the electrophiles [38, 105, 106]. In a series of investigations, Hoveyda has documented the use of chiral nickel catalysts for asymmetric allylic substitution reactions [38]. A brilliant illustration is the conversion of acetal 120 into cyclohexanone 123 (90 %, 92 % ee) by employment of EtMgBr in the presence of a chiral Ni catalyst prepared in situ from diphosphine 121 (Equation 11) [106]. 

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