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Salt-Free Diorganozincs

Zn(OMe)2 is an exceptional precursor of reactive and selective organozinc reagents under salt-free conditions. Historically, the difficulty in preparing organozinc reagents, their highly pyrophoric nature, and the generation of by-products has decreased their widespread use. The use of additives to remove these by-products presents other limitations. Therefore, a method to control the solubility of the by-products so that they can be removed by simple filtration or centrifugation has been developed, and relies on the use of Zn(OMe)2 as a precursor to the organozinc reagent to be formed in solution (Scheme 1). After the active organozinc compound is generated, filtration or centrifugation is followed by the desired organic transformation. An example of the addition of diethylzinc and didecylzinc (which are prepared from the corresponding Grignard reagents) to 2-naphthaldehyde is depicted below.

Scheme 1: Generation of salt-free diorganozincs and subsequent reaction with 2-naphthaldehyde

Scheme 1: Generation of salt-free diorganozincs and subsequent reaction with 2-naphthaldehyde (702684)

The nucleophilic addition of salt-free diorganozinc reagents in the presence of a chiral catalyst is fairly versatile, allowing for asymmetric additions to aldehydes, ketones, and imines, as well as conjugate additions. An example of an asymmetric conjugate addition in the presence of dicyclohexylzinc is shown below (Scheme 2).

Scheme 2:Generation of salt-free dicyclohexylzinc and subsequent reaction with 2-cyclotiexenone

Scheme 2:Generation of salt-free dicyclohexylzinc and subsequent reaction with 2-cyclotiexenone (702684) (665290)

Advantages of Diorganozinc Reagents:

  • Balance between nucleophilicity and basicity
  • Low reactivity of by-products
  • High functional group tolerance
  • Low reactivity in absence of appropriate catalyst
  • Now salt-free diorganozinc reagents are commercially available

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