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All posts Subject: Novel Route to Arylacetones (Chem Commun 1984) Please login to post

Aurelius
(Hive Addict)
05-02-03 18:29
No 430892
      Novel Route to Arylacetones (Chem Commun 1984)
(Rated as: excellent)

A Novel Route to Arylacetones via a Masked alpha-Acylcarbonium Intermediate
Shimon Shatzmiller, Ramy Lidor, Eytan Shalom and Eliezer Bahar
J. Chem. Soc. Chem. Commun. 795 (1984)

The Ag⁺ induced aromatic substitution reaction of bromoacetone O-methyloxime is described.

Abstract:

Oxime ethers can be efficiently metallated at the alpha-carbon atoms and the derived enolate equivalents (1) can then participate in a variety of useful C-C bond forming reactions. ¹ The alpha-carbon might also support a positive charge [i.e. (2)] stabilized by n-type electron delocalization from the oxime group, and this cation could act as a synthetic equivalent of an l-acylcarbonium ion ². In this communication we describe a method for the conversion of oxime ethers into functionalized ketones using reactive intermediates of type (1) or (2), employing acetone O-methyloxime as a model compound.

Example 1:

Litiation of acetone O-methyloxime (3) was achieved with 1.2M solution of ⁿBuLi in THF-hexane in 5 min at –65°C. Addition of lithiated (3) to molecular bromine in THF at –65°C over 15 min resulted in the formation of the Z-bromo-oxime ether, (4). Isomerization of (4) in CHCl₃-HBr gave the thermodynamically favored E-isomer. (5) ⁺ The halogen atom could be exchanged for alkoxy by treatment of (5) with 1.1eq. of MeO[CH₂]₂Ona in THF for 12hr to (6) in 88% yield.

The bromo-oxime ethers (4) and (5) were also converted into a reactive intermediate of type (2) which reacted in aromatic substitution reactions. Addition of 10mmol of either (4) or (5) in 20 ml of dry DCE to a solution of 10 mmol of AgBF₄ and 10mmol of the aromatic compound in 20ml of DCE at 25°C followed by efficient stirring in the dark for 18hr and then work-up with a 10% KCN-H₂ solution gave good (82-91%) yields of the aromatic substitution products (7a-e)   ⁺   as the E-isomers. Acid treatment of (7a-e) (HCl-H₂O-MeOH 1:5:5) for 10 hrs at 65°C and distillation gave the corresponding arylacetones (8a-e) in high (90%) yield.

References:

1. V. Jaeger. H. Grund, and W. Schwab, Agnew. Chem. Int. Ed. Engl., 1979, 18, 78; S. Shatzmiller and R. Lidor, JACS 1981, 103, 5916.

2. J.-P. Begue and M. Charpentier-Morize, Acc. Chem. Res. 1980, 13, 207.

3. G. J. Karbatsos and N. His Tetrahedron 1967, 23, 1079.

4.   S. Shatzmiller, P. Gygax, D. Hall and A. Eschenmoser. Helv. Chim. Acta. 1973, 56, 2961

⁺ The geometry of the oxime ether moiety was elucidated on the basis of the chemical shift of the ketone Me group in the ²Hn.m.r. spectrum (CCl₄) e.g. the methyl hydrogens resonate at 2.01 in (4) but in (5) they are at 1.96. In (6), it is at 1.79 (E-isomer having 1.91) for (7a-e) the resonances are at 1.71, 1.70, 1.70, 1.71 and 1.74 respectively.   Whereas the corresponding Z-isomers are at 1.75, 1.73, 1.74, 1.75 and 1.76 respectively. (see also Ref. 3).

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		Aurelius (Hive Addict) 05-02-03 18:29 No 430892				Novel Route to Arylacetones (Chem Commun 1984) (Rated as: excellent)
						A Novel Route to Arylacetones via a Masked alpha-Acylcarbonium Intermediate Shimon Shatzmiller, Ramy Lidor, Eytan Shalom and Eliezer Bahar J. Chem. Soc. Chem. Commun. 795 (1984) The Ag⁺ induced aromatic substitution reaction of bromoacetone O-methyloxime is described. Abstract: Oxime ethers can be efficiently metallated at the alpha-carbon atoms and the derived enolate equivalents (1) can then participate in a variety of useful C-C bond forming reactions. ¹ The alpha-carbon might also support a positive charge [i.e. (2)] stabilized by n-type electron delocalization from the oxime group, and this cation could act as a synthetic equivalent of an l-acylcarbonium ion ². In this communication we describe a method for the conversion of oxime ethers into functionalized ketones using reactive intermediates of type (1) or (2), employing acetone O-methyloxime as a model compound. Example 1: Litiation of acetone O-methyloxime (3) was achieved with 1.2M solution of ⁿBuLi in THF-hexane in 5 min at –65°C. Addition of lithiated (3) to molecular bromine in THF at –65°C over 15 min resulted in the formation of the Z-bromo-oxime ether, (4). Isomerization of (4) in CHCl₃-HBr gave the thermodynamically favored E-isomer. (5) ⁺ The halogen atom could be exchanged for alkoxy by treatment of (5) with 1.1eq. of MeO[CH₂]₂Ona in THF for 12hr to (6) in 88% yield. The bromo-oxime ethers (4) and (5) were also converted into a reactive intermediate of type (2) which reacted in aromatic substitution reactions. Addition of 10mmol of either (4) or (5) in 20 ml of dry DCE to a solution of 10 mmol of AgBF₄ and 10mmol of the aromatic compound in 20ml of DCE at 25°C followed by efficient stirring in the dark for 18hr and then work-up with a 10% KCN-H₂ solution gave good (82-91%) yields of the aromatic substitution products (7a-e) ⁺ as the E-isomers. Acid treatment of (7a-e) (HCl-H₂O-MeOH 1:5:5) for 10 hrs at 65°C and distillation gave the corresponding arylacetones (8a-e) in high (90%) yield. References: 1. V. Jaeger. H. Grund, and W. Schwab, Agnew. Chem. Int. Ed. Engl., 1979, 18, 78; S. Shatzmiller and R. Lidor, JACS 1981, 103, 5916. 2. J.-P. Begue and M. Charpentier-Morize, Acc. Chem. Res. 1980, 13, 207. 3. G. J. Karbatsos and N. His Tetrahedron 1967, 23, 1079. 4. S. Shatzmiller, P. Gygax, D. Hall and A. Eschenmoser. Helv. Chim. Acta. 1973, 56, 2961 ⁺ The geometry of the oxime ether moiety was elucidated on the basis of the chemical shift of the ketone Me group in the ²Hn.m.r. spectrum (CCl₄) e.g. the methyl hydrogens resonate at 2.01 in (4) but in (5) they are at 1.96. In (6), it is at 1.79 (E-isomer having 1.91) for (7a-e) the resonances are at 1.71, 1.70, 1.70, 1.71 and 1.74 respectively. Whereas the corresponding Z-isomers are at 1.75, 1.73, 1.74, 1.75 and 1.76 respectively. (see also Ref. 3).

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