Übergangsmetallaktivierte organische Verbindungen, XXXVIII. Chemoselektive nucleophile Methylierungen durch In‐Situ‐Blockierung von Aldehydgruppen unter α1‐Phosphonioalkoxid‐Bildung

Transition Metal‐Activated Organic Compounds, XXXVIII [1] . — Chemoselective Nucleophilic Methylation Reactions by In‐Situ‐Blocking of Aldehyde Groups by α1‐Phosphonioalkoxide Formation Herrn Professor Ulrich Schöllkopf zum 65. Geburtstag gewidmet. For the selective methylation of a keto group in the presence of an aldehyde group the complex MeTiCl 3 ·PPh 3 and especially the reagent system [TiC] 4 ·PPh 3 + 0.5 Me 2 Zn] proved to be favourable. For instance, 6‐oxoheptanal ( 8 ) was methylated by [TiCl 4 ·PPh 3 + 0.5 Me 2 Zn] at the keto group with 89% yield and 99:1 selectivity, whereas it was methylated by [TiCl 4 + 0.5 Me 2 Zn] at the aldehyde group with 81% yield and 96:4 selectivity. A selective methylation of benzaldehyde in the presence of heptanal was achieved with MeNbCl 4 ·PPh 3 or [NbCl 5 ·PPh 3 + 0.75 Me 2 Zn] to give (1‐chloroethyl)benzene ( 14 ; yield 92 or 53%; selectivity in each case 93:7) or with [TaCl 5 ·PPh 3 + 1.5 Me 2 Zn] to give mainly 1‐phenylethanol ( 15 ) besides 16 . A 96:4‐selective benzylation (yield 74%) of a ketone in the presence of an aliphatic aldehyde was possible with [TiCl 4 ·PPh 3 + 1 PhCH 2 MgBr]. — The high chemoselectivity is caused by irreversible blocking of aliphatic aldehyde groups by α1‐phosphonioalkoxide formation [1] , whilst the corresponding reaction of keto groups [1] and of benzaldehyde is reversible.