An Unprecedented, Tandem Aldol−Grob Reaction Sequence

The Aldol‐Grob Reaction: Regioselective Synthesis of (E)‐Alkenes from Aldehydes and Ketones with Ytterbium Triflate Catalysis

“…The results are summarised in Table 1. The reaction is regioselective and gave only (E)-alkenes in good yields; the E geometry of the double bond was determined by coupling constants and the multiplicity of the olefinic proton signals, and by comparison of the NMR spectroscopic data with those previously reported. [10] The regioselectively giving exclusively E adducts may be explained by the mechanism proposed by Kabalka and coworkers. [10] When other lanthanide triflates were used as catalysts under the same reaction conditions, a complex mixture of products was obtained and the desired product was recovered in less than 20% yield.…”

“…[10] The regioselectively giving exclusively E adducts may be explained by the mechanism proposed by Kabalka and coworkers. [10] When other lanthanide triflates were used as catalysts under the same reaction conditions, a complex mixture of products was obtained and the desired product was recovered in less than 20% yield. Lower temperatures or shorter reaction times resulted in lower yields which were shown to be not significantly dependent on the position and electronic properties of the aryl substituent, except for nitro Table 1.…”

“…As depicted by the reaction mechanism proposed by Kabalka and co-workers, [10] a nitro group and a complexation of the methoxy or amino group with the metal centre, diminishing its catalytic activity, may be invoked in these cases. Using aliphatic aldehydes as substrates, no alkene was obtained and aldol condensation products (entry 16) or starting materials (entry 17) were recovered.…”

“…[8] In 1993 Sakai [9] and co-workers described a novel BF 3 etherate catalysed one-pot tandem aldol condensationGrob fragmentation sequence. Recently Kabalka and coworkers [10] have published a more detailed study of the aldol-Grob reaction, incorporating the effect of different Lewis acids, solvents and substrate structure on the reaction pathway. [10] The aldol-Grob reaction has been applied successfully in the synthesis of some natural products and is the key step in the synthesis of sarmentosine, [11] pipercide and piperolein A, [12] three amide alkaloids extracted from Piper sarmentosum and P. nigrum.…”

Use of organoboron halides in organic synthesis

“…While examining the effectiveness of a series of Lewis acid catalysts; we discovered an unusual Aldol-Grob reaction sequence initiated by boron triflouride (Scheme 1) [3,4].…”