Gold(I)-Catalyzed Rearrangement of Propargyl Benzyl Ethers: A Practical Method for the Generation and in Situ Transformation of Substituted Allenes

Abstract

A series of benzyl propargyl ethers react with a gold(I) catalyst to furnish variously substituted allenes via a 1,5-hydride shift/fragmentation sequence. This transformation is rapid and practical. It can be performed under very mild conditions (room temperature or 60 degrees C) using terminal as well as substituted alkyne substrates bearing a primary, secondary, or tertiary benzyl ether group. The allenes thus formed can be reacted in situ with an internal or external nucleophile, corresponding to an overall reductive substitution process, to produce more functionalized compounds.

Theoretical Support for the Involvement of a Radical Pathway in the Formation of Allenylzincs from Propargyl Iodides and Dialkylzincs: Influence of Zinc Coordination

“…The NMR data of 1-(propa-1,2-dienyl)benzene (2a), 41 1-(prop-1-ynyl)benzene (2b), 42 1-methoxy-2-(propa-1,2-dienyl)benzene (3a), 43 and 1-methoxy-2-(prop-1-ynyl)benzene (3b) 44 General Procedure D, Radical Reactions with Diethylzinc and Electrophiles. Reactions of Propargyl Iodides, with Diethylzinc and Electrophiles.…”

Gold-catalyzed cyclization of propargylic diynes: Ethers vs acetates – Related products but different pathways

“…Unfortunately this was also the case for substrates bearing nonterminal alkynes at the propargylic moiety (Entries 15e17). In this case the only product that could be identified was allene 7 that can be explained by a 1,5-hydride shift with subsequent elimination of formaldehyde, a sequence that is in analogy to an allene synthesis reported by the Gagosz group [9]. Finally the role of the ether substituent was investigated.…”

Gold-catalyzed cyclization of allenyl acetal derivatives

“…In the presence of added D 2 O, undeuterated 1a gave the product without deuterium content (Scheme 2, reaction 2). The results of these labeling experiments reveal a 1,4-hydrogen shift [20–22] in the d 1 -1a → d 1 -4a transformation.…”

“…The deuterium labeling experiment of the d 1 -1a→d 1 -4a transformation (Scheme 2, reaction 1) indicates that one methylene proton of 4a is derived from the original acetal group. Accordingly, we postulate a 1,4-hydride shift [21–22] for the intermediate transformation B → C . We excluded an alternative route involving the protonation of the fulvene intermediate D because this route would water as a proton source.…”