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Substituent Effects on Fe + ‐Mediated [4+2] Cycloadditions in the Gas Phase
Author(s) -
Schroeter Katrin,
Schalley Christoph A.,
Schröder Detlef,
Schwarz Helmut
Publication year - 1997
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19970800418
Subject(s) - chemistry , substituent , gas phase , phase (matter) , computational chemistry , stereochemistry , organic chemistry
Abstract The Fe + ‐mediated [4+2] cycloaddition of dienes with alkynes has been examined by four‐sector ion‐beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me‐substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e. , 2‐methylbuta‐1,3‐diene, 2,3‐dimethylbuta‐1,3‐diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but‐2‐yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety ( e.g. , penta‐1,3‐diene, liexa‐2,4‐diene) induce side reactions, namely double‐bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4‐dimethylhexa‐2,4‐diene. Similarly, alkynes with larger alkyl substituents (pent‐1‐yne, 3,3‐dimethylbut‐1‐yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the ( E )‐ and ( Z )‐penta‐1,3‐diene ligands as well as for ( E , E )‐ and ( E , Z )‐hexa‐2,4‐diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe + and ethoxyacetylcne/isoprene/Fe + indicate that substituents avoid proximity.