(1 6.1V.97)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-l,3-diene, 2,3-dimethylbuta-l,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-l,3-diene, hexa-2,4-diene) induce side reactions, namely double-bond migration followed by Introduction. -In addition to its widespread analytical applications, in the last two decades mass spectrometry developed to one of the most important techniques for the examination of ion-molecule reactions in the gas phase [l]. The particular notion of investigating processes under these 'artificial' conditions is that here intrinsic features of reactants can be uncovered, while, in the condensed phase, solvents, ligands, and counter ions largely complicate and obscure the mechanistic pattern. Of course, there are also problems even under the otherwise ideal gas-phase conditions ; for example, an in-depth investigation of stereochemical properties by mass spectrometry (MS) is quite challenging [2]. In fact, the differentiation of stereoisomers by MS techniques is often not trivial. While in most cases stereo-differentiation concerns cyclic compounds [2] [3], strategies have been developed recently, which allow for stereochemical probes of acylic compounds by means of 'bare' transition-metal ions [4].-Transition-metal-ion-mediated coupling reactions have been already investigated in the gas phase