Substituent Effects on Fe⁺‐Mediated [4+2] Cycloadditions in the Gas Phase

Abstract

(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

Spectroscopic Characterization of Nonconcerted [4 + 2] Cycloaddition of 1,3-Butadiene with Lanthanacyclopropene To Form Lanthanum–Benzene in the Gas Phase

“…Because it consists of multiple steps, the 1,3-C 4 H 6 + La­(C 2 H 2 ) reaction can only been called a nonconcerted [4 + 2] cycloaddition, not a traditional Diels–Alder reaction. Furthermore, unlike previously reported metal ion-mediated [4 + 2] cycloadditions with both diene and dienophile as initial reactants, − the dienophile in this reaction is a metallacycle produced as an intermediate by the primary reaction.…”

“…Several mass spectrometric studies have reported transition metal ion M + (M = Cr, Mn, Fe, and Co)-mediated [4 + 2] cycloadditions of 1,3-butadiene with acetylene (or propyne) to yield 1,4-cyclohexadiene/M + , which subsequently dehydrogenates to produce benzene/M + as the major product. − Without a dienophile, 1,3-butadiene reactions with metal ions (Fe + and W + ), , metal oxide cluster ions (V x O y + and Ta x O y + ), − or neutral metal oxide clusters (VO 3 (V 2 O 5 ) n ) yielded association, dehydrogenation, or C–C bond cleavage products. In this work, we report spectroscopic characterization of the La atom reaction with 1,3-butadiene in a molecular beam source.…”

Density Functional Study of the Retrocyclization of Norbornadiene and Norbornene Catalyzed by Fe+

“…The theoretical study of organometallic ion chemistry provides important insights into catalytic systems and may allow the design of more selective and benign catalysts in the future. − There are many examples of transition metal cations coordinating to organic substrates and promoting particular reaction pathways. For example, Fe + can promote the retrocyclization of norbornadiene (NBD) and norbornene (NBN) to form CpFeC 2 H 2 + and CpFeC 2 H 4 + , respectively …”