Dihydrogen transfer reactions

Other pericyclic reactions of alkynes that have been studied computationally include the addition of singlet methylene to acetylene [109], the addition of carbon monosulfide to acetylene [110], the [2 + 2] dimerization [100, 111], and the dihydrogen transfer reaction between acetylene and ethylene [112, 113]. 

Figure 1-4 Transition structures of dihydrogen transfer reactions (MP2/6-31G, McKee and Stanbury [112,113]).

Nj. Ab initio SCF calculations at various levels were performed on the symmetric dihydrogen transfer reaction c/s-N2H2 N2 a pericyclic transition structure with synchronous motion of the transferring hydrogen atoms was derived [9]. 

The transfer reaction utilizes a sacrificial alkene to remove the dihydrogen from the pincer or anthraphos complex first, before the oxidative addition of the target alkane. The elementary reaction steps are slightly different from the thermal reaction, which is discussed in the next section, both in their order and their direction. For simplicity, we describe the symmetric reaction where the sacrificial alkene is ethylene and the reactant is ethane (21b). The elementary reaction steps for the mechanism of this transfer reaction involve IVR, IIIR, VIR, VI, III and IV, where the superscript R stands for the reverse of the elementary steps listed in Section III. These reverse steps (IVR, IIIR, and VIR) involve the sacrificial alkene extracting dihydride from the metal to create the Ir(I) species 8, while steps VI, III and IV involve oxidative addition of target alkane, p-H transfer and olefin loss. 

The ability of quaternary ammonium halides to form weakly H-bonded complex ion-pairs with acids is well established, as illustrated by the stability of quaternary ammonium hydrogen difluoride and dihydrogen trifluorides [e.g. 60] and the extractability of halogen acids [61]. It has also been shown that weaker acids, such as hypochlorous acid, carboxylic acids, phenols, alcohols and hydrogen peroxide [61-64] also form complex ion-pairs. Such ion-pairs can often be beneficial in phase-transfer reactions, but the lipophilic nature of H-bonded complex ion-pairs with oxy acids, e.g. [Q+X HOAr] or [Q+X HO.CO.R], inhibits O-alkylation reactions necessitating the maintenance of the aqueous phase at pH > 7.0 with sodium or potassium carbonate to ensure effective formation of ethers or esterification [49,64]. 

DIHYDROGEN BONDS AS INTERMEDIATES IN INTERMOLECULAR PROTON TRANSFER REACTIONS... 

Asymmetric Synthesis by Homogeneous Catalysis Coordination Chemistry History Coordination Organometallic Chemistry Principles Dihydrogen Complexes Related Sigma Complexes Electron Transfer in Coordination Compounds Electron Transfer Reactions Theory Heterogeneous Catalysis by Metals Hydride Complexes of the Transition Metals Euminescence Luminescence Behavior Photochemistry of Organotransition Metal Compounds Photochemistry of Transition Metal Complexes Ruthenium Organometallic Chemistry. 

As Eq. (4.1) emphasizes, dihydrogen is the equivalent of two protons and two electrons. If the reaction is conceived mechanistically as consisting of one proton and two electrons moving as a unit (the hydride ion, H ) and the second proton being moved separately (or omitted entirely), then the process may with justice be called a hydride-transfer reaction. 

Fig. 6.15 An ene reaction drawn as a [a-2s+n2s+n2s] process and a dihydrogen transfer drawn as a [tT2s+(T2s+ n 2s]...

It is possible to observe intermediates in the proton transfer reactions by using low-polar or non-polar media and by varying the concentration and strength of proton donors with the combination of variable temperature IR and NMR spectroscopies. Under suitable conditions, this approach further enables the determination of the thermodynamic parameters associated to each step from the temperature dependence of the corresponding formation constants. It is now experimentally confirmed that the first step of the interaction indeed consists of the dihydrogen bond formation. 

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