12-electron fragment

I have reviewed, in Chapter No. 12, the activation of arenes by the strongly electron-withdrawing 12-electron fragment CpFe+, isolobal to Cr(CO)3 and Mn(CO)3+, and its application to the synthesis of dendritic cores, dendrons, dendrimers, and metallodendrimers, including molecular batteries. 

Scheme 22. Heterolytic C-O cleavage reaction in aryl ether complexes by tBuOK or KOH, induced by the activating 12-electron fragment CpFe+. This reaction is very useful and has been applied to the convenient one-pot synthesis of the pheno l-tri al lyl dendron (see Scheme 23).

Consider now the behaviour of the HF wave function 0 (eq. (4.18)) as the distance between the two nuclei is increased toward infinity. Since the HF wave function is an equal mixture of ionic and covalent terms, the dissociation limit is 50% H+H " and 50% H H. In the gas phase all bonds dissociate homolytically, and the ionic contribution should be 0%. The HF dissociation energy is therefore much too high. This is a general problem of RHF type wave functions, the constraint of doubly occupied MOs is inconsistent with breaking bonds to produce radicals. In order for an RHF wave function to dissociate correctly, an even-electron molecule must break into two even-electron fragments, each being in the lowest electronic state. Furthermore, the orbital symmetries must match. There are only a few covalently bonded systems which obey these requirements (the simplest example is HHe+). The wrong dissociation limit for RHF wave functions has several consequences. 

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

Preparative Organosilane Chemistry of the 14 Electron Fragment [(dtbpm)Pt(O)]... 

Given the ability of 14 electron fragments [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] to activate C-H and C-Si bonds of inert organosilanes under very mild reaction conditions, it was of course no big surprise that Si-H activation reactions of silanes are possible as well. Hydrido-silyl complexes were formed in practically quantitative reactions if 14 or IS were used as precursors for the [(dtbpm)Pt(O)] fragment. Examples of Si-H insertion products, all stable, isolable compounds which could be fully characterized, are 25 - 27, and others have been made. 

One of the remarkable features of these 14 electron fragments, which were developed experimentally on the basis of applied MO theory considerations, is their ability to attack selected C-H and, in particular, unactivated C-Si bonds of various organosilanes. Mechanism studies of these bond activation reactions at this point suggest a new type of a-complexation in a common transition state or intermediate for both C-H and C-Si activation, which has to be further investigated in detail through experiments and by theory. 

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

Loss of a radical from a radical ion creates an even-electron fragment ion, CHs" in this case, which preferably may undergo subsequent loss of a molecule ... 

Example 1,4-Benzoquinone represents the perfect prototype of this fragmentation pattern. The subsequent eliminations of intact molecules causes a series comprising of odd-electron fragment ions only (Fig. 6.35). 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

In general, fragmentations obey the even-electron rule (Chap. 6.1.3). Odd-electron fragments from rearrangement fragmentations behave as if they were molecular ions of the respective smaller molecule. 

Resonance electron capture directly yields the negative molecular ion, M"", whereas even-electron fragment ions are formed by dissociative electron capture and ion-pair formation. Molecular ions are generated by capture of electrons with 0-2 eV kinetic energy, whereas fragment ions are generated by capture of electrons from 0 to 15 eV. Ion-pair formation tends to occur when electron energies exceed 10 eV. [77]... 

Chloroadamantanes (149) and (150) reacted with CH2COPh to afford the monosubstitution products (151) and (152) as intermediates, the intramolecular electron-transfer reaction of the radical anion intermediate being a slow process. Product (151) with chlorine in the 1-position reacted further to give (153), whereas (152) with chlorine in the 2-position is unreactive, showing that the 1-position is the more reactive. 1,2-Diiodoadamantane (154) reacted with CH2NO2 to give the monosubstitution products (155) and (156). This implies that the intramolecular electron-transfer reaction of the radical anion is a slow process. The fact that (155) was formed as major product and (156) was the minor product shows that, when (154) accepts an electron, fragmentation occurs faster at the 1-position than the 2-position. 

The reaction of organometalhc compounds with O2 may produce more or less stable dioxygen complexes. An early and unambiguous example of this kind of transformation was provided in the report by van Asselt et al. of the isolation of a series of stable peroxo alkyl complexes of the type Cp Ta( -02)R (R = Me, Et, Pr, Bn, Ph) [5]. As shown in Scheme 1, O2 presumably oxidatively adds to the 16-electron fragments Cp 2TaR, which are in rapid equihbrium with the 18-electron olefin hydrides or alkylidene hydrides. 

For a given it electron system, there are two stereochemical alternatives, illustrated for the four-electron fragment of a 2 + 4 addition by Structures 2 and 3,... 

Another approach for the construction of rings is to use reactions which start with two acyclic compounds and produce cyclic products. There are many of these processes, but the most used and most useful is the Diels-Alder reaction. This is a reaction between a diene and an olefin to give a new six-membered ring. It is also termed a 4 + 2 cycloaddition because one partner (the diene) containing four 7r electrons adds to a two-electron fragment (the olefin) containing two it electrons to yield a ring. 

---