Redox isomerizations

In the case of a substrate bearing a second hydroxyl group, the aHenylidene can undergo nucleophilic attack at the y-carbon to re-form a vinylidene [15]. In such an event, the newly formed vinylidene can then undergo the reconstitutive condensation shown in Equation 1.14 [16]. This reaction provides access to the spiroketal of calyculin A, a nanomolar inhibitor of serine/threonine phosphatases. 

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Scheme 1 Scheme of squares for cis-trans redox isomerism. 

Figure 23. The proposed redox isomerism of the ene-l,2-trithiolate complex, Cp2Mo S3C2(2-quinoxaline)[C(0)Me] (96).

In special circumstances the metal and a ligand can compete for the spin in a paramagnetic complex. As this alternative involves oxidation-state changes, it is referred to as valence tautomerization or redox isomerization [78]. Such behavior is observed for o-semiquinone complexes of cobalt and manganese [78] recently, a copper(I)-semiquinone-copper(II)-catecholate equilibrium system (7) of biochemical interest has been analyzed by temperature-dependent ESR [79]. 

The rich, synthetically valuable chemistry of these carbanions is mainly due to the independent efforts of two groups ". As outlined in equation 129 addition of a ketone to the carbanion is the basis for the preparation of cyclobutanones obtained after acid treatment of the intermediate oxaspiropentanes. In contrast, aldehydes as electrophiles give isolable bromoalcohols, which undergo redox isomerization to cyclopropyl ketones. 

Application of this catalyst system to propargyl alcohols provides a, 3-unsaturated aldehydes (Equation 1.17) and ketones (Equation 1.18) [18]. The ease of accessibility of the substrates by simple addition of terminal alkynes to aldehydes followed by this redox isomerization constitutes a highly chemoselective and atom economic strategy to these unsaturated carbonyl compounds. The chemoselectivity problems of the direct aldol condensation and the poor atom economy of olefination methods make this new strategy the most efficient and reliable approach to these units. 

A significant improvement in the catalyst adds further utility to this strategy. Using 1-5 mol% of indenyl complex 15 along with indium triflate and camphorsulfonic acid (CSA) as cocatalysts, the redox isomerization in Equation 1.19 was completed in 20 min compared with 1.5 h under the indium chloride cocatalyst conditions. 

Trost, B.M. and Kulawiec, R.J. (1993) Chemoselectivity in the ruthenium-catalyzed redox isomerization of allyl alcohols. Journal of the American Chemical Society, 115, 2027-2036. 

Trost, B.M., Gutierrez, A.C. and Livingston, R.C. (2009) Tandem ruthenium-catalyzed redox isomerization-O-conjugate addition an atom-economic synthesis of cyclic ethers. Organic Letters, 11, 2539-2542. 

Isomerization. Efficiency for the transformation of allyl alcohols to saturated ketones by intramolecular redox isomerization is improved by using these complexes [vs. CpRu(PPh3)2Cl-NH4PF6], but there is the disadvantage of limited substitution patterns on the substrates. The catalyst is readily prepared from RuCl3-3H20. 

Redox isomerization. The combination of InCl, and a ruthenium complex in the presence of EtjNH PFg in refluxing THE promotes the conversion of propargylic alcohols to enals and enones. (For enals, the addition ofNHJPF is beneficial.) The process is highly chemoselective, as functionalities such as isolated carbonyl groups, free alcohol, alkyne, and terminal alkene are not affected. 

Hydrogen Transfer and Redox Isomerization. Catalytic hydrogen transfer from a suitable hydrogen donor to a substrate is a useful way to reduce... 

Redox isomerization of allylic alcohols is an attractive way of producing carbonyl compounds (Scheme 13). This reaction replaces the traditional two-step ox-idation/reduction procedures, emplojnng environmentally harmful oxidants. Furthermore, it is also applicable in cases of sensitive substrates that cannot tolerate harsh conditions of oxidation and subsequent reduction. Redox isomerization of allylic alcohols is intensively studied (131), and several catalysts have already... 

The AT-heterocyclic carbene complexes of Ru(II), that is [RuCl2(bmim)(p-cymene)] and [RuCl(bmim)(p-cymene)(PTA)], were also found active in the redox isomerization of allylic alcohols. These catalysts had to be activated by H2. 

The water-soluble phosphine, PTA, was grafted to dendrimers and reacted with [RuCl2(p-cymene)]2- The resulting dendritic complexes were applied to the redox isomerization of l-octen-3-ol in water/re-heptane biphasic systems at 75°C. Albeit the reactions were slow, there was a large positive dendritic effect The catalytic activity increased with increasing size of the dendritic support, and the catalyst derived from the third generation dendrimers afforded 98% conversion whereas use of the soluble monomeric catalyst led to only 38% conversion under the same conditions. The catalyst was recycled in the aqueous phase by decantation with no or negligible loss of the catalytic activity (138). 

The generation of mthenium carbenoid intermediates can also arise from propargyl alcohols. In the presence of the CpRuCl(PPh3)2 complex, p-oxomthenium carbenoids were presumably formed via redox isomerization of propargyl alcohols. This process applied to enynols allowed the synthesis of [3.1.0] and [4.1.0] bicyclic frameworks by [2+2] cycloaddition of the ruthenium carbenoid species with the double bond of enynols followed by reductive elimination [127] [Eq. (60)]. 

There are a lot of examples that can support this proposition. I find the results we obtained in our laboratory constitute a compelling example of this point of view. The discovery that some metal complexes may undergo redox isomeric interconversion upon irradiation at cryogenic temperatmes opened a challenging topic in materials science because of the potential application of the phenomenon for designing memory devices [26-31]. In particular, we are investigating some cobalt-catecholato derivatives that exhibit a photoinduced interconversion... 

The so-called redox isomerizations represent a dass of internal redox reactions tvherein the overall redox state of the starting material remains unchanged. Again, this type of reaction is ideally suited to couple two oxidoreductase-catalyzed reactions with opposite cofactor demand. We [83] and others [84] have demonstrated the redox isomerization of allylic alcohols into the corresponding saturated ketones by coupling an ADH with an enoate reductase (ER) (Scheme 8.18). 

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