Transition metals sulfinates

Aryl- and heteroaryl halides can undergo thermal or transition metal catalyzed substitution reactions with amines. These reactions proceed on insoluble supports under conditions similar to those used in solution. Not only halides, but also thiolates [76], nitro groups [76], sulfinates [77,78], and alcoholates [79] can serve as leaving groups for aromatic nucleophilic substitution. 

Thiols are susceptible to oxidation by peroxides, molecular oxygen, and other oxidizing processes (e.g., radical-catalyzed oxidation) (Fig. 67). Because thiols easily complex with transition metals, it is believed that most thiol autoxidation reactions are metal-catalyzed (108). Autoxidation of thiols is enhanced by deprotonation of the thiol to the thiolate anion. Thiol oxidation commonly leads to disulfides, although further autoxidation to the sulfinic and, ultimately, sulfonic acid can be accomplished under basic conditions. Disulfides can be reduced back to the thiol (e.g., upon addition of a reducing agent such as dithiothreitol). Thiols are nucleophilic and will readily react with available electrophilic sites. For a more thorough discussion, see Hovorka and Schoneich (108) and Luo et al. (200). 

In light of these results, hypothetical mechanisms for the oxidation of Ni thiolate complexes by 02 may be discussed. The only well-characterized mechanism for the oxidation of thiolates to sulfinates in transition metal complexes involves 0 as an oxidant and proceeds via the stepwise formation of sulfenates (Scheme 1) (90, 91). 

From the evidence cited above it has been assumed that the formation of the O-sulfinates is a common feature of the insertion reactions of coordinatively saturated transition metal alkyls and aryls. The kinetic studies discussed in Section III refer to the scission of the M—R bond which yields the appropriate metal sulfinate. 

An important type of SO2 interaction with metal complexes results in conversion of metal-carbon bonds to O- or S-sulfinates. This type of reaction is usually referred to as an insertion , although (as will be discussed below) the mechanism almost certainly does not involve conventional insertion into the M-C bond. The area has been investigated intensively, and we refer the reader to excellent reviews of the topic. We shall limit ourselves to a few illustrative examples with d- and f-element complexes and also mention some interesting cycloaddition and rearrangement reactions of transition metal unsaturated hydrocarbon complexes with SO2. 

Sulfur dioxide reacts generally with transition metal alkyl, aryl, and a-allyl complexes to give sulfinate complexes. The reaction, first described in 1964 by Wojcicki and Bibler, resembles well-known insertion reactions of CO, C2F4, SnCl2, tetracyanoethy-lene, and other unsaturated species into metal-alkyl bonds, but there are important stereochemical and mechanistic differences Sulfur dioxide insertion into metal-alkene and metal-alkyne bonds have not been reported. However, PdCl2 has been used as a catalyst for copolymerization of ethylene and SO2 to polysulfones and insertion into a Pd-ethylene bond is a conceivable reaction step. 

Sulfinate ligands bind to transition metals as S-sulfLnate, O-sulfinate, or 0,0 -sulfi-nate isomers, the first two types having been demonstrated by x-ray crystallography A fourth structural type, M-S(0)-0R, has also been reported ... 

The various sulfinate isomers are usually distinguishable by the S-O stretching frequencies Although O- and 0,0 -sulfinate isomers are generally isolated from SO2 insertion reactions with non-transition and early transition metals such as Ti and Zr for the mid and late transition elements S-sulfinates predominate. However, exceptions exist, e.g. the O-sulfinate compound [Ni(02SCH3)(p3)](BPh4) . Work by Wojcicki and others has proven that many (if not all) SO2 insertions with Mo, Mn, and Re complexes proceed via O-sulfinate intermediates, which can subsequently rearrange to more stable... 

There is a great range in reactivities of transition metal-alkyls towards SO2. Numerous studies of SO2 insertion reactions have been reported for CpFe(CO)2R complexes, which react rapidly in refluxing sulfur dioxide (—10 °C) to give S-sulfinates,... 

A type of SO2 insertion reaction which has been reported among transition metal complexes exclusively with certain zirconium(IV) cyclopentadienyls is insertion into the M-Cp bond to give polymeric products characterized as O- and 0,0 -sulfinates on the basis of r(SO) values. An example is the reaction... 

Antongst other confounds which function as accelerators with diazonium salts are thiourea dioxide (formamldine sulfinic acid), and p-tolylhydrazine. whilst copper is the most efficient of the metal ions, catalytic qviantities giving a large Increase in R, some other transition metals have a marked effect. These include titanic sulfate and vanadyl sulfate. 

However, hydroperoxide decomposers may act by much more complicated mechanisms. Many sulfur compounds, like the thiodipropionate esters (DRTPs) or the metal dialkyldithiocarbamates (MRDCs) are oxidised to sulfur acids (sulfinic, sulfonic and SO3) which are ionic catalysts for the non-radical decomposition of hydroperoxides. The MRDCs are particularly important since, unlike the phosphites, they also contain complex transition metal ions and when M is a transition metal ion e.g. Ni) they are also UVAs. 

The insertion of SOj can lead to two different products. In most cases, the kinetic product of SOj insertion is an 0-bound sulfinate, whereas the ttiermodynamic product is an S-boimd sulfinate (Equation 12.23). O-bound sulfinates have been observed spectroscopically as intermediates in the reactions of SO with CpFe(CO)jR, CpMo(CO)jR, Mn(CO)jR, and Re(CO)5R. - In other cases of the insertion of SOj into related middle-to-late transition metal-carbon bonds (Equations 12.24 and 12.25 ), only the S-bound sulfinate was observed. In contrast, reactions of SO with hard, early transition metal-alkyl complexes, which are more oxophilic than the softer late transition metal-alkyl complexes, tend to... 

These and other results are best explained, at least when the reaction is carried out in liquid SOj, by the mechanism shown in Figure 12.2. Initial 5 2 attack by the electrophilic sulfur of SOj inverts the configuration at carbon and forms an ion pair. Collapse of the ion pair to form an 0-bound sulfinate occurs reversibly, and the more stable S-bound sulfinate is eventually formed. The cation [CpM(L)(L )] is slow to invert, leading to retention of stereochemistry at the metal. Sulfur electrophiles that are similar to SOj, such as N-sulfinyl-sulfonamides (RS02)NS0 and sulfur bis(sulfonylimide)s (RSOjN)jS, also insert into transition metal-carbon a-bonds with inversion of stereochemistry at carbon, - probably by a mechanism analogous to that shown in Figure 12.2. 

Nucleophilic aromatic substitution reactions of haloarenes complexed to transition metal moieties with oxygen-, sulfin-, and nitrogen-containing nucleophiles allows for the synthesis of a wide variety of aryl ethers, thioethers, and amines. These metal-mediated reactions proceed under very mild conditions and allow for the incorporation of a number of different functional groups. Nucleophilic substitution reactions of chloroarenes complexed to the cyclopentadienyliron moiety have been the focus of many studies directed toward the design of functionalized organic monomers. ... 

The following section takes into account only reactions not involving transition elements. The nucleofiigal leaving group is generally a halide but may also be a sulfinate or an alkoxide. We have to differentiate between two distinct reaction modes, a two-step addition/elimination process on the one hand and a metal-assisted direct displacement of the nucleofuge from a carbenoid intermediate on the other. 

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