SO2 insertion

SO2 inserts into metal-carbon a bonds18). The reaction is stereospecific with respect to the configuration at the a-carbon atom of the alkyl group19) and proceeds with inversion of configuration at the a-carbon atom, as shown on threo-erythro isomeric derivatives of C5H5Fe(CO)2CHDCHDC(CH3)3 by H NMR spectroscopy20 70). 

This aspect of the SO2 insertion will be considered in detail in Section III, B. 

Although SO2 insertion is a clean reaction for many metal-carbon complexes, some metal alkyls, e.g., PhCH2Cr(H20)s (4), decompose rapidly to unidentified materials upon treatment with sulfur dioxide. At the other end of the reactivity scale, a number of metal-carbon a-bonded compounds are inert to SO2. These include, in particular, perfluoro-alkyls and -aryls such as CF3Mn(CO)s (66) and CpFe(CO)2CeF5 (13), as well as other complexes with electron-withdrawing substituents in the o-bonded carbon fragment (71). 

Limited information is available on how the reactivity toward SO2 varies within a given triad, all other factors being constant. The SO2 insertion, like the CO insertion (J37), proceeds much faster with CpMo(CO)3R than with CpW(CO)3R (57, 87). Furthermore, the complexes CpFe(CO)2R (R = Me and CHgPh) react with SO2 more rapidly than do their CpRu(CO)2R counterparts (71, 76). Based on the above comparisons, the observed reactivity order MeRe(CO)5 > MeMn(CO)5 (71, 76) comes as a surprise. It would be desirable to correlate the rates of SO2 insertion with the relative M—C bond strengths unfortunately, the latter data are not available. 

Compared in this section are the activation parameters and the relative reactivities toward SO2 of the various types of transition metal alkyl and aryl complexes. As shown in Table IV, the values of AS and AH range from — 63 to — 43 e.u. and from 2.7 to 8.7 kcal/mole, respectively. These entropies of activation are more negative than those for the solvent-assisted carbon monoxide insertion [—33 to —17 e.u. (1J7)] or for the SO2 insertion into the Sn—R (R = Ph and CHaPh)... 

Gross stereochemical changes occurring in the SO2 insertion have been noted for rranr-CpMo(CO)2(PPhg)Me (57) and m-MeMn(CO)4-PPhg (68). These reactions yield tram- CpMo(CO)2(PPhg)S(0)2Me and... 

Sulfur dioxide insertion with the aryl complexes also proceeds initially to the corresponding 0-sulfinates. A moderate p value of —4.3 is to be compared with p = —1.87 for SO2 insertion into the Sn—R bonds of MeaSnR (51,52). It reflects some stabilization by the substituent X of the positive charge developing on the ring during the reaction. A transition state approximating (X) (or a corresponding closed structure), similar to the classical a complex of electrophilic aromatic substitution reactions (103), seems reasonable on this basis. 

Neither SO2 insertion nor desulfination studies have been reported for these elements. 

The O-bonded MeS(0)0Re(C0)5, formed initially in SO2 insertion with MeRe(CO)6, displays moderate stability in solution and was studied spectroscopically (73). A kinetic study of its formation was also conducted... 

There have been no reports of SO2 insertion into a metal-carbon bond of the lanthanide and actinide elements. 

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... 

Unlike some M-SO2 and all ligand-bound SO2 adducts, most SO2 insertion products do not contain reversibly bound SO2. The SO2 in a sulfinate complex is strongly bound into the organic framework and its extrusion usually requires degradation of the com-pound However, notable exceptions to this observation exist The first reported example of this type was the indicated Pt(II) sulfinate compound, which in turn... 

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,... 

Mechanistic aspects of SO2 insertion reactions have been studied by several groups, espedally for CpFe(CO)2R systems. While the intimate details of the mechanism are still not explained totally, some aspects are well understood. On the basis of kinetic studies, Jacobson and Wojdcki proposed a bimolecular electrophilic process in dilute organic solvents, first order in SO2 and CpFe(CO)2R, to rationalize observed kinetics. The reaction is characterized by large negative zlS values (typically -50 to -60 e.u.), which have been attributed to developing charges in the transition state and to extensive solvation by S02 The proposed mechanism, shown below, involves electrophilic... 

The stereochemistry of SO2 insertion into Fe, Mn, and W compounds has been investigated by several groups and shown to be in complete accord with the above mechanism. These studies have shown conclusively by NMR, CD, and X-ray methods that, in the cases studied, the SO2 insertion reaction proceeds with high stereospedfidty at both the metal (retention) and at carbon (inversion) 5i-i54) fo,- these systems... 

Interesting rearrangement reactions have been reported to accompany some SO2 insertion reactions with a-allyl complexes of Mn(CO)5, CpFe(CO)2, and CpM(CO)3 (M = Mo, W) moieties An example is the following reaction. 

Yet another type of SO2 insertion reaction occurs with transition metal complexes containing a cyclopropane ring. The reaction opens the ring to give a cyclic sulphone. 

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... 

Insertion of isocyanides R NC (R = MesC or MesSi), which are isoelectronic to CO, appears to be quite similar (Scheme 10) this step has been used to achieve overall hydrocyanation (see Section 3.9.3.4.3). C—S and C—N bonds may also be generated. SO2 inserts with retention of configuration to yield [Cp2ClZr02SR], while NO leads to A(-nitrosohydroxylamine derivatives [Cp2ClZrj ON(NO)R) ]. Neither of the last two has been exploited synthetically. 

There are many examples " of SO2 insertions, and the mechanism is better understood. Several modes of insertion are possible ... 

The S-sulfinate or 1,1-insertion product, reviewed in 11.3.2.3, is the most common. These products are characterizedby vs-o of 1100-1250 and 1000-1100cm in the IR spectrum. Other isomers have two vs-o absorptions below 1085 cm The 1,2-insertion products, 0-sulfinites, are the initial species generated on SO2 insertion even when the product is an S-sulfinate. For oxophilic metals this product may be stable. Insertion into Sb-Ph and Ti-Ph bonds have been claimed to yield 0-sulfinates and tin alkyls, [reaction (i)], as have zinc alkyls . 

Mechanistic studies of SO2 insertion reactions show that even when the S-sulfinate product forms, an 0-sulfinate complex forms first. Experiments have shown the... 

An O-bonded sulfinate is a likely intermediate en route to the sulfinate-S product. An alternative radical chain mechanism has been proposed to explain SO2 insertion reactions in some alkylcobaloxime complexes that also proceed with inversion of configuration at the alkyl group. It is interesting to contrast the stereochemistry of SO2 (inversion at carbon) with that of the weak electrophile CO2 (retention at the alkyl carbon). 

Further evidence for the stereochemistry of the SO2 insertion reaction [reaction (j)] has been obtained . The insertion of carbon dioxide into metal-carbon bonds is being developed with renewed interest, in part because of its role in the greenhouse effect . 

Although SO2 undergoes a variety of insertion reactions the most common involves insertion into metal-carbon insertions into metal-metal bonds occur to form products in which the sulfur atom is bound to each of the metal atoms. ... 

A backside attack of C02> on the alkyl carbon, analogous to the SO2 insertion mechanism was ruled out by studies of the a-carbon stereochemistry upon insertion (25). The insertion of CO2 into the metal carbon bond of threo-cis-W(CO) (L)(CHD-CHD-Ph) (L = CO and PMc3) proceeds with retention of configuration at the a-carbon (Scheme 1) (26). This is in contrast to the inversion of configuration at the alpha carbon found in backside SO2 insertion reactions. 

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