SO2 Insertion Reactions

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,... 

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... 

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 . 

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. 

A review of transition metal SO2 complexes contains a section on SO2 insertion reactions. The insertion of Se02 and Te02 into the Mo—R bond, equation (49), has been accomplished by activation of Se02 and Te02 in a metal evaporator and condensation in an ether matrix at... 

Yamamoto, Shono, and Shinra in 1967 181) reported the reaction of the benzylpentacyanide with SO2 to give the benzylsulfinate complex. Since then insertion reactions of the general type... 

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). 

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). 

Sulfur dioxide has been employed as the solvent in most of the investigations of its insertion reactions. The presence of moisture in SO2 was reported markedly to accelerate the rate of the insertion with MeCo(dmgH)2py (S2) on the other hand, added water had no effect on the reaction of CpFe(CO)2CH2Ph (71, 75). 

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. 

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. 

O-sulfinates formed by insertion reactions carried out in hquid SO2 also derives from Lewis acid complexation (by SO2) of unbound sulfinate oxygen ... 

Acyclic ( -pentadienyl)Fp complexes also behave as Diels-Alder dienes in some cases, although two other reaction pathways compete. With TCNE and maleic anhydride, (17 R = H, Me) undergoes cycloaddition in moderate yields. Highly electron-deficient alkynes, on the other hand, undergo Fe bond insertion reactions reminiscent of SO2 reactions with ( -allyl)Fp complexes, to give (30). Finally, when the pentadiene is disubstituted at C-5, the Diels-Alder route is effectively blocked, and [3 + 2] cycloadditions result with TCNE at the 2,3-double bond. ... 

Some of the substances that lead to 1,1 insertion reactions into metal-carbon ff bonds are carbon monoxide, CO alkyl- or arylisocyanide, CNR, carbenes CR2 sulfur dioxide, SO2 and nitrogen oxide NO. Most studied are CO, CNR, and SO2 therefore a more detailed account of them is presented. 

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 . 

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