S-bound sulfinate

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. 

Sulfur dioxide is a much stronger electrophile than CO2 and also needs no vacant site. If SO2 electrophilically attacks the a carbon of an 18e alkyl from the side opposite the metal, an alkyl sulfinate ion is formed with inversion at carbon. Since the anion has much of its negative charge on the oxygens, it is not surprising that the kinetic product of ion recombination is the O-bound sulfinato complex. On the other hand, the thermodynamic product is usually the S-bound sulfinate, as is appropriate for a soft metal binding. This sequence constitutes a 1,2 (O bound sulfinate) or a 1,1 insertion of SO2 (S bound). 

Mutarotation at S is also reported to be catalyzed by light,1041 possibly via an O-bound sulfenate intermediate (equation 162). Adamson and coworkers report that photodecomposition to Co j is the final outcome.1049 For sulfinate complexes donor S to O isomerization occurs1020,1049 and red O-bound (N,0)[Co(cyst02)(en)2](C104)2-H20 has been isolated.1029 This process (equation 163 hv 350-450 nm) is thermally reversible (r1/2 600h, r.t.) but which O atom is involved, axial or equatorial, is not known. The O-bound form is now asymmetric, and some specificity might be expected. 

Cheng et al. [316] synthesized the polymer-bound 2-sulfony-l, 3-dienes from the corresponding support-bound suhblene, which upon thermal S02-extrusion gave the highly reactive diene (345), which could be trapped with dienophiles. The synthesis on a polystyrene support begins with polymer-bound lithium phenyl-sulfinate. S-Alkylation with trans-3, 4-dibromosulfolane (343) in the presence of pyridine gave the resin-bound 3-(phenylsulfonyl)-3-sulfolene. Thermal SO2 extrusion had to be performed in xylene under reflux, which restricts the use of this reaction to PS/DVB-resins. Neither the IRORY-Kans [317] nor the Synphase Crowns [199], which are made of polypropylene, are compatible with these reaction conditions, in which polypropylene is dissolved (Scheme 72) ... 

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