Sulfur dioxide, insertion

This chapter is concerned entirely with the insertion of carbon monoxide into transition metal-carbon cr-bonds. Sulfur dioxide insertion 154, 239), also common among transition metal-carbon complexes, will be treated in a complementary review, which is to appear later. Subject to the restrictions given at the beginning of Section VI, an attempt has been made at a complete literature coverage of the insertion of CO. Particular emphasis focuses on recent results, especially those of a kinetic and stereochemical nature. 

Xlld does not involve the chiral center, so if the reaction takes place by this pathway, the migration of the alkyl group from sulfur to palladium (with the concomitant or subsequent loss of sulfur dioxide) must take place with inversion of configuration at carbon. Inversion of configuration at carbon has been observed in the reverse-type reaction, the sulfur dioxide insertion into a carbon-iron sigma bond (49). Nucleophilic displacement at carbon in compounds of type Xld is unusually difficult, so the reaction via the sulfite intermediate Xlld would appear to be more likely. Conversion of the tosylate of l-phenyl-2,2,2-trifluoroethanol to the corresponding chloride, a reaction which takes place in the presence of tetra- (n-butyl) ajnmonium chloride with inversion of configuration at carbon, requires 100°C for 24 hrs in dimethylsulfoxide. 

Organogold(I) complexes undergo a number of insertion reactions with unsaturated molecules, such as olefins, acetylenes, and sulfur dioxide. Insertion of carbon monoxide or carbon dioxide has not been achieved, although the reverse reaction has been observed with C02 (7/). 

II. Sulfur Dioxide Insertion. General Considerations and Background... 

SULFUR DIOXIDE INSERTION. GENERAL CONSIDERATIONS AND BACKGROUND... 

Sulfur dioxide insertion reactions may be represented by the equation... 

Sulfur dioxide insertions can be either intermolecular or intramolecular. For example, in the reaction (3) (73) precoordination of the inserting... 

Sulfur dioxide insertion is not limited to metal-carbon a bonds, although M—C is by far the most common substrate involved. Reactions have been reported which lead to insertion of SOa metal-carbon TT (or polyhapto) bonds 26, 102, 130-132), as well as transition metal-transition metal 112), transition metal-Group IVB metal 14, 19), and metal-oxygen 9,58) linkages. Moreover, reaction (8) (97) where M = Rh... 

Kinetic studies on the sulfur dioxide insertion have been few and most have utilized liquid SO2 as the reaction medium. The most exhaustive investigation in neat SO2 was conducted on the systems CpFe(CO)2R and various (R Cp)Fe(CO)2R (77, 74, 75) studied cursorily were CpMo(CO)3R, CpRu(CO)2R, RMn(CO)5, and RRe(CO)5 71, 76). Kinetic studies in organic solvents have been confined to a few CpFe(CO)2R complexes 71, 77). Of the many SO2 reactions of the main group metal-carbon systems, only those involving some MesSnR aryl and benzyl compounds were examined kinetically 51, 52, 99). 

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. 

Surveyed in this section are known SOg insertion and desulfination reactions. Kinetic and stereochemical results already discussed in Sections III-V, as well as the cycloaddition processes treated in Section V, have been given a cursory mention for the sake of completeness. The insertion reactions involving bonds other than M—C are omitted. Also excluded are all aspects of the chemistry of metal sulhnate complexes except sulfur dioxide insertion and desulfination. Readers interested in alternative methods of synthesis and in the physical and chemical properties of these and related compounds are referred to a recent review 128). 

Sulfur dioxide insertion has been effected with CpCr(NO)aR [R = Me (61), CHaPh (61, 71, 76), and Ph (71, 76)] in neat SOg the first two compounds also undergo this reaction in hexane solution at room temperature (61). The isolated iS-sulfinate with R = CHaPh does not eliminate SOa upon heating or photolysis (61). 

The reaction of CpNi(PR3)Me (R = Ph or -Bu) with neat SOg to give CpNi(PR3)S(0)2Me 107) represents the only known sulfur dioxide insertions into a Ni-C bond. 

Palladium(II) chloride catalyzes reaction of SO2 with ethylene [Eq. (36], the mechanism of which is postulated to involve sulfur dioxide insertion into a Pd—C bond (55), viz., Eq. (37). 

---