Sulfur dioxide, insertion reactions involving

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. 

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). 

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). 

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). 

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed... 

The dimeric Pd(I) complex Pd2(dpm)2X2 involves two planar metal centers linked by a metal-metal bond. The Pd-Pd separation in these molecules is about 2.7 A. (16,17) These dimers readily undergo insertion of small molecules to give molecular A-frames via reaction (10). (14,18-22) Carbon monoxide and sulfur dioxide... 

Somewhat similar considerations apply to the insertion of carbenes into a bonds, except that in this case the reaction can only involve four electrons, and there is no 6-electron alternative.739 We shall return to the carbene insertion reactions later when we discuss periselectivity why carbenes choose to react with a double bond by the nonlinear approach even with dienes, which would make a 6-electron linear approach analogous to the sulfur dioxide reaction 6.158 allowed. 

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