S-Sulfinates

The induced diastereoselectivity is determined by the chiral sulfinyl moiety of the substrate and not by the menthyl chirality, since a similar but opposed d.r. is obtained in sulfoxide 1 (and ee in the oxidized allenylsulfone 2) when the (-)-menthyl (S/ )-sulfinate is used rather than the ( — )-menthyl (SS)-sulfinate. The induced diastereoselectivity is fair to good, as deduced from the optical purity of the sulfones 2, however, inadvertent resolution of the diastereomers during the chromatographic separation of the allenic and acetylenic sulfoxides may have affected the figures. 

The reaction of photogenerated singlet oxygen with 2,5-dimethylthiophene results in ring opening (70TL791, 70TL795) the c/s-sulfine (149) and the rrans-diketone (150) are the main products. The mechanism of this reaction has been speculated upon. Since (149) itself is photoinert, the two products must arise by different pathways. The key intermediate is (148), formed by cycloaddition of the thiophene with 02 this can lead to the two observed products as shown in Scheme 28. An alternative pathway for the sulfine would be by attack of 02 on sulfur to yield (151), followed by the transformations shown. 

Sulfur dioxide is capable of reacting with metal alkyl, aryl or u-allyl complexes in an insertion-type reaction to yield S-sulfinate (4), O-sulfinate (5) or 0,< -sulfinate complexes (6).13 It can also insert into the metal-metal bond in the cobalt complex (7) to give the S02-bridged complex (8).38... 

The reactions of the other sulfinyl chlorides (n-Pr, z -Pr, p-Tol) with DAG using z -Pr2NEt as base led to the formation of (S)-sulfinate esters as the major isomers with high selectivity (89 to >95%) instead of (/ -)sulfinate esters predominantly obtained when pyridine was used as base (70 to > 95% de, Table 20). 

Moreover the diastereomeric excess can be easily and accurately determined in each case by H NMR analysis of the crude mixtures, because of the differences in chemical shifts of H-l and H-2 in the two diastereomeric sulfinate esters ( Table 21). All of the sulfinate esters obtained up to now are stable and can be stored for several days in the refrigerator without any decomposition. An interesting and common feature of all of these sulfinate esters is the sign of specific rotation (R)-sulfinates are dextrorotatory, while (S)-sulfinates are /evorotatory. 

The intriguing and useful base effect on the stereocourse formation of sulfinate esters of DAG raises several questions. The first and most important is the origin of such effect. In order to answer this question it is essential to know the stereoelec-tronic prerequisite of a base so as to direct the main formation of R- or S-sulfinate. For this study, and taking into account the major sulfinate formed in this reaction,... 

Sulfur dioxide can insert in various ways as shown in (21-XLVI). The usual product is the S-sulfinate (21-XLVIa), although the other isomers are known. The insertion of S02into M—OR bonds, e.g., of Ir(OR)(CO)L2, gives oxygen-coordinated sulfite complexes.211... 

Further studies on these systems have shown that the S-sulfinate is probably formed by the pathways depicted in Eq. (13) (73). This has... 

No desulfination has been reported for metal [Pg.56]

The above reactions are thought to proceed as shown in Eq. (25) (8, 92). Consistent with this general scheme, the 3-alkynyl complex CpFe(CO)aCH2CH2C=CMe affords the corresponding S-sulfinate when treated with sulfur dioxide (124),... 

The alkyls CpMo(CO)3R (R = Me, Et, and CHaPh) readily insert SOa (57) these reactions were the subject of a kinetic study (71, 76). When R = CHaPh, the 0-sulfinato intermediate was detected spectroscopically (72, 73). Contrasting with the above behavior is the apparent lack of reactivity of CpMo(CO)3CFa (57) and CpMo(CO)3CHaCN (124) toward neat SOa- However, the dicarbonyl CpMo(CO)a(PPh3)Me does afford the corresponding >S -sulfinate, the reaction proceeding much more rapidly than that of its parent tricarbonyl, CpMo(CO)3Me (57). The photochemically induced desulfination of CpMo(CO)3S(0)a-CHaPh furnishes CpMo(CO)3CHaPh (35% yield) in contrast, irradiation of CpMo(CO)3S(0)aMe yields [CpMo(CO)3]a (57). 

An important type of SO2 interaction with metal complexes results in conversion of metal-carbon bonds to O- or S-sulfinates. This type of reaction is usually referred to as an insertion , although (as will be discussed below) the mechanism almost certainly does not involve conventional insertion into the M-C bond. The area has been investigated intensively, and we refer the reader to excellent reviews of the topic. We shall limit ourselves to a few illustrative examples with d- and f-element complexes and also mention some interesting cycloaddition and rearrangement reactions of transition metal unsaturated hydrocarbon complexes with SO2. 

The structure of the S-sulfinate complex (Me5Cp)Fe(S02CH2CH=CHPh)(C0)2 is shown in Fig. 32, and clearly shows the tetrahedral sulfur configuration and the terminal nature of the S-O linkages ... 

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... 

Fig. 32. (Me5Cp)Fe(S02CH2CH=CHPh)-(CO)2. (Reprinted, with permission, from Crystallographic Studies on Sulfur Dioxide Insertion Compounds. III. The Structure of an S-Sulfinate Derivative, [w-Cs(CH3)5]-Fe(C0)2S02CH2CH=CH(C6Hs), Churchill, M. R., Wormald, J. Inorg. Chem. 10, 575, Fig. 1, copyright, 1971, by the American Chemical Society)...

S-sulfinate isomers Conversion between isolable sulfinate isomers has been reported for several systems ... 

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,... 

At room temperature the zwitterionic intermediates readily convert to S-sulfinates, in some cases with concomitant formation of the cycloaddition side product CpFe(C0)2(CHCH2S(0)2tH2). 

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... 

Equations 8.22 and 8.23 illustrate the results of some studies designed to elucidate the stereochemistry involved in S02 insertion into iron complexes. In 8.22, the threo Fe complex undergoes insertion to yield the corresponding erythro S-sulfinate with a high degree of stereospecificity. Unlike CO insertion, retention of configuration occurs at the metal center. Equation 8.23 illustrates an experiment designed to correlate stereochemistry of reactant and insertion product. 

The IR spectra of Bi(02SAr)3 show strong bands, due to Vas(S02) and Vs(S02), around 943-880 cm , suggesting that the sulfinate ligands bind to the bismuth center as 0-sul finales and, thus, ruling out the possibility of S-sulfinates or ionic structure. Low solubility of Bi(02SAr)3 supports their polymeric nature in the solid state [72AJC2107]. 

Taylor and Evans extended the substrate for the MIRB to other nucleophiles including methoxide, thiol, and amines in place of de Waard s sulfinate.26 For instance, Michael addition of benzylamine to bromo-vinylsulfone 27 gave the adduct as bromo-sulfone 28, which upon treatment with a base led to the Ramberg-Backlund reaction product 29 in 83% yield. 

The migration (insertion) reaction may also occur in the case of many other unsaturated molecules such as SO2, CO2, N2, NO, nitriles, isonitriles, and ketones. The insertion of sulfur dioxide may proceed very rapidly. The NMR and IR measurements show that the reaction proceeds through O-sulfinates which subsequently undergo rearrangement to give stable S-sulfinates ... 

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