Selenoxides sigmatropic rearrangements

Nishibayashi Y, Uemura S (2000) Selenoxide Elimination and [2,3] Sigmatropic Rearrangements. 208 201-233... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

As already mentioned, the ry -elimination of selenoxides was discovered around 197010 and had a major impact on the development of organoselenium chemistry. This reaction is about three orders of magnitude more rapid than the elimination of the corresponding less polar and less basic sulfoxides. Sigmatropic rearrangements proceed at markedly lower temperatures. These reactions are discussed in detail in Section 9.11.2.5. 

The use of allylic selenides 166 in oxidation reaction leads to intermediate selenoxides 167, which can undergo [2,3]sigmatropic rearrangements to the corresponding allylic selenenates 168. These componds will lead to allylic alcohols 169 after hydrolysis (Scheme 48). This is also a versatile procedure for the synthesis of optically active allylic alcohols, provided that either an asymmetric oxidation or an optically active selenide is used for the rearrangement. Detailed kinetic and thermodynamic studies of [2,3]sigmatropic rearrangements of allylic selenoxides have also been reported.290-294... 

The [2,3]sigmatropic rearrangement of allylic selenides has proven to be a useful method for the preparation of allenic alcohols. Selenide 170 was obtained by a free-radical selenosulfonation of the corresponding enyne. Oxidation with mCPBA afforded the allenic alcohol 171 in 89% yield via an intermediate selenoxide (Scheme 49).295... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

It has long been known that appropriately substituted [2.2]paracyclophanes are chiral, chemically stable and do not racemize under normal reaction conditions. With these seemingly ideal prerequisites for use in chiral synthesis, it is perhaps surprising that only three examples have appeared in the literature, all of them in recent years (Scheme 8). Reich employed [2.2]paracyclo-phane-derived selenides such as 24 to administer chirality transfer in selenoxide [2,3] sigmatropic rearrangements. Using this methodology, he was able to synthesize optically active linalool 25... 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

Stereoselective syntheses of di- and trisubstituted olefins, (5, 400-402 6, 30-31). The stereochemistry of [2,3] sigmatropic rearrangements has been reviewed (103 references). The review covers the rearrangements of allyl sulfoxides and allyl selenoxides, as well as Stevens and Wittig rearrangements. ... 

The following example demonstrates, however, that there can be differences in the behavior of the selenoxide diastereomers. In the oxidation of a steroid selenide, a 1 1 ratio of allylic alcohol and diene was obtainedlf>. One of the diastereomers, probably the (Sef )-diastereomer, underwent sigmatropic rearrangement, the other diastereomer displayed the syn elimination which seems to be favored because the rearrangement is suppressed as a result of steric hindrance. 

In contrast to the few examples17-19 for the stereoselective sigmatropic rearrangements of acyclic allylic selenoxides, many examples9 16 20-29 have been reported for cycloalkenyl se-lenides. Table 8 shows examples demonstrating the pronounced selectivity for the anticipated suprafacial course of the rearrangement of cycloalkenyl selenoxides. 

---