Stereochemistry, olefin

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

Both the olefin stereochemistry observed and the results of cross-over experiments (added m-chlorobenzaldehyde) confirm the faster rate of reversibility (to ylide and aldehyde) of cis-(34) compared to trans-(34) and indicate that this difference is greater than earlier work had suggested. Even more interesting is the observation of a synergistic effect (leading to excessively enhanced amounts of (E)-alkene) when deliberately prepared mixtures of ervthro- and threo-(33) are decomposed. The volumes of activation... 

The products consistently exhibit retention of the original olefin stereochemistry and are probably formed in a concerted manner. An exciplex formed from singlet excited benzene and the ground state olefin (allowing relaxation of the orbital symmetry requirements for concerted 1,3- and 1,4-cycloaddition) has been proposed to account for these products/126 Srinivasan and Hill reported an unusual photochemical addition to benzene to form cycloadduct (52)<74) ... 

Silyloxide eliminations (Petersen olefination) also proceed readily and regiospecifically to give olefins. When base is used to produce die oxyanion, the elimination occurs widi syn stereochemistry. If an acid is used to promote the elimination, it occurs in an anti fashion, leading to die opposite olefin stereochemistry. This is a very useful way to generate either a Z or E olefin from die same starting material. 

The lack of stereospecificity in the cycloaddition reaction and the partial stereospecificity in the ene reaction (Table 3) indicate that at least a portion of the ene reaction occurs via a reaction pathway different from the pathway leading to cycloadducts. A probable intermediate in the pathway to cycloadducts is a long-lived diradical such as 63 which randomizes olefin stereochemistry. The long-lived diradical may also be an intermediate in the stereorandom component of the ene reaction. 

T he Criegee (1) mechanism of ozonolysis postulates that unsymmetrical olefins should give two zwitterions and two carbonyl compounds and hence postulates the possible formation of three different ozonides. This prediction has now been realized in a number of cases (2-9). It has also been shown that in many cases the ozonide stereoisomer ratio depends upon olefin stereochemistry in both normal (3, 6-12) and cross (6-9) ozonides. Since the original Criegee mechanism did not provide for these stereochemical results, a number of additional suggestions for the mechanism have been made (6,9,13, 14), all of which retain the fundamentals of the Criegee mechanism. 

Since the same product or mixture of products was obtained from both cis and trans olefins (8 and 13), it is not possible to determine whether there is any influence of olefin stereochemistry on diperoxide stereochemistry in this case. 

We hope to work with a stereoisomeric pair of olefins of type 2 where the substituent sizes are sufficiently different to permit an evaluation of the influence of olefin stereochemistry on the reaction. Similar considerations will have to be kept in mind when choosing a more complex olefin of type 3. 

We have shown that cross diperoxides can be formed by various ozonolysis procedures. We now hope to parallel the work done where cross ozonides were produced—i.e., to examine the influence of olefin stereochemistry and other reaction variables. 

Substituted vinylphosphonates (195) and allylphosphonates (196) with E-olefin stereochemistry have been prepared for the first time via intermolecular olefin cross-metathesis (CM) using ruthenium alkylidene complex (197) in good yield. A variety of terminal olefins, styrenes and geminally substituted olefins has been successfully employed in these reactions (Scheme 49). ... 

Bisalkoxycarbonylcarbenes have been generated from several different sources. In the photochemical reaction of diazomalonic esters, the adducts to alkenes have been obtained with substantial retention of stereochemistry when the singlet species is allowed to react. On the other hand, an almost complete loss of olefin stereochemistry is observed in the reaction of triplet species Under the catalytic conditions, the cyclopropanation occurs stereospecifically. The major side reactions in these cyclopropanations are allylic C-H... 

Similar methodology was used in a total synthesis of the angularly fused triquinane pentalenene (53) [65]. The known phenol 271 was used as the starting material. In this and the previous instance (viz. with 269) constant-current electrolyses were performed, this time converting 271 to a mixture of the desired adduct 272 (64%) as well as a material epimeric at the CH20Ac-bearing carbon (16%). The major adduct corresponds to one wherein the original olefin stereochemistry has been maintained. Conversion of 272 to the natural product followed chemistry similar to that used in the synthesis of silphinene (56). 

Inversion of Olefin Stereochemistry The preparation of alkenes via inversion of the double bond geometry is an important synthetic transformation. For example, interconversion of the (Z)-alkene to the (ff-isomer depicted below involves treatment of the (Z)-epoxide with the nucleophilic LiPPh2 followed by phosphorus alkylation to furnish the betaine,which undergoes 5yn-elimination to produce the (Ef-alkene. The alkene inversion works for di-, tri-, and tetra-substituted olefins. 

---