Olefin dihydroxylation reaction model

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The results of many dihydroxylation reactions have resulted in the compilation of a mnemonic device for the prediction of the direction of attack with catalysts based on each alkaloid (Scheme 8.17). Although this model is very useful, there can be some ambiguity as to which group is the large one and which is the medium (especially with trans-disubstituted olefins) and electronic cbaracteristics cannot be ignored [63]. This is a byproduct of the lack of an unambiguous group to orient the molecule cf. the AE reaction. Scheme 8.1). 

The present review has outlined the efforts to develop biomimetic non-heme iron and manganese catalysts for alkane hydroxylation, olefin epoxidation, and cis-dihydroxylation reactions. However, the examples reviewed here are mostly presented as reported in the literature, since the various reaction conditions involved in the catalytic oxidations hamper a direct comparison and critical evaluation of the data. The survey has not only illustrated a rich variety of iron and manganese complexes that lead to the successful structural modeling of important non-heme iron and manganese enzymes, but also significant features of the oxidation reactions catalyzed by these complexes in combination with dihydrogen peroxide. 

As is apparent from the preceding discussion, a full understanding of the observed diastereoselectivity in dihydroxylation reactions of acyclic allylic alcohols remains elusive. Thus, the use of any one model is insufficient, and a careful analysis of the steric and electronic particulars of a given substrate must be conducted. Nevertheless, an impressive number of diastereoselec-tive dihydroxylations of acyclic olefins in complex molecule synthesis attest to the central role of this transformation [42, 43], Selected examples of stereodivergent dihydroxylations reported by Danishefsky are showcased in Schemes 9.36 and 9.37 [201]. Dihydroxylation of ( )- and (Z)-unsaturated esters 287 and 290, respectively, thus proceeded with excellent diastereoselectivity. Danishefsky has proposed a transition state model based on the ground state conformations of the starting materials as determined by X-ray analysis. The dihydroxylations were thus postulated to occur from the sterically less hindered faces of the olefins, as depicted in 288 and 291. Diol 292 was subsequently converted into N-acetylneuraminic acid (293). 

The correlation between bulky substituents and stereoselectivity is graphically shown in Figure 3, depicting the possible transition states in the dihydroxylation of a monosubstituted olefin by osmium tetroxide derivatives. This reaction is known to be selective [54], and the selectivity depends on whether the olefin substituent takes a position of type A or B in the transition state. The problem with calculations on a model system where the bulky base is replaced by NH3 is that the positions A and B are completely symmetrical, and thus, they yield the same energy. In other words, the reaction would not be selective with this model system. 

Two reaction mechanisms have been proposed for these dihydroxylations (pathway a or b, Figure 7.23), either a concerted [3+2] cycloaddition of the olefins on osmium-diamine complex 7.33 or a stepwise reversible [2+2] cycloaddition followed by a rearrangement [559,1350], An X-ray crystal structure of the resulting osmic ester 2.89A shows its symmetrical structure. Houk s calculations [1351] are in favor of a concerted reaction, and his transition state model is reactant-like, with steric interactions dictating the face selectivity of osmylation. 

The high-valent iron-oxo sites of nonheme iron enzymes catalyze a variety of reactions (halogenation and hydroxylation of alkanes, desaturation and cyclization, electrophilic aromatic substitution, and cis-dihydroxylation of olefins) [lb]. Most of these (and other) reactions have also been achieved and studied with model systems [Ic, 2a-c]. With the bispidine complexes, we have primarily concentrated on olefin epoxidation and dihydroxylation, alkane hydroxylation and halogenation, and sulfoxidation and demethylation processes. The focus in these studies so far has been on a thorough analysis of the reaction mechanisms rather than the substrate scope and catalyst optimization. 

---