Enantioselection simple olefins

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Jacobsen et al. reported that a different type of dintrogen ligand (48), fe[(2,6-dichlorophenyl)-methylideneaminojcyclohexane, was an efficient chiral ligand for copper-mediated asymmetric aziridination (Scheme 35).154 The reactions of conjugated c/.v-olefins show high enantioselectivity with this catalyst, but enantioselectivity of the reactions of simple olefins such as styrene and indene is moderate. 

Catalytic Enantioselective Epoxidation of Simple Olefins by Salen Complexes... 

Dioximato-cobalt(II) catalysts are unusual in their ability to catalyze cyclopropanation reactions that occur with conjugated olefins (e.g., styrene, 1,3-butadiene, and 1-phenyl-1,3-butadiene) and, also, certain a, 3-unsaturated esters (e.g., methyl a-phenylacrylate, Eq. 5.13), but not with simple olefins and vinyl ethers. In this regard they do not behave like metal carbenes formed with Cu or Rh catalysts that are characteristically electrophilic in their reactions towards alkenes (vinyl ethers > dienes > simple olefins a,p-unsaturated esters) [7], and this divergence has not been adequately explained. However, despite their ability to attain high enantioselectivities in cyclopropanation reactions with ethyl diazoacetate and other diazo esters, no additional details concerning these Co(II) catalysts have been published since the initial reports by Nakamura and Otsuka. 

Some efforts were made in order to obtain good enantioselectivities in the epoxidation of simple olefins using methyltrioxorhenium (MTO), urea hydrogen peroxide (UHP) and six different chiral non racemic 2-substituted pyridine ligands, some of which are novel UHP was chosen as the hydrogen peroxide source in order to avoid unfavourable competition from water for vacant sites on the metal. However, poor enantioselectivity was reached (3-12% ee). 

The present system was applied to the enantioselective epoxidations of various simple olefines. 1,2-Dihydronaphthalenes which contained no function groups were converted into the corresponding optically active epoxides in good yields with good enantioselectivities (52-72% ee. Entries 1-4). The enantioselective aerobic epoxidation of l,2-benzo-l,3-... 

The enantioselective hydrocyanation of alkenes has the potential to serve as an efficient method to generate optically active nitriles, as well as amides, esters, and amines after functional group interconversions of the nitrile group. As in asymmetric hydroformylation, asymmetric hydrocyanation requires control of both regiochemistry and stereochemistry because simple olefins tend to generate achiral terminal nitrile products. The hydrocyanation of norbomene will give a single constitutional isomer and was studied initially. However, modest enantioselectivities were obtained, and the synthetic value is limited. ... 

Iridium-Phosphinooxazoline Catalysts. Asymmetric hydrogenation of simple olefins with chiral Rh or Ru catalysts generally shows low reactivity and unsatisfactory enantioselectivity (198,248,249). However, several unfunctionalized olefins can be hydrogenated in high yields and excellent enantioselectivity by using iridium catalysts with chiral phosphinooxazoline ligands (60,186,187, 189,191-194,250) (Fig. 56). To avoid catalyst deactivation, the extremely weakly coordinating anion tetrakis[3,5-bis(trifiuoromethyl)phenyl]borate has to be used (182,251). 

The origins of enantioselection in the Ir-catalyzed asymmetric hydrogenation of simple olefins are now reasonably well understood as a result of intensive experimental and computational studies. This helps in development of new effective catalysts and synthetic methods. Similar progress is wanted for the recently discovered highly effective Ir-catalyzed asymmetric hydrogenations of aromatic heterocycles, ketones, and keto-esters. 

Methyltrioxorhenium (MTO) has perhaps been the most successful homogeneous catalyst for the epoxidation of simple olefins with hydrogen peroxide, however, attempts to modify the catalyst to achieve enantioselective epoxidation or anchoring to a solid support have met with only moderate success, probably because... 

Comito, R. J., Finelli, F. G., MacMillan, D. W. C. (2013). Enantioselective intramolecular aldehyde a-alkylation with simple olefins direct access to homo-ene products. Journal of American Chemical Society, 135, 9538-9361. 

Brown s discovery of the hydroboration reaction in the 1950s opened new avenues for the selective functionalization of olefins. The recognition that acyclic olefins can have well-defined conformational biases allowed the development of diastereoselective, substrate-controlled processes and the advent of the principles of acyclic stereocontrol. Given the central role the hydroboration of olefins has played, it is hardly surprising that the earliest examples in this field involve such transformations. For the organic chemist, diastereoselective and enantioselective hydrometalation reactions have rapidly become an indispensable tool equally useful in simple olefin functionalizations and in the stereoselective construction of highly complex molecules. 

---