Alkenes selective with oxygen

Compounds lb and 2b were the Urst fluorinated ligands tested in Mn-catalyzed alkene epoxidation [5,6]. The biphasic Uquid system perfluorooc-tane/dichloromethane led to excellent activity and enantioselectivity (90% ee) in the epoxidation of indene with oxygen and pivalaldehyde (Scheme 1, Table 1). In addition, the fluorous solution of the catalyst was reused once and showed the same activity and selectivity. This represents a considerable improvement over the behavior in the homogeneous phase, where the used catalyst was bleached and reuse was impossible. Unfortunately, indene was the only suitable substrate for this system, which failed to epoxidize other alkenes (such as styrene or 1,2-dihydronaphthalene) with high enantioselectivity. The system was also strongly dependent on the oxidant and only 71% ee was obtained in the epoxidation of indene with mCPBA at - 50 °C. 

It should be stated here that the discussion to be followed would remain the same if the selectivity-determining step is the reaction of an adsorbed alkene formed from a surface alkyl. In that case, dehydrogenation products are observed if the alkene desorbs, and oxygen-containing products are observed if it further reacts with a lattice oxygen. (See Scheme III.)... 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

Oxygen may also be used as the oxidant for epoxidation. For example, a selective epoxidation, in the absence of metal ions, has been performed in excellent yield by treatment of alkenes with oxygen in the presence of a large excess of benzaldehyde36. Photooxygenation of alkenes, in the presence of transition metal catalysts, is a very general and synthetically useful reaction which leads to hydroxy-epoxidation (equation 4)37-39. 

Terminal alkenes can be selectively oxidized to aldehydes by reaction with oxygen, using a palladium-copper catalyst in tertiary butanol (equation 35)160. This reaction is contrary to the normal oxidation process which yields a ketone as the major product. The palladium(II) oxidation of terminal alkenes to give methyl ketones is known as the Wacker process. It is a very well established reaction in both laboratory and industrial synthesis161162. The Wacker oxidation of alkenes has been used in the key step in the synthesis of the male sex pheromone of Hylotrupes bajulus (equation 36)163. 

In Tables 10 to 12 we show the heats of formation calculated by the various methods, together with their deviation from the experimentally observed values for alkanes and cycloalkanes, alkenes and cydoalkenes, and acetylenes and aromatic compounds. Table 13 shows a comparison of heats of formation of hydrocarbon radicals calculated by the MINDO methods. Finally, in Tables 14 and 15 we show the results of MINDO/1 calculations on a selection of oxygen- and nitrogen-containing compounds. 

However, the chemical properties of an alkene are dramatically affected by the presence of the double bond. Recall that a carbon-carbon pi bond is considerably weaker than a carbon-carbon or carbon-hydrogen sigma bond. It is possible to selectively cause a reaction to occur at a pi bond under conditions that do not affect the sigma bonds. The pi bond is the weak spot of an alkene, and it is there that most chemical reactions occur. This is why unsaturated fats spoil more readily than saturated fats. Their pi bonds provide a place for reaction with oxygen to occur, which leads to spoilage. 

Although this reaction is general for ester-stabilized Wittig reagents, (fcHmylmethylene)triphenylphos-phorane was reacted with a ppmose (125) to form the expected ( )-alkene (126 Scheme 23). Reduction of (126) and (127) supplied ( )- and (Z)-allylic alcohols selectively. Precise rules for the prediction of ( )- and (2J)-selectivity with a-oxygenated substrates are difficult to formulate. 

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