Olefins carbon-oxygen bond formation

A criss-cross flexibility of O2 in such a complex was reported" previously by Bartlett and coworkers. Carbon-oxygen bond formation and C—H bond breaking would then follow such an intermediate. Since the formation of the C—Oa bond requires a closer approach to the olefin than the C—H bond breaking, a transition state favouring C—Oa... 

Nitrone-Nitrile Oxide Cycloaddition. Unsaturated sugars have been used for the simultaneous formation of carbon-carbon and carbon-oxygen bonds in a cir-relation. One of the best ways to achieve this transformation is the cycloaddition of nitrone or nitrile oxides. The cycloaddition of nitrones with olefins has been reviewed [133]. The regioselec-tivity is almost complete when using activated double bonds, such as enone, enelactone (see compound 98, Scheme 35), or esters. 

Abstract The formation of carbon-oxygen bond upon addition of O-nucleophUes to unsaturated molecules is very attractive as it represents an atom economical strategy to prepare a variety of saturated compounds from olefins and vinylic derivatives from aUcynes. Group 8 metals, especially ruthenium have provided an important contribution in this field. We report here on iron- and ruthenium-catalyzed addition of nucleophiles to unsaturated systems. As additions to alkenes are still scarce with these metals and the use of iron catalysts is limited, the main part of the chapter is dedicated to addition of carbamates, carboxylic acids, alcohols and water to triple bonds with ruthenium catalysts. 

The initial bond formation between the -> ir excited carbonyl compound and an alkene can occur by interaction of the half-filled n -orbital of the [I CO] with the ir-system of the alkene, in a sense transferring a tt-electron to the -orbital and making a bond between an alkene carbon and the carbonyl oxygen. In this process (common for electron rich olefins) the plane formed by the alkene carbons and their four substituents is perpendicular to the plane of the carbonyl groups and its two substituents (Figure 1). In the... 

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

---