Double bonds cleaving reaction

One of the most useful and widely used applications in the synthesis of natural product derivatives relies on the efficient photoaddition of RS-H onto a double bond (a reaction known as thiol-ene coupling) [55], The reaction exploits the weakness of the S—H bond that can be cleaved homolytically under irradiation (atca. 254nm). The electrophilic sulfur-centered radical attacks a nucleophilic double bond, thus starting a radical chain reaction. 

The cyclic intermediate, called an osmate ester, is not isolated instead, the osmium-oxygen bonds are cleaved by using a reagent such as sodium sulfite, Na2S03, resulting in the formation of a 1,2-diol. (The mechanistic details of the cleavage step need not concern us.) Because both the electrophilic and nucleophilic oxygens are attached to the same metal atom, both are delivered from the same side of the plane of the double bond—the reaction is a syn addition. 

A widely applied strategy for the synthesis of various difunctionalized organic molecules, e.g. diols, dialdehydes, etc., relies on the oxidative cleavage of olelinic double bonds. Besides transition metal catalysis for asymmetric synthesis, periodate oxidation and ozonolysis are the standard tools for oxidative bond cleaving reactions. For economic and safety reasons, technically applicable alternatives to osmium-based chemistry and ozonolysis are of great interest. 

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

Suzuki et al. reported a unique C-C bond cleaving reaction of cyclopentadiene with a triruthenium cluster 82 (Scheme 7.31) [46]. One of the ruthenium centers inserted into the C(sp )-C(sp ) bond and the remaining two were coordinated with the C=C double bonds to afford ruthenacyclohexadiene 83. Subsequent rearrangement furnished 2-methylruthenacylclohexadiene 84. The triruthenium cluster 82 also cleaves a C-C bond of pent-l-ene [47]. 

The reaction of olefins with ozone constitutes an important method of cleaving carbon-carbon double bonds. " This reaction is a useful degradative tool and also finds some use in synthesis. Recent years have seen the application of detailed low-temperature spectroscopic techniques to the study of the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have put early mechanistic ideas on a firmer basis and have elaborated many additional details of the reaction mechanism. ... 

Cyclopropane rings are opened hydrogenolytically, e.g., over platinum on platinum dioxide (Adam s catalyst) in acetic acid at 2 - 4 bars hydrogen pressure. The bond, which is best accessible to the catalyst and most activated by conjugated substituents, is cleaved selectively (W.J. Irwin, 1968 R.L. Augustine, 1976). Synthetically this reaction is useful as a means to hydromethylate C—C double bonds via carbenoid addition (see p. 74f. Z. Majerski, 1968 C.W. Woodworth, 1968). 

Alkenes are cleaved to carbonyl compounds by ozonolysis This reaction IS useful both for synthesis (preparation of aldehydes ketones or car boxyhc acids) and analysis When applied to analysis the carbonyl com pounds are isolated and identified allowing the substituents attached to the double bond to be deduced... 

Bleaching and decolorization can occur by destroying one or more of the double bonds in the conjugated chain, by cleaving the conjugated chain, or by oxidation of one of the other moieties in the conjugated chain. The result of any one of the three reactions is an increase in the energy gap between the... 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

Epoxides are normally hydrogenated in preference to saturated ketones but double bonds are usually reduced under these conditions. It is possible in some cases to selectively cleave an epoxide without saturating double bonds by the use of the deactivated catalysts recommended for the partial reduction of acetylenes (see section IV) or by the addition of silver nitrate to the palladium-catalyzed reaction mixture. " ... 

The biological activity of calicheamicin 4 (simplified structure) is based on the ability to damage DNA. At the reaction site, initially the distance between the triple bonds is diminished by an addition reaction of a sulfur nucleophile to the enone carbon-carbon double bond, whereupon the Bergman cyclization takes place leading to the benzenoid diradical 5, which is capable of cleaving double-stranded DNA." ... 

The carbon-carbon double bond of an alkene 8 can be cleaved oxidatively, by a dihydroxylation reaction-glycol cleavage sequence ... 

The reactions of olefins with peracids to form epoxides allows for the selective oxidation of carbon-carbon double bonds in the presence of other functional groups which may be subject to oxidation (for example, hydroxyl groups). The epoxides that result are easily cleaved by strong acids to diols or half-esters of diols and are therefore useful intermediates in the synthesis of polyfunctional compounds. 

In all the alkene addition reactions we ve seen thus far, the carbon-carbon double bond has been converted into a single bond but the carbon skeleton has been left intact. There are, however, powerful oxidizing reagents that will cleave C=C bonds and produce two carbonyl-containing fragments. 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

In an effort to make productive use of the undesired C-13 epimer, 100-/ , a process was developed to convert it into the desired isomer 100. To this end, reaction of the lactone enolate derived from 100-) with phenylselenenyl bromide produces an a-selenated lactone which can subsequently be converted to a,) -unsaturated lactone 148 through oxidative syn elimination (91 % overall yield). Interestingly, when 148 is treated sequentially with lithium bis(trimethylsilyl)amide and methanol, the double bond of the unsaturated lactone is shifted, the lactone ring is cleaved, and ) ,y-unsaturated methyl ester alcohol 149 is formed in 94% yield. In light of the constitution of compound 149, we were hopeful that a hydroxyl-directed hydrogenation52 of the trisubstituted double bond might proceed diastereoselectively in the desired direction In the event, however, hydrogenation of 149 in the presence of [Ir(COD)(py)P(Cy)3](PF6)53 produces an equimolar mixture of C-13 epimers in 80 % yield. Sequential methyl ester saponification and lactonization reactions then furnish a separable 1 1 mixture of lactones 100 and 100-) (72% overall yield from 149). 

A Lewis acid is involved in the reaction media when RCu BF3 or R3Al is used to cleave an acetal or ketal framework, and the resulting enol ether contains a E double bond. This is quite reasonable, since the overall reaction proceeds in an anti-SN2 manner. When a v>- -SN2 process is involved, the formation of products containing a Z double bond is observed60 (Table 2). The reaction of alkyllithium reagents with a./J-ethylcnic acetals and ketals proceeds in a. H7i-SN2 manner without assistance of Lewis acids, giving mainly the Z-products61-63. 

---