Carbon-selenium bonds formation

Diselenolylium ions have been prepared by acid induced cyclization and dehydration of oxo esters of AT,AT-dialkyldiselenocarbamates (75JOC746, 77CC505, 80CC866, 80CC867). As shown in Scheme 14, protonation of the carbonyl oxygen results in carbon-selenium bond formation by electrophilic attack on selenium. Acid induced dehydration generates the... 

Little C—Se bond formation by palladium catalysis has been reported, although these reactions should be feasible considering the ability of palladium complexes to catalyze C—S, C—P, and C—As bond formation. However, nickel complexes have been used for carbon-selenium bond formation. In 1985, Cristau and co-workers used bis(bipyridyl)nickel(ll) bromide to form aryl selenides from aryl halides and sodium benzeneselenoate. ... 

Carbon-Oxygen, -Sulfur and -Selenium Bond Formation (Table 1)... 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Addition Reactions with Formation of Carbon-Suifur or Carbon-Selenium Bonds... 

FORMATION OF CARBON-SELENIUM BONDS FROM ADDITIONS TO -ir-BONDS... 

A variety of synthetic methods for preparing selenocarbonyl compounds have been developed, and recent reviews disclosed details of each compound in Scheme 1 [2b, 6, 8c, 9,11]. Accordingly, the synthetic methods are classified based on the reaction patterns rather than the types of compounds in this review. In particular, attention has been paid to the step of the formation of the carbon-selenium bond. The selenium atom has generally been introduced electrophilically or nucleophilically to the organic molecules. Additionally, heterocumulenes involving selenium atoms have been used as a starting material. 

Addition Reactions with Formation of Carbon-Nitrogen Bonds Addition Reactions with Formation of Carbon-Sulfiir or Carbon-Selenium Bonds Addition Reactions with Formation of Carbon-Halogen Bonds Cleavage Reactions... 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

Selenium-containing six-membered ring heterocycles have proved to be useful catalysts in a variety of transformations. The Baylis-Hillman reaction involves the reaction of alkenes containing electron-withdrawing groups such as a,/3-unsaturated carbonyl compounds with aldehydes leading to carbon-carbon bond formation (Equation 79). The reaction is promoted by tertiary amines such as l,4-diazabicyclo[2.2.2]octane (DABCO), or tertiary phosphines and Lewis acids. Unfortunately, the Baylis-Hillman reaction is severely limited because it proceeds only very slowly <1998CC197>. Much research has been carried out in attempts to increase the rate of this reaction. 

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

Formation of Carbon-Sulfur, Carbon-Selenium and Carbon-Tellurium Bonds via Organomagnesium Compounds... 

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