1-Alkenes, 1-iodo-trans

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

Admixture of BAIB with two equivalents of trimethylsilyl isothiocyanate in dichloromethane leads to the rapid formation of thiocyanogen and iodoben-zene, presumably via [bis(thiocyanato)iodo]benzene (Scheme 10) [32]. The addition of alkenes to such solutions affords vzcmaZ-dithiocyanatoalkanes. Cyclohexene and its 1-methyl analog were converted exclusively to the transadducts under these conditions, while dihydropyran gave a 1 1 mixture of cis-and trans-isomers. 

Diphenyl diselenide is an especially useful co-reagent with [bis(acetoxy)-iodo]benzene. For example, the BAIB/PhSeSePh (2 1) combination has been employed for trans, Markovnikov additions of PhSeOAc and PhSeOH to alkenes [35]. Such formal additions appear to be regulated by seleniranium intermediates, and were extended to intramolecular cyclizations of olefinic alcohols, carboxylic acids, and / -dicarbonyl compounds (Scheme 12). 

The aliphatic iodane C3F7I(02CCF3)2 was reactive towards alkenes for example, with 1-hexene it gave 1,2-bis-trifluoroacetoxy-hexane (68%) with cyclohexene the quantitative formation of trans-1 -iodo-2-trifluoroacetoxy adduct occurred, at 0°C [59]. 

Iodine azide, generally made in situ from ICl and NaNa in MeCN at 0 C, adds readily to alkenes by a similar heterolytic mechanism to INCO. Whereas the trans stereochemistry is generally well established,the regiochemistry of the adduct with 1-phenylcyclohexene has been queried recently it was originally formulated as the l-azido-2-iodo compound (Scheme 86), but base treatment was subsequently shown to yield what appeared to be 6-azido-l-phenylcyclohexene, which would have arisen from the l-iodo-2-azido isomer. However, it has very recently teen shown by 3(X) MHz NMR that the elimination product is in fact 3-azido-l-phenylcyclohexene, derived ultimately (Scheme 86) from the originally proposed l-azido-2-iodo- structure. 

Decarboxylation precedes dehydrohalogenation, however, as noted by analysis of the gas formed in the preparation of 2-butene. Decarboxylation occurs at 20-30° in sodium carbonate solution. Butene is then evolved at higher temperatures. The reaction is important in the preparation of cis and tra s-2-alkenes from cis- and trans-alkylacrylic acids, RCH= =CR COjH, respectively. ° Either the )3-iodo or /3-bromo acids prepared by the addition of hydrogen halide are suitable sources. 

Because detailed mechanistic information is not available, a tentative mechanism is proposed here. It is assumed that iodine first adds to the double bond to form an iodonium cation in a process similar to the bromination of alkene, then nucleophilic attack from acetate leads to the formation of the ra/25 -o -iodoacetate. In the presence of silver acetate, a-iodo leaves to give a dioxoanyl carbocation, which is attacked by the second acetate anion to give 1,2-diacetate. Upon hydrolysis or LiAlH4 reduction, diacetate is transformed into trans-l,2-diol. The displayed mechanism is similar to the one proposed by Sudalai. ... 

Selective hydrogenation of a -unsaturated carbonyl compounds can be carried out by reduction with iron pentacarbonyl and a small amount of base in moist solvents. The method is applicable to oc -unsaturated aldehydes, ketones, esters, and lactones with negligible over-reduction of the carbonyl group and is susceptible to the steric environment of the olefin. Spectroscopic evidence suggests that solutions of equimolecular amounts of iodine and thiocyanogen contain an appreciable concentration of iodine thiocyanate. Addition of alkenes results in tran.r-addition to yield jS-iodo-thiocyanates which in base suffer rapid hydrolysis of the thiocyanate followed by ring closure to the episulphide (516). As a synthetic procedure this does not appear to be applicable to acyclic olefins. ... 

It has been reported that t-butyl hypoiodite adds to alkenes via a bridged iodonium ion intermediate in a trans manner in benzene to give P-iodo-ethers in the dark. Unlike the addition in nitrite photolysis, thermal ionic additions may take place in hypoiodite photolysis. The additions in the steroids, even under photolytic conditions, can be rationahzed in terms of an ionic trans-addition of alkyl hypoiodites via bridged iodonium ions, although a homolytic mechanism cannot be excluded. 

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