1.1- diiodo-l-alkenes

A synthesis of potentially useful 1-iodo-l-bromo-l-alkenes has thus been developed that offers complete stereocontrol. These methods have also proven to be highly efficient for the preparation of 1,1-diiodo-l-alkenes and 1,1-dibromo-l-alkenes, which are very useful synthetic intermediates (Scheme 7.26) [155]. 

Soc. Perkin Trans. 11998, 2413-2423. Barluenga, J. Rodriguez, M. A. Campos, P. J. Asensio, G. Synthesis of 2-functionalized 1,1-diiodo-l-alkenes. Generation and reactions of... 

Diodo-l-alkenes.1 This reagent (1) in combination with a wide variety of nucleophiles reacts with 1-iodoalkynes to afford 2-substituted 1,1-diiodo-l-alkenes (2) in 50-85% yield. The products can be converted to a variety of alkenes by reaction of both the iodine atoms.2... 

Kadota, I. et ah, A simple and practical method for the stereoselective synthesis of (Z)-l-iodo-l-alkenes from 1,1-diiodo-l-alkenes, Tetrahedron Lett., 44, 8645, 2003. 

Negishi and coworkers reported an alternative method for 1,2-disubstituted cyclobutene derivatives from the reaction of t-BuLi and 1,4-diiodo-l-alkenes (Eq. 37) [35], which are isolated from treatment of zirconacyclopentes with iodine. However, the use of t-BuLi does not tolerate other functional groups in the starting diiodides. 

A novel and highly efficient synthesis of 1,1-diiodo-, 1,1-dibromo-, and mixed (Z)- or ( )-1-iodo-l-bromo-l-alkenes using the 1,1-heterobimetallic reagents obtained by hydrozirconation of stannylacetylenes has also been described [155], The hydrozirconation and halo-genolysis steps were carried out at room temperature in THF under a nitrogen atmosphere. It is noteworthy that the developed route is compatible with various functionalities... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Dioxetanes. These unstable compounds have been isolated from the reactions of alkenes and singlet oxygen (5,487-488). Kopecky et al have reported a nonphotochemical route to 1,2-dioxetanes from (S-halo hydroperoxides, prepared by reaction in ether of an alkene, 98% HjOj, and 1,3-dibromo- or I, .l-diiodo-5,5-dimethylhydantoin (1, 208). Treatment of jS-halo hydroperoxides derived from trisubsLiluted olefins with sodium hydroxide in methanol at low lemperatures gives 1,2-dioxetanes, as formulated fortrimethyl-l,2-dioxetane (1) ... 

In attempts to circumvent these difficulties, a related but conceptually discrete tail-to-head construction protocol has recently been developed. For discussions of some intricate aspects, it is useful to distinguish two modes of oligoisoprenoid construction, that is, head-to-tail (H-to-T hereafter) and tail-to-head (T-to-H hereafter) modes, where the head and tail of the isoprene unit are defined as the disubstituted (head) and monosubstituted (tail) ends of a trisubstituted alkene moiety, respectively. Thus, the protocol developed in 1980 represents the H-to-T mode of isoprenoid construction. The recently developed T-to-H construction protocol entails one-pot homologation cycles using (E)- and (Z)-l,4-diiodo-2-methyl-l-butenes (1 and 2, respectively, in Scheme 3). More specifically, it involves (i) Pd-catalyzed... 

The iterative and convergent protocol using (E)- and/or (Z)-l,4-diiodo-2-methyl-1-butenes is efficient as well as regio- and stereoselective, requiring hardly any isomeric separation even in the synthesis of a decameric isoprenoid. It permits both iterative and convergent modes of construction of oligomeric isoprenoids in any desired ratio of the two modes of operation to best suit a given synthetic task. At any point of synthesis, either the E- or Z-trisubstituted Cs-alkene unit can be incorporated. This synthetic method promises to find many additional applications in the area of isoprenoid synthesis. 

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