Gem-dibromoalkene

Reduction of gem-dibromides. Triethyl phosphite and triethylamine reduces gent-dibromocyclopropanes and gem-dibromoalkenes to the corresponding monobromides. 

Reduction of bromides. gem-Dibromocyclopropanes and gem-dibromoalkenes are reduced to the monobromides by (C2H50)2P(0)H and N(C2H5)3.1 Under these conditions gem-bromochlorocyclopropanes are reduced to chlorocyclopropanes.2 l,l-Dibromo-2-tri-methylsilyloxycyclopropanes or a-bromo-a,p-enones are reduced in this way to 0,y-enones.3 Other examples indicate that only activated halogen atoms are subject to this reduction. 

Reduction of gem-dibromoalkenes to monobromoalkenes with diethyl phosphite and triethylamine. 

Example 2.16 Synthesis of Ynol Ethers from gem-Dibromoalkenes [193]. 

The conversion of alcohols into ynol ethers has been achieved [188, 189]. This methodology requires the use of powerful bases thus, the potential for functional group incompatibility needs to be addressed prior to implementation of this approach Ynol ethers have been generated from gem-dibromoalkenes through initial formation of a brominated vinyl ether followed by elimination [193] Terminal aUcynes are convenient starting points for this chemistry and several excellent approaches are known [187, 191, 192]... 

Gem-dibromoalkenes can be converted into vinylphosphine oxides (Scheme 4.296) [462]. Nickel bromide was used as the catalyst in this reaction along with 2,2-bipyridine as the solubilizing/stabilizing ligand. As with many of the other systems described in this section, the chemistry was highly regioselective however, the process consistently generated mixtures of stereoisomers. The chemistry predominately formed the fi-isomer however, the Z-isomer was still formed in most cases. 

Aryl substituted alkynylphosphonates (322) have been synthesised in the metal and ligand free, Cs2C03.promoted reaction of gem-dibromoalkenes with triethyl phosphite. In a practical, one-pot synthesis valuable alkynylphosphonates (322) were obtained in 67-97% yields (Scheme 89). ... 

Scheme 1-4. Stereocontrolled debromination of gem-dibromoalkenes and ge/w-dibromocyclopropanes by halogen/metal permutation.

The benzoic acid derivative 457 is formed by the carbonylation of iodoben-zene in aqueous DMF (1 1) without using a phosphine ligand at room temperature and 1 atm[311]. As optimum conditions for the technical synthesis of the anthranilic acid derivative 458, it has been found that A-acetyl protection, which has a chelating effect, is important[312]. Phase-transfer catalysis is combined with the Pd-catalyzed carbonylation of halides[3l3]. Carbonylation of 1,1-dibromoalkenes in the presence of a phase-transfer catalyst gives the gem-inal dicarboxylic acid 459. Use of a polar solvent is important[314]. Interestingly, addition of trimethylsilyl chloride (2 equiv.) increased yield of the lactone 460 remarkabiy[3l5]. Formate esters as a CO source and NaOR are used for the carbonylation of aryl iodides under a nitrogen atmosphere without using CO[316]. Chlorobenzene coordinated by Cr(CO)j is carbonylated with ethyl formate[3l7]. 

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