Gem-Dibromides

Treatment with hydrazine to give the hydrazone and then CuBr2/f-BuOLi generated the gem-dibromide. " Oximes gives gem-dichlorides upon treatment with chlorine and BF3-OEt2, and then HCl. Some dithianes can be converted to gem-... 

Titanium-based reagents generated by the reduction of gem-dihalides with low-valent metal species are widely used for the alkylidenation of carbonyl compounds (Scheme 14.28). As in the case of methylidenation, the system gem-dibromide/TiCl4/Zn/TMEDA... 

For an indirect method of converting ketones to gem-dibromides, see Napolitano Fiaschi Mastrorilli Synthesis 1986, 122. 

Tributyltin hydride reduces selectively bromocyclopropanes without affecting the triple bond (equation 167)231. The reduction of gem-dibromides is stepwise, which enables convenient access to the monobromoacetylenes and further, via elimination of HBr, to 3-(cyclopropylethynyl)cyclopropene. 

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

Phenols are transformed into aryl chlorides by PhPCl4779. Aromatic and aliphatic aldehydes give, in high yields, gem dibromides with combination of triphenylphosphite and bromine780. Brominative deoxygenation of ketones is achieved by 2,2,2-tribromo-l,2,3-benzodioxaphosphone781. Allylic alcohols are transformed into allylic iodides by reaction with P2I4782. 

Vinylidenes have been transferred from a variety of precursors to olefins to produce methylenecyclopropanes . Because of ready intramolecular hydrogen shifts to give terminal acetylenes, the addition of vinylidene to olefins is rather limited to 2,2-disubstituted species. The methodologies so far developed include (1) gem-dibromides, 7, with MeLi, (2) vinyl halides " or vinyl triflates, 8 " with r-BuOK, (3) the fluoride ion promoted decomposition of vinylsilanes, 92 2,243 4 thermolysis of mercuric derivatives, 10, at 250 (5) decomposition of vinylazo compounds, 11, at 25 (6) the alkaline... 

The gem-dibromide, MejSiCHBrj, reacts with a Zn-Cu couple in THF at 60°C to give the gem-bis(bromo)zinc compound, Me3SiCH(ZnBr)2. 

In certain cases, dimsylsodium (24) will undergo selective nucleophilic substitution with bromides thus, v/c-dibromides (29) and gem-dibromides (30) are preferentially debrominated to the products (31) and (32) (Scheme 13). 

The reactions of [W(N2)2(diphos)2] with RBr are clearly catalyzed by visible light. The homolytic fission of the R— Br bond that takes place at the metal center is preceded by the loss of one N2 molecule. The resulting C—bond is formed by an alkyl radical attack on the remaining coordinated dinitrogen. Product distribution in these photocatalyzed reactions depends on the solvent and the stability of the free radical. This mechanism is strongly supported by flash photolysis experiments. When gem-dibromides are used in the photo-catalyzed reaction, diazoalkanes are produced. With CH2Br2 for example, the diazomethane complex [WBr(diphos)2(N2CH2)]Br is obtained. More recently it has been shown that some of the diazoalkanes do not react with protonic acids, but that the unique carbon atom is attacked by nucleophiles such as MeLi to yield diazenido complexes. ... 

A drawback of the Tebbe and related reagents is that they are generally suitable only for methylenation and do not permit the formation of higher alkyl analogues. However, the alkenylation of esters (or amides) has been found possible using the Oshima-Lombardo conditions in the presence of TMEDA (tetramethylethylene-diamine) (2.106). This chemistry requires the prior formation of the alkyl gem-dibromide and a more-convenient method, using a dithioacetal, has been reported (2.107). ... 

Transition metal ketene complexes have been derived from various metals through the study of CO reduction relating to the Fischer-Tropsch synthesis. The old preparative method for the ketene-metal complex is carbonylation of carbene complex, in which gem-dibromide is used as a common source of carbene. The reaction is carried out in the presence of Zn powder as a reducing agent. Among the examined metals such as Pd, Ni, Pt, and Co, Pd catalyst is shown to be inferior to the other metals, especially Co complex. Some typical examples using Pd catalyst are summarized in Table... 

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

The AgNOs-mediated debromination of benzylic dibromides in hot aqueous ethanol affords benzaldehydes in high yields. However, poor to moderate yields of ketones are obtained from the corresponding gem-dibromides.Silver nitrate is used as the HI scavenger in the synthesis of multisubstituted pyrroles from the condensation of a-iodoketones, primary amines, and -dicarbonyl compounds under high-speed vibration milling conditions. ... 

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