1.1- Dibromoalkenes

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]. 

Alkenylstannanes react with 1,1-dibromoalkenes to give enynes.204 These reactions are thought to involve elimination of the elements of HBr prior to reductive elimination. 

When the organocuprate methodology is applied to l,l-dibromo-2-methylprop-l-ene, octamethyl[4]radialene (94) and decamethyl[5]radialene are the major products73. While this method does not offer any advantage over the Ni(0)-mediated syntheses of 94 (see above), it constitutes the only known synthesis of the permethylated [5]radialene (see Section II.C). For the sake of completeness, we mention that the Ni(0)-mediated dehy-drohalogenation/cyclotetramerization of 1,1-dibromoalkenes is not an efficient route to [4]radialenes66. 

When 1,1-dichloro- and 1,1-dibromoalkenes with different /1-substituents (R 7 R ) are submitted to a halogen-lithium exchange, the problem of stereoselectivity arises. The substituents R and R may differ in their steric demand and in their ability to chelate the lithium atom. For the former case, there is a tendency in the sense that the bulky group seems to favor lithiation in the Z position - - , as illustrated by equation 26 . 

Shortly after Negishi s original work, Oku and coworkers described reactions of triorganozincates with 1,1-dibromoalkenes 9, which gave different products upon protonolysis depending on the reaction temperature (equations 4 and 5)3. 

TABLE 1. Reactions of 1,1-dibromoalkenes with triorganozincates (equation 5)... 

Following the report on reactions of 1,1-dibromoalkenes 9 with triorganozincates, the same group reported reactions of 1,1-dibromocyclopropanes 14 (equation if. 

Electrosynthesis of Sulfur-Containing Organic Fluorine Compounds from Trifluoromethyl-Substituted 1,1-Dibromoalkenes Using a Sacrificial Sulfur-Graphite Electrode ... 

Analogous to the Wit tig olefmation, triphenylphosphine and carbon tetrabromide react to give an intermediate dibrominated phospho-nium ylide. Ylide 33 extends the chain of aldehyde 11 with elimination of triphenylphosphine oxide to give 1,1-dibromoalkene 12.17... 

Transition structures for the lithium-bromine exchange reaction of 1,1-dibromo-alkenes with methyllithium have been located by both the B3LYP and the MP2 levels of theory with the 6-31+G basis set.67 The reaction with methyllithium dimer gave similar results with lower activation energies. These calculations predict both the kinetic and the thermodynamic stereoselectivity correctly. It has been found that predominantly the sterically more constrained bromine atom of 1,1-dibromoalkenes reacted with alkyllithium (dimer) in the kinetic condition. 

RCHO - RCmCH. A new two-step method for conversion of RCHO to RC=CH involves conversion first to a 1,1-dibromoalkene, RCH=CBr2, followed by debromination. The most convenient reagent for this second step is magnesium ia refluxing THF. 

Z,E)-2-Bromo-l,3-dienes. 1,1-Dibromoalkenes and E-vinylboronic acids couple to (Z,E)-2-bromo-1,3-dienes in the presence of this Pd(0) catalyst and TlOH as base, conditions developed earlier for coupling of vinyl bromides with vinyl-... 

Application of the Corey-Fuchs olefination to thioxanthone affords the 1,1-dibromoalkene 366 from which the 1,1-bis(trimethylsilylethynyl)alkene can be obtained by a double Sonogashira coupling reaction. Desilylation yields the 1,1-diethynylalkene (Scheme 86) <2004JA3108>. 

Alkylation of a 1,1-dibromoalkene with a zincate synthesis of 1-methoxy-4,4-dimethyl-2-phenyl-2-pentene (Structure 8)5b... 

Grandjean, D. Pale, P., Selective debromination of 1,1 -dibromoalkenes a new access to di- ortrisubsti-tuted alkenes. Tetrahedron Letters (1993), 34(7), 1155-8. 

Lithium trialkylzincates (8, 515-516).4 1,1-Dibromoalkenes react with LiR3Zn by a Br/Zn exchange at - 85° to form a 1-bromoalkenylzincate (a), which undergoes. in intramolecular alkylation in the presence of acetic acid at 0°. The alkylation involves at least partial inversion at the carbenoid carbon. 

Fig. 11. 29. Aldehyde — alkyne chain elongation via [1 -rearrangement of a vinyl carbenoid (Corey-Fuchs procedure). The aldehyde and phosphonium ylide A generated in situ undergo a Wittig reaction and form the 1,1-dibromoalkene. In the second stage, the dibromoalkene is reacted with two equivalents of n-BuLi and the vinyl carbenoid C is formed. The carbenoid undergoes H migration to form the alkyne B. The alkyne B reacts immediately with the second equivalent of w-BuLi to give the lithium acetylide.

Dibromoalkenes (R R C = CBr2) have been prepared analogously by Appel reaction of the requisite ketone with carbon tetrabromide/triphenylphosphane. - ... 

A rather unusual ylide, 85, can be readily generated by the interaction of carbon tetrabromide with triphenylphosphine. Reaction of 85 with aldehydes furnishes 1,1-dibromoalkenes 86a. The latter compounds, under the action of BuLi, are transformed into lithium acetylides 86b and ultimately into substituted acetylenes 86c. This sequence is widely used as a reliable method for the preparation of various acetylenes from readily available aldehydes. 

Geminal substituted dibromoalkenes can be prepared by the alkylation of dibromomethyllithium with a-chloroalkyl methyl ethers. Deprotonation of the alkylation products results in the elimination of methanol and the formation of the corresponding 1,1-dibromoalkenes. Despite the lower acidity of 1 -bromo-1 -chloroalkyllithiums relative to the dibromo analogs they exhibit similar nucleophilic proper-ties. Alkyl dichloroacetates can also be deprotonated with lithium diethylamide and alkylated with a range of alkyl halides. ... 

When an organotin compound is the nucleophile, the coupling is called Stille coupling after J. K. Stille of Colorado State University, the inventor.26 In chapter 15 we saw how organo-tin compounds are sources of radicals, but with palladium catalysis the reaction changes. We shall start with an example which may look trivial but is very important for this chapter. Reaction of readily prepared 1,1-dibromoalkenes 184 with tributyltin hydride under Pd(0) catalysis gives Z-l-bromoalkenes 185 with high stereoselectivity.27... 

Chain extension of aldehydes to 1,1-dibromoalkenes followed by elimination to alkynes by means of BuLi or RMgX. 

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