2- Bromo-l,3-dienes

Use of Alkylzincs and 2-Bromo-l,3-dienes, 2-Bromo-l,3-enynes, or a-Bromostyrenes... 

SCHEME 38. Pd-catalyzed coupling reactions of 2-bromo-l, 3-dienes with or without stereoinversion... 

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-... 

Backvall et al. utilized a stereoconvergent palladium-catalyzed SN2 reaction of a-allenic acetates 39 for the synthesis of (Z, )-2-bromo-l,3-dienes 40 [29]. 

Phosphorus tribromide Bromallenes and 2-bromo-l,3-dienes from allenealcohols... 

Intermolecular cyclization of 4-dialkylamino-2-aza-l,3-diene-1,1-dicarbonitriles proceeds on treatment with hydrogen bromide in acetic acid to yield 2-bromo-3-cyanopyrazine <90HCA1210>. Tetraalkyl or -arylpyrazines are synthesized by reductive cyclization of the corresponding nitriles with titanium tetrachloride and zinc in modest to good yields (Equation (18)) <90S70l>. 

The reaction with 2-bromo-1,3-dienes to form allene products is more remarkable. Similarly, 3-bromo-3-alken-l-ynes are susceptible to transpositional substitution, forming butatriene products. In this reaction, 2,2 -bis(diphenylphosphino)-l,l -biphenyl is the added ligand. ... 

Hayashi found that methylene-7r-allylpalladium 635 can be generated from 2-bromo-1,3-diene, which is prepared by the Pd-catalyzed cross-coupling of 1,1-dibromo-l-alkene 633 with vinylzinc reagent. Thus, the reaction of l-phenyl-2-... 

To a solution of 2-bromo-l,6-diene (0.627 mmol) in acetonitrile (5 mL) in a screw-capped Pyrex bottle were added Pd(OAc)2 (5 mg, 3.5 mol%), PPhj (12.8 mg, 8 mol%), silver carbonate (207 mg, 0.75 mmol) and the respective dienophile (0.74 mmol). The solution was purged with argon, and then stirred in the sealed bottle at 90 °C for 45 min. After the mixture had been cooled down to ambient temperature, the reaction mixture was filtered through a bed of charcoal and celite, and the solvent was removed under reduced pressure. The residue was purified by chromatography on silica gel (6.0 g, column 1.7 x 6 cm). For performing the second step under high pressure, the cooled mixture was filtered directly into a Teflon tube that was sealed and subjected to 10 kbar at 25 °C for 3 d colorless oil. 

The heteroaryne 1 is trapped as the Diels-Alder adduct when 6-bromo-l,3/4, 5,2,4-trilhia-diazepine is treated with various dienes in the presence of N./V-diisopropylethylamine or sodium methoxide, e.g. trapping of 1 with furan to give 2.391... 

Chloroprene (2-chloro-l,3-butadiene 105), which is a mass-produced, inexpensive industrial material, is an excellent precursor to a variety of terminal allenes 107 [97]. The palladium-catalyzed reaction of 105 with pronucleophiles 106 in the presence of an appropriate base gave the terminal allenes 107 in good yields (Scheme 3.53). The palladium species generated from Pd2(dba)3-CHC13 and DPEphos was a good catalyst for these reactions of chloroprene. In contrast, (Z)-l-Phenyl-2-chloro-l,3-buta-diene, which is isostructural with the bromo-substrate 101, was nearly inert under these conditions. There is no substituent at the vicinal ris-position to the chloride in 105, which allows oxidative addition of the C-Cl bond in 105 to the Pd(0) species. 

If photolyzed with light of the intensity I, HBr adds to propadiene (la) in the gas phase with a rate given by v=kexp[HBr]I<). This transformation affords within the detection limit (GC) 2-bromo-l-propene (5a) as sole reaction product (Table 11.1). The conversion of methyl-substituted allenes, such as lc and If, under these conditions follows the same kinetic expression [37]. Results from competition experiments indicate that the reactivity of an allene towards HBr increases progressively with the number of methyl substituents from propadiene (la) (= 1.00) to 2,4-dimethylpenta-2,3-diene (If) (1.65). In all instances, Br addition occurs exclusively at Cp to furnish substituted allyl radicals, which were trapped in the rate determining step by HBr. 

Treatment of phosphorus trichloride with isoprene in the presence of acetone leads to 1-chlorophospholen 1-oxides (33).33 Evidence has been presented that the diene reacts first with the trichloride to produce (34), which then reacts with acetone.33 l-Bromo-3,4-dimethyl-A3-phospholen (35) does not form a spiro-compound with 2,3-dimethylbuta-l,3-diene, although the bisphospholen derivative (36) has been isolated after hydrolytic work-up.34... 

Benzene-l,4-diols are oxidized to quinones by benzyltrimethylammonium tribromide under mild conditions in almost quantitative yields [6]. With an excess of the tribromide further reaction produces the 2-bromo-l, 4-quinones. This oxidation is in contrast to the analogous reaction of phenols, which produces bromophenols (see Section 2.3). Hindered 4-methyl-phenols are oxidized to the corresponding benzyl alcohols, benzaldehydes, bromomethyl derivatives and 4-bromo-4-methylcyclo-hexa-2,5-dien-l-ones [7]. Benzylic alcohols are oxidized under neutral or basic conditions to yield the corresponding aldehydes (>70%) oxidation with an excess of the reagent produces the benzoic acids (>90%) [8],... 

An extensive review of 4 - - 3-cycloaddition reactions has been presented. The 1,3-difluorooxyallyl intermediate obtained from l-bromo-l,3-difluoropropan-2-one undergoes 4 + 3-cycloaddition with cyclopentadiene and ftiran to give difluorobicyclo[3.2.1]octenones. The use of 4 + 3-cycloaddition reactions of cyclic oxyallyls in the synthesis of natural products has been extensively studied. The intramolecular 4 + 3-cycloaddition of allylic sulfones (111) possessing a diene in the side-chain in the presence of Lewis acids yield cycloadducts (112) in good to excellent yields (Scheme 43). ... 

The products are useful precursors to 1-alkynes. The l-bromo-1,3-dienes can be converted to alkenyllithiums by tert-butyllithium (2 equiv.) at — 120° in THF/pentane. These products react with aldehydes with retention of olefin geometry. 

Reaction of 3-bromo-l,5-di-(4-tolyl)-l,5-diazapenta-l,3-diene 99 with hydroxyamine affords 2-substituted pyrazole 1-oxides 100 (1995JCS(P1)2773) (Scheme 29). 

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

Similarly, 1-halo-l-sulfonylallenes (139) have been prepared by heating in toluene at 80 °C of propargyl esters (138) via [2,3]sigmatropic rearrangement of the latter (Scheme 32) [51]. 1-Bromo-l-sulfonylallenes 139, when treated with bromine, undergo attack on central allenic carbon with formation of intermediate carbenium bromide followed by hydrogen bromide eUmination, and afford stereospecifically the 2,3-dibromo-l-sulfonyl-l,3-dienes 140. 

In vfc-dihalocyclopropanes with two different halogens, the bond opposite to the better leaving group is opened. For example, 2-bromo-l,l-dichloro-3,3-dimethylcyclo-propane undergoes rearrangement in quinoline at 160°C to give l,l-dichloro-3-methyl-buta-1,3-diene in 64% yield. ... 

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