Pentane, 1,5 dibromo

To a solution of hexamethyldisilane (2.5 mmol) in HMPA (CAUTION— CANCER SUSPECT AGENT) (3 ml) at 0-5 °C was added methyl lithium (2.5 mmol, 1.5 m MeLi.LiBr complex in ether) dropwise. After being stirred for 3 min, the red solution was treated with Cul (2.5 mmol) in Me2S (1 ml), the resulting black reaction mixture was stirred for 3 min. and 2,3-dibromo-propene (1 mmol) was added rapidly via a syringe. The reaction mixture was allowed to warm to room temperature, and was stirred for 1.5 h. It was then poured into pentane (25 ml) and saturated ammonium chloride solution (25 ml, buffered to pH 8 by the addition of ammonium hydroxide), and the mixture was stirred vigorously for 1 h. The aqueous phase was re-extracted with pentane, and the combined organic extracts were dried. Removal of... 

B. (3-Bromo-3,3-difluoropropyl)trimethylsilane. A 1-L, four-necked flask is equipped with a mechanical stirrer, thermometer, Claisen adapter, septum inlet, reflux condenser (the top of which is connected to a calcium chloride drying tube), and a solid addition funnel. The flask is charged with (1,3-dibromo-3,3-difluoropropyl)trimethylsilane (78.3 g, 0.25 mol), and anhydrous dimethyl sulfoxide (200 mL), and the solid addition funnel is charged with sodium borohydride (11.5 g, 0.30 mol) (Notes 7 and 8). The stirred solution is warmed to 80°C, and sodium borohydride is added at a rate sufficient to maintain a reaction temperature of 80-90°C (Note 9). Toward the end of the addition, an additional portion of dimethyl sulfoxide (200 mL) is added via syringe to lower the viscosity of the reaction mixture. After the addition is complete, the mixture is cooled in an ice-water bath, diluted with 100 mL of pentane, and cautiously quenched with 12 M hydrochloric acid until no further gas evolution occurs. The mixture is transferred to a separatory funnel and washed with three, 100-mL portions of 5% brine. The pentane extract is dried over calcium chloride and the solvent removed through a 15-cm Vigreux column. Further fractionation yields 41.5 g (72%) of 3-bromo-3,3-difluoropropyltrimethylsilane, bp 139-141 °C (Note 10). 

A. 1,3-Diacetylbicyclo[1.1.1]pentane (2). I1.1.1]Propellane is generated from 50 g (0.167 mol) of 1.1-dibromo-2,2-bis(chloromethyl)cyclopropane (Note 1) in pentane (Note 2) according to the procedure of Lynch and Dailey.3 To the solution of [1.1.1 Jpropellane, 1 (Note 3), is added 15 mL of freshly distilled 2,3-butanedione and the mixture is irradiated with a 450 W medium pressure UV lamp (Ace Glass Co, catalog no. 7825-34) at -10 5°C for 8 hr (Note 4). Solvents are evaporated on a rotary evaporator. The resulting crystalline material is washed three times with cold 2 1 pentane diethyl ether to give 16.95 g of 1,3-diacetylbicyclo[1.1.1]pentane (2) (Note 5). Another 1 g of the diketone is obtained upon concentration and crystallization of the pentane/diethyl ether rinses. Thus the total yield of 2 is 17.95 g [70% from 1,1-... 

Pentane 1.3-Dibromo-2,5.5.5-tetrachloro-1.1.2-Lrtflaoro-EI0b2, 133/1 35 (Educt)... 

Exercise 5-13 Write structures for all the configurations possible for 2,4-dibromo-pentane. Which stereoisomers are enantiomers Which are diastereomers What combination of isomers would give a racemic mixture Which isomer is achiral ... 

The second piece of evidence for the intermediacy of the dimeric peroxide, 32, stems from the low temperature ozonolysis of trans-2,3-dibromo-2-butene, 23. In contrast to the usual reactions which were carried out at ca. —35°C, the ozonolysis of 23 in pentane at —78°C yielded a solid white precipitate which exploded violently when we tried to isolate it. When the low temperature ozonolysis was carried out in methylene chloride, on the other hand, no precipitate appeared. However, when the methylene chloride solution of the ozonolysis product was allowed to warm up gradually, an exothermic reaction set in at around —50 °C, and the originally colorless solution showed the typical color of dissolved bromine. It is assumed that the explosive material is the dimeric peroxide, 32, which undergoes spontaneous debromination to form diacetyl peroxide, 33. 

Reduction of a,a -dibromo ketones. Reduction of 2,4-dibromo-2,4-dimethyl-pentane-3-one (1) with ultrasonically dispersed mercury in the presence of a ketone such as acetone leads to a 4-methylene-l,3-dioxolane (2) in about 50% yield. A similar reaction has been observed in the reduction of dibromo ketones with zinc in dimethylformamide (5, 221). 2-Oxyallyl cations such as a have been invoked as intermediates in reductions of dihalo ketones with various metals. 

Let s illustrate a stepwise procedure for finding all possible stereoisomers using 2,3-dibromo-pentane. 

Figure 1.7. Structural formula of 2-methyl-2,3-dibromopentane (a) pentane is the major chain therefore, there is a straight saturated five-carbon chain as the major backbone (b) 2-methyl- there is a methyl group on the number two carbon (c) -2,3-dibromo dibiomo means two bromine atoms, and they are on the number 2 and 3 carbons (d) the rest of the bonds are not specified therefore, they are all bonded to hydrogen thus we have 2-methyl-2,3-dibromopentane.

C6H12Br2 1,2-dibromo-3,3-dimethyl butane 640-21-1 301.65 21.161 2 8654 C6H13Br 2-bromo-3-methyl pentane 62168-41-6 186.68 9.535 2... 

C6H12Br2 1,5-dibromo-3-methyl pentane 4457-72-1 301.65 21.161 2 8657 C6H13Br 3-bronno-3-methylpentane 25346-31-0 180.95 9.167 2... 

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