September 2011 - Career Point

oxymercuration - demercuration of 3, 3-dimethyl-1-butene is a striking example illustrating this feature.It is in direct contrast to the hydration of 3, 3-dimethyl-1-butene.CH3CH 3C| − CH = CH|CH33, 3-Dimethyl-1-butene2(1)Hg(OAc) 2 / THF − H2O⎯⎯⎯⎯⎯⎯⎯⎯→(2) NaBH4, OHCH3|CH3C — CHCH| |CH OH3,3-Dimethyl-2-butanol (94%)Analysis of the mixture of products by gaschromatography failed to reveal the presence of any2, 3-dimethyl-2-butanol. The acid-catalyzedhydration of 3, 3-dimethyl-1-butene, by contrast,gives 2, 3-dimethyl-2-butanol as the major product.A mechanism that accounts for the orientation ofaddition in the oxymercuration stage, and one thatalso explains the general lack of accompanyingrearrangements. Central to this mechanism is anelectrophilic attack by the mercury species, H + gOAc ,at the less substituted carbon of the double bond (i.e.,at the carbon atom that bears the greater number ofhydrogen atoms), and the formation of a bridgedintermediate.Hydroboration : Synthesis of AlkylboranesHydroboration of an alkene is the starting point for anumber of useful synthetic procedures, including theanti-Markovnikov syn hydration procedure.Hydroboration was discovered by Herbert C. Brown,and it can be represented in its simplest terms asfollows :C = C + H — B hydroboration — C — C —AlkeneBoron hydride33HBAlkylboraneHydroboration can be accomplished with diborane(B 2 H 6 ), which is a gaseous dimer of borane (BH 3 ), ormore conveniently with a reagent prepared bydissolving diborane in THF. When diborane isintroduced to THF, it reacts to form a Lewis acid–base complex of borane (the Lewis acid) and THF.The complex is represented as BH 3 : THF.B 2 H 6 + 2 ODiboraneTHF(tetrahydrofuran)H– +2H – B – OHBH 3 : THFSolutions containing the BH 3 : THF complex can beobtained commercially. Hydroboration reactions areusually carried out in ethers : either in diethyl ether(CH 3 CH 2 ) 2 O, or in some higher molecular weightether such as “diglyme” [(CH 3 OCH 2 CH 2 ) 2 O,diethylene glycol dimethyl ether]. Great care must beused in handling diborane and alkylboranes becausethey ignite spontaneously in air (with a green flame).The solution of BH 3 : THF must be used in an inertatmosphere (e.g., argon or nitrogen) and with care.Stereochemistry of Hydroboration :H B+HH — BHWecan see the results of a syn addition in our examplesinvolving the hydroboration of1-methylcyclopentene ring :Space Quick Facts1. There has only been one satellite destroyed by ameteor, it was the European Space Agency’sOlympus in 1993.2. The International Space Station orbits at 248miles above the Earth.3. The Earth orbits the Sun at 66,700mph.4. Venus spins in the opposite direction comparedto the Earth and most other planets. This meansthat the Sun rises in the West and sets in theEast.5. The Moon is moving away from the Earth atabout 34cm per year.6. The Sun, composed mostly of helium andhydrogen, has a surface temperature of 6000degrees Celsius.7. A manned rocket reaches the moon in less timethan it took a stagecoach to travel the length ofEngland.8. The nearest known black hole is 1,600 lightyears (10 quadrillion miles/16 quadrillionkilometers) away.XtraEdge for IIT-JEE 35 SEPTEMBER 2011