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Abeer Zahid

Lit Assignment #2

H.C. Brown and B. Subba Rao (1) discoered the regiose!ectiit" and

stereochemistr" o boron h"drides. $t is whi!e the" were running e%&eriments to use

boron h"drides as reducing agents that it was obsered that boron attached to the a!'ene

in a er" s&eciic wa". hen &erorming the reduction o an unsaturated ester (eth"!

o!eate) with aBH*and cata!"tic A!C!+, Subba Rao witnessed that an sur&!us number o

mo!e e-uia!ents o boron h"dride were consumed 2.+/ ( s. 2.00 or the reaction o

saturated eth"! stearate) . hen this act was e%amined more thorough!" the" ound that

BH was in act adding to the a!'ene. his then became a common in a reaction that is

'nown as 3h"droboration4 toda".

Later on it was obsered that the h"drogen a!wa"s added on to the more

substituted end o the carbon. he more substituted end meant the h"drogen wou!d add

on to the carbon with the ewest attached h"drogens. his chemica! characteristic o this

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reaction, h"droboration became 'nown as the anti5ar'oni'o ru!e. he &!a" o

stereochemistr" was a!so obsered in that both h"drogen and boron added to the same

ace o the a!'ene. Subse-uent!", this act became 'nown as the s"naddition.

$n addition to these obserations, there were no rearrangements o an" 'ind.

Rearrangements did not occur een in s"stems that were strained and wou!d usua!!" be

e%&ected to rearrange. 6inene was one such mo!ecu!e that was strained and wou!d be

e%&ected to rearrange, howeer, no rearrangement was seen in &inene h"drboration.

7urthermore, &rotic so!ents do not a!!ow the reaction to &roceed as norma!.

$nact, &rotic so!ation !eads to h"drogen gas orming and borane brea'ing down. So

&rotic so!ents cannot tra& the nuc!eo&hi!ic substituents or aect the reaction without

disintegrating the borane.Another, noteworth" obseration made was that when e!ectronwithdrawing

substituents were added to the a!'ene and reduction in the se!ectiit" or the anti

5ar'oni'o &roduct was seen. ithout these e!ectronwithdrawing substituents the

se!ectiit" was much higher or the anti5ar'oni'o &roduct.

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Reerence

Brown, Herbert C., and BC Subba Rao. 8A new techni-ue or the conersion o o!eins

into organoboranes and re!ated a!coho!s.8Journal of the American Chemical Society/9,no. 21 (1:;