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Date: 5-3-2017
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Date: 28-10-2018
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Date: 6-1-2019
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Group 13: Boron
Organoboranes of type R3B can be prepared by reaction 1.1, or by the hydroboration reaction mentioned above.
(1.1)
Trialkylboranes are monomeric and inert towards water, but are pyrophoric; the triaryl compounds are less reactive. Both sets of compounds contain planar 3-coordinate B and act as Lewis acids towards amines and carbanions. Reaction 1.2 shows an important example; sodium tetraphenylborate is water-soluble but salts of larger monopositive cations (e.g. K) are insoluble. This makes Na[BPh4] useful in the precipitation of large metal ions.
(1.2)
Compounds of the types R2BCl and RBCl2 can be prepared by transmetallation reactions (e.g. equation 1.3) and are synthetically useful (e.g. reaction 1.4).
(1.3)
(1.4)
The bonding in R2B(µ-H)2BR2 can be described in a similar manner to that in B2H6. An important member of this family is 1.1, commonly known as 9-BBN, which is used for the regioselective reduction of ketones, aldehydes, alkynes and nitriles.
(1.1)
By using bulky organic substituents (e.g. mesityl = 2,4,6-Me3C6H2), it is possible to stabilize compounds of type R2B_BR2. These should be contrasted with X2B_BX2 where X = halogen or NR2 in which there is X → B π- overlap. Two-electron reduction of R2B_BR2 gives [R2B=BR2]2-, an isoelectronic analogue of an alkene. The planar B2C4 framework has been confirmed by X-ray diffraction for Li2[B2(2,4,6-Me3C6H2)3Ph], although there is significant interaction between the B=B unit and two Li centres. The shortening of the B_B bond on going from B2(2,4,6-Me3C6H2)3Ph (171pm) to [B2(2,4,6-Me3C6H2)3Ph]2- (163 pm) is less than might be expected and this observation is attributed to the large Coulombic repulsion between the two B- centres.
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