Boron Chemistry topic 4 Flashcards
Knowledge check:
Boron forms three bonds to other elements in neutral complexes. What shape does it adopt?
Trigonal planar
Boron is sp2 hybridized, with an empty p orbital orthogonal to the plane of the three sp2 hybrid atomic orbitals.
Knowledge check:
Which of the following would form an ate complex and hence stabilise BH3? Tick all that apply
dimethyl sulfide
pyridine
triphenylphosphine
cyclohexane
THF
dimethyl sulfide
pyridine
triphenylphosphine
THF
These contain atoms with lone pairs that are able to donate and therefore can act as lewis bases to stabilise the BH3 lewis acid.
Knowledge check:
Which would you expect to have the longer bond length, B(OH)3 or B(OH)4¯, and why?
B(OH)3, because the oxygen lone-pair can overlap with the empty p-orbital on boron.
B(OH)3, because it is overall neutral.
B(OH)4¯, because it is overall negatively charged.
B(OH)4¯, because there is no p-pi p-pi bonding.
B(OH)4¯, because there is no p-pi p-pi bonding.
p-pi p-pi bonding in B(OH)3 would reduce the B-O bond length. It would have some double bond character. In B(OH)4¯, there is no empty p orbital on B for the oxygen lone pair to overlap with, and so the bond length would be longer than in B(OH)3.
Knowledge check:
What is/are the role(s) triethanolamine in the crotylation/ allylation reaction? Tick all that are correct.
It acts as a Brønsted acid.
It cleaves the boronate ester to liberate the homoallylic alcohol.
It acts as a Brønsted base.
It acts as a ligand for boron.
It acts as a Lewis base.
It acts as a Brønsted acid.
It cleaves the boronate ester to liberate the homoallylic alcohol.
It acts as a ligand for boron.
It acts as a Lewis base.
The only option does not apply is that it acts as a Brønsted base (i.e. in this transformation it does not accept a proton, even though it is an amine). It does act as a Brønsted acid because it is acting as a proton donor i.e. the H of the alcohol comes from triethanol amine.
Knowledge check:
In the chair-like transition state for the addition of an (E)-crotylborane to an aldehyde, the methyl group on the double bond:
(pick one)
is pseudoaxial, because of the fixed double bond geometry
is pseudoaxial, to avoid a 1,2-gauche interaction with the pseudoequatorial aldehyde carbon substituent
is pseudoequatorial, because groups prefer to be equatorial on chair conformations
is pseudoequatorial, because of the fixed double bond geometry.
is pseudoequatorial, because of the fixed double bond geometry.
There is no choice where the groups attached to the alkene are on the chair transition state, it is fixed by the double bond geometry. For an E-double bond, the two hydrogens must be on opposite sides, and on adjacent carbons on a chair transition state they would therefore be both pseudoaxial, making the methyl group pseudoequatorial.
Knowledge check:
What is the major factor which determines regioselectivity in hydroboration?
Sterics- boron is bigger than hydrogen and it adds to the more accesible, less substituted end of the double bond.
Electronics are also considered important but not nearly as much as sterics.
Knowledge check:
Why is a thexylborane a good choice of borane in many transformations that undergo a carbonylation reaction?
It is sterically hindered, can be sequentially reacted with two (relatively unhindered) alkenes because of the rate of hydroboration slows down sufficiently after the first alkene addition .. The thexyl group has a low migratory aptitude in the subsequent conversion to a ketone, thereby avoiding competing ketone products.
Knowledge check:
Knowledge check:
Arrange the following in terms of decreasing Lewis acidity, most Lewis acidic first:
B(OEt)3 : B(SEt)3 : BEt3
BEt3 > B(SEt)3 > B(OEt)3
BEt3 is the most Lewis acidic because there is no heteroatom attached to boron which can reduce Lewis acidity through mesomerism (p-pi - p-pi bonding). Although sulfur is less electronegative than oxygen, the overlap of the S lone pair with the vacant orbital on B will be less effective than the overlap of the oxygen atom lone pair (B and O are in the same period so orbitals are of similar energy) i.e. the mesomeric effect is more important than the inductive effect on the Lewis acidity.
Lewis acidity:
Does mesomerism (p-pi - p-pi bonding) decrease or increase lewis acidity generally?
decrease
eg BEt3 which does not exhibit mesomerism is more Lewis acidic than B(SEt)3 because there is no heteroatom attached to boron.
The heteroatom reduces Lewis acidity through mesomerism (p-p - p-p bonding).
Boranes basics:
What is the hybridisation of a borane?
sp2
Boranes basics:
what shape does a borane take?
trigonal planar
Boranes basics:
what is the structure of a borane?
trigonal planar with an empty pz orbital.
Boranes basics:
Are boranes lewis acids or lewis bases? why?
Lewis acids. they have an empty p orbital which can accept an electron pair from a donor compound.
Boranes basics:
What makes a good lewis base to be used to form a “ate” complex with a borane?
a good lewis base will have an atom in it that contains a lone pair that it can donate. eg pyridine has an N with a lone pair, DMS has an S with a lone pair.
