Kinetic studies provide valuable information in the areas of both mechanistic and synthetic chemistry concerning the effects of substituents in alkenes and alkynes.The effects of substituents that donate or withdraw or polarize electrons of C=C or CŁC provide information regarding the mechanism of hydrobora-tion.On the other hand,relative rates of hydroboration of substituted or un-substituted C=C or CŁC give synthetic chemists improved means of predicting the selective hydroboration of C=C or CŁC or their functionalized derivatives. 9-BBN has proven to be the best candidate for the investigation of mechanism and kinetics of hydroboration because:
1.It has high thermal stability and purity.
2.It is convenient to handle compared to other boranes as it has lower sensitiv-
ity to oxygen and water vapors.
3.With only one center per boron,its overall reaction with an alkene involves
only one dissociation step and one hydroboration step;in contrast,borane
(BH
3)has three consecutive addition reactions,three redistribution equilib-
ria,and five monomer–dimer equilibria[1].
4.9-BBN reactions can be studied in solvents such as carbon tetrachloride,
cyclohexane,and benzene whereas9-BBN exists exclusively as dimer,thus eliminating complexation[2]with solvents and simplifying the kinetics. 5.9-BBN is highly regio-and stereoselective,which assures the study of only
one reaction.
6.In addition,the progress of the reaction can easily be monitored via IR
where disappearance of the1,570cm–1absorption of bridged B–H bonds occurs.
Consequently,kinetic studies of9-BBN have yielded very useful data for its relative reactivity toward various types of unsaturation and the effect of solvent on the reaction.
reaction kinetics mechanism期刊It is significant to mention that in compounds that display first-order kinet-
ics,the values differ in THF and CCl
4solvents.The first-order rate constant in
THF is about10times that in CCl
4solvent.This is due to the catalytic effect of
THF[3]on the(9-BBN)
2dimer that breaks the B–H bridge bond[4].
4Kinetic Studies
4 Kinetic Studies 20This mechanism leads to the following kinetic expression (Eq.4.6)utilizing the usual steady state approximation.
(4.6)
If 1/2k 2[olefin]>>k –1[9-BBN]Eq.4.6reduces to Eq.4.1,the reaction be-haves like a unimolecular reaction and exhibits first-order kinetics.However,if 1/2k 2[olefin]<<k –1[9-BBN],Eq.4.6reduces to Eq.4.2.The reaction exhibits three-halves-order kinetics.Consequently,kinetics reveals that the hydrobora-tion with dimeric 9-BBN of representative alkenes proceeds through prior dis-sociation of the dimer to the monomer,leading to simplified kinetic expression,as expressed in Eqs.4.1and 4.2for reactive alkenes and for less reactive alkenes,respectively.However,for olefins like 2-methyl-2-butene and cis -3-hexene 1/2k 2[olefin]≈k –1[9-BBN]2and the kinetics fail to follow the simplified rate expres-sion Eqs.4.1and 4.2.
To test the proposed dissociation mechanism with (9-BBN)2,a detailed study is conducted using quantitative IR spectrometry [3].(9-BBN)2exhibits a very strong IR absorption at 1,570cm –1due to B–H bridges [4].This method is conve-nient and more reliable and thus has an advantage over tedious GLC analyses.The hydroboration kinetics are studied by addition of alkenes to the solu-tion of (9-BBN)2in the solvent maintained at 25°C.The reaction mixtures are pumped through a sodium chloride IR cell.The rates of the disappearance of B–H bridges of (9-BBN)2at 1,570cm –1are monitored by quantitative IR spec-trometry.The absorbance is recorded on chart paper.
In IR studies,an exponentially decaying curve of the absorbance of B–H bridges is noticed which also r
eveals that the reaction follows first-order kinet-ics.Six representative points on the exponentially decaying curve are calculated (Table 4.3)[3].
(4.7)
where [(9-BBN)2]o is the initial concentration of (9-BBN)2and [(9-BBN)2]t is its concentration at time t ;a ,is the initial absorbance;c ,is the absorbance at time t ;and b is the background absorbance
(4.8)
where b is the initial concentration of cyclopentene and b –2x ,is the concentra-tion at time t.
19
(4.3) where b is the initial concentration of olefin and b–2x is the concentration at time(t).
The first-order rate constants are quite similar in solvents other than THF, which are1.52in CCl
4
,1.97in hexane,1.52in cyclohexane,and1.95in benzene, and2in diethylether.However,in THF, k
1
is considerably larger,at10.8.
The less reactive olefins such as cyclohexene,1-methylcyclohexene,and2,3-dimethyl-2-butene exhibit rates that are slower and vary with concentration and the structure of individual olefins.The kinetics establishes these reactions to be first order in olefins and one-half order in the9-BBN dimer(Eq.4.2).The calculated three-halves-order rate constants also do not change as the reaction proceeds.The rate constants observed both for the first-order and three-halves-order kinetics are summarized in Table4.2[1].
These studies unequivocally establish that the hydroboration of alkenes with (9-BBN)
2
proceeds through the prior dissociation of the dimer to the monomer (Eq.4.4),followed by the reaction of monomer with the unsaturated substrate (Eq.
4.5).
(4.4)
(4.5)
Table4.2First-order-and three-halves-order rate constants for the hydroboration of representa-tive olefins with(9-BBN)
2
in carbon tetrachloride at25°C[1]
Olefin a104k
1
,s–1104k
3/2
,l1/2M–1/2s–1
1-Hexene  1.54
2-Methyl-1-pentene  1.53
3,3-Dimethyl-1-butene  1.45
Cyclopentene b  1.52
Cyclohexene b0.323
1-Methylcyclohexene0.051
2,3-Dimethyl-2-butene0.02
a Rate constants in table are for initial concentration of olefin(0.4M),and(9-BBN)2(0.2M).
b Variation of the initial concentrations of the olefin and(9-BBN)2do not change the observed rate constants,significantly:cyclopentene(0.4M),(9-BBN)
2
(0.1M),104k
1
1.58;cyclopentene (0.2M),(9-BBN)
2
(0.1M),104k
1
1.58;cyclohexene(0.4M),(9-BBN)
2
(0.1M),104k
3/2
0.324; cyclohexene(0.2M),(9-BBN)
2
(0.1M),104k
3/2
0.345.
4.1Hydroboration Kinetics of Alkenes
4 Kinetic Studies 18
4.1
Hydroboration Kinetics of Alkenes
The hydroboration reactions of(9-BBN)
2with more reactive olefins have been
found to be the first-order kinetics(Eq.4.1)[1].
(4.1) and three-halves kinetics,with less reactive olefins(Eq.4.2)[2].
(4.2) The kinetics for hydroboration of alkenes are conducted in various solvents such as carbon tetrachloride,hexane,cyclohexane,benzene,and THF.9-BBN
exists predominantly as the dimer(9-BBN)
2[2].After the addition of olefins,
at25°C,the aliquots from the reaction mixture are removed after appropri-ate intervals of time,quenched with an excess methanol,and analyzed by GLC for residual olefin.All operations are per
formed under nitrogen until identical rates are observed for more reactive olefins such as1-hexene,2-methyl-1-pen-tene,3,3-dimethyl-1-butene,and cyclopentene,and variation of olefin concen-tration does not alter the rate.These results establish that the reaction is first order(Eq.4.1).Typical data for cyclopentene and cyclohexene are presented in Table4.1[1].
Table4.1Rate data and rate constants for the hydroboration of cyclopentene(0.400M)and cyclo-hexene(0.400M)with(9-BBN)
2
(0.200M)in carbon tetrachloride at25°C[1]
Time(s)Cyclopentene a
(M)104k
1
b s–1Time(s)Cyclohexene a
(M)
104k
3/2
b
l1/2
mol–1/2s–1
00.40000.400
2980.382  1.506,0010.3390.321 1,2050.332  1.5415,3800.2620.343 2,7130.263  1.5521,6050.2250.344 4,5400.202  1.5142,4940.1480.338 6,2970.153  1.5261,7690.1080.336 9,0010.102  1.5272,0070.0960.324
a Concentration of(9-BBN)2is one half that of olefin.
b Calculated from the equations

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