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6-1-2001 Hydroboration of 4-phenyl-1-butene with rhodium trichloride, a catalytic approach Ahmad Al-Dheiby
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Recommended Citation Al-Dheiby, Ahmad, "Hydroboration of 4-phenyl-1-butene with rhodium trichloride, a catalytic approach" (2001). Thesis. Rochester Institute of Technology. Accessed from
This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. Hydroboration of 4-Phenyl-l-Butene With Rhodium Trichloride,
A Catalytic Approach
By
Ahmad S. AI-Dheiby
In partial completion for the requirements of the Degree of Master of Science
Terence Morrill Thesis Advisor
Department Head
Library
Department of Chemistry Rochester Institute of Technology, Rochester, New York, United States of America Title of thesis: Hydrboration of 4-Phenyl-l-Butene With Rhodium Trichloride, A
Catalytic Approach.
I, Ahmad S. AI-Dheiby, hereby grant permission to the Wallace Memorial Library of the
Rochester Institute of Technology to reproduce my thesis in whole or in part. Any reproduction will not be for commercial use or profit.
Signature of Author..;..: _ Dedication
This work is dedicated to my parents for their support in continuing my education, my
siblings, my friend Zaher Al-Dheiby, and my life Susan R. Barnard for her invaluable
support and help when I first came to RIT. itself." Do not see the silver lining in the clouds, but see the beauty in the cloud
Susan R. Barnard Table of Contents
1 . History and Introduction 1
2. Experimental 32
a. Overfed Hydroboration Procedure 32
b. Starved Hydroboration 33
c. Catalyzed Procedure 33
d. High Temperature Hydroboration 34
3. Results and Discussion 35
4. Summary and Conclusion 55
5. Acknowledgements 56
6. References 57 History and Introduction
One of the most important reactions in organic chemistry is the hydroboration reaction, which was first discovered by H.C. Brown of Purdue University in 1956. Since then, extensive research has been conducted using different reagents and catalysts. Brown first discovered that treating an olefin with a borohydride, like sodium or lithium borohydride, followed by oxidation, gives the corresponding alcohol. The oxidation occurs via anti-
Markovnikov and syn regiochemistry. Oxidative hydroboration involves two steps: the first is addition of borane to the alkene to produce a trialkylborane, and the second is the
oxidation of this organoborane to alcohol:
(" H^Q^0H-
^^ THF:BH, ^ ^ )3B , 3 CHXHPCH?OH
Figl
The alkylboranes are usually not isolated but they are transformed directly to alcohol by
addition of hydrogen peroxide in an aqueous base .
R3B HA ^ 3 RQH + Na3Bo3 NaOH 25C
Hg2 Mechanism of Hydroboration:
H.C. Brown and B.C Subra Rao proposed a reasonable mechanism for the hydroboration
step2.
B + H H--B H B
Fig3
The mechanism of the oxidation step involves the addition of the hydroperoxide ion
(HOO") to the electron-deficient boron atom.
R R R
O OO H -? RB R B + "O R r H ? OR
A R R
OH-
f2"00H
0R 0H-
* OR B B033- "* -* + 3R0H OR
These mechanistic steps are consistent with the observed anti-Markovnikov
regiochemistry and syn stereochemistry.
Many hydroboration reactions have been run to see how the product composition depends
upon the nature of the reactants; the following results were obtained: 2 1. Straight-chain terminal olefins such as 1-pentene and 1-hexene yield 94% of the
boron on the 1- and 6% on the 2- position of the chain.
2. No appreciable product change was observed upon simple branching of the alkyl
chain.
3. A methyl group on the 2-carbon atom results in the formation of 99% primary anti-
Markovnikov addition derivative.
4. Phenyl substituents result in a significant change -80% primary and 20% secondary
alcohols from styrene3.
5. No rearrangement of the carbon skeleton was observed.
The reactions written on the following page illustrates the above observations.
As seen on page 4, alkyl branching has no major effect on the position of the boron atom in the alkylborane product. There is, however, a significant effect when a phenyl ring is present in the olefin (page5). Brown and his co-workers suggested that electronic effects play a major role in producing the significant percentage of the minor secondary alcohol form styrene. Substitution on the phenyl ring affected the position of hydroboration, depending on the type of the substituting group. BH, +
0H-, H902^2 OH
94% 6% 1-Pentene
OH
BH, + OH-,H202 OH
6% 1-Hexene 94%
OH
BH,
OH-, H202 OH
6% 3-Methyl 1-Butene 94%
OH
BH, UH",H202
4,4-Dimethyl 1-pentene 7%
OH HO
BH, // \ // \ / \ OH,H202
2-phenyl ethanol Styrene 1 -phenyl ethanol-1 80% 20% BH3, THF
OH- H20,2"2
p-chlorostyrene p-Chlorophenyl ethanol p-Chlorophenyl ehanol-1
35% 65%
-OH OH
CH30 CH30 CH30
styrene p-Methoxy p-Methoxyphenyl ethanol 1 -p-Methoxyphenyl ethanol-1
90% 10%
Electron withdrawing groups (Cl) decreases the yield of the secondary alcohol in the styrene system, while electron donating groups (CH3O) increases the amount of secondary alcohols.
Brown suggested an intermediate that explains electronic effect in the styrene system:
H H
-B H C = CH2 6H2
B H +
The phenyl group, as noticed in the above benzyl anion, can stabilize a negative charge in the a-position. These results were obtained when different solvents such as ethyl ether,
THF, or diglyme were used. Moreover, the different hydroborating reagents such as lithium borohydride, sodium borohydride, or diborane, gave the same distribution of the isomeric products.
Long chain terminal olefins, as stated earlier, can undergo hydroboration to produce the corresponding primary alcohol. However, disubstituted olefins, such as 2-hexene yields
when* 3- organoboranes which, oxidized directly without prior isolation, yield 2-and hexanol in equimolar amounts. If, however, the organoborane is heated in refluxing diglyme for a period of time (3-4 hours) at 160C prior to oxidation, the product obtained is pure 1-hexanol.
Furthermore, 2-octene, mixed with 2,3,4,and 5- decenes, and a similar mixture of
tetradecenes, have been converted into 1-octanol, 1-decanol, and 1-tetradecanol. Brown
BH, +
HEAT (160C)
t
OH-,H202
Almost Pure (99%) thought that a sequence of eliminations, followed by re-addition, until the boranes converted to the linear terminal trialkylborane, which is more stable. The following scheme illustrates the above idea, using 2-hexene as an example:
If isolation of the organoborane is done prior to oxidation, then the distribution of the
hydroxyl group on the second and third carbon of the disubstiuted olefin (chain olefin
such as 2-pentene) after oxidation will be recognizable. In the case of 2-pentene, the
products obtained using this procedure were 63% 2-pentanol, and 37% 3-pentanol. If the
2- hydroboration product is oxidized in situ, without isolation, an equimolar mixture of
and 3-pentanol is produced. During earlier distillation a partial shift of the boron atom
from the 3-to the 2-position had occurred. Hennion and his co-workers first reported
isomerisation of 2-alkyl boranes when they tried to synthesize tri-t-butylborane by the
reaction of t-butyl magnesium chloride with boron trifluoride4. They obtained instead,
sec- and t-butyl boranes; they found that tri-sec-butylborane was isomerized to the
primary tri-n-butyl borate in about 20 hours at 200-2 15C.
Brown discovered that the isomerization of oganoboranes is solvent dependant; he
observed that isomerization occurred more rapidly in diglyme solution, being complete in
hours' 2 to 4 refluxing in that solvent at 160C; the diglyme solution, which is an ether
solution, has a catalytic effect on the isomerisation reaction. BH, +
2-Pentene prior to Oxidation
HEAT (160C)
t
OH-,H202 -^-
Pentanol-1
Almost Pure (99%)
2- Oxidation without prior isolation of the organoborane yielded nearly 50% each of
pentanol, and 3-pentanol.
The same hydroboration procedure done at a higher temperature of 190-220C on a
mixture of 2-, 3-, 4-, and 5-decenes yielded only pure 1-decanol. However, branched
decenes, like tetradecene, required more time for the isomerisation to occur (4-8 hours);
the yield of the corresponding primary alcohol was 59%, at longer time (18 hours) the
yield increased to 70%.
A reasonable mechanism for isomerisation proposed by Brown and his co-workers is the
following: H H H H H H H H H
CH RPc RC=C RC C CH CH ^ \ I rr H H /B H H H H
I
H H H H H H
CH C=CH
)B H H RT H
Under conditions of the alkylborane reaction, a high concentration of added olefin would cause the displacement of the alkene formed in the elimination stage. That means, if tri pentylborane were treated individually with 1-hexene, 1-octene, and 1-decene in a flask of a fractionating column, 1-pentene, would be formed in each case. The 1-pentene is collected as a distillate, and the tri- alkylboranes of the corresponding alkenes would be collected as yields in 85-93%. The following table summarizes the above mentioned
results:
Organoboranes By The Reaction Of Olefins With Tri-Pentylborane
Olefin Organoborane % Yield of % Yield of 1-
Organoborane Pentene
1-Hexene Tri-hexyl 93% 90%
1-Octene Tri- octyl 90% 90%
1-Decene Tri-decyl 85% 93% Gallagher and Field, of the University of Sydney, reported that the hydroboration of
1,2-dimethylcyclopentene with BH3'THF produced a structure in which boron migrates
stereospecifically at low temperature at 25C into the cyclopentane ring5. Previous
studies6 of this reaction at higher temperatures (110C) revealed high stereospecificity in
the thermal rearrangement of the organoborane formed upon in situ hydroboration of the similar 1,2-dimethylcyclohexene. Under the above conditions, the rearrangement gave, after oxidation, the two primary alcohols shown below in the ratio of 97:3.
1 BH3.THF H 2110C H CH2OH CH2OH 3 H202/OH- CH3 CH3'-H
97% 3%
However, direct hydroboration of the exocyclic olefin below, under the same conditions, produced the same alcohols above, but in the ratio of 30:70 as shown below:
1 BH3.THF
H CH2OH 2 110C H CH2OH 3 H.cyoH- CH3 CH3"H
30% 70%
The above results indicate that simple rearrangement to the exocyclic alkene cannot be important for the hydroboration of 1,2-dimethylcyclohexene because the product ratio for
the last two reactions would be the same. Gallager and Field found that there was a kind of similar internal rearrangement in the hydroboration of 1,2-dimethylcyclopentene. They
10 hydroborated this compound and found that the products were the major exo- and the
endo- alcohols in the ratio 99% to 1% respectively:
BH3.THF
50C
H202/OH- H3C H
99% -1%
Hydroboration of 1,5-dimethylcyclopentene under the same conditions produced the
same products but in the ratio of 20:80, respectively.
H, /\ H<
BHg.THF CHg'
50C
Hp^OH- H3C H
20% 80%
Hydroboration using rhodium compounds as catalysts was first reported by Noth and
Mannig, and a new facet of hydroboration was revealed. Many scientists have worked on
these catalyzed hydroborations of alkenes, and among the many alkenes hydroborated,
we review vinyl arenes and long chain alkenes.
11 Jinfand and Zhang his co-workers at the Shanghai Institute of Organic Chemistry reported that the hydroboration of styrene using catecholborane7(CB) in the presence of
Wilkinson's catalyst promoted the production of the a-alcohol as the major product of
the reaction; i.e., the Markovnikov product was the major one7.
O
,B H
O
Rh+1
(Wilkinson's Catalyst)
Styrene a-alcohol j3-alcohol
Major minor
86 : 14
Yield was 87% when R=H
When vinylarenes were treated with CB under different conditions, different product
ratios were obtained; for example, treatment of styrene with CB in THF using
Wilkinson's catalyst at 5C such that the mole ratio of CBxatalyst is 1:0.02, 98% of the
a-alcohol was obtained.
X> B H
Rh+1 5C
ratio: catecholeborane:catalytst 98 2 1 : 0.02
91% yield
a-alcohol (3-alcohol
12 Addition of a Lewis acid such as SnCl2, ZnCl2, or BH3 increased the ratio of a: (3 alcohols to 98:2
Other vinylarenes, such as indene and dihydronapthalene, produced similar ratios under the same conditions when catalyzed by Wilkinson's catalyst:
Indene Dihydronapthalene
Hydroboration of these vinylarenes produced the Markovnikov alcohol (substitution
nearer to the phenyl ring).
Another neutral rhodium catalyst, RhCl(COD)2, was examined. The absence of the tertiary phosphine ligand (which is part Wilkinson's catalyst), or the presence of a Lewis
acid, the major product was still the |3-alcohol. For example, in the presence of SnCl2, the a:|3 ration was 33:67. Nevertheless, upon addition of the triphenylphosphine (2 mole %, a potential ligand), the ratio changed to be 59:41. With 4% phospine, the regiochemistry
reversed completely to an a:j3 ratio of 98:2 .
O H
,B H
^ O H
W ilkin son 's C ata lyst
M a jo r M in o r
O H
,B H O H OO W ilkinson's C ata lyst
M ajor Minor
13 The mechanism suggested for catalyzed hydroboration was the following:
rCH^CH, CH, =CH
M ArCH M ArCH=CH,
ArCHXH,
ArCHCH, ArCH,CH B \ M B- / M b;
secondary insertion primary insertion
1=unsaturated RhCIL2 species
B= B02C6H4 (catechole derivative)
Uncatalyzed hydroboration of styrene produces the terminal alcohol after oxidation.
Therefore, the catalyzed and the uncatalyzed hydroboration reactions are complementary.
Differences between the two reactions include instances of inverted regioselection
(uncatalyzed), and another includes examples of inverted stereoselection as shown in the
hydroboration (catalyzed) of 1,1 disubstituted allylic alcohol derivatives8,as shown
below:
14 X
B Rh(l) + H HO V R ox R R
syn isomer
R \ B H
_B:
ox
Evans et al. reported labeling studies to demonstrate that the catalytic hydroboration of
olefins is reversible and the analysis of the stereochemical course of this kinetically
complex reaction is highly substrate dependent.
In order to perceive the degree of reversibility of the olefin binding and the hydride
migration steps illustrated by the above Mannig-Noth mechanism, Evans, et al., ran
catalyzed hydroboration reactions of alkenes with deuteriocatecholborane as illustrated
by the following reactions:
15 1-Decene:
0.02 Rh(PPh3)CI m%) octyl ~ ? H0^^ Xoctyl 2Q \_D (14%)
D distribution, %
Specific conditions of the above reaction were maintained to test the degree of
reversibility of the hydride shift in the Mannig -Noth mechanism (0.1 equiv borane, 0.2
catalyst % , THF, 20C ),
JS% ^ 0.02 Rh(PPh3)a (8g%) octyl + HO_^octy| D f 0.1IL dl (14%)
D distribution, %
[olefin] : [alcohol]= 1:2
It was found that deuterium is incorporated not only in the product alcohol, but also in the recovered alkene, as shown above.
The presence of deuterium in the recovered 1-decene explains that migration (to form either primary or secondary alkyl rhodium) and olefin binding are reversible, although the rate of reductive elimination is faster than the rate of reversible binding.
The following scheme illustrates the stated argument:
16 On Rh(PPh3)CI B D
Ox ^^octyl
, octyl "\ t D
octyl IX octyl
Rh / D alkene dissociation hydride migration octyl D ^ D A IX octyl D ~n-Oct .octyl Rh. B B Rh H B Rh B Rh / H
reductive elimination ,
D D
octyl , "octyl D, "octyl B B B Rh
D
B octyl
4-[(tert-butyldimethylsilyl):
17 Reversible hydride migration and reversible olefin binding to catalyst are observed in the hydroboration of the 4-[(tert-butyldimethylsilyl)oxy]cyclohexene ether of 4- hydroxycyclohexene as shown here9:
OTBS OTBS OTBS
0.002 Rh(PPh3)3CI
0-1 fi "T D D OH,D CO
Olefin : Alcohol = 2 : 1
CH,
TBS= (CH3)3C Si O
CK
Styrene:
As shown earlier, hydroboration of styrene using Wilkinson's catalyst involves high
enantioselectivity and generates nonracemic secondary alcohols if the ligands on the
catalyst are chiral. In contrast to the above stated cases, studies of catalytic styrene
hydroboration provided no evidence of reversible olefin binding and hydride migration;
when excess styrene was treated with deuteriocatecholborane under standard conditions
18 of temperature and l-phenyl-2- pressure, only the deuterium containing product of deuterioethanol was observed:
0.002 Rh(PPh)CI
^J? .0 o<^ 100% D -1OC0>-
Another reaction that throws some insight on the reversibility of hydride migration is the catalytic hydroboration of 2-methyl-3-[(tert-butyldimethylsilyl)-oxy]butene-l. The
reaction is illustrated below:
OTBS OTBS 0.002 Rh(PPh3),CI 17% I HO^^r83%Me Me O Me 0.1 B D Me
O-
D distribution, %
From these reactions, it was found that the relative rates of the elementary steps in the hydroboration catalytic cycle were highly substrate dependent. Thus, the alcohol
and as the regioselectivity-determining step was different for 1-decene styrene, following
scheme indicates:
19 (RO)2B H
Rh(PPh3)CI
octyl Ph
Ph ,octyl
(RO)2B (RO)2B Rh Rh / / H
regioselectivity determining step
H Ph
H (RO)2B (RO)2B Rh xocty| Rh
regioselectivity determining step
H Ph
H octyl (RO)2B B(RO)2
20 et Evans, al., reported some hydroboration reactions to compare the stereoselectivity of catalyzed vs. uncatalyzed processes; furthermore, he found that monosubstituted alkene hydroborate faster than 1,2-disubstiuted alkenes and 1,2-disubstituted cycloalkenes. Here are some reactions that show stereoselectivity in hydroboration:
Cyclohexanol Derivatives:
OR uncatalyzed
,OH R2BH OR +
2-A 2-S
OR OR
catalyzed (RO)2BH + 'OH 'OH
3- A 3-S
Catalyst: Wilkinson's catalyst A= anti S=syn
Hydroboration of Cyclohexene Derivatives
R Conditions Overall %Yield 2-A 3-A 3-S 2-S
H uncatalyzed 86 83 5 10 2
catalyzed 84 18 72 9 1
CH2Ph uncatalyzed 73 68 13 19 0
catalyzed 87 7 72 13 8
SitBuMe2 uncatalyzed 70 74 13 13 0
catalyzed 79 2 86 11 1
21 Catalyzed Hydroboration under Mild Conditions:
Hydroboration Rh2+ reactions using a catalyst [dirhodium(H) carboxylate and carboxyamide], have been reported11. Rh2(OAc) proved to have a high catalytic activity for hydroboration with catecholborane, and to increase the facility with which olefin isomerization can occur.
0.5mol % l l HA Rh2(OAc)4 ^*- , /h^ OH- CH2CI2 l^J i; OH
1.1 equiv exo endo
99% 1%
yield = 80%
Similar effective hydroboration-oxidation reactions were run for different types of
alkenes under the same conditions with different solvent such as THF. The most suitable solvent was CH2C12; THF coordinates with dirhodium(II) compounds to inhibit the catalytic hydroboration reaction11. Uncatalyzed hydroboration by catecholborane accounted for less than 5% of product formation. Hydroboration reactions followed by oxidation with substituted acyclic alkenes using the same catalyst produced different
isomers of alcohols with different ratios as shown below:
22 OH catecholborane OH CH2CI2 Rh2(OAc).
39% 43%
OH catecholborane + \. CH2CI2 Rh2(OAc)4
21% 9%
Hydroboration of allylbenzene with catalytic amounts of Rh2(OAc)4 ( 1.0% mol) and
catecholborane (3.0% mol) resulted in its complete isomerization to Z- and E-
1- phenyl-1-propene after for 22 hours. Neither the catalyst nor the borane alone could
cause this isomerization.
3 mol %
catecholborane
^^ 1 mol % ^^ Rh2(OAc)4
E:Z = 19:81
23 Rhodium Trichloride (RhCl3) has been used as a catalyst in many reactions such as hydrogenation of aromatic compounds and hydroboration of olefins. Hydrogenation aromatic compounds is usually performed at high temperature and high pressure, but the quite mild conditions were used with rhodium as per the following examples:
X X
Ni
H2 100C, 1 atm
X X
Ru
H2 150C, 100atm
X X
Rh
H2 RT 2-4 atm
X is an alkyl group or a halide.
Later, it was discovered that aromatic compounds were easily hydrogenated under very mild conditions to produce cyclohexane derivatives. Wescott and his co-workers used the ion-pair formed from aqueous RI1CI3 and aliquat 336 in dichloroethane to perform
such reactions:
24 RhCI3 Aliquat 336
H2 1 atm 30 oq
51% yield
CH, CH,
RhCI3 Aliquat 336
H2 1 atm 30C
16% yield
The same group found that hydrogenation of naphthalene in the presence of RhCl3 and aliquat 336, produced 1,2,3,4-tetrahydronaphthalene (tetralin) at 30C and hydrogen
12 pressure of 1 atm
RhCI3 Aliquat 336
1 atm H2 30C
81 % yield
25 No one had reported catalytic hydroboration with RhCl3 before the T.C.Morrill research
has group at RIT carried out such studies. He, in collaboration with his research students, run many hydroboration reactions using RhCl3 catalyst; novel and positive results were obtained. We shall now review the research done by a number these students.
Lu Yang, an RIT graduate student under Professor Morrill's supervision, ran many hydroboration reactions using RhCl3 and catecholborane for different olefins, and found that RhCl3 promoted hydroboration via Markovnikov addition. The following reactions illustrate the results obtained:
R /^ OX CH2CI2,RhCI3,Aliquat 336 R- V -I 2 H202,NaOH OH O'
?%'o
R ^ OH
Aliquat is: [CH3(CH2)7]3NCH3CI 336 ?%
R is CH3(CH2)4
Lu Yang ran hydroboration reactions using 1-octene and catecholborane under different conditions, and she obtained different yields of primary and apparently, secondary alcohols. Amazingly, the major product was a secondary alcohol in most of these
reactions:
26 0, 1 CH2CI2,RhCI3,Aliquat 336 B 95% H ? 2 EtOH, H202,NaOH 0
5%
The results are summarized in Table 1.:
Tablel: Hydroboration of 1-Octene with Catecholborane and RhCl3 using Aliquat 336 using CH2C12 as a solvent
Moles of Moles of Moles of Moles of T(C) hours Product yields RhCl3 al336 CB 1-octene ratio 1: 2
2.4x10" 1.1x10" 0.01 0.01 33 10 5:95 10.3%
2.4x10" 1.1x10" 0.05 0.01 33 10 5:95 18%
2.4x10" none 0.03 0.01 21 5 87:13 35%
2.4x10" 1.1x10" 0.03 0.01 21 10 33:67 65.4%
2.4x10" 1.1x10" 0.03 0.01 21 5 5:95 59.3%
Using THF as a solvent, the results in Table 2 were obtained:
Table2: Hydroboration of 1-Octene with Catecholborane and RhCl3 using Aliquat 336 using THF as a Solvent
Moles of Moles of Moles of Moles of T(C) hours Product yields RhCl3 al336 CB 1-octene ratio 1: 2 2.4x10" 1.1x10" 0.03 0.01 21 10 50:50 53.5%
2.4x10-" 1.1x10" 0.03 0.01 21 10 10:90 63.6%
2.4x10" 1.1x10" 0.03 0.01 60 5 5:95 11%
The mechanism proposed by Yang is shown below:
27 0, H
;b q+ci- H + Q+ -LKC|
RhCI Q+CI" 3 + z^: QRhCI,
0X H Cl\ /Cl \ -/ a Ck XI o/ + RhCI4 cr -\ ^F
Cl\ /Cl Cl OH EtOH /Rh Cl RK NaOH VCI R - u It seemed that at higher temperature (60C) at shorter periods of time., hydroboration was driven more towards the Markovnikov addition. Catalytic hydroboration of styrene under the same conditions above produced the primary alcohol as the major product. 28 ,0, Aliquat 1 RhCL,CH,CI2, 336 B H minor 2. EtOH, NaOH, o H202 major that The reason that the primary alcohol is the major product is part of the mechanism \-/H -Cl x RhCl4 + Cl Rh o CL \ ^^ Cl Cl / Cl Rh^ follows: 29 A steric effect due to the presence of the phenyl ring is apparently the reason behind the Markovnokov addition in this hydroboration reaction. Progress in this research at this chemistry department lead to the use of BH3'THF (borane complex) as the hydroborating reagent in an attempt to increase the yield and the % of the desired product: Tony Sampognaro14, under the direction of Professor Morrill, ran hydroboration reactions 1- of 1-octene using BH3'THF as the hydroborating reagent, he obtained isomers of octanol, 2-octanol, 3-octanol, and 4-octanol with different ratios depending on the reaction condition: + 3.8% 10mg RhCI3, BH3.THF THF H20,H202,NaOH (2 hours) 37.3% 41.9% OH 3- 4- Sampognaro found that Lu Yang's GC samples of 2-octanol actually contained and mentioned in the experimental section of this octanol. The use of the new instrument paper lead to the above clarification. 30 Experimental Procedures a. Overfed Hydroboration 10 ml volume of tetrahydrofuran (THF) (Aldrich Chemicals Inc.) was placed in a 100ml three-neck round bottom flask, and lOmg of rhodium trichloride bought also from (Aldrich Chemicals Inc.) were added to the solvent and stirred thoroughly with a (IO2 magnetic stirrer until most of the rhodium catalyst was dissolved. Then 1.5 ml moles) of 4-phenyl-l-butene were added and the mixture was stirred for about 20 (6xl03 minutes. 6ml 1M moles) of BH3'THF complex was injected in the mixture using a 10 ml syringe. The BH3 was introduced dropwise for 20 minutes. The total injection time was about 30 minutes. Then, the mixture was stirred for different periods of time at different temperatures according to the data indicated in the Results and Discussions Section. Oxidation was done by carefully adding 7ml of 30% hydrogen peroxide dropwise, followed by dropwise addition of 7ml of 3M sodium hydroxide and 10ml of distilled water. The oxidation reactions were first run for two hours at 0C and at room temperature. The solution was then filtered by gravity filtration followed by extraction with 25ml of 3M NaOH, 15ml of saturated NaCl and 50 ml of dry ether. The solvent was added in portions to get better yield of the final product. Furthermore, the organic layer magnesium chloride for was separated by a separatory funnel and dried over anhydrous evaporator. The about 10 min, gravity filtered, and distilled by a rotatory resulting GC-MS model spectrometer. The organic product was analyzed using a Hewlett-Packard Gas Chromatograph. yield was determined using the GOW-MAC and the stir Low temperature hydroborations were run at 0C by letting the THF RhCl3 in of the olefin. The olefin was refrigerated an ice bath for 30 minutes before the addition borane was added dropwise to maintain before it was added to the THF solvent. The 31 constant temperature of 0C and to obtain a better isomerization of the 4-phenyl-l-butene the system; temperature was intermittently monitored and it was in a range of 0-lC. The low temperature oxidation reactions were also carefully performed by refrigerating the 30% hydrogen peroxide, the 3M sodium hydroxide, and the distilled water. The temperature rose to 10C upon the addition of the first drops of the peroxide solution then it lowered to the desired range. In fact, it did not matter whether the oxidation reaction was done at 0C or at room temperature; both cases gave the same final product mixtures in all the reactions. b. Starved Hydroboration Reactions In the starved hydroboration reactions, the same exact procedure above was followed, but 5x10" moles of BH3'THF were added, instead. As for the mild hydroboration reactions, 3xlO"3 moles of BH3'THf were added. c. Catalyzed Procedure When RhCl3 was added to the THF solvent and stirred for 20 minutes, the color of the solution turned red brown; this color gradually changed when the borane was added dropwise. First, the color became light black upon adding few drops of the borane complex, then the color became dark black when all the 6 ml of borane were added and a black solid was formed. Oxidation did not change the black color of the solution and the black solid remained. However, this was not the case for starved hydroboration, in which the solution turned light black after the addition of 0.5ml of borane. This is because only a small amount of borane was added and that was not enough to turn the color to dark black and form the black solid. On the other hand, upon oxidation of the reaction, the color of the solution changed to light yellow. When extracting the final solution, two layers appeared in the separatory funnel: an aqueous layer and an organic layer. The aqueous layer was discarded in the waste bottle and the organic layer was colorless; the 32 final product obtained after evaporation of the ether was light yellow for the alcohols and colorless for the olefins. The original 4-phenyl-l-butene purchased from Aldrich Chemicals was light yellow and it was 99.98% pure. d. High Temperature Starved Hydroboration Reaction The same procedure was followed in the starved hydroboration of 4-phenyl-l- butene at reflux temperature, 64C. The RhCl3 was dissolved in 15 ml of THF instead of 10ml to as much solvent keep as we could after reflux, since some of the solvent would be lost due to evaporation. The rhodium catalyst was first dissolved, then the olefin was added and the mixture was stirred for 30 minutes, followed by heating under reflux for 10 minutes before 0.5ml of borane were added. The reaction was run under reflux for different periods of time and the results above were obtained. As stated above, all the chemical products of the hydroboration reactions were analyzed using a Hewlett-Packard GC-MS spectrometer. The oven temperature of the instrument was set between 100 to 150C; the running time was 25minutes for all the analyses performed. The method used in the GC-MS gave the following retention times for the different products: Product Retention time(minutes) 1-phenylbutane 9.46 Trans 4-phenyl-2-butene 9.70 Cis 4-phenyl-2-butene 10.15 1 -phenyl-1 -butene 11.54 4-phenyl-2-butanol 15.38 4-phenyl-3-butanol 16.30 1 -phenyl-1 -butanol 19.85 The spectra of the peaks that correspond to the above retention times are shown on the following page. 33 GC/MS Spectrum of Starved hydroboration Reaction at 0C IIC: HB36.D 9:02 6500000: 6000000; 5500000; 5000000; J0.15 4000000- 9.7 3500000- 3000000- 2500000, 2000000- 1500000- I I 1000000^ 500000; I V J LL 12.00 13.00 14.00 15.00 16.00 17:00 18.00 19100 6 00 7.oo 8.00 9.00 10.00 11-00 34 GC/MS Spectrum of Starved hydroboration Reaction at 25C bundance TIC:HB3BT3~ 9 71 1.05e+07j 1e+07j 9000000- 8500000- 8000000 I 7500000; 7000000- 6500000- 6000000 : 10.15 5500000 ' i 5000000^ 4500000- 4000000- 3500000 3000000. :i !! 2500000; i 2000000- 1 500000 11.53 1000000; i 9.0-P;45 500000; 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 HiOO 15.00 16.00 17.00 18.00 19.00 20.00 35 GC/MS Spectrum of Starved hydroboration Reaction at 64C jndance "TIC: HB50.D 9 72 i 9500000- 9000000- 8500000; : 8000000 i 7500000; 7000000- 6500000 6000000- 5500000- 5000000- 4500000 10.16 4000000 11.56 3500000- 3000000- 2500000- 2000000; 1500000- 1000000- 9.029-47 500000; i I !! I II II: 'V 6.00 7.00 8.00 9.00 10^0 11.00 12100 13.00 14100 15100 16.00 17.00 18.00 19.00 36 GC/MS Spectrum of Overfed hydroboration Reaction at 0C Abundance "TIC:HB19.D 9148 | 8500000 | | 8000000- 7500000] 1 7000000-' 6500000 19.84 6000000 ; 15.43 5500000 I I 5000000^ 4500000, 4000000; 16.34 I 3500000; I 3000000! 2500000 . 2000000- 1500000: 1000000-1 500000; d I 111,* ll LAJJki Tjme-> 6.00 7.00 8.00 9.00 10.00 1ll00 12.00 13.00 14l00 15100 16.00 17.00 18.00 19.00 20l00 37 GC/MS Spectrum Overfed hydroboration Reaction at 25C Abundance Tlc7HB58~ir 15.44 1e+07j I ! 9500000; ! 9000000 ! Ij I : 8500000] I I ! i ' 8000000; i i i I 7500000] i ; 16.34 7000000] :j 6500000- i i l 6000000] :: i t 5500000; 5000000- 4500000; I j ! 4000000: i i ; I 9.46 3500000: 3000000- i 2500000 I 2000000- 19.77 i 1 1500000- i 1 000000 15.90 14.80 ' 500000 16.51 lJ.d Time-> 6.00 7.00 8.00 9.00 mOO 11100 12.00 13.00 14100 15100 16.00 17.00 18.00 19100 20l00 21.00 22.00 38 GC/MS Spectrum Overfed hydroboration Reaction at 64C Abundance I IC: HB60.D 4600000 15142 16.34 4400000 i ! 4200000; ! j j 4000000, j 3600000 j | 3400000; i i I i: 32000001 i i ' 3000000 | 2800000 i 2600000 | 2400000 15.9211 I i 2200000 ' 2000000: ' ', 1 800000 i I I , 1600000 9.46 ' 1400000 1200000 : 1000000 ; 800000 i . 600000 19.77 li i 400000 !i 200000 Wul 1 1 r, IL L Time-> 6.00 7.00 8.00 9.00 10100 11.00 12100 13.00 14.00 15.00 16.00 17100 18.00 19.00 20.00 39 Results and Discussion My research involves the catalytic hydroboration of 4-Phenyl-l- butene; hydroboration of this system using RhCl3 as a catalyst was carried out looking for high proportions of secondary alcohol isomers. 4-Phenyl- 1 -butene First, an attempt to isomerize the above system to the corresponding internal alkenes was performed. The system was treated with RhCl3, without the hydroborating agent, using different solvents. The system isomerized to different alkenes in different ratios: RhCI, -Phenyl-2- 4-Phenyl-1 -butene-1 1 butene 1-phenyl-1 -butene (E,Z) Pi 40 Table 3 illustrates the results obtained for different reactions under different conditions: Table3: Percent Ratio of Phenyl-butene Isomers in Different Solvents. Exp# MassofRhCl3 Solvent Time of Rx Pi:P2:P3 (hours) 1 lOmg THF 3 6 : 60:34 2 lOmg CH2C12 3 0 : 47 : 11 3 lOmg Ethanol 3 16 : 57: 28 4 lOmg Ethanol 5 0 : 72 : 28 All the reactions were run at room temperature under nitrogen. Classical hydroboration (hydroboration without any catalyst) 4-phenyl-l-butene gave the results shown: HO ( ll r ! 1) BH3.THF THF 2) H20,H202,NaOH 91% 9% BH3.THF 1) CH2CI2 2) H20,H202,NaOH 93% 7% 41 as a Using RhCl3 catalyst, hydroboration reactions were run for our system. Various isomers of alcohols were and the observed, ratio, as well as the yield, differed according to the solvent used: 1 BH3.THF 64% RhCla CH2CI2 2 H20,H202,NaOH ' 3 0% 20% Overall Yield 57% Using THF as a solvent, and under the same conditions otherwise, we obtained the following results: 42 1 BH3.THF RhCla THF 2 H20,H202,NaOH P3 P4 43% 36% Overall Yield 45% Both hydroboration reactions were 3 hours long at room temperature, followed by 2 hours of oxidation. The solvent might have an effect on the yield of the reaction; this, however, was not a major effect, since the overall yield of all the hydroboration reactions ranged between 40 and 58%. Conditions to establish isomerization: > Stir RhCl3 in solvent until it dissolves almost completely. > Add the olefin. for 20 and then > Inject the borane using a syringe: First, let the borane drop min, minutes' complete injection dropwise in a 20 time. > Color was observed during addition of the borane. while solution in an > For the oxidation process, add the peroxide dropwise the is ice bath, since the oxidation reaction is highly exothermic. compared to the mole ratio of the olefin and If only a small amount of borane is added, in a period of half an followed that of the RhCl3 (l%mole RhCl3, 3% mole borane), hour, isomers of the olefin are by oxidation using a stoichiomertric amount of peroxide, only 43 observed; only a trace amount of alcohol was observed. C. D'Souza suggested this type of hydroboration and he called it: "Starved Hydroboration". Using 1-, 2-, 3-, 4-octenes, the following results were obtained by Mr D'Souza. 1-Octene0.5% 1)RhCL 0.3ml BH3.THF 2-Octene 39.9 % 2) Oxidation 0.5ml Octene 3-Octene 42% 4-Octene 17.6% First-step Time =3 Hours 10mg RhCI3 Using the 4 octenes as starting materials, each octenes gave the same results with the same ratios Starved hydroboration of 4-Phenyl-l- butene, gave the following: 4-Phenyl-1 -Butene 1-Phenyl-2-Butene 1-phenyl-1 -Butene (E,Z) Pi 44 Results of different reactions at different periods of time are illustrated in Table 4. Table 4: Phenylbutene Isomers Produced at Different Times Exp# Moles Moles Moles of Time Ratio % Yield RhCl3 borane olefin (hours) P1:P2:P3 5xl0"5 5xl04 IO'2 1 2 0: 80:14 57 5xl0"5 5xl0"4 IO2 2 5 0: 45: 22 40 5xl05 5xl04 IO2 3 1 2: 83: 8 47 In all reactions, a significant yield of 1-Phenylbutane was obtained. Running a starved hydroboration followed by complete hydroboration, we got the Starved-Overfed Hydroboration with results illustrated in table 5. Table 5: Star*ved-overfed Hydrboration reaction at different Temperature Exp# Moles Moles Moles of Time Temp Ratio % RhCl3 borane olefin (hours) (C) (alcohol) Yield P1:P2:P3:P4 1 5xl05 5xl05- 102 2 RT 0:17:17:40 59 9.5xl03 IO2 2 5xl05 5xl05- 2 0 0:18:16:48 67 9.5xl03 IO2 3 5xl0"5 5xl05- 2 50 0:4:59:20 67 9.5xl03 Hydroboration at Different Temperature and periods of time: Many starved hydroboration reactions were run at 0C and room temperature for different periods of time to investigate the time and temperature effects on the migration of the double bond; the following tables illustrate the obtained results. 45 Table 6. Results Obtained Running Starved Hydroboration at 0C. 4-phenyl-1- Time trans- -phenyl-2- 1 c/s -1-phenyl-2- 1-phenyl-1- 4-phenyl butene (hours) butene butene butene butane 0.5 41 25 27 1 3 1 43 25 27 1 3 1.5 47 26 27 0 0 3 41 26 27 2 3 4 42 26 27 2 3 In the above table, the trans and the cis olefin were classified. The distributions of the above products are shown in the graph below: Starved hydroboration of 4-phenyl-1- butene at OoC 50 40" .2 (0 DC 30 - A A A A "ft - w w o --t=0.5h -- o 20 1=1 h o -^-t=1.5h * 10- x -t=3h -x-t=4h ^^^ * X )^_ _-_ U n I I I C) 2 4 6 Length of Reaction (h) The results of starved hydroboration reactions at room temperature are summarizes in table 7. Table 7: Phenylbutene Isomers Obtained From Starved Hydroboration at Room Temperature cis- trans- -phenyl-2- 1-phenyl-1- -phenyl-2-butene 1 Time(hours) 4-phenyl-1 -butene 1 4-phenyl butane butene butene 0.5 9 50 35 4 2 1 4 57 29 6 4 1.5 8 52 34 4 2 2 2 62 28 5 3 3 3 61 28 5 3 4 8 53 34 3 2 8 3 63 28 5 2 46 The above data are illustrated in the graph follows: Staved Hydroboration of 4-phenyl-1- -4-phenyl butene-1 butene at Room Temperaute 70 - cis- 1- 60 o phenylbut *- 3 50 -m ene-2 40 55 -A trans- 1- A A A 30 as=a- | TT phenyl 2 20 - butene-2 10 ^ 4-phenyl butene-3 Length of Reaction (h) -3K-4-phenyl butane Starved hydroboration reactions were run 64C (boiling point of THF) for different periods of time; the results are summarized in table 8. Table 8. Isomers of Phenylbutene From Starved Hydroborations at Reflux(64C): frans- 4-phenyl-2- cis- Time(hours) 4-phenyl-1 -butene 4-phenyl-2-butene butene 1 -phenyl-1 -butene4-phenylbutar 0.5 2 48 16 34 1 1.5 2 50 15 27 2 3 2 52 20 17 2 4.5 1 55 18 26 1 6 2 55 20 20 3 12 2 49 15 27 2 The above results are illustrated in the following graph: 4-phenyl-1- Satrved Hydroboration of 4-phenyl-1 -butene at Reflux butene 60 cis- 1- phenyl-2- 50 o butene 4-. trans- 1- TO 40 rr phenyl2- u 30 D butene D 1 -phenyl-1 - O 20 butene 10 4-pheny 0 butane 0 5 10 15 Length of Recation (h) 47 Complete (overfed) hydroboration reactions were run at 0C for different periods of time; the results are in table 9. Table 9. Results of Overfed Hydroboration Reactions at 0C Time(hours) 4-phenyl butane (%) 4-phenyl-3- 4-phenyl-2-butanol(%) 4-phenyl-1-butanol(%) butanol(%) 1 10 10 8 58 2 21 3 5 68 3 25 5 5 64 4 15 9 10 64 5 21 5 6 54 The graph illustrating the results of Table 9 is shown below: Hydroboration of 4-Phenyl-1 -butene at 0C .2 80 ?4-phenyl butane 60 I x x g 40 4-phenyl-3- butanol p 20 is 4-phenyl2- butanol 2 4 4-phenyl-1- Length of Reaction(h) butanol Overfed hydroboration reactions were completed at 25C and 64C for different periods of time to investigate the difference in the product ratios of the alcohol yielded; the results are included in table 10 and 1 1 respectively. Hydrboration" Table 10. Alcohol Isomers From "Overfed Reactions at 25C Time(hours) 4-phenyl butane(%) 4-phenyl-3-butanol(%) 4-phenyl-2-butanol(%) 4-phenyl-3-butanol(%) 0.5 30 52 10 29 53 29 54 10 4.5 13 29 48 Hydrboration" Table 11. Alcohol Isomers From "Overfed Reactions at 64C; Time(hours) 4-phenyl butane(%) 4-phenyl-3-butanol(%) 4-phenyl-2-butanol(%) 4-phenyl-3-butanol(%) 40 0.5 11 J6 16| 51 11 40 15 4.5 34 48 Discussions of all the results above are included in the later. Another isomerisation reaction was run with rhodium trichloride at higher temperature. In this reaction 2- ethoxyethyl ether (B.P. 185C) was used as a solvent, the reaction was run for 1.5 hours and repeated for 4.5 hours. Isomerisation occurred with a good percentage of trans- and cw-4-phenyl-2-butene (40:20 % respectively), 25% of 1 -phenyl-1-butanol, and 15% of Phenyl butane. a. Mild Hydroboration of 4-phenyl-1-butene at 25 and 64C in THF. Mild hydrboration was run for 1.5 and 3 hours in THF at 25 and 64C(reflux). In each reaction, 3ml of borane were added, and oxidation was performed exactly as for starved and overfed hydroboration reactions. A mixture of olefin and alcohol isomers was 4-phenyl- -4-phenyl-2- obtained according to the following figures: l-butene(l%), trans butene (8%), cis-4-phenyl-2-butene (2%), 1 phenyl-1-butene (12%), 4-phenyl-3-butanol (37%), 4-phenyl-2-butanol (2%), 4-phenyl-1-butanol (3%), Phenyl-butane (12%), and other minor products. The two reactions at different temperatures yielded the same products with approximate product ratios. Discussion All the performed hydroboration reactions gave almost the same percent yield, whether reactions produced starved or overfed, which ranged between 45%to 55%. Different such as different yields that fell in that range. Many attempts were done, changing material the same but all temperature and mole fraction of the added (using solvent), same. When methylene chloride was the proved fruitless; the yield came out the used, was calculated a GOW-MAC GC column yield dropped down to 40%. The yield using rate of helium was 30ml/min. The packed that was provided in our lab. The flow injection of the analyzed products. All the instrument was turned on 4 hours before and 15 min. The overall percent area of the products came at retention times between 0 49 peaks was obtained. First, samples of pure alcohols were run to determine the retention times of all the products. The yield was determined by obtaining the % area for the compound of interest and dividing it by the total area for all products. This ratio was then multiplied by the total product mass and divided by the mass expected for 100% yield. Isomerisation of olefins using transition metal catalyzed starved hydroboration has been observed in a number of different alkenes under different conditions; the migration of the double bond in the alkene system depends on the function of the catalyst. Cramer and Lindsy pointed out different characteristics of isomerisation reagents, one of them being the fact that transition metal compounds become catalysts for isomerisation only when hydrides15 they are converted to . Isomerisation proceeds by addition of metal hydride to coordinated olefin followed by isomerisation elimination. One of the reactions available olefin1 for hydride generation is the displacement of hydrogen form coordinated . Isomerisation can lead to an equilibrium distribution of isomers, but initial products are kinetically controlled, a fact that permits wide variation in product composition. For example, in the isomerisation of 1-hexene using rhodium trichloride, the percentage of the cw-2-hexene reaches a maximum of about 45% and then falls to 16 %as the 17 isomerisation continues . 4-phenyl- with rhodium as the data 1 -butene, was able to isomerize trichloride; however, shown in the table earlier, the isomerisation depends on the solvent used. For example a all polar solvent isomerised the compound more than a non-polar solvent did. In cases, the time needed to yield the isomerized product was long. (see table 1) seemed to provide a hydride ion to the The catalytic amount of the borane used catalyst, faster. causing the isomerisation to go 50 As discussed in the experimental section, there was a black solid formed upon the addition of the borane; the is a big possibility that the solid produced was a rhodium- hydride species. We were not able to analyze that solid, but we speculated the formation of the hydride catalyst, and the mechanism that will follow is based on our speculations. In long- general, the mechanism that involves addition and elimination of a kinetically lived metal hydride according to the following reaction18: M H + RCH2CH=CH2 ^ [RCH2CHCH3] M y M H + RCH=CHCH3 M is a transition metal. As noted earlier, the metal catalyst needs to be supplied as a hydride for this mechanism. The hydride is formed when a co-catalyst is present. The co-catalyst can be a source of protons like protonic acids or an alcohol; these co-catalysts are known to convert transition metal compounds to hydrides15. In our case, the co-catalyst seems to be the borane, in which the hydrogen atom is cleaved and forms the rhodium hydride, keeping the oxidation state of rhodium unchanged. However, the presence of the chloride ion cannot be ignored here, since it has been reported that isomerisation does not occur in the absence of the chloride ion . The following mechanism illustrates the process of isomerisation: 51 RhCL ^ ,H Cl Rh1b; insertion y reductive elimination < // Rh" Cl Rh' 2.BH, 1. B / \ H H The above mechanism accounts for the formation of the three olefin isomers: 4-phenyl- 1- butene, 4-phenyl-2-butene, and 1 -phenyl-1-butene. The three isomers were produced in different ratios according to the conditions of each reaction. The starved hydroboration reactions at 0C did not effectively isomerise the olefin; the 4-phenyl- major product was the starting material, 1 -butene. It has been reported that 1- migration from the 2-to the 3- position is much slower than from the to the 2-position; 52 this permits the formation of a single positional isomer to be in high yield19. Furthermore, temperature has a great effect on the migration of the double bond. In general, at low temperature, migration is not favored, and the kinetic, less stable product is formed. In our reaction, most of the starting olefin remained unisomerized (40%), compared to other isomers produced. Also cis- and trans-l-phenyl-2-butenes were produced almost in equal amounts (26 %) each; this accounts for the fact that the two products have equal stability at low temperature. At 25C, the product ratios are different from those at 0C; most of the starting material isomerised to cis- and trans-l-phenyl-2-butene in the average ratio of 31 to 57% respectively. At reflux temperature (64C), the product ratios changed; 52 % of the product were cis, and 17% were trans; however, the ratio of 1 -phenyl-1-butene 1- increased to 24% (average) unlike all other reactions, in which the percentage of phenyl-1 -butene was about 5%. From all the tables and the graphs included in the experimental section, it can be reactions. The obviously noticed that time did not have any effect on the isomerisation periods of time. On the product ratios were the same in every reaction run at different product ratios differed at other hand, temperature had a great effect on the reactions; the occurred between starved each temperature, as the graphs illustrate. The other difference added. At hydroboration reactions at 25 and 64C with respect to the amount of borane lxlO"4 5xl0"4 added at 64C for the 25C, borane were added, whereas, moles were 5xl0"4 moles of borane at 64C did not isomerisation reaction to proceed. Addition of change the product ratios. since the alkene -butene be the most stable It is supposed that 1 -phenyl-1 to isomer, and it should have double bond is conjugated with respect to the phenyl ring, therefore, reactions. Our reactions did not produce the been the major product in the isomerisation have been carried out to increase the production of the most stable isomer; many attempts 53 1-pheny-l all -butene, but proved fruitless. The only one that increased the yield was the reflux reaction, and that is discussed above. The overfed hydroboration produced results not much different from the starved hydroboration reaction. As stated in the experimental section, the borane was added very slowly so that the different isomers of the alcohol would be produced; when the borane was injected quickly, very little isomerisation occurred and the reaction proceeded in the classical Markovnikov way. When rhodium trichloride was mixed with 2-ethoxy diethyl ether(boiling point is 185C) and heated until reflux at 185C with no borane added, the product was different, about trans- 1- 40% of the product was and 20% was cw-l-phenyl-2-butene; about 24% was 4-phenyl- phenyl-1-butene and 16% was 1 -butene, the starting material; again, no borane was added. Before the mixture was heated, the color of the solution was red brown. 2- However, upon heating the reaction, and when the temperature reached reflux for the ethoxy diethyl ether, the color of the solution became light black, the same color of the complete hydroboration with borane added. These observations prove that at high temperature, rhodium trichloride change to something else, most probably rhodium of the hydrogenated hydride, as suspected in the hydroboration reactions. No formation olefin was observed. 4-phenyl- of the Overfed hydroboration reactions of 1 -butene produced isomers The alcohols were produced with different corresponding alcohols, phenyl butanols. each reaction. As can be noticed in the product ratios according to the conditions of effect on the percent ratio of the produced corresponding tables, time did not have any produced 4-phenyl-1-butanol as the alcohols. For example, overfed hydroboration at 0C was the anti-Markovnikov major product (62% average). The major product primary effective at low temperature. As in the case of the alcohol; the catalyst did not prove very 54 starved hydroboration, the major product was the thermodynamically stable product, the 4-phenyl- 1-butanol. Eventually, the other alcohol isomers were produced as a result of the isomerisation step, which took place prior to the completion of the hydroboration reaction. In other words, when the borane was injected dropwise into the solution that contained THF, RhCl3 and 4-phenyl-l-butene, isomerisation of the olefin took place when the first 0.5ml borane was added. This caused the olefin to isomerize to the other products as indicated above in the starved hydroboration reaction. Complete addition of the 6ml borane caused the already formed isomers to hydroboarte and oxidize to the corresponding alcohol when the oxidation reaction took place. The mechanism of the overfed hydroboration can be interpreted as follows: Rh111 Rh"! Rh" 2. 3MNaOH 3. H20 + + + 55 As stated in the introduction, hydroboration mechanism involves the formation of tri- alkyl boranes. Therefore, each alkyl borane is, in fact, a tri-alkyl borane, where the olefin is to bonded boron from three sides. An example of one of the tri-alkyl borane formed in our reaction is shown below: tri-4-phenyl-2-butyl borane Similarly, other phenyl butyl boranes were formed. Obviously, we know that 1 -phenyl-1- butene is the most stable conjugated alkene among the other two; however, this alkene could not by produced with the highest yield. Correspondingly, 1 -phenyl-1-butanol was not the major product in the hydroboration-oxidation reaction. Another reason for the -phenyl- 1-phenyl- unsuccessful production of 1 1-butanol is that the intermediate, the tri- 1-butyl borane, is a very bulky and sterically hindered; thus decreasing the yield of the alcohol. Kinetically, and keeping in mind that the tri-alkyl borane is hindered, the alkyl borane that corresponds to the primary alcohol would be the more stable and the major product at room at 0C, allowing the primary alcohol to be the major product. However, product is more stable. The internal temperature, the case is different; the thermodynamic the major as alkene is more stable than the terminal one; therefore, it would be product, tri-4 phenyl-2-butyl and stated before. This internal alkene allows for the formation of the 4- tri-phenyl-3-butyl boranes, which lead to the formation of 4-phenyl-2-butanol, and phenyl-3-butanol as major products. 56 c. Mild Hydroboration The mild hydroboration reactions (in which 3ml of 1M borane was added) produced a mixture of olefin and alcohol isomers of the phenyl butene system. The reason is that not enough was borane added to account for the complete production of the alcohol isomers. In other 3 1M of words, ml, borane were not enough to produce the alcohols; twice as much is need for the overfed hydroboration to occur. As discussed earlier, there is no difference between room and reflux temperature in terms of isomer production in overfed hydroboration reactions. The case was the same in mild hydroboration reactions. From the above pieces of information, we can conclude, that kinetically speaking, only temperature has no effects on overfed and mild hydrboration reactions; however, in case of starved hydroboration reactions, yields were different at 0C from those at 25 and 64C. When the solvent was changed to di-ethoxy ethyl ether, the isomerisation reaction yielded the same products as starved hydroboration at 64C with almost the same percent yield except for the hydrogenation product, 4-phenyl butane, which was not observed. yield of 1-pheny-l Only at high temperatures did we obtain comparatively higher -butene, which, supposedly, should have been produced in the highest yield. Time had no effect whatsoever on all starved, mild and overfed hydrboration reactions or did not change the run. A reaction, be it of any kind, run for 0.5 hours 12 hours product ratios. Most of the plots in the graphs shown in the previous section are straight ratios were almost the same. lines, meaning that at different time periods, the products -butene could not be formed in the highest percent It is still vague to why 1-pheny-l yield, the olefin isomers. We were although, theoretically, it is the most stable product among not able to increase its percent ration to higher than 27%. 57 Temperature an had effect on all hydroboration reactions. As discussed above, product yields were different at 0C from those at 25 and 64C. At higher temperatures there was no major effect; however, at 0C, the difference is noticeable. The product ratios obtained at different temperature can be noticed in the corresponding tables in the previous sections. Borane concentration did have an effect on all hydroboration reactions; however, the catalyst concentration did not. When a catalytic amount of borane was added, isomers of the corresponding alcohol were not obtained; only olefin isomers were obtained instead. 5xl0"4 When the amount of borane increased from to 3xl0"3moles, a mixture of olefin and alcohol isomers was obtained in all reactions at different temperatures and different periods of time. The increase of the amount of borane to 6xl0"3moles produced only 2.4x10" 3x10" alcohol isomers, with no olefins. On the other hand, the addition of and moles of rhodium trichloride did not change the percent ratio of all the olefin and the alcohol isomers in all hydroboration reactions. 4-phenyl- Davis et.al, reported the isomerisation of 1 -butene using palladium 4-phenyl- -butene catalyzed complexes15. He discovered that isomerisation of 1 by 4-phenyl-2- bis(benzonitrile)dichloropalladium(II) involved a rapid conversion into both 1- major product in that isomerisation reaction was butene and 1 -phenyl-1-butene. The to our isomerisation reactions, in which the major products phenyl-1 -butene, contrary nature of the catalyst plays a role in the were l-phenyl-2-butene isomers. Perhaps, the isomerisation involves an olefin complex with isomerisation reaction. The mechanism of occurs1 isomerisation there is an the transition metal with which the ; therefore, and the product from that intermediate involving a metal olefin coordination, resulting 58 intermediate depends on the stability of the intermediate. For example, Davis found that the yield of czs-l-phenyl-2-butene was higher than that of ?ran5-l-phenyl-2-butene, a reason Davis attributed to the fact that olefin-metal complex intermediate of the cis- product is more stable than that of the frans-product. Other cases where the yield of the ris-was higher than that of the trans-were reported20. The same phenomenon applies to 4-phenyl- GC-MS the isomerisation of 1 -butene using rhodium trichloride. The instrument lists the peaks in order of increasing boiling points; in other words, the noticed from the spectra retention time is higher for a higher boiling liquid. It can be corresponds obtained for the starved hydroboration reactions that the retention time that a fact that to the trans- is higher than that of the cz's-isomer, with lower percent ratio; referred The cis- and trans-olefin isomers or agrees with previously reported data below. l-phenyl-2-butene are shown below: cr / \ c/s-metal-olefin complex frans-metal-olefin complex isomerisation reactions movement of the double bond in It has been maintained that the Davis reported that the metals is stepwise in all cases21. However, catalyzed by transition stepwise and is the rate carbon 2 to carbon 3 is movement of the double bond from carbon 4 is rapid due to the fact movement from carbon 3 to determining step, and the with its other isomers due to conjugation that 1-pheny-l-buene is the most stable among the phenyl ring. 59 As stated above, the percent ratios of the alcohol isomers depend on the stability of the alkyl borane intermediates. The phenylbutene system involves a tri-phenyl borane intermediate upon hydroboration; an example of this intermediate is shown above. The of this intermediate stability depends on two things: First, the position of the boron atom, and the second, position of the rhodium atom with respect to the phenyl ring. As for the position of the boron atom, it has been discussed above that there will a steric hindrance that prohibits the formation of the corresponding alcohol, where the hydroxyl group is on carbon 3 or 4(4-phenyl-3-butanol, and 1 -phenyl-1-butanol). The second reason arises from the fact that if rhodium is close to the phenyl ring, there will be an orbital interaction between the d-orbitals of rhodium and the d-orbitas of the phenyl ring. In the Lu Yang mechanism shown earlier, the latter reason was proposed for the formation of the secondary alcohol as a minor yield, a fact that contradicts what Brown found when he use Wilkinson's catalyst in the hydroboration of styrene. As shown in the tables, most of the hydroboration reactions produced the corresponding alkanes in a minor yield. The reason might be due to the formation of the rhodium hydride species. There might be a competition between hydroboration and reduction reactions in which hydroboration wins, thus little reduction product might be obtained. It is not know for sure why the reduction takes place in the hydroboration reactions and the above ideas are only speculations. Another possibility is that a black rhodium metal is formed, and since rhodium trichloride is a little hydrated, a reaction between BH3 and moisture might have occurred and produced hydrogen gas in a very small amount, accounting for the reduction reaction to take place. observations of the refluxed starved hydroboration versus Comparing with the physical color of the reaction the low and room temperature one, we found that the turned dark contrast with the red black after 0.1ml of the borane were added, in brown color of the 60 reaction upon addition of 0.5ml of borane at low and room temperature. This observation made us think that 0.5ml of borane would lead to an overfed hydroboration, since the color was comparable to exactly that of the overfed reaction. However, that was not the we got case; instead, the same product as the low and room temperature starved except hydroboration, for the production of a little amount of the alcohols. To convince ourselves of what was happening in the reaction, we had to run the reflux reaction and add as little borane as possible, just to turn the color to that of the room temperature reaction. Amazingly, we found that upon addition of 0.05ml of borane at reflux, the color of the reaction changed to red brown, exactly as the room temperature reaction; in addition, the product ratio was very comparable to that which was obtained when 0.5ml of borane were added at reflux. This led us to the conclusion that very little amount of borane is necessary for the starved hydroboration at reflux. Summary and Conclusions Hydroboration of olefin was widely studied using different catalysts(Wilkinson's, Rhn) 4-phenyl- and under different conditions. Hydroboration of 1 -butene is of special interest; it was expected that hydroboration using a catalytic amount of BH3 (starved hydroboration) might yield the most stable conjugated isomer, 1 -phenyl-1-butene. However, starved hydroboration reaction under various conditions did not support those expectations. On the contrary, 4-phenyl-2-butene was the major product and the cis-was 1-pheny- in higher yield than the zrarcs-isomer. Similarly, 1-butanol was expected to be 4-phenyl-2- the major product in overfed hydroboration, but experiments proved that butanol 4-phenyl-3-butanol were the major products. The products are obtained by a the to be series of addition elimination reactions causing OH group distributed along the not 1 -phenyl- the results alkyl chain. Since the major product was 1-butanol, are clearly 61 not the result of a single thermodynamically controlled series of phenyl-substituted alkenes. 4-phenyl- The starved hydroboration reaction of 1 -butene is of particular importance. It has been listed that 4-phenyl-2-butene and 1 -phenyl-1-butene are two very expensive compounds; therefore, it would be of great commercial benefit if these two isomers were produced by catalytic hydroboration reactions. The starting materials are incredibly cheap 4-phenyl-l- compared to the products, and it has been shown that the two isomers of butene were produced in good yield in our lab. 62 Acknowledgements I gratefully thank Professor Terrence C. Morrill for his invaluable guidance and support when conducting my research. I also thank my graduate committee: Professor James Worman, Professor Massoud Miri, and Professor Gerald Tackas for their patience in reviewing my thesis. Furthermore, The chemistry department at the Rochester Institute of Technology is gratefully acknowledged and thanked for the financial support it provided me to complete this work. I also thank my research mates for keeping company with most of the time. 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