Olefin isomerization process

Internal olefins in the 4 to 30 carbon number range are isomerized to alpha olefins, utilizing cyclopentadienyltantalum compounds in either a catalyst or reagent mode. Under a preferred practice of the process of this invention, internal olefin is contacted with the reagent to form a first complex, alpha olefin is liberated from said first complex upon contact with carbon monoxide to yield a second complex, and reagent is regenerated from said second complex upon contact with water.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the conversion of internal 
olefins having a carbon number in the range from 4 to about 30 to terminal 
or alpha-olefins of corresponding carbon number. 
Olefins in the C.sub.4 to C.sub.30 range have well recognized commercial 
utility, for instance, in the synthesis of surfactants, lubricants, and 
plasticizers. From the standpoint of most recognized uses for the olefins, 
there is a definite economic incentive for the isomerization of olefins 
having an internal double bond position to alpha olefins with a terminal 
double bond. 
Thermodynamically, however, isomerization in the reverse direction is 
favored, and many processes are known for the conversion of alpha olefins 
to internal olefins. 
For the conversion of internal olefins to alpha olefins, U.S. Pat. No. 
3,131,225 to A. J. Rutkowski and U.S. Pat. No. 3,173,967 to H. C. Brown 
describe the use of alkylborane compounds. Amoung the drawbacks to use of 
this process are relatively high temperatures and long reaction times and 
difficulties in recovering and recycling active alkylborane. 
In the isomerization process of the present invention, use is made of 
certain cyclopentadienyl compounds of tantalum. J. Schwarz et al have 
reported (Angew. Chem. Int. Ed. Engl., 15 (1976), p.333) that 
cyclopentadienyl compounds of zirconium, of the form Cp.sub.2 Zr(H)Cl, can 
be applied to olefin isomerization. An alkylzirconium species is formed by 
reaction of an internal olefin with the Cp.sub.2 Zr(H)Cl. Subsequent 
cleavage of this species results in a net isomerization of the internal 
olefin starting material to alpha olefin. This route suffers substantial 
disadvantage, however, in the measures which must be taken to liberate the 
olefin. Schwarz et al found that olefin was not released from the 
alkylzirconium complex upon heating or upon treatment with another olefin, 
e.g., ethylene. Isomerized olefin was obtained only through treatment of 
the alkylzirconium species with trityltetrafluoroborate, a procedure 
accompanied by substantial conversion of the trityltetrafluoroborate to 
triphenylmethane by-product. 
In U.S. Pat. No. 4,125,567, R. L. Kidwell et al also report that zirconium 
complexes of the same sort investigated by Schwarz et al can be applied to 
olefin isomerization. In apparent contradiction to the findings of 
Schwarz, Kidwell et al disclose that alpha olefin can be liberated from 
the alkylzirconium species by a treatment for exchange of the bound olefin 
with another olefin (for example, ethylene) of different carbon skeletal 
structure. It is suggested that formation of an alkylzirconium species 
from an internal olefin and subsequent exchange with another olefin 
results in a net isomerization of internal to alpha olefin. Experiments 
repeating the work of Kidwell et al have not indicated, however, that the 
overall process fails to achieve such an isomerization in any significant 
degree. Under the procedures of U.S. Pat. No. 4,125,567, the 
cyclopentadienyl-zirconium compound apparently accomplishes a separation 
of alpha olefins from mixtures with internal olefins, rather than a 
conversion of internal olefins to alpha olefins. 
With specific regard to reagents useful in the process of the present 
invention, cyclopentadienyl-tantalum compounds are known materials (U.S. 
Pat. No. 3,288,829 to G. Wilkinson; M. L. H. Green et al, J. Chem. Soc. 
4854 (1961); and A. H. Klazinga et al, J. Organometal. Chem., 157 (1978), 
413). Such compounds are not known to be recognized for utility in olefin 
isomerization. The Klazinga publication does report that the reaction 
between Cp.sub.2 TaCl.sub.2 and either n-BuMgCl or s-BuMgCl yield the 
complex Cp.sub.2 Ta(H) (1-butene). 
SUMMARY OF THE INVENTION 
It has now been found that certain cyclopentadienyl compounds of tantalum 
are useful as reagents for the isomerization of C.sub.4 to C.sub.30 
internal olefins to alpha olefins. Unlike related zirconium compounds, 
complexes of olefins with the tantalum undergo cleavage upon treatment 
with mild liberating agents to result in the desired net isomerization 
with respect to double bond position. 
In its broadest sense, the invention is a process comprising steps for 
contacting a compound of the formula Cp.sub.2 TaH.sub.3, wherein Cp 
represents an optionally alkyl-substituted cyclopentadienyl, indenyl or 
fluorenyl radical, with an internal olefin having a carbon number in the 
range from 4 to about 30 to form an alkyl-tantalum complex, and liberating 
alpha olefin from said complex. 
Preferably liberation of the alpha olefin is accomplished through contact 
of the complex with a liberating agent. The liberating agent is suitably 
the same internal olefin which serves as starting material for the 
isomerization process, in which case the two steps can be accomplished in 
the same reaction mixture with the Cp.sub.2 TaH.sub.3 functioning in the 
nature of a catalyst. Alternatively, use may suitably be made of other 
liberating agents, for instance, oxygen (or air), hydrogen, carbon 
monoxide, and olefins other than the process starting material or product, 
in which case the Cp.sub.2 TaH.sub.3 compound functions as a reagent. Of 
particular interest is a process employing a carbon monoxide liberating 
agent, and comprising as an additional, third step a reaction between the 
resulting Cp.sub.2 Ta(H)CO complex and water to regenerate recycleable 
Cp.sub.2 TaH.sub.3. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is applicable to the isomerization of any internal 
mono-olefin having a carbon number in the range from 4 to about 30. The 
process is most efficiently applied to olefins of the lower carbon 
numbers, preferably to those in the C.sub.4 to C.sub.20 range, more 
preferably to those in the C.sub.4 to C.sub.15 range, and most preferably 
to those in the C.sub.4 to C.sub.10 range. Preference may further be 
stated for an internal olefin starting material of linear (straight-chain) 
molecular structure, although branched olefins are suitable. The 
particular position of the internal double bond in the olefin molecule 
(i.e., whether at the 2, 3, 4 position, etc, in the carbon chain) is not 
found to materially influence its suitability as starting material for 
isomerization under the invention. 
A necessary process step in any embodiment of the invention is the contact 
of the internal olefin with a reagent of the formula Cp.sub.2 TaH.sub.3, 
wherein each of the two Cp substituents is an optionally-substituted 
cyclopentadienyl, idenyl, or fluorenyl radical. Preferably, both of the Cp 
substituents are cyclopentadienyl radicals of the formula C.sub.5 R.sub.5, 
wherein each of the five R moieties individually represents a hydrogen 
atom or an alkyl group. As examples of such preferred Cp substituents, 
mention may be made of pentaalkylcyclopentadienyl groups (in which all 
five R moieties represent the same of different alkyl substituents) such 
as pentamethylcyclopentadienyl, pentaethylcyclopentadienyl, 
pentabutylcyclopentadienyl, pentaoctylcyclopentadienyl, 
ethyltetramethylcyclopentadienyl, diethyltrimethylcyclopentadienyl, 
methylethylpropylhexyloctylcyclopentadienyl; tetraalkylcyclopentadienyl 
groups (in which four of the five R moieties represent the same or 
different alkyl substituents while the fifth represents hydrogen) such as 
tetramethylcyclopentadienyl, methyltriethylcyclopentadienyl, and 
methylethyldipentylcyclopentadienyl; trialkylcyclopentadienyl groups 
(three R moieties represent the same or different alkyl substituents while 
the remaining two represent hydrogen) such as trimethylcyclopentadienyl, 
methylethylpropylcyclopentadienyl, and methyldibutylcyclopentadienyl; 
dialkylcyclopentadienyl groups (two R moieties represent the same or 
different alkyl substituents while the remaining three represent hydrogen) 
such as dimethylcyclopentadienyl and ethylbutylcyclopentadienyl; and 
(mono)alkylcyclopentadienyl groups (four R moieties represent hydrogen 
while the fifth represents an alkyl substituent) such as 
methylcyclopentadienyl and heptacyclopentadienyl. The particular alkyl 
substituents may be positioned at any location and in any order on the 
carbon ring. Thus, 1-methyl-2-ethyl-3-propylcyclopentadienyl is suitable, 
as is 1-methyl-3-ethyl-4-propylcyclopentadienyl, and as is 
1-methyl-2-propyl-3-ethylcyclopentadienyl. For most applications 
contemplated for the invention, each alkyl group represented by R in the 
above formula preferably has no more than about 10, more preferably no 
more than about 6, and most preferably no more than about 3 carbon atoms. 
Particularly preferred for use in the invention is the compound 
trihydridobis(h.sup.5 -cyclopentadienyl)tantalum (V), wherein each R 
substituent the cyclopentadienyl ring is a hydrogen atom. 
The Cp substituent of the compound may also suitably represent an 
optionally-substituted indenyl radical of the formula C.sub.9 R.sub.7 or a 
fluorenyl radical of the formula C.sub.13 R.sub.9. Again, each individual 
R is suitably a hydrogen atom or an alkyl group, and preferences, as 
indicated above, may be expressed for an R alkyl group of limited carbon 
number or for a hydrogen atom. 
The Cp.sub.2 TaH.sub.3 compounds utilized in the invention are known 
materials. The preparation may be accomplished according to methods 
described by G. Wilkinson in U.S. Pat. No. 3,288,829 the relevant 
teachings of which are incorporated herein by this reference. Further 
discussion of the synthesis of such compounds is provided in a publication 
by M. L. H. Green et al (J. Chem. Soc. (1961), p.4854). In general, 
preparation is suitably accomplished by combining (i) a tantalum compound 
(e.g., a halide salt such as TaCl.sub.5), (ii) a substituted or 
unsubstituted cyclopentadienide salt (e.g., NaCp,LiCp), and (iii) a 
reducing agent (e.g. NaBH.sub.4, NaAlH.sub.2 (OCH.sub.2 OCH.sub.3).sub.2), 
in a solvent such as tetrahydrofuran. Yields of the desired Cp.sub.2 
TaH.sub.3 are typically on the order of 5 to 10%. 
The tantalum atom of the reagent is considered critical for purposes of the 
invention. Cyclopentadienyl complexes of zirconium are not found to 
accomplish isomerization of internal to alpha-olefins under like 
processing conditions. Likewise, niobium agents of the form Cp.sub.2 
NbH.sub.3 are ineffective for the desired isomerization. 
Distinctions between the isomerization performance of the Cp.sub.2 
TaH.sub.3 tantalum compounds for purposes of this invention and analogous 
zirconium compounds are believed to relate to the stability of the 
complexes formed upon their reaction with olefin. It is considered to be 
possible to realize the desired isomerization under the invention because 
the resulting tantalum complex is less stable than the zirconium complex. 
Relative instability of the tantalum complex facilitates liberation of the 
isomerized olefin. 
The three hydrogen atoms of the cyclopentadienyl-tantalum compound (that 
is, the three hydrogens apart from any which form a part of the Cp 
radicals) are similarly critical. Replacement of one or more of these 
hydrogen atoms, for instance, by a halogen, analogous to the 
cyclopentadienylzirconium compounds of the art, results in a compound not 
suitable for the isomerization process of the invention. 
It is of substantial advantage that the process of the invention achieves 
isomerization of internal to alpha olefins without producing significant 
changes in carbon structure of the olefin. The process results in little 
or no branching in the olefin structure. Similarly, little or no 
dimerization of olefin is encountered, as is the case, for instance, when 
olefins are contacted with corresponding niobium compounds. 
The cyclopentadienyl-tantalum compound may be applied for isomerization as 
a catalyst. For instance, contact of the compound with internal olefin 
under suitable temperature conditions results in conversion to alpha 
olefin. Internal olefin starting material liberates alpha olefin from a 
complex previously formed, and at the same time is consumed in the 
formation of a complex through which it will undergo isomerization. In 
distinction to the disclosure of U.S. Pat. No. 4,125,567, an exchange 
between olefin molecules of different carbon skeletal structures is 
unnecessary. Performance as a catalyst, however, it inherently limited by 
theromdynamic and equilibrium principles. Accordingly, preference is given 
to the use of the Cp.sub.2 TaH.sub.3 as a reagent. In this case, 
stoichiometric reaction between the reagent and the internal olefin 
starting material yields an alkyl-tantalum first complex. Subsequent 
treatment with liberating agent releases alpha olefin from said first 
complex. Suitable liberating agents for this purpose are substances which 
preferentially (relative to the alpha olefin product) ligate or react with 
the cyclopentadienyltantalum compound. In other words, the displacement of 
alpha olefin from its complex with the reagent is an exothermic reaction. 
Examples of preferred liberating agents include oxygen (or air) the oxides 
of carbon, nitrogen, and sulfur (particularly carbon monoxide), hydrogen, 
and phosphines (particularly alkyl phosphines such as triethyl phosphine). 
Most desirably the process includes a further step for regeneration of 
recycleable Cp.sub.2 TaH.sub.3 from the products of the alpha olefin 
liberation step. In this regard it has been found to be of advantage to 
make use of carbon monoxide as the liberating agent, in a step which 
yields the complex Cp.sub.2 Ta(H)CO. Treatment of this carbon monoxide 
complex with water regenerates Cp.sub.2 TaH.sub.3. Overall, this three 
step process embodiment, including steps for olefin complex formation, 
olefin liberation, and reagent regeneration, in effect superimposes a 
water gas shift reaction upon the isomerization to give a net exothermic 
cycle. 
For all process embodiments of the invention, formation of the complex 
between the Cp.sub.2 TaH.sub.3 compound and the internal olefin is 
suitably accomplished at temperatures in the range from about 0.degree. to 
250.degree. C. Temperatures in the range from about 10.degree. to 
200.degree. C. are preferred, while temperatures between about 15.degree. 
and 150.degree. C. are more preferred and those between about 20.degree. 
and 100.degree. C. are considered most preferred. The lower temperatures 
within these ranges are particularly preferred for minimizng isomerization 
of starting material to olefin of different molecular structure. Similar 
preferences apply to temperature of the alpha olefin liberation and 
reagent regeneration steps. Process pressure is not critical, but should 
be sufficient to maintain olefin starting material and product in the 
liquid phase. Pressures higher than atmospheric are preferred for alpha 
olefin liberation when hydrogen is used as the liberation agent. 
It is generally of advantage to carry out the process in a solvent for 
Cp.sub.2 TaH.sub.3 and its complexes. As examples of preferred solvents, 
mention may particularly be made of ethers and aromatic hydrocarbons, and 
more particularly of tetrahydrofuran, benzene, toluene, and xylene. Apart 
from a requirement that the solvent be essentially inert to other 
components of the processing mixture, however, the choice of solvent is 
not critical. The use of a substantial quantity of solvent is often 
desirable. For instance, with benzene as solvent, Cp.sub.2 TaH.sub.3 is 
soluble only to the extent of about 5 percent by volume, and reaction 
solutions of lesser concentration, e.g., about one percent by volume, are 
often found to result in the most effective use of the compound.

EXAMPLE 1 
The compound trihydridobis(h.sup.5 -cyclopentadienyl)tantalum (V), of the 
formula Cp.sub.2 TaH.sub.3 wherein C.sub.p is an unsubstituted 
cyclopentadienyl radical, was prepared by the following procedure. To a 
1000 ml flask were added 0.18 gram mol of sodium borohydride and 0.53 
gram mol of sodium cyclopentadienide in 210 ml tetrahydrofuran solvent. 
The mixture was cooled to ice bath temperature and 0.06 mol of TaCl.sub.5 
was added over a period of 30 minutes. After refluxing overnight, the 
solution was cooled to room temperature, transferred to another flask and 
evaporated under vacuum to yield a thick paste. The paste was poured into 
a crystallizing dish and evaporated to dryness to produce a brown solid. 
After grinding to a fine powder, the solid was sublimed to yield 2.2 g of 
the desired product, a 10% yield based on tantalum. 
EXAMPLE 2 
A series of experiments were carried out in accordance with the invention 
for the conversion of 2-butene to 1-butene using the trihydrobis(h.sup.5 
-cyclopentadienyl)tantalum reagent. In each case about 80 ml (gas volume 
at standard temperature and pressure) of 2-butene (45% cis-2-butene and 
55% trans-2-butene) was mixed (in a flask which had been previously purged 
with nitrogen to remove air) with 10 ml of a solution containing 10 mg of 
the CpTaH.sub.3 reagent in a benzene, tetrahydrofuran, or xylene solvent. 
After a reaction of the desired duration, oxygen (air) was bubbled through 
the mixture to liberate alpha-olefin. Analysis of the product was made by 
ozonolysis. Isomerization results, in terms of percent of the 2-butene 
converted to 1-butene under various conditions of reaction temperature and 
time and with different solvents, are summarized in the following table: 
ISOMERIZATION OF 2-BUTENE TO 1-BUTENE 
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conversion (%) 
time (hours) 
solvent temperature (.degree. C.) 
5 24 29 48 53 72 
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benzene 20 1 4 5 8 8 8 
benzene 60 1 4 6 9 9 9 
benzene 100 2 5 6 10 11 11 
tetrahydrofuran 
20 1 5 5 8 9 10 
tetrahydrofuran 
60 1 5 6 10 10 10 
tetrahydrofuran 
100 1 6 7 11 12 12 
xylene 20 1 5 5 9 9 9 
xylene 60 1 5 6 8 10 10 
xylene 100 1 5 6 10 10 10 
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EXAMPLE 3 
A series of experiments in accordance with the invention were carried out 
for the isomerization of trans-5-decene. For each experiment, 60 to 140 
milligrams of trihydrobis(h.sup.5 -cyclopentadienyl)tantalum, 10 ml of 
benzene, and 25 microliters of trans-5-decene were added to an 8-dram 
vial. After one to four days of reaction at room temperature, the product 
mixtures were analyzed by ozonolysis. Conversions to 1-decene were on the 
order of about 5% when reagent quantities exceeded about 100 milligrams. 
With lesser quantities of the Cp.sub.2 TaH.sub.3 reagent, results were 
inconsistent, apparently a result of reagent deactivation by impurities 
(e.g., oxygen) in the reaction system. 
COMATIVE EXAMPLE 
The compound Cp.sub.2 Zr(C.sub.3 H.sub.7)Cl was prepared by bubbling 
propylene through a solution of 3.22 grams (12.5 millimols) of Cp.sub.2 
Zr(H)Cl in 100 ml of dry, oxygen-free tetrahydrofuran. Propylene was 
bubbled through the stirred solution at a moderate rate for about two 
hours, until all solid Cp.sub.2 Zr(H)Cl had disappeared. The resulting 
bright yellow solution was bubbled with nitrogen for 15 minutes to remove 
unreacted propylene. 
An unsuccessful attempt was made to convert internal dodecenes to alpha 
dodecenes according to the procedures of U.S. Pat. No. 4,125,567 using the 
CpZr(H)Cl reagent. To the solution prepared as described above were added 
2.35 grams of a mixture of internal dodecenes and 15 ml of 
tetrahydrofuran. The mixture was heated to 80.degree. C. and maintained at 
that temperature under a flow of nitrogen overnight, then cooled to room 
temperature. Volume was reduced to 40 ml by evaporating under vacuum and 
the remaining mixture was transferred to an 83 ml autoclave. The autoclave 
was pressurized to 12.2 atm at -40.degree. C. by addition of propylene. 
The stirred autoclave was maintained at 80.degree. C. for 11 hours then 
cooled to room temperature. Gas chromatographic analysis of the resulting 
liquid product failed to reveal any 1-dodecene product. This result is 
consistent with the suggestions of Schwarz et al (Angew. Chem. Int. Ed. 
Engl., Vol. 15 (1976), no.6, p. 333). 
EXAMPLE 4 
To exemplify a preferred practice under the invention, in which the 
Cp.sub.2 TaH.sub.3 is regenerated, the procedure of Example 3 is followed, 
substituting carbon monoxide for oxygen as the liberating agent. 
Conversion of trans-5-decene to 1-decene is again about 5%. Exchange 
between the carbon monoxide and the alpha olefin yields a complex of the 
form Cp.sub.2 Ta(H)CO from which olefins are separated using standard 
distillation and/or chromatographic techniques. The Cp.sub.2 Ta(H)CO is 
then contacted with water at a temperature of about 50.degree. C. and 
under alkaline conditions (e.g., with pH adjusted to greater than about 
7.5 by addition of a base such as potassium hydroxide) to regenerate 
Cp.sub.2 TaH.sub.3. The reagent is separated from the regeneration 
mixture, for example, by drying followed by solvent (e.g., benzene) 
extraction or, more preferably, by sublimation. Regenerated Cp.sub.2 
TaH.sub.3 is recycled to contact with additional internal olefin and the 
cycle repeated. 
In addition to the isomerization of internal alpha olefins, the process of 
the invention has further been found to effect the isomerization of trans 
olefins to cis olefins. Advantage can be taken of this aspect of the 
invention, for example, in the preparation of cis-dialkylcyclohexanes via 
a Diels-Alder reaction involving a cis olefin and an appropriate 
dienophile. Such structural components are found in terpenes and steroids 
which have utility in drug and pesticide manufacturing. This aspect of the 
invention is illustrated in the following example. 
EXAMPLE 5 
Under the procedures of Example 2, isomerization of trans-2-butene to 
cis-2-butene was accomplished with results as set out in the following 
table: 
ISOMERIZATION OF TRANS-2-BUTENE TO CIS-2-BUTENE 
______________________________________ 
conversion (%) 
time (hours) 
solvent temperature .degree.C. 
5 24 29 48 53 72 
______________________________________ 
benzene 20 3 12 15 21 20 13 
benzene 60 3 15 18 25 23 11 
benzene 100 4 17 21 25 23 15 
tetrahycrofuran 
20 2 22 24 15 12 5 
tetrahydrofuran 
60 3 26 26 13 10 4 
tetrahydrofuran 
100 3 27 25 13 12 4 
xylene 20 2 10 10 18 18 16 
xylene 60 3 10 12 20 20 17 
xylene 100 3 14 16 23 22 16 
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