This invention provides a process for hydroesterification of an alpha-olefin with carbon monoxide and hindered thiol compound, in the presence of a catalyst which is a stabilized complex of palladium and tertiary phosphine ligand.

BACKGROUND OF THE INVENTION 
Catalytic carbonylation of olefinic and acetylenic compounds to form 
oxygenated derivatives with an increased content of carbon atoms is a 
well-established technology. Various developments and improvements are 
described in U.S. patents such as U.S. Pat. Nos. 2,768,968; 2,863,911; 
2,876,254; 3,040,090; 3,455,989; 3,501,518; 3,507,891; 3,652,655; 
3,660,439; 3,700,706; 3,723,486; 3,746,747; 3,755,419; 3,755,421; 
3,793,369; 3,856,832; 3,859,319; 3,887,595; 3,906,015; 3,917,677; 
3,952,034; 3,992,423; 4,102,920; 4,245,115; 4,246,183; and references 
cited therein. 
Of particular interest with respect to the present invention is the 
chemical literature relating to hydroesterification of alpha-olefins to 
yield alkanoate esters. 
In J. Org. Chem., 41, 793(1976) and J. Org., Chem., 41, 2885(1976) there is 
reported the synthesis of linear carboxylate esters from alpha-olefins in 
the presence of a homogeneous platinum complex catalyst: 
##STR1## 
U.S. Pat. No. 3,933,884 describes a process for preparing thioloesters by 
the interaction of an alpha-olefin with carbon monoxide and a thiol 
compound in the presence of a catalyst composed of a noble metal halide 
and a Group IVB metal halide and a Group VB donor ligand. 
There is continuing development effort directed to improvement of processes 
and catalysts for carbonylation and hydroesterification of olefinic 
substrates to yield oxygenated or sulfurated derivatives of increased 
carbon content via monomeric and dimeric reaction mechanisms. 
Accordingly, it is a main object of this invention to provide an improved 
process for conversion of aliphatic alpha-olefins into fatty acid 
derivatives. 
It is another object of this invention to provide a process for producing 
alkyl thioloalkanoate by hydroesterification of 1-alkene with improved 
conversion and selectivity. 
It is a further object of this invention to provide a stabilized palladium 
catalyst solution adapted for hydroesterification of olefinic 
hydrocarbons. 
Other objects and advantages of the present invention shall become apparent 
from the accompanying description and illustrative processing data. 
DESCRIPTION OF THE INVENTION 
One or more objects of the present invention are accomplished by the 
provision of a process for hydroesterification of 1-alkene which comprises 
(1) reacting 1-alkene with carbon monoxide and hindered thiol compound in 
a liquid medium containing a halide-free catalyst complex of palladium and 
tertiary phosphine ligand; and (2) recovering alkyl thioloalkanoate 
product. 
The term "1-alkene" is meant to include aliphatic alpha-olefins which 
contain between about 2-12 carbon atoms, and which can contain heteroatoms 
such as oxygen, sulfur, nitrogen and halogen which do not interfere with 
the invention process hydroesterification reaction. Illustrative of 
suitable alpha-olefins are propene, 1-butene; 1-pentene; 1-hexene; 
1,4-hexadiene; 6-chloro-1-hexene; 6-methyl-1-heptene; vinylcyclohexane; 
1-dodecene; and the like. Normal 1-alkene compounds are preferred because 
they can be converted to straight chain fatty acid derivatives which are 
adapted for application as synthetic lubricants. 
The present invention process is highly selective in reactivity, and is 
restricted to the hydroesterification of alpha-olefins. For example, 
1-hexene reacts efficiently under the processing conditions, while 
2-hexene is inert under the same conditions. 
An important aspect of the present invention is the use of a hindered thiol 
compound in the hydroesterification reaction. The term "hindered thiol" is 
meant to include secondary and tertiary thiol compounds which are reactive 
with 1-alkene compounds under the hydroesterification conditions. Thiol 
compounds which are not "hindered" have little or no reactivity with 
1-alkene compounds for purposes of hydroesterification. Thus, 
tertiary-butylthiol reacts smoothly with 1-hexene under the processing 
conditions, while 1-butanethiol is essentially unreactive under the same 
conditions. 
Illustrative of suitable hindered thiol compounds are secondary and 
tertiary thiols containing between about 3-30 carbon atoms and 1-2 thiolo 
groups, such as 2-propanethiol; 2-butanethiol; 1,1-dimethylethanethiol; 
2,4-pentanedithiol; 2-decanethiol; 3-tridecanethiol; 2-eicosanethiol; 
cyclohexanethiol; 1,1,1-triphenylmethanethiol; and the like. 
The thiol and 1-alkene and carbon monoxide coreactants can be employed in 
essentially any proportions as dictated by practical considerations of 
economy and convenience. The presence of the three coreactants per se in a 
reactor system satisfies the stoichiometry of the process, notwithstanding 
that any one coreactant may be present in molar excess relative to the 
other coreactants. 
It is preferred that the carbon monoxide is introduced into the process 
reaction system up to a partial pressure of between about 300 and 2000 psi 
of carbon monoxide. The carbon monoxide environment in the process system 
can contain one or more inert gases such as nitrogen, helium, argon, and 
the like. For optimal results it is essential that the process is 
conducted in a deoxygenated environment, so as not to affect adversely the 
1-alkene conversion rate and the selective yield of alkyl thioloalkanoate 
product. 
The liquid medium in the first step of the process can include a solvent 
diluent, in addition to the other liquid constituents in the 
hydroesterification reaction system. Suitable solvents include propane, 
butane, pentane, cyclopentane, hexane, cyclohexane, heptane, octane, 
tetradecane, petroleum refinery light hydrocarbon mixtures, benzene, 
chlorobenzene, nitrobenzene, toluene, xylene, mesitylene, tetrahydrofuran, 
dimethylformamide, methyl ethyl ketone, the thioloester product, and the 
like. 
A further aspect of the present invention is the provision of a stabilized 
catalyst which is highly selective for hydroesterification of alpha-olefin 
compounds. Thus, in another embodiment the present invention provides a 
catalyst composition consisting of a solvent solution of solute components 
comprising a halide-free complex of palladiun salt and tertiary phosphine 
ligand which is in contact with a stabilizing quantity of thiol compound. 
The "solvent" in the said stabilized catalyst composition can comprise an 
inert solvent diluent of the type previously described, and/or 1-alkene 
and/or tertiary phosphine, and the like. The said catalyst composition can 
be preformed prior to introduction into a hydroesterification zone, or it 
can be formed in situ by the separate introduction of the palladium salt, 
tertiary phosphine ligand and thiol components into the carbonylation 
reaction zone. 
The palladium component of the catalyst composition preferably is 
introduced in the form of a palladium-containing compound such as 
palladium acetate, palladium propionate, palladium acetylacetonate, 
bis-(1,5-diphenyl-3-pentadienone) palladium(o), palladium nitrate, 
palladium sulfate, and the like. The palladium can be in either a plus two 
or zero valent state. 
It is highly preferred that the catalyst composition is halide-free, e.g., 
any halide-containing salt such as palladium(II) chloride is excluded. An 
important advantage of a "halide-free" catalyst complex is the prevention 
of a highly corrosive reaction environment. 
With reference to the tertiary phosphine ligand, the term "phosphine" is 
meant to include corresponding phosphite derivatives. Illustrative of 
suitable tertiary phosphine ligands are triisopropylphosphine, 
tri-n-butylphosphine, triisobutylphosphine, tri-n-octylphosphine, 
tricyclohexylphosphine, triphenylphosphine, tritolylphosphine, 
tribenzylphosphine, and the corresponding phosphite compounds. The 
substituents in the tertiary phosphine ligands can be the same or 
different, and mixtures of tertiary phosphine ligands can be employed. 
Illustrative of a ligand mixture is one containing about 70-99 mole 
percent trialkylphosphine (e.g., triisopropylphosphine) and about 1-30 
mole percent triarylphosphine (e.g., triphenylphosphine). A preferred 
class of tertiary phosphine ligands are trialkylphosphines in which each 
alkyl group contains between 2 and about 8 carbon atoms. 
It appears that a specific type of palladium/tertiary phosphine complex 
catalyst exhibits a superior combination of properties with respect to 
hydroesterification of 1-alkene in comparison with a complex of palladium 
and some other tertiary phosphine ligand, i.e., the preferred catalyst 
contains a trialkylphosphine ligand which has a .DELTA.HNP basicity 
between about 70-350 and a steric parameter .theta. between about 
136.degree.-190.degree.. Illustrative of this category of 
trialkylphosphines are triisopropylphosphine, tri-secondary-butylphosphine 
and triisobutylphosphine. 
For example, palladium/triisopropylphosphine complex provides a better 
balance of conversion and selectivity as a catalyst in the present 
invention process than does any of palladium/tri-n-propylphosphine 
complex, palladium/tri-n-butylphosphine complex, 
palladium/diethylphenylphosphine complex, palladium/tricyclohexylphosphine 
complex, or palladium/triphenylphosphine complex, respectively. 
By the term ".DELTA.HNP" is meant the difference in the half neutralization 
potential between the liquid under consideration and 
N,N'-diphenylquanidine as determined in accordance with the procedure 
described in Analytical Chemistry, 32, 985-987 (1960). The .DELTA.HNP of 
24 tertiary phosphines are listed in U.S. Pat. No. 3,527,809. 
By the term "steric parameter .theta." is meant the apex angle of a 
cylindrical cone, centered 2.28 .ANG. from the center of the phosphorus 
atom, which touches the Van der Waals radii of the outermost atoms of the 
hydrocarbyl substituents [C. A. Tolman, J. Amer. Chem. Soc., 92, 2953 
(1970); Ibid, 92, 2956 (1970); and Ibid, 96, 53 (1974)]. 
It appears that the superior catalytic properties of a 
palladium/triisopropylphosphine type of catalyst complex are attributable 
to the specifically inherent basicity and steric structure of 
triisopropylphosphine as a complexing ligand. It is believed that the 
physicochemical properties of triisopropylphosphine favor the formation of 
a highly active form of complexed palladium catalyst for the purposes of 
hydroesterification of 1-alkene compounds. 
The catalyst complex of palladium salt/tertiary phosphine is provided in 
the hydroesterification reaction medium in at least a catalytic quantity, 
and the mole ratio of 1-alkene to catalyst complex preferably is at least 
1:1 or higher. 
The palladium and tertiary phosphine ligand in the hydroesterification zone 
liquid reaction medium typically are provided in a ratio between about 
1-20 moles of tertiary phosphine ligand per gram atom of palladium metal. 
The palladium and thiol compound in the hydroesterification zone liquid 
reaction medium typically are provided in a ratio between about 1-100 
moles of thiol compound per gram atom of palladium metal. 
It has been observed that the reactivity of the catalyst complex and the 
reaction rate are enhanced if the pH of the liquid medium is maintained in 
a mildly acidic range during the hydroesterification reaction, e.g., a pH 
in the range between about 1-6. In addition, the hydroesterification 
proceeds in a more predictable and reproducible manner when the pH of the 
reaction medium is in the acidic range. It is believed that the acidic pH 
promotes the presence of a favorable catalyst species. 
A convenient means of establishing a desirable acidic pH range is by the 
inclusion of a soluble organic acid in the liquid medium, e.g., acetic 
acid, p-toluenesulfonic acid, or the like. It appears that optimal 
reactivity of the catalyst complex is favored by controlling the acidic pH 
with an acidic compound which is characterized by a poor ligating anion, 
e.g., a carboxylate anion. 
It is highly preferred that the stabilized catalyst complex in the reaction 
system is "halide-free". Among the disadvantages of a catalyst complex 
containing a halide component (e.g., in the form of palladium (II) 
chloride) is the consequential corrosion of metal surfaces in the reactor 
system containing the catalyst halide component. 
It is also preferred to conduct the hydroesterification step of the 
invention process in the presence of a polymerization inhibitor, e.g., 
hydroquinone. If an inhibitor is not included in the reaction system then 
there is an increased incremental loss of 1-alkene to polymeric 
byproducts. When a polymerization inhibitor is employed, the yield of 
byproducts can be limited to less than about 10 percent. 
The temperature for the first step hydroesterification reaction can vary in 
the range between about 50.degree. C. and 180.degree. C., and preferably 
is in the range between about 80.degree. C. and 130.degree. C. 
The pressure in the first step reaction zone can vary in the range between 
about 300 and 3000 psi, and preferably is in the range between about 500 
and 1500 psi. As previously indicated, it is advantageous to provide a 
carbon monoxide partial pressure in the range between about 300 and 2000 
psi in the first step reaction zone. 
In a typical batch type process, the reaction time for the 
hydroesterification step will average in the range between about 0.5 and 
50 hours, as determined by temperature and pressure parameters and the 
reactivity of the palladium-phosphine complex catalyst. 
After the completion of the first step hydroesterification reaction, the 
liquid product mixture is cooled to room temperature or lower. Any high 
molecular weight polyene byproducts in the reaction product mixture tend 
to precipitate out during the cooling stage. As necessary, the reaction 
product mixture can be filtered to remove polymeric precipitate. 
The product mixture is then fractionated by a conventional method such as 
distillation to recover the alkyl thioloalkanoate product. It is highly 
advantageous to leave some alkyl thioloalkanoate as a residual solvent 
medium for the catalyst complex which is in solution. The said solvent 
solution of catalyst can be recycled to the carbonylation step of the 
process. 
In a batch type process, it is convenient and advantageous to perform 
several hydroesterification runs successively in the same reactor system, 
without recovery of alkyl thioloalkanoate product between the respective 
runs. The accumulated product is recovered after the completion of the 
last run. 
In another embodiment, this invention contemplates a continuous process for 
producing and recovering alkyl thioloalkanoate. Illustrative of a specific 
application of the continuous process, a solution of palladium-phosphine 
complex and thiol is fed continuously to a first reaction zone of an 
elongated reactor system, simultaneously with the introduction of 
1-alkene. In the first reaction zone, the feed materials are admixed 
efficiently with each other and with carbon monoxide which is present at a 
partial pressure of at least 300 psi (e.g., 400-700 psi). The admixture is 
passed into a second reaction zone of the reactor system, and the 
temperature and flow rates are controlled in the second reaction zone so 
that optimal proportions of 1-alkene and carbon monoxide are reacted. 
A product stream is removed continuously from the end of the second 
reaction zone. The product stream is distilled to remove a portion of the 
alkyl thioloalkanoate product. The residual solution of product and 
catalyst is recycled to the first reaction zone of the hydroesterification 
system. 
In a typical run, the 1-alkene conversion is 60-65 percent and the 
selectivity to alkyl thioloalkanoate is 80-85 percent. 
At 100.degree. C. and 750 psi carbon monoxide pressure, tertiary-butyl 
thioloheptanoate can be produced from 1-hexene and tertiary-butylthiol 
with a space-time yield of 13-31 grams per liter-hour, a Linear/Branched 
ratio of about 20/1, and about 5 percent 1-hexene isomerization to 
internal hexenes.

The following example is further illustrative of the present invention. The 
reactants and other specific ingredients are presented as being typical, 
and various modifications can be devised in view of the foregoing 
disclosure within the scope of the invention. 
All catalyst solutions were prepared under prepurified nitrogen employing 
standard anaerobic techniques. A standard 300 cc magnedrive 316 SS 
autoclave from Autoclave Engineers was used for pressure reactions. Gas 
was fed into the autoclave from a one liter storage vessel through a 
pressure regulator to maintain constant autoclave pressure. 
The autoclave was also equipped with two 150 ml cylinders to allow addition 
of liquids into the autoclave while under pressure. The autoclave was 
evacuated to &lt;2 mm Hg before each experiment to collect foreign 
condensable materials into a -196.degree. C. trap. The reactor and tubing 
were flushed with carbon monoxide before each run. 
EXAMPLE 
This Example illustrates a typical procedure in accordance with the present 
invention with respect to the production of t-butyl thioloheptanoate. 
Into a nitrogen flushed flask was sequentially placed palladium(II) acetate 
(0.9 g, 4.0 mmole), 20 ml of dry deoxygenated tetrahydrofuran and 
triisopropylphosphine (0.8 ml, 0.7 g, 4.5.times.10.sup.-3 mole) and 0.1 ml 
acetic acid. Upon stirring this mixture at room temperature for 15 
minutes, a deep red-brown solution resulted which constituted the catalyst 
solution. 
The reactants 1-hexene (37.4 ml, 25.2 g) and t-butylthiol (25.0 ml, 20.0 g) 
were added, and tetradecane (12.5 ml, 4.8.times.10.sup.-3 mole) was 
included as a g.c. internal standard. 
A 300 ml 316 SS magnedrive autoclave was flushed with CO and charged at 
room temperature with the reaction solution. The CO pressure in the 
reactor was maintained at 750 psi fed from a one liter storage vessel. The 
reactor temperature was brought to 100.degree. C. as quickly as possible 
(about 0.5 hour) and stabilized at this temperature. 
The reaction was followed as a function of time by observing both the 
change in pressure in the one liter storage vessel and the appearance of 
products by g.c. A 1 ml sample was taken from a bottom liquid sampling tap 
on the autoclave at a given time. This sampling line was washed with 
pentane and flushed with nitrogen after each sample was taken. 
Reaction products were separated and isolated by prep g.c. on a 10-foot, 
3/8 inch, aluminum column packed with 8% Dexil 300 on Anakrom Q 60/80 
mesh(Supelco Inc.). 
A typical plot for the appearance of t-butyl thioloheptanoate and t-butyl 
2-methylthiolohexanoate is shown in the FIGURE. The ratio of t-butyl 
thioloheptanoate to t-butyl 2-methylthiolohexanoate was about 11/1 
throughout the run. Data corresponding to the FIGURE are summarized in the 
Table. 
The initial charge consisted of 0.30 mole of 1-hexene, 4 mmole of 
Pd(OAc).sub.2 /(isopropyl).sub.3 P and 750 psia carbon monoxide, and the 
reaction temperature was 100.degree. C. 
Other experimental efforts demonstrated that the rate of 
hydroesterification of 1-alkene with carbon monoxide and hindered thiol 
compound was not inhibited by an excess of thiol reactant, but the rate 
was reduced when the thiol reactant was kept at a minimum in the reaction 
medium. 
The results indicated that the reaction order in thiol reactant was 
positive, and that the selectivity to linear thiolester product was not 
thiol concentration dependent. 
It was also observed that at a higher reaction rate (conducted over a 
period of 21 hours) the Linear/Branched ratio of thiolester products was 
about 10-12 while at a lower reaction rate (conducted over a period of 44 
hours) the said L/B ratio was in the range of about 20-24. 
TABLE 
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t-Butyl t-Butyl 
Time 2-methylthiolohexanoate 
thioloheptanoate 
Linear/ 
(hour) 
(moles) % (a) (moles) 
% (a) Branched 
______________________________________ 
0.00 0 0 0 0 -- 
0.08 0 0 0 0 -- 
1.08 0 0 0.007 3.3 -- 
2.08 0.002 0.9 0.025 11.3 12.5 
3.08 0.004 1.7 0.041 18.7 10.2 
4.08 0.005 2.2 0.057 26.1 11.4 
5.08 0.006 2.8 0.071 32.2 11.8 
6.33 0.008 3.5 0.087 39.4 10.9 
8.83 0.010 4.5 0.113 51.3 11.3 
21.33 0.015 7.1 0.173 78.8 11.5 
______________________________________ 
(a) Based on tbutylthiol initial (0.22 mole).