Page 249 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011A-544 July 25, 2001 18:30

536 Organometallic Chemistry

a binding group such as OH is present in the alkene, the CH 3 CH CHCH 3 = CH 3 CH 2 CH CH 2 (30)
catalyst will bind to this group and then add the H 2 to the
CH 3 CH 2 CH CH 2 + H 2 + CO
C C bond. This alters the stereochemistry of the product
= CH 3 CH 2 CH 2 CH 2 CHO (31)
in a useful way. A simplified mechanistic scheme for the
reaction is shown for ethylene and H 2 in Eqs. (21–24). An Alkene hydrocyanation, the addition of HCN across an
oxidative addition of H 2 leads to the dihydride. The ethy- alkene, is useful in the industrial preparation of adiponi-
lene then binds and inserts into one M H bond to give the trile [Eq. (32)], the key intermediate in the manufacture of
metal ethyl. The ethyl group then reductively eliminates nylon.
with the remaining H ligand to give ethane. The final re-
CH 2 CHCH CH 2 + 2HCN
ductive elimination is the reverse of the initial oxidative
addition, but involves a C H rather than an H H bond. = NC CH 2 CH 2 CH 2 CH 2 CN (32)
Note that the catalyst, symbolized by L n M, is regenerated
Hydrosilation, the addition of a silane, R 3 Si H across
in the final step so that it can react with a second and
an alkene is useful in the industrial preparation of silicone
subsequent molecules of the reagents. Many thousands or
polymers and materials.
millions of catalytic turnovers can be carried out, which
means that very little catalyst needs to be present. (33)
RCH CH 2 + R 3 Si H = RCH 2 CH 2 SiR 3
(21)
L n M + H 2 = L n MH 2 Polymerization of alkenes is an important area in which
L n MH 2 + H 2 C CH 2 = L n MH 2 (H 2 C CH 2 ) (22) homogeneous catalysts, in the form of Ziegler–Natta and
metallocenesystems,havebeenveryimportant.Thesecat-
L n MH 2 (H 2 C CH 2 ) = L n MH(CH 2 CH 3 ) (23)
alysts are formed from an early metal compound, such as
L n MH(CH 2 CH 3 ) = L n M + H 3 C CH 3 (24) Cp 2 ZrCl 2 or TiCl 4 , an aluminum alkyl, such as Me 2 AlCl,
Hydroformylation, Eqs. (25) and (26), is useful for the and a trace of water. They are believed to operate by suc-
preparation of aldehydes and, by hydrogenation of the cessive insertion reactions, as shown in Eq. (34).
aldehydes in situ, alcohols. L n M(CH 2 CH 3 ) + H 2 C CH 2 = L n M(CH 2 CH 2 CH 2 CH 3 )
RCH CH 2 + CO + H 2 = RCH 2 CH 2 CHO (25) (34)
RCH 2 CH 2 CHO + H 2 = RCH 2 CH 2 CH 2 OH Metal-carbon multiple bonds are found in a number
(26) of situations. The simplest is illustrated in Scheme 12,
which shows the synthesis of a carbene complex. When
The mechanism is similar to that for hydrogenation, but at
a heteroatom is present, α to the carbene carbon, the
the alkyl hydride stage a carbonyl insertion reaction takes
M C bond is not a full double bond because both res-
place to lead to the sequence shown in Eqs. (27)–(29).
onance structures (Scheme 12) are thought to contribute.
L n MH(CH 2 CH 3 ) + CO Carbenes without heteroatom substituents have been pre-
pared, and these seem to have a true double bond. One
= L (n −1) M(CO)H(CH 2 CH 3 ) (27)
example is Cp 2 Ta( CH 2 )Me, formed by deprotonation of
L (n −1) M(CO)H(CH 2 CH 3 ) the Cp 2 TaMe cation.
+
2
= L (n −1) MH(COCH 2 CH 3 ) (28)
Me
L (n −1) MH(COCH 2 CH 3 ) MeLi MeI
(CO) 5 M CO (CO) 5 M C
= L (n −1) M + HCOCH 2 CH 3 ) (29)
OLi
Some hydroformylation catalysts are also alkene iso-
Me Me
merization catalysts; that is to say they move the position
(CO) 5 M C (CO) 5 M C
of the C C double bond along a chain, as shown in Eq.
(30). This can happen much faster than hydroformylation OMe OMe
itself. Advantage can be taken of this effect if one of the Scheme 12.
isomers reacts faster in the hydroformylation sequence
M C double bonded intermediates are believed to be
than the others. In the case shown in Eqs. (30) and (31)
important in alkene metathesis Eq. (35), a useful com-
the terminal alkene reacts faster and forms the linear
mercial process that has been applied to organic synthe-
aldehyde shown. Of three possible aldehydes, only one
sis, polymerization, and to changing the molecular weight
is formed in significant quantities if the right catalyst is
distribution of a mixture of alkenes (Eq. 35).
used. This high reaction selectivity is a useful property
of homogeneous catalysts. RCH CH 2 = RCH CHR + CH 2 CH 2 (35)