Chapter Ί Metal Catalysis in Organic Chemistry
Inorganic complexes arefindingincreasing use in organic chemistry, both as reagents and as catalysts f...
Inorganic complexes arefindingincreasing use in organic chemistry, both as reagents and as catalysts for carrying out a variety of syntheses.14 In a number of cases, metal catalysis is uniquely suited for effecting reactions which are not otherwise possible, including such conceptually simple transformations as: CH 2 =CH 2
+ HOAc + o 2
(i) 5 - 7
CH2=CH-OAc + H 2 0
(2) 8 · 9
CH3OH + CO
H CM
3 \jCy c"3 + ° 2 2 CH,C=CH
CH 2 =CHCH 3 + NH 3 + 0 2
" H O * c ^O^ c °: H + H^° P)1 CH,
(4)11.12 (5)13.14
■ CH,=CHCN + H , 0
(6)15,
3 HC=CH
<] + r
COjEt
2 CH 3 CH=CH 2
/
^.C02Et
(7)1
—>^y — ► CH 2 =CH 2 + CH 3 CH=CHCH 3
+ HOAc
O + 2H O
16
+
(8)18a'
(9) 19
b
2
1, Metal Catalysis in Organic Chemistry
Metal catalysis is important in industrial chemistry20 since it allows for high selectivity and economic efficiency in such processes as: Hydrogenation 2H 2 + HOCH2C=CCH2OH
Despite the large number and variety of important catalytic processes extant, many of the major processes are understood only in general outline, and others are hardly understood at all. This situation is a natural consequence of the difficulty of studying catalytic reactions in which the steady state concentrations of the reactive intermediates are perforce low. To promote further developments in this field, a mechanistic understanding of the chemical interactions between the metal complex and the organic substrate
1. Metal Catalysis in Organic Chemistry
3
is desirable and important. Organometals, in which there is bonding between metal and a carbon-centered ligand, play key roles as reactive intermediates in a number of these systems. However, there is surprisingly little that is quantitatively known about how these organometal intermediates are formed and how they undergo further reaction. There are two principal driving forces to consider in the reactions of inorganic complexes: ligand coordination and oxidation-reduction of the metal center. Although these factors are not necessarily mutually exclusive properties,35-37 we will consider them largely as separate. Collman38 has pointed out that a vacant coordination site is a most important property of a catalyst, for it allows the substrate to be brought close to the metal. Retardation and inhibition as well as the requirement of thermal and photochemical activation can often be traced to the necessity of expelling a ligand to generate an active catalyst.39 The optimum coordination number in a transition metal complex with dn configuration is (18 — n)/2. For example, in those metal centers with d6, d8, and d10 spin-paired configurations, full saturation in a metal complex is characterized by 6-, 5-, and 4-coordination, respectively. Coordinative unsaturation is most commonly effected in a metal complex by either loss of a ligand, e.g. 40 ^ 1 PdCl42~ : PdCl3- +CH 2 =CH 2 ;
PdCl3- + C P
(19)
PdCl3(CH2=CH2)- - ^ — CH3CHO
(20)
42 43
dissociation of a bridged dinuclear species, e.g. '