Reactive Intermediates Volume 1
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89
Metal Atoms as Reactive Intermediates
In the case of Ge atoms, oxidative addition of CCl 4 occurs, but the resulting Cl 3 CGeCI abstracts CI atoms from excess CCI 4 , resulting in a respectable yield of CI 3 CGeCI 3 • Chloroform and silicon tetrachloride reacted similarly as shown below 110:
~(....:.C..:..;H,-C...:Ia,-
Ge
r
ClaCGeCIa 20%
~
ClaSiGeCla
10%
B. Acyl Halides with Metal Atoms (Macroscale Co condensation) Acyl chlorides oxidatively add to palladium atoms and nickel atoms.27i.95 In all the examples studied, the resultant RCOMCI species liberated CO very
o 1/
RCCI
+
o II
M atom - - + RC-M-CI
M = Ni, Pd R = CHa, CFa, C.H s , C.Fs , n-CaF7
readily. By carrying out low-temperature trapping experiments with Et 3 P we were ahle to reach some conclusions about thermal stabilities and decomposition pathways. Table 10 summarizes our findings,27i.95 and Figure 20 illustrates the
TABLE /0. Acyl Chloride-Metal-Atom Reactions, Products, and Comments271.95
Reactants Pd, CF 3 COCI Pd, CF aCF 2 CF 2 COCI Pd, C 6 FsCOCl Pd, C 6 H sCOCI Pd, CH 3 COCI Ni, CF 3 COCI
Comments CFaCOPdCl eliminates CO at -80T, at which point Et 3 P addition yielded almost equal quantities of (Et 3 PhPdCI(CF 3 ), (Et 3 P),PdCI(COCF 3 ) and (Et 3 P),PdCI 2 CF 3 CF 2 CF 2 COPdCI stable above -80 v C (but -
:C:
+
stable fragments
(7)
one must choose an energetic carbene in which the fragment(s) are of sufficiently low energy to provide a thermodynamic driving force. A carbene which appears to meet the above criteria is tetracyclo[3.2.0.02.7.04.6]-heptane-3-ylidene (1), which is reported to generate atomic carbon by the sequence shown in equation (8).42 Pyrolysis of the tosylhydrazone lithium salt 2 produces benzene and a carbon atom. The carbon atom then reacts N- N-Ts
tb II
+
C
J:;
(8)
2
with benzene to produce toluene, a known reaction of atomic carbon. 43 That the toluene is not simply formed by the rearrangement of 1 is established by adding benzene- l4 C and observing radioactive toluene. Although other reactions characteristic of atomic carbon are observed in this system, the high reactivity of carbon toward benzene renders investigations of these reactions difficult. A more satisfactory carbene precursor to atomic carbon appears to be 3. Thermal decomposition of 5-tetrazoyldiazonium chloride (4) by the reaction sequence shown in equation (9) generates atomic carbon.44 Reactions of atomic
8
Philip B. Shevlin N 2 +C1-
NAN/ \
C
H
/
N=N
N/ "N
~
II
\\
-----* -HCl
N-N
4
-- C
+
2N2
(9)
3
carbon are investigated in this system by pyrolyzing 4 in the presence of a gaseous reactant. A number of reactions of these chemically generated carbon atoms have been reported and will be discussed in subsequent sections of this review. Investigations of reactions involving the thermolysis of 4 have the advantage of generating carbon atoms at convenient temperatures (~JOO°C) with little excess kinetic energy. However, it has not been definitely established that free carbon atoms are produced in these systems
