tivities taken after 1 or about 80 h are the highest (<1h or <f;
Table 5) for the Me2SnCy2 complexes.
Although the results are somewhat fragmented, the results
from this systematic study of the hydrogenolysis of the Sn–C
bond on Rh/SiO2 has brought to light the importance of
fivefold coordination and steric effects as determining factors.
References
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Scheme 3
Concerning the kinetics of the reaction and the selectivity,
there are a few points that deserve to be mentioned (Table 5).
(i) The Sn–Me bond always reacts more rapidly than the Sn–
Bu bond. (ii) The proportion of methane or butane given off
after 1 h of reaction (MeH1h/Me or RH1h/R) is proportional to
the number of Sn–Me bonds in the starting compound. (iii)
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reacted after ca. 80 h.
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bonds can be easily explained by the higher steric constraints
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any steric effect observed.
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We have studied the hydrogenolysis of organometallic deriva-
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pounds examined can be divided into three families of the
following general formula: (i) SnMexBu(4 ꢀ x), where x = 0–4;
(ii) SnMe3R, where R = tBu, Np, Cy; (iii) SnMe2R2, where
R = tBu, Np, Cy
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(ii) During the first hour, as after 80 h, only methane and
hydrocarbons corresponding to the cleavage of the Sn–C bond
are observed. This implies that there is no cleavage of C–C
bonds. This regioselectivity of the hydrogenolysis is probably
governed by the difference in the Sn–C and C–C bond en-
ergies.45,46
(iii) The stoichiometry of the surface species after 1 h shows
the presence of more alkyl groups than after 80 h of hydro-
genolysis, which supports a step-wise hydrogenolysis of the
Sn–C bond.
(iv) The first step of the reaction could form a five-coordi-
nate tin intermediate, of D3h symmetry, where the metallic
surface plays the role of the electron-donating coordinate and
the bulkiest R3 group is apicophilic.51
(v) Once grafted, the five-coordinate surface tin species can
eliminate an alkyl group via a concerted hydrogen transfer-
elimination mechanism.52
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(vi) Among the alkyl groups studied (R = Me, Bu, tBu, Np,
Cy), the Cy group is the most stable, since both the regioselec-
1536
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 5 3 1 – 1 5 3 7