"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Radical Chemistry on Silicon Surfaces 213
8.5.4 ADDITION OF ALKENES ON Si(100) SURFACES
The method of heating at 200 8C without radical initiator has been extended to
the monolayer formation on a Si(100) surface (see Figure 8.3) [66]. The reaction
works also well with v-functionalized alkenes and allows further manipulation
of the monolayers, such as the removal of protecting groups of acids or alcohols.
A major limitation of the above mentioned hydrosilylation procedures, is the
large excess of alkene required since the freshly prepared fluoride ion-etched
silicon hydride surface was placed in pure liquid alkene. It was reported that it is
possible to work in solution with dilute alkene but the choice of solvent has an
influence on the molecular ordering of the alkyl monolayers [67]. Mesitylene was
the only solvent in the study that provided well ordered monolayers and it was
suggested that it is too large to fit in pinholes of the forming film and thus, it
cannot interfere with the monolayer formation process.
It is worth underlining two fundamental aspects of organosilicon hydrides in
solution that may be also important to surface radical chain reactions: (i) The
dissociation enthalpy of H w Si bond in the surface should be strongly dependent
on the nature of the surface. Based on the available thermochemical data of
organosilicon hydrides reported in Chapter 2, the H w Si(111) is expected to be
about 10 kJ/mol weaker than the Si w H bond in dihydride-terminated Si(100)
surfaces. Consequently, the rate constant for hydrogen abstraction should vary
substantially depending on the nature of the silicon surface (cf. Chapter 3). (ii)
The hydrogen donor abilities of solvent could also play an important role like
that observed in reduction reactions (cf. Chapter 4). For example, it would be
not very surprising to discover that mesitylene is also a mediator in the hydrosilylation
of dihydride-terminated Si(100) surface with 1-alkenes. Therefore,
attention should be paid to extrapolating a mechanistic scheme from one to
another silicon surface or in comparing two similar reaction products derived
from different silicon material.
Under ultrahigh vacuum conditions and exposure to hydrogen atoms it is
possible to produce the so-called Si(100) 2 1 dimer surface, in which the
Si w H surface bonds decrease from two per silicon to only one (Scheme 8.12)
[46,48]. It was shown that the addition of styrene to H w Si(100) surface could
be initiated from isolated surfaces silyl radicals created using the tip of the
scanning tunnelling microscope [68]. Sequence of STM images showed that by
increasing the exposure to styrene, growth lines up to 13 nm long (corresponding
to 34 adsorption sites) were observed. However, growth of the lines was
observed to stop at pre-existing defects of the surface. It was proposed that the
reaction proceeds through a surface radical chain reaction as shown in Scheme
8.12. The mechanism consists of the addition of surface silyl radical to styrene
followed by a 1,5 hydrogen shift to generate another surface silyl radical that
continues the chain. The interesting consequence of the anisotropic nature of
the H w Si(100) surface is the preferential growth along one edge of the silicon
dimer row, whereas in the analogous H w Si(111) surface experiment compact