Transcript Group 14 - University of Ottawa

Group 14
General Features Electron precise species
four coordination - greater steric congestion
lack of low energy LUMO - hydrolytically resistant
much less susceptible to nucleophilic attack vs.
group 13
These elements have electronegativity closer to C
Element
C
Decreasing E-C strength down the group
Si
Pb-R homolysis on heating about 100ºC.
Ge
Sn
bond enthalpy of approx 150kJ/mol vs.
Pb
Si-C of approx 320 kJ/mol
low polarity to the E-C

2.55
1.9
2.01
1.96
2.33
Group 14
General
Carbon is a special member of the group. For example, p bonding
much more important to this element than to other members.
Homoleptic alkyls are possible in both the +2 (ER2) and +4 (ER4)
oxidation states
ER4 are generally very stable.
The inert pair effect leads to higher prevalence of R2E down the
group.
Synthesis
One of the first organometallic compounds (Frankland, J. Chem
Soc. 1849, 2, 263) :
Sn + 2EtI  Et2SnI2
Followed up by reactions with Et2Zn:
2 ZnR2 + SnCl4  SnR4 + 2 ZnCl2
ZnR2 + SnCl2  SnR4 + ZnCl2
With the discovery of Grignard, this synthetic method replaced by
RMgX metathesis.
Synthesis
The homoleptic ER4 compounds can be made by metathesis,
hydrometallation, and coupling reactions :
SnCl4 + LiR4  SnR4 + 4 LiCl
SiH4 + H2C=CH2  SiEt4
Coupling reactions combine direct and metathesis reactions:
GeCl4 + 4 RX + 8 Na  GeR4 + 4 NaCl + 4 NaX
4 RX + 8 Na  4 NaX + 4 NaR
4 NaR + GeCl4  GeR4 + 4 NaCl
Industrial preparation of tetrabutyl tin
3 SnCl4 + 4 R3Al  3 SnR4 + AlCl3
Synthesis
An interesting reaction for Sn is the transmetallation with Li
reagents :
R3SnR’ + LiR”  R3SnR” + LiR’
This reaction is particularly useful when R’ = allyl or benzyl.
In this case it is difficult to directly make LiR’ due to the reaction of
LiR’ with R’X:
Li + R’X  LiR’ + LiX
LiR’ + R’X  R’-R’ + LiX
Structures and Stability of ER4
Compounds
These compounds typically exist as monomers.
The coordinative saturation and low electrophilicity prevent
dimerization or interaction with a donor solvent.
The reactivity of these compounds is not enhanced by the addition
of a donor solvent or atom.
With the exception of lead compounds, these organometallics are
stable in air at room temperature. Bond strengths determine their
lability to thermolysis..
The inert pair effect leads to more stable +2 oxidation state which
also increases the lability of R4E down the group.
PbEt4 Synthesis
“Leaded Gas” contains the antiknock agent PbEt4.
It is produced by the disproportionation of Pb(II) acetate in the
presence of Et3Al:
6 Pb(OAc)2 + 4 AlEt3  3 Pb + 3 PbEt4 + 4 Al(OAc)3
Two key features of organolead compounds
toxic (but 1/10 that of Pd!)
much weaker M-C – thermal and light.
Organoleads will decompose by Pb-C bond homolysis and bhydrogen elimination/reductive elimination to produce lead, H2,
alkanes and alkenes.
For R4Pb, the stability follows:
Me > Et > iPr
Synthesis of the Mixed Alkyl Halides
Mixed halo alkyl species are synthetically useful and more reactive than
homoleptic alkyls
Redistribution reactions of the homoleptic alkyls are employed commercially to
prepare halosilanes (AlCl3 Lewis acid catalyst)
n R4M + (4-n) MX4  4 MRnX4-n
Often non-statistical product distribution - subtle bonding and steric effects can
favor particular product.
Metathesis reactions can be employed using LiR or Grignard:
SiCl4 + x LiR  SiR4-xClx + x LiCl
Transmetallation can be used with divalent species
(Sn(II)/(IV) with Hg(II/0))
SnCl2 + Ph2Hg
Ph2SnCl2 + Hg
Structures of the Group 14 Alkyl Halides
Presence of halide leads to rich structural chemistry for Sn and Pb
owing to M-X-M bridges The tendency toward aggregation
increases with diameter.
Silicon and germanium compounds are typically monomers.
Alkyl monohalides of Sn and Pb show polymeric
aggregation in the solid phase,
bridging through the halides:
tbp
Me3SnF
Ph3PbX
Sterics of R play a key role
Alkyl Dihalides of Sn and Pb
Dihalo derivatives tend to display octahedral centers.
Me2SnCl2
Me2SnF2
All of these are monomers in the gas and solution phases.
Reactivity of Si-X
Protonolysis
Convenient route to Si-O bonds (siloxanes), Si-N (silazanes from N-H), Si-S
(from S-H)
Note that the less polar E-C are not as reactive
For the reaction with water –
Initial reaction is formation of silanol but a strong tendency to form Si-O-Si
linkages leads to H2O elimination and formation of siloxanes
dihalosilane – rings and chains
RSiCl3 can lead to more elaborate three-dimensional structures.
Siloxanes undergo redistribution to yield silicones.
Reactivity of R3ECl
(R3E)3N
(R3E)2O
R3EOH
H2O
L-ER3Cl
Li3N
R3E-ER3
or NaSnR3
Li or Na
L
R 3ECl
R'OH
LiAlH4
R3EH
R3EOR'
R'Li
R 3ER'
It is common for silicon to dehydrate and couple (less so for Sn
and Ge):
2 R3SiOH  H2O + (R3Si)2O
2 R3SiSH  H2S + (R3Si)2S
2 R3SiNH2  H3N + (R3Si)2NH
Reactivity of R3ECl
Mixed species are particularly useful starting materials for
metathesis reactions.
The mechanism of these reactions appear to be associative and
second order overall with a dependence of k on identity of
entering group.
Intermediate is likely a five-coordinate species
Stereochemical studies indicate that both inversion and retention
of E configuration can be observed
Lewis Acid Character
These compounds are still have enough electrophilic behaviour to
accept nucleophiles and produce anions in solution:
Bu3SnCl + Cl-  Bu3SnCl2Me2SnCl2 + 2 Cl-  Me2SnCl42Me2SnCl2 + 2 O=SMe2  Me2SnCl2.2 O=SMe
This amphoteric behaviour allows these compounds to eliminate a
halide and form anions when reduced with sodium.
Rochow Process (Direct)
Organosilicon dichlorides (specifically) can be made by the
Rochow process:
2 RCl + E/Cu  R2ECl2 + Cu
generalizes over Si, Ge, and Sn
Cu is a necessary catalyst in this process – shuttles between
Cu/CuCl and transfers Cl and Me to Si.
This process allowed access to silicones.
Polysiloxanes (Silicones)
Once organosilicon chlorides were widely available, allowed largescale silicone production
Silicones are made by the hydrolysis of organosilicon chlorides
and subsequent dehydration and redistribution :
R4-xSiClx + x H2O  x HCl + R4-xSi(OH)x  [R4-xSi-O]n + n H2O
Depending on the number of chlorides, the resulting silicone can
be linear or highly branched.
They are strong, flexible polymers
Stability of Si-O-Si
R3Si
O
SiR3
Si-O-Si exhibits low Lewis basicity and large angle suggest a role for p-d-p
bonding (p) or overlap with the s* on Si
Increased flexibility of this linkage due to decreased directionality of the Si-O
bonds
Planarity in N(SiH3)3 has also been explained by similar delocalization of the N
lone pair (weakly basic)
A related observation is the relative ease of deprotonation of SiCH3 by strong
bases (carbanions) - conjugate base stabilization via delocalization to Si
Polystannoxanes
Due to the weaker Sn-O bond, polystannoxanes don’t show as
strong a backbone, and thus as unreactive a polymer as the
silicon analogue:
R2SnCl2 (OH-, H2O) R2Sn(OH)2 (-H2O) [R2Sn-O-]n
This willingness to datively bond to another polymer chain is due
to the weaker p interactions due to tin’s size and low charge
density.
Reactivity of Polystannoxanes
Polystannoxanes are subject to decomposition by acids and
bases, unlike silicones:
Again, the weak Sn-O-Sn backbone allows attack of a nucleophile
at the tin, or by protons at the oxygen linkage to dehydrate.
Anions
The monohalides of group 14 can be reduced with electropositive
metals to produce anions:
Ph3SiCl + 2 Li  Ph3SiLi + LiCl
Tin and lead need to be reduced in liquid ammonia, due to their
amphoteric behaviour:
R3EX + 2 Na (NH3, -78oC) NaER3 + NaX (E = Sn, Pb)
This is analogous to similar carbon chemistry, and tertiary group
14 compounds with electron-withdrawing moieties stabilizes these
anions.
Commercial uses of Organotin
Compounds
Catalysis, stabilizers, biocidal agents:
Bu3SnOAc (Bu3SnCl and NaOAc) – antifouling agent and applications to
catalysis (polymerization)
Bu2SnOAc2 PVC stabilizer
(cyclo-C6H11)3SnOAc – insecticide in orchards and vineyards
Bu3SnOSnBu3 (hydrolysis of Bu3SnCl) – algicide and antifouling
Ph3SnOSnPh3 - antifouling
(Bu2SnS)3 from Bu2SnCl2 with Na2S – PVC stabilizer
Electronegativity - Hydrides
Element
C
Si
Ge
Sn
Pb

2.55
1.9
2.01
1.96
2.33
"hydride"
DH (kcal/mol)
CH4
105
Me3SnH
95
Bu3GeH
88.6
Bu3SnH
78.6
Because hydrogen has an electronegativity of 2.20, Si, Ge, and
Sn hydrides can react in a “hydridic” fashion.
Note that since germanium and hydrogen are very close in
electronegativity, electron-donating groups can allow the
germaninum hydride compound to react to release a proton:
R3GeH + RLi  RH + R3GeLi
Hydrides - Silane Synthesis
Silane is made by the reaction of lithium aluminium hydride with a
polychlorosilane.
It requires a silicon chloride species, which can be made by direct
reaction:
n Si + (n+1) Cl2  SinCl2n+2 (n = 1-6)
This step goes through SiCl4 and subsequently reacts with
additional silicon. The amount of excess silicon determines n.
SinCl2n+2 + xs LiAlH4  SinH2n+2
This step produces some AlCl3, but it is very difficult to get all of
the hydrides on LiAlH4 to exchange.
Hydrosilation
Hydrosilation is an important organic reaction. Regioselective to
anti-Markovnikov products.
Unlike hydroboration, it can selectively reduce a carbonyl group:
R
R
O
R3Si
O
C
H
R
R3Si
C
H
R
O
R
SiR 3
C
H R
Here the hydrogen is the nucleophile due to the low
electronegativity of silicon. Thus, this is a hydride transfer.
Hydrogermylation and hydrostannylation are also known.
Ge/Sn Hydrides
Radical based reduction reactions:
e.g. R3SnH + R’X (hu) R3SnX + R’H
Due to the weakness of the Sn-H bond (300 kJ/mol), it is possible
to break organotin hydrides into radical species:
R3SnH (hu) R3Sn• + H•
Bu3GeH is about 10x slower than Bu3SnH
(interestingly (Me3Si)3SiH is comparable to Bu3SnH in many
cases)
Used to prepare GeCl2 from GeCl4
Lead Hydride
Lead hydride decomposes at room temperature due to the
weakness of its Pb-H bond (~205 kJ/mol) .
It will form R3Pb-PbR3 compounds and H2 from a radical coupling
reaction.
Hydroplumbation adds readily at low temperature to alkenes and
alkynes to gve stable Pb(IV) compounds.
Aside from reaction with lithium aluminium hydride, it is common
to synthesize lead hydride (at low temperature) by metathesis
using another group 14 hydride:
nBu PbX
3
+ Ph3SnH (-78oC) nBu3PbH + Ph3SnX
Use in Organic Reactions
Group 14 organic compounds are commonly used in C-C bond
forming reactions in organic chemistry.
Si-C reacts as a carbanion equivalent
Mukiyama aldol
Hosomi-Sakurai
Hiyama coupling
Sn used in Pd catalyzed coupling reactions
Stille coupling
Use in Organic Reactions
Silyl enolates:
silyl enol ethers as an enolate equivalent in Lewis acid-catalyzed
aldol additions
Trichlorosilyl enolateoffers a route free of
catalysts
Hosomi-Sakurai Reaction
Lewis acid-promoted allylation of various electrophiles with
allyltrimethysilane. Activation by Lewis acids is critical for an
efficient allylation to take place.
Initial step of proposed mechanism:
Only catalytic amounts of Lewis acid are needed in the
newer protocols (allylsilyl chlorides instead of
allyltrimethylsilane)
H. M. Zerth, N. M. Leonard, R. S. Mohan, Org. Lett., 2003, 5, 55-57.
Stille Coupling
The Stille Coupling is a versatile C-C bond forming reaction
between stannanes and halides or pseudohalides, with very few
limitations on the R-groups.
The main drawback is the toxicity of the tin compounds used, and their low
polarity, which makes them poorly soluble in water.
Stannanes are stable, but boronic acids and their derivatives undergo much the
same chemistry in what is known as the Suzuki Coupling.