1. 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 low polarity to the E-C Decreasing E-C strength down the group Pb-R homolysis on heating about 100ºC. bond enthalpy of approx 150kJ/mol vs. Si-C of approx 320 kJ/mol

2. Group 14 General Carbon is a special member of the group. For example,  bonding much more important to this element than to other members. Homoleptic alkyls are possible in both the +2 (ER 2 ) and +4 (ER 4 ) oxidation states ER 4 are generally very stable. The inert pair effect leads to higher prevalence of R 2 E down the group.

3. Synthesis One of the first organometallic compounds (Frankland, J. Chem Soc. 1849, 2, 263) : Sn + 2EtI  Et 2 SnI 2 Followed up by reactions with Et 2 Zn: 2 ZnR 2 + SnCl 4  SnR 4 + 2 ZnCl 2 ZnR 2 + SnCl 2  SnR 4 + ZnCl 2 With the discovery of Grignard, this synthetic method replaced by RMgX metathesis.

4. Synthesis The homoleptic ER 4 compounds can be made by metathesis, hydrometallation, and coupling reactions : SnCl 4 + LiR 4  SnR 4 + 4 LiCl SiH 4 + H 2 C=CH 2  SiEt 4 Coupling reactions combine direct and metathesis reactions: GeCl 4 + 4 RX + 8 Na  GeR 4 + 4 NaCl + 4 NaX 4 RX + 8 Na  4 NaX + 4 NaR 4 NaR + GeCl 4  GeR 4 + 4 NaCl Industrial preparation of tetrabutyl tin 3 SnCl 4 + 4 R 3 Al  3 SnR 4 + AlCl 3

5. Synthesis An interesting reaction for Sn is the transmetallation with Li reagents : R 3 SnR’ + LiR”  R 3 SnR” + 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

6. Structures and Stability of ER 4 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 R 4 E down the group.

7. PbEt 4 Synthesis “Leaded Gas” contains the antiknock agent PbEt 4. It is produced by the disproportionation of Pb(II) acetate in the presence of Et 3 Al: 6 Pb(OAc) 2 + 4 AlEt 3  3 Pb + 3 PbEt 4 + 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  - hydrogen elimination/reductive elimination to produce lead, H 2, alkanes and alkenes. For R 4 Pb, the stability follows: Me > Et > i Pr

8. 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 (AlCl 3 Lewis acid catalyst) n R 4 M + (4-n) MX 4  4 MR n X 4-n Often non-statistical product distribution - subtle bonding and steric effects can favor particular product. Metathesis reactions can be employed using LiR or Grignard: SiCl 4 + x LiR  SiR 4-x Cl x + x LiCl Transmetallation can be used with divalent species (Sn(II)/(IV) with Hg(II/0))

9. 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: Ph 3 PbX Me 3 SnF tbp Sterics of R play a key role

10. Alkyl Dihalides of Sn and Pb Dihalo derivatives tend to display octahedral centers. All of these are monomers in the gas and solution phases. Me 2 SnF 2 Me 2 SnCl 2

11. 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 H 2 O elimination and formation of siloxanes dihalosilane – rings and chains RSiCl 3 can lead to more elaborate three-dimensional structures. Siloxanes undergo redistribution to yield silicones.

12. Reactivity of R 3 ECl It is common for silicon to dehydrate and couple (less so for Sn and Ge): 2 R 3 SiOH  H 2 O + (R 3 Si) 2 O 2 R 3 SiSH  H 2 S + (R 3 Si) 2 S 2 R 3 SiNH 2  H 3 N + (R 3 Si) 2 NH

13. Reactivity of R 3 ECl 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

14. Lewis Acid Character These compounds are still have enough electrophilic behaviour to accept nucleophiles and produce anions in solution: Bu 3 SnCl + Cl -  Bu 3 SnCl 2 - Me 2 SnCl 2 + 2 Cl -  Me 2 SnCl 4 2- Me 2 SnCl 2 + 2 O=SMe 2  Me 2 SnCl 2.2 O=SMe This amphoteric behaviour allows these compounds to eliminate a halide and form anions when reduced with sodium.

15. Rochow Process (Direct) Organosilicon dichlorides (specifically) can be made by the Rochow process: 2 RCl + E/Cu  R 2 ECl 2 + 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.

16. Polysiloxanes (Silicones) Once organosilicon chlorides were widely available, allowed large- scale silicone production Silicones are made by the hydrolysis of organosilicon chlorides and subsequent dehydration and redistribution : R 4-x SiCl x + x H 2 O  x HCl + R 4-x Si(OH) x  [R 4-x Si-O] n + n H 2 O Depending on the number of chlorides, the resulting silicone can be linear or highly branched. They are strong, flexible polymers

17. Stability of Si-O-Si Si-O-Si exhibits low Lewis basicity and large angle suggest a role for p-d-p bonding (  ) or overlap with the  * on Si Increased flexibility of this linkage due to decreased directionality of the Si-O bonds Planarity in N(SiH 3 ) 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 SiCH 3 by strong bases (carbanions) - conjugate base stabilization via delocalization to Si

18. 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: R 2 SnCl 2 (OH -, H 2 O)  R 2 Sn(OH) 2 (-H 2 O)  [R 2 Sn-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.

19. 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.

20. Anions The monohalides of group 14 can be reduced with electropositive metals to produce anions: Ph 3 SiCl + 2 Li  Ph 3 SiLi + LiCl Tin and lead need to be reduced in liquid ammonia, due to their amphoteric behaviour: R 3 EX + 2 Na (NH 3, -78 o C)  NaER 3 + NaX (E = Sn, Pb) This is analogous to similar carbon chemistry, and tertiary group 14 compounds with electron-withdrawing moieties stabilizes these anions.

21. Commercial uses of Organotin Compounds Catalysis, stabilizers, biocidal agents: Bu 3 SnOAc (Bu 3 SnCl and NaOAc) – antifouling agent and applications to catalysis (polymerization) Bu 2 SnOAc 2 PVC stabilizer (cyclo-C 6 H 11 ) 3 SnOAc – insecticide in orchards and vineyards Bu 3 SnOSnBu 3 (hydrolysis of Bu 3 SnCl) – algicide and antifouling Ph 3 SnOSnPh 3 - antifouling (Bu 2 SnS) 3 from Bu 2 SnCl 2 with Na 2 S – PVC stabilizer

22. Electronegativity - Hydrides 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: R 3 GeH + RLi  RH + R 3 GeLi "hydride"DH (kcal/mol) CH 4 105 Me 3 SnH95 Bu 3 GeH88.6 Bu 3 SnH78.6

23. 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) Cl 2  Si n Cl 2n+2 (n = 1-6) This step goes through SiCl 4 and subsequently reacts with additional silicon. The amount of excess silicon determines n. Si n Cl 2n+2 + xs LiAlH 4  Si n H 2n+2 This step produces some AlCl 3, but it is very difficult to get all of the hydrides on LiAlH 4 to exchange.

24. Hydrosilation Hydrosilation is an important organic reaction. Regioselective to anti-Markovnikov products. Unlike hydroboration, it can selectively reduce a carbonyl group: 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.

25. Ge/Sn Hydrides Radical based reduction reactions: e.g. R 3 SnH + R’X (h  )  R 3 SnX + R’H Due to the weakness of the Sn-H bond (300 kJ/mol), it is possible to break organotin hydrides into radical species: R 3 SnH (h  )  R 3 Sn + H Bu 3 GeH is about 10x slower than Bu 3 SnH (interestingly (Me 3 Si) 3 SiH is comparable to Bu 3 SnH in many cases) Used to prepare GeCl 2 from GeCl 4

26. Lead Hydride Lead hydride decomposes at room temperature due to the weakness of its Pb-H bond (~205 kJ/mol). It will form R 3 Pb-PbR 3 compounds and H 2 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: n Bu 3 PbX + Ph 3 SnH (-78 o C)  n Bu 3 PbH + Ph 3 SnX

27. 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

28. Use in Organic Reactions Silyl enolates: silyl enol ethers as an enolate equivalent in Lewis acid-catalyzed aldol additions Trichlorosilyl enolate- offers a route free of catalysts

29. 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. Only catalytic amounts of Lewis acid are needed in the newer protocols (allylsilyl chlorides instead of allyltrimethylsilane) Initial step of proposed mechanism: H. M. Zerth, N. M. Leonard, R. S. Mohan, Org. Lett., 2003, 5, 55-57.

30. 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.