1

Chapter 13 Silicon reagents

General features

Two other highly important properties of silicon

Reactions of organosilanes 1,2 rearrangements (Brook rearrangement)

Silicon as a protecting group for OH

Peterson olefination(Si-stabilised carbanions)

Silylenolethers

Alkylsilanes

Vinyl- or Alkenylsilanes

Allyl Silanes

Aryl Silanes

2

General Features

•Silicon is directly below carbon in the periodic table, and shows

some similarity in bonding. It forms 4 bonds in neutral molecules

and is tetrahedral.

• Silicon does not form very stable multiple bonds, as the large 3p

orbital on silicon does not overlap well with the 2p orbital on carbon,

oxygen or nitrogen.

• Carbon is more electronegative than silicon

•Silicon is a very versatile element, and you will find silicon reagents

in 2 major roles;

•As protecting groups for OH

•In reactions for C-C and C=C bond formation

3

4

Two other highly important properties of silicon

Si-stabilised carbanions are important in the Peterson olefination reaction (see later)

Donation of the negative charge into a * antigonding orbityal of an adjacent Si-C bond stabilises the anion.

This donation is rendered more effective due to silicon’s lower electronegativity compared to carbon. Therefore the * antibonding orbital has a greater orbital coeffient on silicon. What this really means is that the antibonding orbital is a bit larger on the silicon side and leads to better overlap with the orbital containing the negative charge.

Stablisation of α-carbanions

5

Two other highly important properties of silicon

Stablisation of β-cations (-effect of silicon )

C C

Si

+

E.g allyl and vinyl silanes react with electrophiles via the following mechanisms

6

ipso-Substitution: a consequence of -effect of silicon

7

Reaction of organosilanes (Brook rearrangemen

t)

When silylcarbinols are treated with base or active metals the silyl gro

up is known to migrate from the C to a neighbouring O atom e.g. Bro

ok rearrangement, generating silyl ethers

8

• Various protecting groups are available and the resulting silyl ethers

can be cleaved under a variety of conditions.

• Silyl protecting groups are typically put on under basic conditions

Reaction of organosilanes (Silicon as a protecting grou

p for OH)

R OHTBSClBase R OTBS

Base = Et3N, imidazole

9

The stability of the silyl ether towards cleavage depends on a numbe

r of factors:

Increasing steric bulk raises stability.

EWGs on silicon increase acid stability and decrease base stability and v

ice versa.

The ease of cleavage with F- parallels the ease of basic hydrolysis.

Reaction of organosilanes (Silicon as a protecting

group for OH)

With bulkier groups, such as TBDMS, it is possible to distinguish between primary

and secondary alcohols.

10

Reaction of organosilanes (Silicon as a protecting group for O

H)Deprotection:

Selective deprotection

11

Reaction of organosilanes (Peterson olefination)

Generation of Si-stabilised carbanions

Deprotonation with a strong base

Metal-halogen exchange

Addition of organometallics to vinyl silanes

12

Reaction of organosilanes (Peterson olefination)

• Key compounds are β-hydroxysilanes generated from the reactio

n of Si-stabilised carbanions with carbonyl compounds

13

Reaction of organosilanes (Peterson olefination)

The next step is the elimination of OH and SiMe3, which can

generally be done in two ways. Each method is stereospecifi

c.

Acidic hydrolysis proceeds via

an anti-elimination

14

Reaction of organosilanes (Peterson olefination)

Basic conditions - syn elimination

Therefore, in principle the Peterson olefination can be used to make single

geometric isomers of alkenes. However, this is complicated by the difficult

y in preparing pure diastereomers of the β-hydroxysilanes.

15

Example:

The Peterson olefination is often less sterially demanding than the Wittig reaction

W. Adam, C. M. Ortega-Schulte, Synlett, 2003, 414-416.

Recent Literature:

16

M. Iguchi, K. Tomioka, Org. Lett., 2002, 4, 4329-4331.

A. Barbero, Y. Blanco, C. Garcia, Synthesis, 2000, 1223-1228.

17

Reaction of organosilanes (Silyl Enol Ethers )

• Some reactions of silyl enol ethers

• Preparation of silyl enol ethers

OSiMe3 OBr

OLi+

MeLi

Me4Si

OSiMe3 O O

I

Me3

N+CH2

Ph

PhCH2N+Me3F

-

FSiMe3

Activated enolate

O

OSiMe3

OSiMe3

1. Me2CuLi2. Me3SiCl

Et3SiHCat. (Ph3P)3RhCl

18

In the presence of a lewis acid

19

Genaration:

Silyl Enol Ethers: 2-Trimethylsilyloxybuta-1,3-dienes

O O

SiMe3

Me3SiCl

NEt3

O O

SiMe3

Me3SiCl

NEt3

OMe OMe

ZnCl2

Danishefsky's diene

O

SiMe3

OMe

+ O

O

O

O

O

OH

HMe3SiO

OMe

Via endo TS

H3O+

O

O

OH

HO

Reaction:

20

Alkylsilanes

• Almost all acyclic Cl, Br, and I silanes react with all nucleophi

les by an SN2 type mechanism leading to an inversion of confi

guration at the Si atom.

• In many cases the pentacovalent Si is thought to be an interme

diate rather than a transition state.

21

Vinyl silanes

• Preparation using a variety of methods

22

Vinyl silanes

• The reactions of vinyl silanes: electrophilic substitution of the silyl gr

oup. This is highly regioselective and the substitution occurs with reten

tion of alkene geometry.

23

Vinyl silanes

24

Allyl silanes

Preparation:

The reaction of allyl organometallic reagents with silylating agents.

The Wittig Reaction

25

The reactions of allyl silanes

Allyl silanes are more reactive than vinyl silanes and much more reactive th

an simple alkenes

26

The reactions of allyl silanes

Allyl silanes react with electrophiles with high regioselectivity. The electrophile

attacks at the other end of the allylic system, and no bond rotation is required.

27

Allyl silanes react with a wide range of electrophiles in the presence of

Lewis acids such as TiCl4

28

29

Aryl silanes

30

Exercise 4

• Drawing the structures of the intermediate and the final products.

• Outline at least two kinds of enzymes or whole-cell systems used in the redu

ction of ketones to secondary alcohols.

1.O

H

+ TMSClEt3N/C6H6 CO2Et

2.+

O

HPh

OTMS i) TBAFii) H2O