The main points about sulphur 1. A range of oxidation states are available – predominantly 2, 4 and 6. 2. Quite nucleophilic in its divalent state, even more so as S–. 3. Hydrogens alpha to sulphur atoms are noticeably more acidic than they would be if the sulphur were absent. The carbocation generated by deprotonation is stabilised inductively by the sulphur, … Read more
A widely used method of protecting hydroxyl groups in organic molecules is to turn them into silyl ethers. A protecting group is one which reduces the reactivity of the functional group it protects so that chemistry may be carried out elsewhere on the molecule without fear of interference from the protected group. A silyl ether is quite easy to make – all that … Read more
Recall from the first page of this chapter that silicon stabilises carbocations β to it and carbanions α to it. Thus vinylsilanes can undergo both electrophilic substitution and nucleophilic addition. Electrophilic Substitution For maximum stabilisation of the carbocation, the C-Si bond must be in the same plane as the empty p orbital (of the carbocation) as this gives maximum overlap of the C-Si σ bond into … Read more
Whereas enols are seen only as a small proportion of the keto-enol tautomerism, silyl enol ethers are stable and very useful sources of regiochemically pure enolate ions. They are easily prepared – following the route to the enolate ion then treating with a silyl reagent like Tri-Methyl-Silyl-Chloride (‘TMSCl’). For example; If the ketone is asymmetrical then there are two possibilities for deprotonation, and the proton that … Read more
1. Silicon forms very strong bonds to oxygen and fluorine (particularly to oxygen) – this provides a driving force for much of silicon’s chemistry. 2. Silicon is more electropositive than carbon, so when bonded to carbon it often represents an attractive site for nucleophilic attack (i.e. it has δ+) – particularly if a halogen is attached. 3. The mechanism for nucleophilic substitution is … Read more
Some assorted reactions to show the versatility of phosphorus and to exemplify the points made on page 1. 1. i. Shows the potency of P=O as a driving force – in the penultimate step a carbanion is created (fairly unstable) while the P=O is formed. ii. Ph-CH2– is the only possible carbanion is the system with any stabilisation (see below), so it … Read more
The first step of the Wittig Reaction – attack of the phosphonium ylid on the carbonyl – can be reversible or irreversible depending on the nature of the reactants, and this can have an important effect on the stereoselectivity of the process (if both reactants are asymmetrical). If the ylid is stabilised in some way – usually by an Electron … Read more
The main points about phosphorus i. Has two oxidation states readily available; (+/-) 3 and (+/-) 5. ii. In its trivalent state (e.g. R3P:) it is very nucleophilic. iii. Phosphorus-Oxygen bonds (particularly P=O) are exceedingly strong and often provide a driving force for a reaction. The Wittig Reaction Probably the most common use of phosphorus in organic chemistry is … Read more
The empty p orbital on the boron makes it susceptible to nucleophilic attack. Alkyl migration can then occur if the nucleophile has a leaving group or electron sink of some sort – this process can be very useful in syntheses. This pattern of addition-then-migration dominates the reactions of alkylboranes. Oxidation In general; Hydrogenation Full cis addition of H2 to an alkene – an alternative to the metal catalysed … Read more
The highly important thing to note about boron is that in its common trivalent state it has only 6 electrons and therefore is quite electron deficient. It can be used to perform many useful synthetic operations, which often follow unusual mechanisms. The Hydroboration Reaction The most common of Boron’s reactions in organic chemistry (the exact details … Read more