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
It was noted previously (here) that there were great similarities in SN and E in terms of the starting materials and reagents – both involve a molecule containing a leaving group, and a reagent that is either a nucleophile or a base (it was also noted in the Nucleophilic Substitution chapter that these two share similar properties). So why do some reactions go as … Read more
The issue of which proton is removed when there is a choice is not as simple in E2 as it is in the other mechanisms. Two empirical rules have been formulated which apply to different systems, and essentially say the opposite things. These rules were developed by Hofmann and Saytzev (working separately) in the 19th century. Hofmann’s work applied … Read more
The stereochemistry of E1 and E1CB is relatively easily to dissect. In terms of the first step the regiochemistry is reasonably clear (and there should be no stereochemistry to consider) – in E1CB the most acidic proton (adjacent to the leaving group) is taken and in E1 the leaving group leaves! From the charged intermediate there are two possible olefinic outcomes and the more … Read more
E1 1. The similarity in rate determining steps between E1 and SN1 helps to explain why the same factors govern whether a mechanism is E1 or E2 (E1CB disregarded for the time being) – those being; possibility of a stabilised cation and a polar solvent which can solvate said cation. Therefore, the E1 order of reactivity for alkyl halides runs tertiary > secondary > primary. 2. Where there … Read more
An elimination reaction involves the removal of two species from a molecule: The most common type of elimination is a so-called 1, 2- elimination where the two species are removed from adjacent carbon atoms, usually causing the formation of a multiple bond. For example; This 1, 2- style elimination can happen in one of three distinct mechanisms, … Read more
There are a number of fairly commonly encountered electrophilic additions that have their own very particular mechanisms, some of which have been collected here. Some sections of the mechanisms may be unfamiliar; if they are, simply focus upon the electrophilic addition step that is important for this chapter – the other aspects of the mechanism should become more clear as later … Read more