Chemistry Reference
In-Depth Information
stable and easier to handle than its monomeric form. When an anion opens the triox-
ane ring, the resulting anion degrades, as shown below, with release of two equiva-
lents of formaldehyde. The resulting species is essentially that resulting from reaction
of the initial phosphonate anion with formaldehyde itself. Please note the net incor-
poration of only one carbon atom and only one oxygen atom. Additionally, the pot-
assium cation is omitted from the remainder of the illustrations for clarity.
Again, referring to the ability of anions to undergo further transformations, we must
recognize that phosphorus is a unique element with a strong affinity for oxygen.
Furthermore, the phosphorus-oxygen double bond bears much of the same reactiv-
ity of a carbon-oxygen double bond and will accept addition of a nucleophile into
the system as shown below. The illustrated four-membered species is known as a
phosphetane.
As phosphorus exhibits a strong affinity for oxygen, phosphetane rings are known to
undergo further reactions. As illustrated below, the negative charge on the oxygen is
capable of breaking the adjacent carbon-phosphorus bond and transferring the nega-
tive charge to the carbon atom. Carrying this cycle forward, the negatively charged
carbon atom participates in an E2 elimination (Chapter 6) with formation of a new
double bond and cleavage of the adjacent carbon-oxygen bond. The resulting two
species are an olefin and a phosphate anion.
This reaction, known as a Horner-Emmons olefination, was presented to illustrate
that through consideration of the electronic nature of a given starting material and
the transient species involved in reactions with this material, products of more
complex reactions may be identified. However, it is important to note that while
this sequence appears complex, each step involved utilizes principles of arrow
pushing easily applied from material presented in this topic.
