Formation of Carbon-Phosphorus Double Bonds

The field of carbon-phosphorus double or triple bonds has been covered in several recent reviews [1-4a]. 

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The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

One of the subjects of our investigations involved the interaction of allenes with the P=E derivatives. This work provided some very interesting, unexpected results that may well be of use in synthetic organophosphorus chemistry. Thus, in attempting to study the (2+2)-cycloadditions of the phosphinimine and the (methylene)phosphine systems with the allenic C=C bond, we established that the reactions take a different and more interesting course — that of an ene" reaction (eq 1). In the first step of the reaction, the electrophilic phosphorus center apparently attacks the nucleophilic central carbon of the allene system. Then, instead of undergoing nucleophilic attack on the incipient carbonium ion, the anionic center (E) abstracts a proton from the terminal C-H bond, leading to the formation of a new double bond in the phosphorus-substituted 1,3-butadiene derivatives (3). 

The final example is slightly different from the previous protocols in two ways. First, the IMP is generated by Sn2 displacement of a benzotriazole moiety. The IMP then opens an epoxide, which generates a betaine with an extra carbon atom between phosphorus and the negatively charged oxygen. Collapse of this betaine results in the formation of an aziridine as opposed to a carbon-nitrogen double bond. 

By analogy, the formation of a double bond between carbon and phosphorus, which in our opinion, however is improbable, is assumed to be the third stage. Finally, this is followed by the addition of water and tautomeric rearrangement to the primary phosphine. In this way, phosphine, which is present in technical acetylene, and because of its good solubility in actone concentrates in the commercial steel cylinders, forms with acetone, isopropylphosphine oxide and possibly, secondary products... 

Phosphorus ylides are very important because of their use in the well-known Wittig reaction (1954) for the synthesis of alkenes. In the Wittig reaction, a phosphorus ylide (1) reacts with an aldehyde or ketone to yield the corresponding alkene (16) (Scheme 7). The reaction involves nucleophilic attack by the ylide (1) on the electrophilic carbonyl carbon atom to yield the betaine intermediate, which then collapses with elimination of the phosphine oxide and formation of the alkene (16). The driving force of the Wittig reaction is the production of the very strong phosphorus-oxygen double bond in the phosphine oxide (Scheme 7). 

Because of the radical mechanism for SET reactions, introduction of both a perfluoroalkyl group and a heteroatom moiety to the carbon-carbon double [17-20] and even triple [21] bonds is possible. The initially generated perfluoroalkyl radicals add first to olefins to form a new radical intermediate (23), which then couples with anions (22) to form new anion radicals (24). The formation of the product (25) and the chain propagation via electron transfer from anion radicals (24) to perfluoroalkyl halides constitutes a chain reaction as shown in Scheme 2.38. Sulfur [19], selenium [20], tellurium [21], and phosphorus [22] anions (22) have been employed for these reactions [23]. 

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