Two-coordinate phosphines

During the past decade, many different examples of one- and two-coordinate, trivalent phosphorus compounds have been synthesized and isolated as thermally stable, but chemically reactive, products. The two-coordinate derivatives, in particular, have been found to undergo a variety of interesting and synthetically useful transformations, a few of which are illustrated below. Although several examples of addition, cycloaddition, oxidation, and metal-coordination reactions of two-coordinate phosphines are readily found in the literature (7), there is still a great deal of work to be done before the full potential of these new reagents can be realized. 

Me3Si)2NP=NSiMe3 (2). It is known that the heteroatomic double bond in these two-coordinate phosphines is highly polar in nature with a partial positive charge on phosphorus, thus making it an electrophilic center. The sp hybridized... 

Since dialkyl acetylenes contain two sp hybridized carbon centers, they have the potential, like allenes, to undergo ene reactions. This prompted us to investigate the reaction of 2-butyne with the two-coordinate phosphines (eq 3). Initially, we found that this reaction was extremely slow at room temperature. Due to the high volatility of 2-butyne, we then carried out the reactions in sealed glass ampoules at 80 °C without solvent. In this case, the reactions were completed after 10 - 15 days and the novel allenic phosphines (5) were isolated in good yields. The simplicity of this reaction makes it a very useful method for the synthesis of here-to-fore unknown allenic phosphines. 

These results prompted us to investigate the reactions of two-coordinate phosphines with propargyl chloride (eq 8). In direct contrast to the results described above, this reaction proceeded not by chloride displacement by the nucleophilic phosphine, but by electrophilic addition of the terminal C-H bond to the P=N linkage. This reaction, therefore, took the same course as was found... 

We also obtained interesting results in studying the reactivity of acetylenic alcohols with the two-coordinate phosphines (eq 9). Depending on the position of the OH group relative to the triple bond, different types of products are obtained. In all cases, however, the first step of the reaction is addition of the 0-H group to the P=N center to yield the three-coordinate phosphine. In the case of a-acetylenic alcohols, this addition is followed by a rapid acetylene-allene rearrangement and a... 

Our results and related literature data (7) clearly show that the P=E double bond in two-coordinate phosphines undergoes facile addition of polar reagents to give three-coordinate P derivatives. It is also well known that the three-coordinate pin center is relatively nucleophilic and that, in molecules containing an electrophilic site, further chemical transformations are usually observed. Thus, in certain reactions of the two-coordinate phosphines, it is possible to find both types of reactions electrophilic addition to the P=E bond, followed by nucleophilic attack by the three-coordinate P atom on an internal electrophilic center. For example, in the enol form of a ketone, the O-H moiety is capable of undergoing addition to the P=E double bond of the two-coordinate phosphines. Acetylacetone and related reagents react rapidly with the phosphinimine at low temperature (ca. -50 C) and with the (methylene)phosphine at 0 °C to give the new 2,3-dihydro-... 

Allenic compounds, reactions with two-coordinate phosphines, 77-79... 

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