Phospha‐alkenes and Phospha‐alkynes, Genesis and Properties of the (p‐p)π‐Multiple Bond

Abstract

Dedicated to Professor Herbert Griinewald on the occasion of his 60th birthdayJust as the octet rule had prevented the discovery of the inert-gas compounds, the systematic search for phosphorus-carbon-and phosphorus-nitrogen-compounds having (p-p)nmultiple bonds was hindered by the double bond rule. The first successful synthesis of mesomeric-stabilized phosphorus-carbon compounds with coordination number 2, seventeen years ago, was followed nine years later by the preparation of the imino-phosphanes whose intensive investigation has continued to the present day. A dramatic development has now begun in the phospha-alkene and -alkyne fields, and a large variety of preparative methods have been found. Several compounds of this type are amazingly stable, however they do not participate in typical ylide reactions such as the Wittig reaction. In contrast, the PC double bond is more comparable to that of olefins, which has been confirmed by the occurrence of E,Z-isomers and pericyclic reactions of phospha-1,s-hexadienes.

Cycloaddition reactions of heterophospholes

“…The ❍ ❍ ✟ ✟C P moiety in the heterophospholes, as in acyclic phosphaalkenes [26][27][28][29], acts as dienophile and undergoes [2 + 4] cycloaddition with a variety of 1,3-dienes, such as 2,3-dimethylbutadiene, 1,4-diphenylbutadiene, isoprene, cyclopentadiene, 1,4-diphenylazabutadiene and o-quinones [13,[30][31][32][33][34][35][36][37][38][39]. The reaction with unsymmetrical diene such as isoprene or 1,4-diphenylazabutadiene proceeds regioselectively, the regioselectivity in many cases being 100%.…”

About the structure of phosphorus ylids: Electron distribution and geometry

“…We first compared (see Table 1 C15-19)) triphenylphosphonio-methanide (l), the most common Wittig reagent, with one imidelcarbanide-hybridized, 1,3-dipolar zwitterion (2) and three typical "phospha-alkenes" [20] pure "ylid" form should give rise to a signal at the high-field end of the nmr scale [21]. Obviously the true phosphorus species behaves in this respect very much like a P-C zwitterion.…”

η1:η2-P-Pyrazolylphosphaalkene Complexes of Ruthenium(0)

“…After almost half a century of study, the chemistry of low-coordinate phosphorus continues to fascinate both organic and inorganic chemists alike [1][2][3][4][5][6][7][8]. Dominated by the isolobal and isoelectronic relationship between phosphorus and the "CH" fragment, the chemistry of phosphacarbons is often familiar from their carbo-centric and nitrogenous counterparts, yet they simultaneously embody appreciable dichotomy in terms of their underlying electronic and chemical nature.…”