This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

A new class of π-conjugated macromolecule, poly(p-phenylenephosphaalkene) (PPP), is reported. PPPs are phosphorus analogues of the important electronic material poly(p-phenylenevinylene) (PPV) wher...

Phosphorus Copies of PPV: π-Conjugated Polymers and Molecules Composed of Alternating Phenylene and Phosphaalkene Moieties

Publisher Summary
This chapter begins with an explanation of heterophospholes as five-membered 6 π-systems. It discusses the six ring systems with just one heteroatom in addition to phosphorus and adjacent or non-adjacent to it, the 1,2- and 1,3-azaphospholes, the 1,2- and 1,3-oxaphospholes and the 1,2- and 1,3-thiaphospholes. The remaining 42 systems with two and more heteroatoms besides phosphorus are also considered. A heterophosphole may also contain a second two-coordinate phosphorus atom in the ring. The resulting aza- and thiadiphospholes, diazadiphospholes, azaphosphaarsoles and azaphosphastiboles are covered in the chapter. If phosphorus atoms are the only heteroatoms (possibly together with an arsenic or antimony atom) the five-membered ring as a rule must be negatively charged to establish a 671-system. 1, 2- and 1,3-Diphospholides are discussed together with the other ring systems with two adjacent and non-adjacent heteroatoms. In phospholes that contain more than one two-coordinate phosphorus atoms in the ring, the three-coordinate phosphorus atom attains planarity more easily and consequently even neutral oligophospholes may be aromatic. Oligophospholes and oligophospholides with three, four and five phosphorus atoms are described. A brief summary of the history of heterophospholes is also provided.

4.3 – Heterophospholes

The cycloaddition reactions namely, [2 + 4], [4 + 2], [2 + 3] and [1 + 4] given by the heterophospholes, which are five-membered 6π aromatic heterocycles containing at least one σ2, λ3 phosphorus, are reviewed in the present article. Different aspects of these reactions such as stereoselectivity, regioselectivity, relative reactivities and theoretical correlations thereof are included. The literature has been surveyed up to December 2003. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:271–287, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20002

Cycloaddition reactions of heterophospholes

First Brønsted Acid-Induced Ring Expansion of an Oxaphosphirane Complex: A Combined Experimental and DFT Study

The 5-alkylidene-4,5-dihydro-3H-1,2,4(λ3)-diazaphospholes (4-phosphapyrazolines) are thermally much more stable than related compounds without the exocyclic double bond. Thermolysis reactions typically occur in the range 110-150°C in toluene solution, and different, mostly competing, reaction pathways are observed. Thermal extrusion of nitrogen from 8a-g gives rise to β-phosphanaylsiloxyalkenes 10, benzo[c]-phosphole derivatives 11, 14 and 115, (β-siloxyalkylidene)-Phosphiranes 12, and dihydro-1,3-oxaphospholes 13. The thermolysis of 5-alkylidene-4,5-dihydro-4-trimethylsilyl-3-trimethylsilyloxy-3H-1,2,4-dizaphospholes 17 afforded three products, including the highly substituted and stable 2-phosphabutadienes 18 formed by nitrogen extrusion and rearrangement. Finally, the 4-chloro-3-trimethylsilyloxy-substituted heterocycle 21 was transformed at 170°C into 4H-1,2,4-diazaphosphole 23. The structures of 13c and 18a were determined by single-crystal X-ray diffraction.

Thermal Reactions of 5‐Alkylidene‐4,5‐dihydro‐3H‐1,2,4(λ3)‐diazaphospholes (4‐Phosphapyrazolines) – A Route to Various PHeterocycles and to 2‐Phosphabutadienes

Cycloadducts from Diazocumulenes and 1,2,3(λ3)‐Diazaphospholes: Thermolysis Generates Products Derived from 3‐Alkenylidene‐1,2,3(λ5)‐diazaphospholes

Polycyclic Isophosphindole Derivatives by Intramolecular Trapping of 2-Phospha-1,3-diyls or Isomeric Bis(methylene)phosphoranes

Photochemical Isomerizations of 5‐Alkylidene‐4,5‐dihydro‐3H‐1,2,4(λ3)‐diazaphospholes (4‐Phosphapyrazolines): (5→4) Ring Contraction Generates Azomethineimine Dipoles

5-Alkylidene-5,7a-dihydro-3H-[1,2,4]diazaphospholo[4,3-c][1,2,3]diazaphospholes 10a−c were obtained from (1-diazo-2-oxoalkyl)silane 8 and 2-acetyl-2H-1,2,3-diazaphospholes 9a−c. Thermally induced elimination of molecular nitrogen from 10 generated polycyclic isophosphindole (benzo[c]phosphole) derivatives 13 by means of intramolecular [3+2] cycloaddition of an intermediate 2-phospha-1,3-diradical or an isomeric bis(methylene)phosphorane onto a benzene ring.

Polycyclic Isophosphindole Derivatives by Intramolecular Trapping of 2‐Phospha‐1,3‐diyls or Isomeric Bis(methylene)phosphoranes.

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