Phospholide ion

Phospholide ions can form metal complexes with different coordination numbers / -complexation is characteristic for their aromatic behavior. The first 7 -complex with Mn(CO)3 (7) was reported in 1976. The ring in some cases (e.g., in 7) is not strictly planar. In 7, the P atom is displaced by 0.041 A from the plane of the carbon atoms. The large number of... 

Structural data of phospholide ions themselves are scarce. The lithium salt of the tetramethylphospho-lide ion, which is in fact an y -complex, and the K salt of the 2,4,5-tri-terf-butyl-l,3-diphospholide an-ion have been reported. Also the structure of the Li salt of the 2,5-bis(ferf-butyl)-l,3,4-triphospholide ion has been obtained In all these structures the bond lengths are equalized (CC, 1.396—1.424 A CP, 1.690-1.752 A). 

A final remark to close the section dealing with five-membered rings is that Mathey recently reported a one-pot procedure converting a phospholide ion (which is highly aromatic, as discussed above) to the six-membered ring phosphabenzene (which is also highly aromatic, as will be discussed below) as shown in Scheme 49 [278],... 

It is important to recall that the reactivity pattern of phosphoies is very different from that of the related S, N, and O ring systems due to their limited aromatic character. For example, electrophilic substitution takes place only with a handful of phosphoies that have been specifically tailored via increasing the bulkiness of the P substituent (see Section 3.15.10.4, Scheme 83). In fact, electrophiles react at the phosphoms atom affording a panel of neutral and cationic CN 4 derivatives (Scheme 8). Phosphoies are also versatile synthons for the preparation of other heterocyclic systems via Diels-Alder reactions. The cycloaddition can involve the dienic moiety of the phosphole ring or can occur following a 1,5-shift of the P-substituent (Scheme 8). Finally, phosphoies can be transformed into phospholide ions, which are powerful nucleophiles that have found a variety of applications (Scheme 8). All these facets of phosphole reactivity are presented in this section. It should also be noted that CN 3 phosphoies exhibit a rich coordination chemistry toward transition metals (see Section 3.15.12.2). 

Reactions involving chemical modification of the substituents at phosphorus of CN 3 phospholes are extremely rare. The usual reaction sites of these P-containing rings are the nucleophilic P-atom and the dienic system. However, it should be noted that the P-substituent can be displaced either by (1) reductive cleavage leading to phospholide ions, or by (2) nucleophilic attack (see Section 3.15.10.3), or (3) 1,5-sigmatropic processes (see Section 3.15.5.1.2(i)). 

In many respects there is a continuum between the synthesis and chemistry of phospholide ions and of phosphametallocenes. Additionally, most syntheses of phosphametallocenes start from phospholide ions, so a quick glance at their chemistry is desirable. 

All known reactions of phospholide ions with electrophiles (R+, H+...) take place at phosphorus, probably as a result of the high concentration of negative charge at the heteroatom. With transition metal electrophiles (typically metal halides), three reaction pathways can lead either to the desired /f-complexes (Eq. 5), to -complexes (Eq. 6) or to l,l -biphospholes (Eq. 7) ... 

The formation of the jf-c omplexes (Eq. 6) is disfavored by the introduction of bulky substituents at the a-positions of the phosphole ring. For example, rf-phospholyl-cobalt-dicarbonyls can be synthesized only with a-substituents such as phenyl [14] or tert-butyl [15]. In order to avoid redox reactions (Eq. 7), it may be necessary to replace the phospholide ions by tin [16] or lead derivatives [17] (Eqs. 8,9) ... 

Substitution and Cleavage Reactions on Phosphorus Phospholide Ions... 

Chesnut and Quin have extended their calculations to include a treatment of the cause of the very strong deshielding of P that characterizes all known phospholide ions. This theoretical work is presented in Section 2.15.3.2.1. Relevant to the present discussion is the conclusion that, although... 

Table 2 Calculated and experimental molecular parameters for phospholide ion.

Structures for several phospholes with higher coordination numbers at phosphorus have also been reported and are included in Table 3, as are some reports on benzo derivatives. A structure for the 2-coordinate phospholide ion (as a lithium salt) is also included the effects of delocalization are pronounced in this case, consistent with other observations on such ions. The C—C single bond is decreased to 1.424 A, and the C=C double bond is increased to 1.396 A. 

Dramatic downfield shifts accompany the cleavage of P-substituents with formation of phospholide ions. These shifts are considerably further downfield than those for simpler phosphide ions, and have been taken to be a reflection of strong electron delocalization in the phospholide ion (see Section 2.15.2). Data are collected in Table 6, where it will be seen that typical shifts fall in the region -t-70 to -t-100. Once again the 3,4-dimethyl shielding effect is present and indeed is of the largest size (A 21.4 ppm) of all families examined. Phospholide ions with this substitution have shifts in the range -t- 50 to -t- 60. 

Compared to noncyclic phosphide ions, phospholide ions show a definite decrease in basicity that is consistent with the high degree of electron delocalization evident from other properties (see Sections 2.15.2 and 2.15.3.2). The ions (56) and (57) in THF solution were found not to be protonated by ethanol, implying an acidity for the conjugate acid (the 1 fl-phosphole) greater than that of ethanol, pi a 15.9 in water <79CC40l>. 

No other measure of the basicity of phospholide ions has yet been published. For comparison, the p of diphenylphosphine is 21.7. 

Phospholide ions can also be made separately by the cleavage reaction with metals, and then... 

The phospholide ions are also used in the synthesis of 1//-phospholes. However, in a paper that is of great importance in phosphole chemistry, 1//-phospholes were found to be quite unstable, undergoing a [1,5] sigmatropic hydrogen shift to form 2//-phospholes (e.g. (108)) which then dimerize <82TL511> by a [4 + 2] cycloaddition (e.g. to (109)). This reaction can be so rapid that the parent 1//-phosphole and some substituted derivatives can only be observed at low temperatures, and dimerization is complete at room temperature (Scheme 14) <83PS(l8)5i>. 

It was also shown by the Mathey laboratory that the 2//-phosphole moiety could be preserved in the form of a chromium or tungsten (e.g. (221)) complex <84JA826>. This was accomplished by preparing a phospholide ion complex (e.g. complex (220)) and treating it with acid at room temperature P-protonation occurred and the usual rearrangement to the 2//-phosphole followed (Equation (43)). The nature of the complex then prevented the usual dimerization from taking place. This is an excellent example of the power of conducting reactions in the coordination sphere of a metal. 

Another type of reactivity displayed by P-phenyl substituents is their ready cleavage by alkali metals. This is a reaction of great importance in phosphole chemistry as the products are phospholide ions, widely used as precursors of other P-substituted phospholes (see Section 2.15.5.7). The P-methyl group can also be cleaved by metals, as can an ethylene linkage between two phosphole... 

A more versatile method for a group interchange at P consists of cleaving a P-phenyl substituent with a metal (lithium, sodium, or potassium) to form the phospholide ion, which can then be reacted with electrophiles. In every case known the electrophile has attacked exclusively at P, even though electron delocalization in the phospholide ion is extensive (see Section 2.15.2). This reaction was... 

Two phosphole-3-carboxylates with 4,5-diphenyl substituents ((276) and (277)) have been prepared by a rather specialized approach <93BSF695>, outlined in Scheme 85. This method was designed to generate phospholide ions with a C-functional group (e.g. (278)), but the ions can be alkylated on P to form phospholes. The ions are formed by a base-induced retro-Michael reaction. While of value in producing an otherwise inaccessible substitution pattern on the ring, the method is less... 

The sensitivity of metal complexes of phospholide ions to electrophilic substitution has been employed in another approach to phosphole ketones <85JOC467>. The molybdenum complex (282) was acetylated at the 2-position under Friedel-Crafts conditions, and the metallic group then removed with carbon monoxide under pressure. Trimethyl phosphite also can be used for this purpose. 

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