Biphospholes

The reaction between 2,2 -biphosphole and dimanganese decacarbonyl was studied under different conditions [86JOM(316)271]. In boiling xylene and under inert atmosphere, the main product is the isomeric bis(ri -diphospholyl) complex 186, and complexes 187 and 188 are also produced. When performing the reaction in a closed vessel and at 420 K, the new tt complex 189 is formed. 

The discovery that the oxidative coupling of 2-lithiophosphole, obtained from (7a), led to biphosphole (8a) (Scheme 2) was a breakthrough [19], opening the way to oligo(phosphole)s. This very efficient methodology has been... 

Bromo-capped biphosphole (8b) is also the precursor of cychc derivatives, as illustrated by the synthesis of the fully unsaturated macrocycle (11), via a Wittig reaction involving the 5,5 -bis(carboxaldehyde) (10) (Scheme 2) [22]. An X-ray diffraction study revealed that macrocycle (11) is distorted, with an flZZ-trans-disposition of the four P-phenyl substituants. 

Almost no attention has been paid to diphosphine sulfides employed as chiral ligands for palladium-catalysed nucleophilic substitution reactions. In this context, enantiomerically pure diphosphine sulfides derived from 2,2 -biphosphole, which combined axial chirality and phosphorus chiralities, were synthesised, in 2008, by Gouygou et al. through a four-step synthetic sequence. Among various palladium catalytic systems derived from this type of ligands and evaluated for the test reaction, that depicted in Scheme 1.62... 

Scheme 1.62 Test reaction with 2,2 -biphosphole-derived diphosphine sulfide.

It was found that the P-phenyl substituent of phospholes may undergo migration to carbon forming the corresponding 27/-phosphole intermediate [56, 57], The reaction is shown for the double rearrangement of biphosphole 60 to generate intermediate... 

The reduction of octaethyl-l,r-biphosphole, -biarsole, and -bistibole with strontium and barium in THF yields the l,l -dipentelaoctaethylstrontocenes and -barocenes (ECsEt4H)2M (E = P 157, As, Sb). The strontocene crystallizes as a THF adduct whereas the barocene 157 precipitates co-ligand-free but as a coordination polymer (Figure 83).260... 

It is noteworthy that the P atoms of these phosphole oligomers retain the versatile reactivity observed with monophospholes [7]. In particular, biphosphole 9 exhibits a rich coordination chemistry, which allows for the preparation of a variety of neutral and cationic transition metal complexes (Scheme 4.5) [18a,b, 22]. These have been used as precursors for homogeneous catalysts [22] however, their photophysical properties have not been studied. 

Mixed Oligomers Based on Biphospholes with other (Hetero)aromatics... 

Cyclic oligomers based on biphospholes are also available via Wittig reactions involving the 5,5 -bis(carboxaldehyde) 53 (Scheme 4.16) [44]. An X-ray diffraction study of 54 revealed that the four P-phenyl groups have an all-trans disposition and that this fully unsaturated macrocycle is distorted [44]. The cavity of the 24-membered heterocycle 54 is rather large, the diagonal distance between two P atoms reaching 6.1 A. 

Diastereomeric mixtures (three isomers) of 2,2 -biphospholes 162-164 were synthesized by asymmetric alkylation of 2,2 -biphospholyl anion 161 with enantiomerically pure diol ditosylates. The generated 2,2 -biphospholes were converted into the more stable disulfide derivatives 58, 165, and 166 <2005OM5549, 2003CC1154>. Dianion 161 was generated in two steps from l-phenyl-2,3-dimethylphosphole 159 by pyrolysis and subsequent treatment of the formed phosphole tetramer 160 with sodium naphthalene. Structures of the diastereomers of disufides 58 and 165 were established by the X-ray crystallographic data. 

An excellent example of the use of method (2) for the synthesis of phospholyl complexes is given by the preparation of the Ca and Sr complexes 299 and 300. Here, insertion of either of these two metals into the P-P bond of 1,1 -biphosphole 298 occurs readily, affording the corresponding group 2 metal complexes (Scheme 102) <1999IC3207>. 

In order to modulate the rotational and inversion processes associated with BIPHOS, the related P-NPr 2-functionalized 2,2 -biphospholes 103 a and 103 b have been prepared from the corresponding aminophospoles 102 a and 102 b, albeit it in low yield, ca. 10% (Scheme 123) <2002EJ0675>. In order to lock the axial chirality of the biphosphole, it proved crucial to have both the P-NPr 2 and phenyl groups on the ring backbone. Unlike compound 103 a, 103 b exists as two stable diastereoisomers. 

One final example of a miscellaneous expanded porphyrin is the 24-mem-bered tetraphosphole macrocycle 4.213 reported by Mathey and coworkers in 1995. It was synthesized in 60% yield from the Wittig-type reaction between the phenyl-substituted 5,5 -bis(formyl)-2,2 -biphosphole 4.211 and the o-xylylidene bis-ylid 4.212 (Scheme 4.6.7). A single crystal X-ray structural analysis carried out on 4.213 revealed an aW-trans orientation of the four phosphorus lone pairs, and a macrocyclic diameter of ca. 6 A (Figure 4.6.4). Because of the steric influence of the phenyl substituents, macrocycle 4.213 is decidedly non-planar. Nevertheless, this macrocycle is expected to exhibit a rich coordination chemistry, particularly towards soft, heavy transition elements. Unfortunately, no such putative metal complexes have as yet been described in the literature. 

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 stability of phosphaferricinium ions in solution is low cyclic voltammetry reveals that it decreases as the polarity of the medium rises or the degree of ring substitution falls [33]. The parent diphosphaferrocene evolves chemically under electrochemical oxidation in CH2C12, MeCN, and DMSO and is only stable in solvents such as propylene carbonate [33]. Compounds 24 and 25 are reversibly oxidized in dichloromethane but decompose in DMF, probably to the corresponding l,l -biphosphole (Eq. 20) [33] ... 

The biphosphole (370) has been obtained in enantiomerically pure form by spontaneous resolution in the crystallisation of a racemic mixture, without the use of chiral auxiliaries. A new approach to -functionalised phospholes is afforded by metallation at a methyl group of l-phenyl-3,4-dimethylphosphole(in which both phosphorus and the diene unit are protected by coordination to an iron carbonyl acceptor), followed by treatment with electrophiles, to give C-substituted products, e.g., (371). Copper(II) oxidation of the intermediate lith-iomethyl derivative leads to the formation of bridged systems, e.g., (372). ... 

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