Monophosphine-boranes

Scheme 2 Formation of zwitterionic Ni complexes upon coordination of monophosphine-boranes...

Reaction of phosphinite borane 42 (Table 4.2, entry 43) with 1-bromo-l -lithioferrocene afforded the monophosphine borane 43 (Table 4.4, entry 50), which still possesses a bromine atom susceptible to metallation on the non-phosphinated Cp ring. After lithiation, the ferrocenyl anion reacted with the second phosphinite borane 44 (Table 4.2, entry 44), affording the desired optically pure dissymmetric diphosphine borane 45 in 29% yield from 42. During the synthesis impurities such as undesired monosubstituted ferrocene derivatives could be separated by column chromatography. 

Standard cyclisation methodology was used to access the cyclic monophosphinic acid derivative 78 by reaction of ammonium phosphonate and ethyldiisopropylamine, followed by the addition of chlorotrimethylsilane, with 2,2 -bis (bromomethyl)-l,l -biphenyl. Silane reduction of 78 gave the secondary phosphine. The secondary phosphine borane complex 79 could be used in alkylation or Michael addition reactions. For example the Michael adduct 80 was produced in high yield by treatment of 78 with a NaH suspension in THF followed by the addition of diethylvinylphosphonate . 

Examples of iron rj -B=C complexes of a boraalkene are known as well. The Fe(CO)4 complex of 9-fluorenylidene(tetramethylpiperidino)borane (126) has recently been prepared by the thermal reaction of the borane and (2) or by the photolytic reaction of (1) and the borane. The monophosphine/phosphite complexes are prepared from the tetracarbonyl by photolytic CO dissociation, followed by PRb addition. ... 

Lithioferrocene (322) or dilithioferrocene (323) can also be prepared, and then utilized in situ, from the corresponding halo- or dihaloferrocenes by halogen-metal exchange with alkyl lithiums. Alternatively, monohthiation of l,l -dibromoferrocene gives rise to anion (351), which may be reacted with an electrophile in order to afford additional ferrocene derivatives. In this way, monophosphine oxazoline ferrocene (340) is prepared from bromo oxazoline ferrocene (352), a compound that also is used to prepare chiral-at-phosphorus ferrocenes (353). Other chiral-at-phosphoras ferrocenes (354) are made from (323) and a phosphite borane as the electrophile (equation 80). The phosphorus atoms may also be contained in a ring that possesses chirality the sequences used to prepare this family... 

Monoferrocenyllithium was prepared by direct metallation of ferrocene with t-BuLi, a procedure that resulted in the formation of traces of l,l -dilithio-ferrocene, explaining the obtention of the interesting diphosphine borane 40 in 3% yield. This has been exploited to prepare bidentate ligands, as detailed below. Other groups prepared other ferrocenyl monophosphines as can be... 

The same method was applied to u-difluorobenzene tricarbonylchromium with the aim of obtaining or / o-substituted monophosphines and diphosphines (Scheme 5.10). The first nucleophilic attack provided the desired mono-substituted compound (20) in 97% yield, but no traces of diphosphine borane could be detected, even using excess of phosphide borane. Compound 20 possesses a second stereogenic element (a plane) and it was found to be present as a mixture of separable diastereomers in 60% de. When this compound was reacted with MeMgBr it was found that the large steric hindrance of the phosphine borane group inhibited the second nucleophilic attack and therefore the deprotection of the phosphine was required (Scheme 5.10). 

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