Borane phosphino

For the Rh-catalysed amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as catal5dic precursor, the catalytic activity was completely suppressed by the addition of an excess of mercury. But for phosphino-borane, Hg(0) addition had no effect. Consequently, the authors proved that the catalyst nature depends on the substrate using the same catalytic precursor [15]. 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

Aiming at preparing a phosphino-borane ILjPBR with PB doublebond character,43 Karsch reacted the lithium phosphide LiPMes2 with tribromoborane (Scheme 19).44 Instead of the desired compound 25, they isolated the PB 26a that results formally from the 1,2-addition of a C-H bond of a mesityl group across the PB double bond of 25. The 31P and... 

Nucleophilic Attack at Other Atoms.- Interest has continued in the synthesis of phosphine-borane adducts. Complexes of tetramethyldiphosphine with borane and various halogenoboranes have been used to generate a series of open-chain and cyclic phosphino-boranes. Adducts of 1,3,5-diazaphosphorinanes involving two... 

Keywords. Phosphorus, Polymers, Heterophosphazenes, Thionylphosphazenes, Phosphino-boranes... 

Table 6.1 P-NMR resonances and coordination chemical shift values for phosphino boranes...

Reactions. - The reactions of diorganoiodophosphines with varying quantities of iodine have been studied by NMR techniques, which indicate the formation of a dialkyldi-iodophosphonium iodide as the initial product, followed by polyhalide anion formation in the presence of excess iodine. A modified McCormack synthesis of 3-silylated phosphol-3-enes (168) from phenyl-dichlorophosphine and 2-silylated 1,3-butadienes has been described. The reaction is carried out in the presence of a small amount of copper stearate in acetic anhydride at 50 °C, these conditions preventing the complete desilylation observed under standard conditions. The consecutive reaction of lithium phenylacetylide with triphenylborane and diphenylchlorophosphine results in the formation of the intramolecularly coordinated phosphino-borane (169). Treatment of the alkoxyvinyldichlorophosphines (170) with Grignard reagents... 

Recognizing that FLP activation of H2 requires a bulky base and acid combination, we probed the question of whether a bulky imine could act both as the base component of an FLP and as the substrate for catalytic reduction. Indeed, a number of aldimines and ketimines could be hydrogenated with a catalytic amount of B(QF5)3 under H2 in a facile manner (Table 11.2, Scheme 11.13) [47]. As with the phosphino-borane catalyst, dx-triphenylaziridine is also reduced under these conditions to N-(l,2-diphenylethyl)aniline (Table 11.2) [47]. 

Moret M-E, PetersJC. Terminal iron dinitrogen and iron imide complexes supported by a tris(phosphino)borane ligand. Angew Chem Int Ed. 2011 50 2063-2067. 

Another strategy for hydrogen storage is through the use of frustrated Lewis pairs. For example, a phosphonium borate compound has been described that reversibly stores hydrogen in a practical temperature range [152]. The phosphino-borane compound shown on the left-hand side of Equation 16.50 was shown to accept 1 equiv. of hydrogen, at 1 atm and 25°C,... 

In pioneering work, the reversible action of FLPs was demonstrated by Stephan and co-workers [34], Starting from a zwitterionic salt 87, reduction with Me SiHCl provided another zwitterion, an internal phosphonium borohydride 88. This species turned out to be relatively robust, but heating to 150°C forced the loss of to generate the phosphino-borane 86 as an orange-red sohd. 86 reacted with at 25°C to regenerate the zwitterion 88, thus completing the cycle. The phosphino-borane 86 turned out to be monomeric in solution (Scheme 6.15). 

Dinuclear palladium complexes catalyze m-hydroarylation of alkynes with arenes.56 The reaction of 3-hexyne with benzene in the presence of a dinulear palladium complex Pd2R2(M-OH)(//-dpfam) [dpfam = j/V,Ar -bis[2-(diphenyl-phosphino)phenyl]formamidinate, R=/>-Tol] and tri(/z-butyl)borane at 100 °C for 4h affords ( )-3-phenyl-3-hexene quantitatively (Equation (53)). The hydroarylation of 3-hexyne with monosubstituted benzenes ( )-3-aryl-3-hexenes with a 2 1 ratio of the meta- and ra -isomers. This regioselectivity is different from that of the hydroarylation of diphenylacetylene catalyzed by Rh4(GO)12.57... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

In continuation of our efforts to explore the utility of the SAMP/RAMP hydra-zone methodology, we developed the first asymmetric synthesis of a-phosphino ketones via formation of a carbon-phosphorus bond in the a-position to the carbonyl group [70]. The key step of this asymmetric C—P bond formation is the electrophilic phosphinylation of the ketone SAMP hydrazone 87, giving rise to the borane-adduct of the phosphino hydrazone 88 with excellent diastereoselectiv-ity (de = 95-98%). Since these phosphane-borane adducts are stable with respect to oxidation, the chemoselective cleavage of the chiral auxiliary by ozonolysis leading to the a-phosphino ketones (R)-89 could be accomplished with virtually no racemization. Using RAMP as a chiral auxiliary, the synthesis of the enantiomer (S)-89 was possible (Scheme 1.1.25). 

In addition to the results described, enantioselective access to 2-phosphino alcohols could be accomplished, too [71]. Starting from a borane-protected a-phosphino aldehyde hydrazone 91 as the key intermediate and available by two different approaches, the enantioselective synthesis of the desired 2-phosphino alcohols 93 could be accomplished. Thus, the electrophilic phosphinylation of aldehyde hydrazones 90 (via route I with the chlorodiphenylphosphine-borane adduct or via route II with chlorophosphines and subsequent phosphorus-boron bond formation) and the alkylation of phosphino acetaldehyde-SAMP hydrazones 92 (route III) was carried out (Scheme 1.1.26). 

An alternative access was achieved by alkylation of the a-diphenylphosphino acetaldehyde SAMP hydrazone 95, yielding the hydrazone products 96 in good yields (60-63%) and good diastereomeric excesses (die = 68-71%) as EjZ mixtures, from which the major diastereomer was separated and purified by preparative HPLC. Ozonolysis and in-situ reduction with the borane-dimethyl sulfide complex of the aldehydes generated gave the air-stable borane-protected 2-diphenylphosphino alcohols 97 in good yields (67-83%). Reaction with DABCO afforded the unprotected 2-phosphino alcohols 98 in very good yields (85-91%) and excellent enantiomeric excesses (ee > 96%) (Scheme 1.1.27). 

The regioseledivity of the last reaction in Scheme 5.13 is not only because of the greater acidity of the methylene group, but also because some secondary and tertiary amides (e.g. /3-arylamides, /3-vinylamides, or /3-(phenylthio)amides, or borane complexes of /3-phosphino propionamides [132, 133]) are deprotonated at the /3 position under kinetic control to yield chelate-stabilized carbanions [58, 134], Illustrative examples of such remarkable metalations are shown in Scheme 5.14. 

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