P-borane

Also, desymmetrization of prochiral hydroxyalkylphosphine P-boranes was successfully performed using similar reagents and conditions. In the case of bis(hydroxymethyl)phenylphosphine P-borane 87, both its acetylation and hydrolysis of the diacetyl derivative 89 gave good results, although in addition to the expected monoacetate 88, the diol 87 and diacetate 89 were always present in the reaction mixture (Equation 42). °°... 

In turn, bis(2-hydroxyethyl)phenylphosphine P-borane 90 underwent acetylation only in the presence of the lipase from Pseudomonas fluorescens, but its stereochemical outcome depended on the solvent used (Equation 43). The absolute configuration of 91 was not determined. 

Polymerization of various monomers was performed with the ligands examined to synthesize a variety of polymeric ligands. The polypyrrole-bound mono- and bisphosphines 73 and 74 were prepared as their P-borane com-... 

A 5 -P-borane-substituted thymidine monophosphate (68) and triphosphate (see Section 2) have been prepared from the boranophosphoramidate (69) which was obtained by reacting the thymidine 5 -(J-cyanoethylphosphoramidite with borane-diisopropylethylamine complex followed by treatment with ammonia. Acid hydrolysis of (69) gave the thymidine monophosphate derivative (68).The compound shows similar stability to normal nucleoside monophosphates, hydrolysis proceeding by P-0 rather than P-B cleavage. Acid phosphatase hydrolysed (68) to thymidine whilst the analogue was not a substrate for alkaline phosphatase. 

Kielbasinski P, Albrycht M, Zurawinski R, Mikolajczyk M (2006) Lipase-mediated kinetic resolution of racemic and desymmetrization of prochiral organophosphorus P-boranes. J Mol Catal B 39 45 9... 

Kwiatkowska M, Krasinski G, Cypryk M, Cierpial T, Kielbasinski P (2011) Lipase-mediated stereoselective transformations of chiral organophosphorus P-boranes revisited revision of the absolute configuration of alkoxy(hydroxymethyl)phenylphosphine P-boranes. Tetrahedron Asymmetry 22 1581-1590... 

The lipase-promoted kinetic resolution of a series of P-boranes (854) which might be treated as phosphinic acid derivatives, proceeded with moderate stereoselectivity to give both the unreacted substrates (855) and their O-acetyl derivatives (856) (Scheme 214). Some additional studies were carried out which proved that the borane reduction proceeded with inversion of configuration at the phosphorus center. ... 

Similar fragmentations to produce S-cyclodecen-l-ones and 1,6-cyclodecadienes have employed l-tosyloxy-4a-decalols and 5-mesyloxy-l-decalyl boranes as educts. The ringfusing carbon-carbon bond was smoothly cleaved and new n-bonds were thereby formed in the macrocycle (P.S. Wharton, 1965 J.A. Marshall, 1966). The mechanism of these reactions is probably E2, and the positions of the leaving groups determine the stereochemistry of the olefinic product. 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Heteroboranes contain heteroelements classified as nonmetals. The heteroatoms known to form part of a borane polyhedron include C, N, Si, P, As, S, Se, Sb, and Te either alone or in combination. In principle, most heteroboranes could have a wide range of skeletal sizes. However, with the primary exception of the carbaboranes, extensive chemistry has emerged only for the thiaboranes and azaboranes, which have the greatest availabiLity and demonstrated scope of chemistry. 

The H3+ ion is the simplest possible example of a three-centre two-electron bond (see discussion of bonding in boranes on p. 157) and is also a model for the dihapto bonding mode of the ligand r] -H2 (pp. 44-7) ... 

Alternative high-yield syntheses of these various boranes via hydride-ion abstraction from borane anions by BBra and other Lewis acids have recently been devised l (see p. 162). 

This works well for all nido- and arachno-boranes but not for the c/o o-dianions, which are much less reactive. Reactions of B2H6 with NH3 are complex and, depending on the conditions, yield aminodiborane, H2B(/r-H)(/r-NH2)BH2, or the diammoniate of diborane, [BH2(NH3)2]-[BH4] (p. 165) at higher temperatures the benzene analogue borazine, (HNBH)3, results (see p. 210). 

Decaborane is the most studied of all the polyhedral boranes and at one time (mid-1950s) was manufactured on a multitonne scale in the USA as a potential high-energy fuel. It is now obtainable in research quantities by the pyrolysis of B2H9 at 100-200°C in the presence of catalytic amounts of Lewis bases such as Me20. B10H14 is a colourless, volatile, crystalline solid (see Table 6.2, p. 163) which... 

The neutral m do-borane B Hio (structure 10) has a basal B-B bond (see p. 159) and this enables it to act as a ligand by displacing ethene from Zeise s salt (p. 930). 

Similarly, reaction of B6H10 with Fe2(CO)9 (p. 1 104) at room temperature results in the smooth elimination of Fe(CO)s to form [Fe(r -B6Hio)(CO)4] as a. stable, volatile yellow solid. Use of these electron-donor propcities of B(,H o towards reactive (vacant orbital) borane radicals resulted in the preparation of several new coti/uncm-boranes. e.g. BnHiy, BuHji and B15H23 (p. 162),... 

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

Derivatives of the boranes include not only simple substituted compounds in which H has been replaced by halogen, OH, alkyl or aryl groups, etc., but also the much more diverse and numerous class of compounds in which one or more B atom in the cluster is replaced by another main-group element such as C, P or S, or by a wide range of metal atoms or coordinated metal groups. These will be considered in later sections. 

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