Chiral diphosphinoterpenes and transition metal complexes thereof

The present invention relates to chiral diphosphinoterpenes and transition metal complexes thereof, to a process for preparing chiral diphosphinoterpenes and oxides thereof. In a further aspect, the invention relates to the use of the chiral diphosphinoterpenes or transition metal complexes thereof in asymmetric syntheses.

Enantiomerically enriched chiral compounds are valuable starting substances for preparing agrochemicals and pharmaceuticals. The asymmetric catalysis for the synthesis of such enantiomerically enriched chiral compounds has gained great industrial significance.

The multitude of publications in the field of asymmetric synthesis shows clearly that transition metal complexes of diphosphorus compounds are particularly suitable as catalysts in asymmetrically controlled reactions. In particular, transition metal complexes of diphosphorus compounds have found use in industrial processes in asymmetric hydrogenations of C═O, C═N and C═C bonds, hydrocyanations and hydroformylations.

Haltermann, Organometallics, 9, (1), 1990, pp. 273-275 discloses (2R,3S)-2-[(diphenylphosphinyl)methyl]-3-diphenylphosphinylbicyclo[3.3.1]heptane and a palladium complex preparable therefrom. No information is given about use in asymmetric synthesis.

There is a need to develop a ligand system which can be varied readily in its steric and electronic properties, and whose transition metal complexes as catalysts in asymmetric synthesis, in particular asymmetric hydrogenations, enable not only high enantioselectivity but also high turnover rates. In addition, there is a need to develop a convenient route for such a ligand system and the corresponding precursors.

Compounds of the formula (I) have now been found

in which

*2 and *3 each mark a stereogenic carbon atom which is in R- or S-configuration, although the substituents are arranged in a cisoid manner based on the connecting carbon bond and R 1 , R 2 , R 3 and R 4 may each independently be: alkyl, arylalkyl or aryl or a heterocyclic radical having a total of 4 to 16 carbon atoms, or R 1 and R 2 and/or R 3 and R 4 each together are alkylene.

The scope of the invention encompasses all radical definitions, parameters and illustrations above and cited hereinbelow, in general or within areas of preference, in any combination with one another, i.e. also between the particular areas and areas of preference.

In the context of the invention, unless stated specifically, aryl is preferably carbocyclic aromatic radicals having 6 to 24 skeleton carbon atoms or heteroaromatic radicals having 5 to 24 skeleton carbon atoms, in which no, one, two or three skeleton carbon atoms per cycle, but at least one skeleton carbon atom in the entire molecule, may be substituted by heteroatoms selected from the group of nitrogen, sulphur or oxygen. In addition, the carbocyclic aromatic radicals or heteroaromatic radicals may be substituted by up to five identical or different substituents per cycle, selected from the group of hydroxyl, fluorine, nitro, cyano, free and protected formyl, C 1 -C 12 -alkyl, C 5 -C 14 -aryl, C 6 -C 15 -arylalkyl, —PO—[(C 1 -C 8 )-alkyl] 2 , —PO—[(C 5 -C 14 )-aryl] 2 , —PO—[(C 1 -C 8 )-alkyl)(C 5 -C 14 )-aryl)], tri(C 1 -C 8 -alkyl)siloxyl and radicals of the formulae (IIa) to (IIf). The same applies to the aryl moiety of an aryl-alkyl radical.

For example, aryl is more preferably phenyl, naphthyl or anthracenyl, each of which is optionally mono-, di- or trisubstituted by radicals which are each independently selected from the group of C 1 -C 6 -alkyl, C 5 -C 14 -aryl, C 1 -C 6 -alkoxy, C 1 -C 6 -alkoxy -carbonyl, halogen, hydroxyl, nitro or cyano.

In the context of the invention, unless stated specifically, alkyl, alkylene and alkoxy are preferably each independently a straight-chain, cyclic, branched or unbranched alkyl, alkylene and alkoxy radical respectively, each of which may optionally be further substituted by C 1 -C 4 -alkoxy radicals. The same applies to the alkylene moiety of an arylalkyl radical.

For example, alkyl is more preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-pentyl, cyclohexyl and n-hexyl, n-heptyl, n-octyl, isooctyl, n-decyl and n-dodecyl.

For example, alkylene is preferably 1,3-propylene, 1,4-butylene, 1,5-pentylene, 1,6-hexylene, S,S- or R,R-2,5-hexylene, 1,4-cyclohexylene, 1,2-cyclohexylene and 1,8-octylene.

For example, alkoxy is preferably methoxy, ethoxy, isopropoxy, n-propoxy, n-butoxy, tert-butoxy and cyclohexyloxy.

In the context of the invention, unless stated specifically, arylalkyl is preferably in each case independently a straight-chain, cyclic, branched or unbranched alkyl radical which is mono- or polysubstituted, more preferably monosubstituted, by aryl radicals as defined above.

In the context of the invention, unless stated specifically, haloalkyl and haloalkylene are preferably in each case independently a straight-chain, cyclic, branched or unbranched alkyl and alkylene radical respectively, each of which may be substituted once, more than once or fully by halogen atoms each selected independently from the group of fluorine, chlorine, bromine and iodine.

For example, haloalkyl is more preferably trifluoromethyl, trichloromethyl, 2,2,2-trifluoroethyl, pentafluoroethyl and nonafluorobutyl; C 1 -C 8 -fluoroalkyl is more preferably trifluoromethyl, 2,2,2-trifluoroethyl, pentafluoroethyl and nonafluorobutyl.

Preferred compounds of the formula (I) are defined hereinbelow.

The compounds of the formula (I) are preferably enantiomerically enriched. R 1 and R 2 or R 3 and R 4 are preferably, in each case in pairs and identically: alkyl or aryl or a heterocyclic radical having a total of 4 to 9 carbon atoms or, in each case together, alkylene.

More preferably, R 1 and R 2 or R 3 and R 4 , in each case as a pair and identically, are: C 3 -C 6 -alkyl, optionally mono-, di- or tri-C 1 -C 6 -alkyl-, —C 1 -C 6 -haloalkyl-, —C 1 -C 6 -alkoxy-, -chlorine- or -fluorine-substituted phenyl or, in each case together, are C 4 -C 6 -alkylene. Very particular preference is given to all four radicals R 1 , R 2 , R 3 and R 4 being identical.

Very particularly preferred compounds of the formula (I) are:

(2S,3S)-3-diphenylphosphinyl-2-[(diphenylphosphinyl)methyl]-6,6-dimethylbicyclo -[3.1.1]heptane, (2R,3R)-3-diphenylphosphinyl-2-[(diphenylphosphinyl)methyl]-6,6-dimethylbicyclo[3.1.1]heptane, (2R,3R)-3-diphenylphosphinyl-2-[(diphenylphosphinyl)methyl]-6,6-dimethylbicyclo[3.1.1]heptane and (2R,3R)-3-diphenylphosphinyl -2-[(diphenylphosphinyl)methyl]-6,6-dimethylbicyclo[3.1.1]heptane.

In addition, the invention also embraces complexes of compounds of the formula (I) with boranes, for example borane or borabicyclononane (BBN-9).

In the context of the invention, the terms stereoisomerically enriched and enantiomerically enriched include stereoisomerically pure and enantiomerically pure compounds and mixtures of stereoisomeric or enantiomeric compounds in which one stereoisomer or enantiomer is present in a larger relative proportion than the other stereoisomer(s) or enantiomer(s), preferably in a relative proportion of above 50% to 100 mol %, more preferably 90 to 100 mol % and most preferably 98 to 100 mol %.

The compounds of the formula (I) can be prepared by a process which likewise forms part of the subject-matter of the invention.

This process is characterized in that

in a step A), compounds of the formula (IIa) or (IIb)

in which *3 has the definitions and areas of preference specified for formula (I)

are reacted with compounds of the formula (IIIa) or (IIIb)

Hal-PR 1 R 2   (IIIa)

Hal-PR 3 R 4   (IIIb)

to give compounds of the formula (IVa) and (IVb)

and

in a step B), the compounds of the formula (IVa) and (IVb), optionally in an organic solvent, are converted by heating to at least 60° C. to compounds of the formula (Va) and (Vb)

and, in a step C), the compounds of the formula (Va) and (Vb) are converted by reacting with a borane and subsequently oxidizing to compounds of the formula (VIa) and (VIb)

and, in a step D), the compounds of the formula (VIa) and (VIb) are converted by reducing to compounds of the formula (VIIa) and (VIIb)

and, in a step E), the compounds of the formula (VIIa) and (VIIb) are converted by reacting with compounds of the formula (VIII)

Hal-O 2 SR 5   (VIII)

to compounds of the formula (IXa) and (IXb)

and, in a step F), the compounds of the formula (IX) are converted by reacting with compounds of the formula (Xa) or (Xb)

HPR 3 R 4   (Xa)

HPR 1 R 2   (Xb)

to the compounds of the formula (I).

In the formulae (IIa), (IIb), (IIIa), (IIIb), (IVa), (IVb), (Va), (Vb), (VIa), (VIb), (VIIa), (VIIb), (IXa), (IXb), (Xa) and (Xb), *2 and *3, R 1 , R 2 , R 3 and R 4 each have the same definitions and areas of preference as have already been described under the formula (I).

Moreover, R 5 in the formulae (VIII), (IXa) and (IXb) is alkyl, fluoroalkyl, arylalkyl or aryl, preferably alkyl and fluoroalkyl.

In the formulae (IIIa) and (IIIb) and (VIII), Hal is in each case chlorine, bromine or iodine, preferably chlorine.

The compounds of the formulae (IVa), (IVb), (Va), (Vb), (VIa), (VIb), (VIIa), (VIIb), (IXa) and (IXb) are hitherto unknown and are therefore encompassed by the invention as indispensible intermediates. The abovementioned definitions and areas of preference for *1, *2 and *3, R 1 , R 2 , R 3 , R 4 and R 5 apply in the same manner.

Also encompassed by the invention are complexes of compounds of the formula (VIIa) and (VIIb) with boranes, for example borane or borabicyclononane (BBN-9).

The invention further provides processes which comprise the steps below as have been defined above:

F) or A) E) and F) or A) and B) D), E) and F) or A), B) and C) C), D), E) and F) or A), B) C) and D) B), C), D), E) and F) or A), B), C), D) and E)

The chemical nature of the individual steps is already known in principle and they may be employed to the inventive compounds in a similar manner.

Preferred compounds of the formula (IIa) and (IIb) are trans-pinocarveol and myrtenol.

A preferred compound of the formulae (IIIa) and (IIIb) is chlorodiphenylphosphine.

Preferred compounds of the formula (IVa) and (IVb) are:

O-diphenylphosphinyl-trans-pinocarveol and O-diphenylphosphinylmyrtenol.

Preferred compounds of the formula (Va) and (Vb) are:

2-(diphenylphosphinoylmethyl)-6,6-dimethylbicyclo[3.1.1]hept-2-ene and (R)-3-(diphenylphosphinoyl)-6,6-dimethyl-2-methylenebicyclo[3.1.1]heptane. Preferred compounds the formula (VIa) and (VIb) are: (2S,3R)-2-(diphenylphosphinoylmethyl)-6,6-dimethylbicyclo[3.1.1]heptan-3-ol and (2R,3R)[3-(diphenylphosphinoyl)-6,6-dimethylbicyclo[3.1.1]hept-2-yl]methanol.

Preferred compounds of the formula (VIIa) and (VIIb) are:

(2S,3R)-2-(diphenylphosphinylmethyl)-6,6-dimethylbicyclo[3.1.1]heptan-3-ol and (2R,3R)[3-(diphenylphosphinyl)-6,6-dimethylbicyclo[3.1.1]hept-2-yl]methanol.

Preferred compounds the formula (IXa) and (IXb) are:

(2S,3R)-2-(diphenylphosphinylmethyl)-6,6-dimethyl-3-methanesulphonyloxybicyclo-[3.1.1]heptane and (2R,3R)[3-(diphenylphosphinyl)-6,6-dimethylbicyclo[3.1.1]hept-2-yl]-1-methanesulphonyloxymethane.

A preferred compound of the formulae (Xa) and (Xb) is diphenylphosphine.

In the manner described, the compounds of the formula (I) are obtainable in high yields in an efficient manner.

The invention further embraces transition metal complexes which comprise the inventive compounds of the formula (I).

Transition metal complexes are preferably those of ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum and copper, preferably those of ruthenium, rhodium, iridium, nickel, palladium and platinum, more preferably those of ruthenium, rhodium, iridium and palladium.

The inventive transition metal complexes are especially suitable as catalysts. The invention therefore also embraces catalysts which comprise the inventive transition metal complexes.

The catalysts used may, for example, either be isolated transition metal complexes or those transition metal complexes which are obtainable by reacting transition metal compounds and compounds of the formula (I).

Isolated transition metal complexes which contain the compounds of the formula (I) are preferably those in which the ratio of transition metal to compound of the formula (I) is 1:1.

Preference is given to the inventive compounds of the formula (XI)

[(I)L 1 2 M][An 1 ]  (XI)

in which (I) represents compounds of the formula (I) with the definition specified there and its areas of preference and

M is rhodium or iridium and L 1 is in each case a C 2 -C 12 -alkene, for example ethylene or cyclooctene, or a nitrile, for example acetonitrile, benzonitrile or benzyl nitrile, or L 1 2 together is a (C 4 -C 12 )-diene, for example bicyclo[2.1.1]hepta-2,5-diene (norbornadiene) or 1,5-cyclooctadiene and [An 1 ] is an anion, preferably methanesulphonate, trifluoromethanesulphonate, tetrafluoroborate, hexafluorophosphate, perchlorate, hexafluoroantimonate, tetra -(bis-3,5-trifluoromethylphenyl)borate or tetraphenylborate.

Preferred transition metal complexes are those which are obtainable by reacting transition metal compounds and compounds of the formula (I).

Suitable transition metal compounds are, for example, those of the formula

M(An 2 ) q   (XIIa)

in which

M is rhodium, iridium, ruthenium, nickel, palladium, platinum or copper and An 2 is chloride, bromide, acetate, nitrate, methanesulphonate, trifluoromethanesulphonate or acetylacetonate and q is 3 for rhodium, iridium and ruthenium, is 2 for nickel, palladium and platinum and is 1 for copper, or transition metal complexes of the formula (XIIb)

M(An 3 ) q L 1 2   (XIIb)

in which M is ruthenium, iridium, ruthenium, nickel, palladium, platinum or copper and An 3 is chloride, bromide, acetate, acetylacetonate, methanesulphonate or trifluoromethanesulphonate, tetrafluoroborate or hexafluorophosphate, perchlorate, hexafluoroantimonate, tetra(bis-3,5-trifluoromethylphenyl)borate or tetraphenylborate and q is 1 for rhodium and iridium, is 2 for ruthenium, nickel, palladium and platinum and is 1 for copper, L 1 is in each case a C 2 -C 12 -alkene, for example ethylene or cyclooctene, or a nitrile, for example acetonitrile, benzonitrile or benzyl nitrile, or L 1 2 together is a (C 4 -C 12 )-diene, for example bicyclo[2.1.1]hepta-2,5-diene (norbornadiene) or 1,5-cyclooctadiene or transition metal compounds of the formula (XIIc)

[ML 2 An 1 2 ] 2   (XIIc)

in which M is ruthenium and L 2 is an aryl radical, for example cymene, mesityl, phenyl or cyclooctadiene, norbornadiene or methylallyl or transition metal compounds of the formula (XIId)

Met 3 q [M(An 4 ) 4 ]  (XIId)

in which M is palladium, nickel, iridium or rhodium and An 4 is chloride or bromide and Me is lithium, sodium, potassium, ammonium or organic ammonium and q is 3 for rhodium and iridium, is 2 for nickel, palladium and platinum, or transition metal compounds the formula (XIIe)

[M(L 3 ) 2 ]An 5   (XIIe)

in which M is iridium or rhodium and L 3 is (C 4 -C 12 )-diene, for example bicyclo[2.1.1]hepta-2,5-diene (norbornadiene) or 1,5-cyclooctadiene and An 5 is a noncoordinating or weakly coordinating anion, for example methanesulphonate, trifluoromethanesulphonate, tetrafluoroborate, hexafluorophosphate, perchlorate, hexafluoroantimonate, tetra(bis-3,5-trifluoromethylphenyl)borate or tetraphenylborate.

Additionally suitable as transition metal compounds are, for example, Ni(1,5-cyclooctadiene) 2 , Pd 2 (dibenzylideneacetone) 3 , Pd[PPh 3 ] 4 , cyclopentadienyl 2 Ru, Rh(acac)(CO) 2 , Ir(pyridine) 2 (1,5-cyclooctadiene), Cu(phenyl)Br, Cu(phenyl)Cl, Cu(phenyl)I, Cu(PPh 3 ) 2 Br, [Cu(CH 3 CN) 4 ]BF 4 and [Cu(CH 3 CN) 4 ]PF 6 or polynuclear bridged complexes, for example [Rh(1,5-cyclooctadiene)Cl] 2 , [Rh(1,5-cyclooctadiene)Br] 2 , [Rh(ethene) 2 Cl] 2 , [Rh(cyclooctene) 2 Cl] 2 .

The transition metal compounds used are preferably:

[Rh(cod)(acac)] where acac is acetylacetonate, [Ir(ethylene) 2 (acac)], [Rh(ethylene) 2 (acac)], [Rh(cod)Cl] 2 , [Rh(cod)Br] 2 , [Rh(cod) 2 ]ClO 4 , [Rh(cod) 2 ]BF 4 , [Rh(cod) 2 ]PF 4 , [Rh(cod) 2 ]ClO 6 , [Rh(cod) 2 ]OTf, [Rh(cod) 2 ]BAr 4 (Ar=3,5-bis-trifluoromethylphenyl), [Rh(cod) 2 ]SbF 6 , RuCl 2 (cod), [(cymene)RuCl 2 ] 2 , [(benzene)RuCl 2 ] 2 , [(mesityl)RuCl 2 ] 2 , [(cymene)RuBr 2 ] 2 , [(cymene)RuI 2 ] 2 , [(cymene)Ru(BF 4 ) 2 ] 2 , [(cymene)Ru(PF 6 ) 2 ] 2 , [(cymene)Ru(BAr 4 ) 2 ] 2 (Ar=3,5-bistrifluoromethylphenyl), [(cymene)Ru(SbF 6 ) 2 ] 2 , [Ir(cod)Cl] 2 , [Ir(cod) 2 ]PF 6 , [Ir(cod) 2 ]ClO 4 , [Ir(cod) 2 ]SbF 6 , [Ir(cod) 2 ]BF 4 , [Ir(cod) 2 ]OTf, [Ir(cod) 2 ]BAr 4 (Ar=3,5-bistrifluoromethylphenyl), RuCl 3 , NiCl 3 , RhCl 3 , PdCl 2 , PdBr 2 , Pd(OAc) 2 , Pd 2 (dibenzylideneacetone) 3 , Pd(acetylacetonate) 2 , CuOTf, CuI, CuCl, Cu(OTf) 2 , CuBr, CuI, CuBr 2 , CuCl 2 , CuI 2 , [Rh(nbd)Cl] 2 , [Rh(nbd)Br] 2 , [Rh(nbd) 2 ]ClO 4 , [Rh(nbd) 2 ]BF 4 , [Rh(nbd) 2 ]PF 6 , [Rh(nbd) 2 ]OTf, [Rh(nbd) 2 ]BAr 4 (Ar=3,5-bistrifluoromethylphenyl), [Rh(nbd) 2 ]SbF 6 , RuCl 2 (nbd), [Ir(nbd) 2 ]PF 6 , [Ir(nbd) 2 ]ClO 4 , [Ir(nbd) 2 ]SbF 6 , [Ir(nbd) 2 ]BF 4 , [Ir(nbd) 2 ]OTf, [Ir(nbd) 2 ]BAr 4 (Ar=3,5-bistrifluoromethylphenyl), Ir(pyridine) 2 (nbd), [Ru(DMSO) 4 Cl 2 ], [Ru(CH 3 CN) 4 Cl 2 ], [Ru(PhCN) 4 Cl 2 ], [Ru(cod)Cl 2 ] n , [Ru(cod) 4 (methallyl) 2 ], [Ru(acetylacetonate) 3 ].

The amount of the transition metal compounds used may, for example, be 25 to 200 mol % based on the compound of the formula (I) used; preference is given to 50 to 150 mol %, very particular preference to 75 to 125 mol % and even greater preference to 100 to 125 mol %.

The catalysts which comprise the inventive transition metal complexes are especially suitable for use in a process for preparing stereoisomerically enriched, preferably enantiomerically enriched, compounds.

Preference is given to using the catalysts for asymmetric 1,4-additions, asymmetric hydroformylations, asymmetric allylic substitutions, asymmetric hydrocyanations, asymmetric Heck reactions, asymmetric hydroborations and asymmetric hydrogenations, more preferably for asymmetric hydrogenations, asymmetric 1,4-additions, asymmetric hydroborations and asymmetric allylic substitutions.

Preferred asymmetric hydrogenations are, for example, hydrogenations of prochiral C═C bonds, for example of prochiral enamines, enamides, olefins, enol ethers, C═O bonds, for example of prochiral ketones, and C═N bonds, for example of prochiral imines. Particularly preferred asymmetric hydrogenations are hydrogenations of prochiral C═C bonds, for example of prochiral enamines and enamides, olefins.

The invention therefore also embraces a process for preparing stereoisomerically enriched, preferably enantiomerically enriched, compounds by catalytic hydrogenation of olefins, enamines, enamides, imines or ketones, which is characterized in that the catalysts used are those which comprise transition metal complexes of compounds of the formula (I) with the definition specified there.

The amount of the transition metal compound used or of the transition metal complex used may, for example, be 0.001 to 5 mol % based on the substrate used; preference is given to 0.001 to 0.5 mol %, very particular preference to 0.001 to 0.1 mol % and even greater preference to 0.001 to 0.008 mol %.

In a preferred embodiment, asymmetric hydrogenations, 1,4-additions and hydroborations may be carried out, for example, in such a way that the catalyst is generated from a transition metal compound and compound of the formula (I), optionally in a suitable solvent, the substrate is added and the reaction mixture is admixed with the reactant at reaction temperature (hydrogen, boronic acids, boranes, etc.).

Suitable solvents for the asymmetric catalysis are, for example, chlorinated alkanes such as methyl chloride, short-chain C 1 -C 6 -alcohols, for example methanol, isopropanol or ethanol, aromatic hydrocarbons, for example toluene or benzene, ketones, for example acetone, or carboxylic esters, for example ethyl acetate.

The asymmetric catalysis is carried out advantageously at a temperature of −20° C. to 200° C., preferably 0 to 100° C. and more preferably at 20° to 70° C.

The inventive catalysts are suitable in a process for preparing stereoisomerically enriched, preferably enantiomerically enriched, active ingredients in medicaments and agrochemicals, or intermediates of these two classes.

The advantage of the present invention is that the compounds of the formula (I) can be prepared in an efficient manner and their electronic and steric properties are variable to a wide degree starting from readily available reactants. In addition, the inventive ligands and their transition metal complexes exhibit good performance in asymmetric syntheses.

General Remarks:

All reactions with air- or moisture-sensitive compounds were carried out under an argon atmosphere using Schlenk technology. The solvents were dried over suitable reagents and distilled under a nitrogen atmosphere. In some cases, the solvents were distilled beforehand on a rotary evaporator. Commercially available reagents were, unless stated specifically, used without further purification. The silica gel mentioned in the text is silica gel 60 (Merck 0.04-0.063 mm, 230-400 mesh), alumina is aluminium oxide (Fluka, treated with water to attain Brockmann grade III), celite is kieselguhr (Merck). The oil-pump vacuum corresponds to about 0.2 mbar. The reaction temperatures relate to internal temperatures.

General procedure 1 for the conversion of phosphine oxides to the corresponding phosphine-borane complexes.

Under an argon atmosphere, 20 ml of anhydrous toluene, 2 ml of polymethylhydrosiloxane (Aldrich) and 1.5 ml of titanium isopropoxide (Acros) were introduced into a dried Schlenk flask. A sample of the reaction mixture was introduced into an NMR tube in order to monitor the reaction. The reduction mixture and the tube were heated at 105° C. for 2-4 h until complete reduction was detected in the 31 P NMR (oxide at 30-40 ppm, phosphines at −20-0 ppm). The reaction mixture is allowed to cool and 1 ml of borane-dimethyl sulphide complex (Aldrich) is added. After a few minutes, the mixture is introduced into a 250 ml Erlenmeyer flask with 5 ml of methanol (hydrogen evolution!). After the gas evolution has abated, the solution is introduced into a Nalgene® bottle with 20 ml of 48% HF and 20 ml of water and stirred at room temperature for 12 h. (This removes the PMHS excess which is otherwise difficult to remove.) The organic phase is removed and the aqueous phase extracted with 15 ml of toluene. The combined organic phases are washed with saturated sodium hydrogencarbonate solution and brine, and dried over magnesium sulphate, and the solvent is removed under reduced pressure. The residue is dissolved in a small amount of diethyl ether and filtered through a 3 cm silica gel layer, the latter is washed with diethyl ether, the solvent is removed and the substance is dried under reduced pressure in order to obtain the phosphine-borane complex as a viscous oil which gradually solidifies. Typical yield range 95-99%.

General procedure 2 for the conversion of phosphine-borane alcohols to the corresponding mesylates and nucleophilic substitution with diphenylphosphine.

Under an argon atmosphere, 5 mM of a phosphine-borane alcohol and 40 ml of dry dichloromethane were introduced into a 100 ml dry Schlenk vessel. The mixture was cooled to −30° C. and 2.4 ml of anhydrous triethylamine were added.

1.2 ml of methanesulphonyl chloride were then added dropwise at this temperature with vigorous stirring. The reaction mixture was left at −30° for 2 h and introduced into 200 ml of anhydrous diethyl ether with stirring at this temperature. After 5 min, the white precipitate was filtered off through a 3 cm silica gel layer. The filtercake was washed with 100 ml diethyl ether and the solvent was removed under reduced pressure down to a residue of 10 ml. The remaining solvent was removed under high vacuum in order to prevent thermal stress. The remaining residue was left under high vacuum for 5 h in order to remove traces of MsCl. The thus obtained mesylate was used in the next step without further purification.

In a 100 ml Schlenk vessel under an argon atmosphere, 2.00 g of t-BuOK in 25 ml of anhydrous THF and 2.32 g (3 equivalents) of diphenylphosphine (Strem) were added. The orange-coloured solution was cooled to −20° C. and the above-described mesylate was introduced slowly in 10 ml of THF. The reaction mixture was allowed to warm to room temperature and then heated to 50° C. for 18 h. After cooling to room temperature, 2.5 ml of borane-dimethyl sulphide complex (Aldrich) were added. The contents of the reaction vessel were introduced cautiously into a 250 ml Erlenmeyer flask with 10 ml of methanol. After the gas evolution had abated, 50 ml of saturated NH 4 Cl solution were added, the organic phase was removed and the aqueous phase was extracted twice with 20 ml each time of dichloromethane. The combined organic phases were washed with saturated sodium chloride solution and dried over magnesium sulphate, and the solvent was removed under reduced pressure. The residue was dissolved in 5 ml of dichloromethane-pentane (1:1) and filtered through a 5 cm alumina layer on a 25 mm (diameter) filter. The majority of the solvent was removed under reduced pressure and the residue was diluted with 15 ml of diethyl ether and left at 0° C. for 12 h. The solid formed was filtered off and dried under reduced pressure.

General procedure 3 for the deprotection of a phosphine-borane with N,N′-bis-(3-aminopropyl)piperazine.

Phosphine-borane (1 mM) was introduced under an argon atmosphere into a 10 ml Schlenk vessel and dissolved in 2 ml of anhydrous toluene. 1 ml of N,N′-bis(3-aminopropyl)piperazine (Lancaster) was added to this solution. The reaction mixture was heated to 105° C. for 2 h, cooled to room temperature and diluted with 10 ml of diethyl ether. Under an argon atmosphere, the mixture was filtered through diethyl ether-moistened silica gel into a flask (very strict oxygen exclusion). The silica gel was rewashed twice with 30 ml each time of diethyl ether. The solvent was removed under reduced pressure. The resulting phosphine is obtained as a white foam or highly viscous oil which solidifies. The product was stored under an argon atmosphere.

General procedure 4 for the hydrogenation of a phosphine oxide over Raney nickel.

6.0 g of Raney nickel (50% water-moist, Acros) was washed by mixing and decanting off three times with 20 ml units of methanol, admixed with 30 ml of methanol and transferred into a 200 ml V4A autoclave. 8 mM of diphenylphosphine oxide were added and the autoclave was charged with hydrogen until 50 bar were attained. The hydrogenation was effected at 50° C. over a period of 48 h, the autoclave was decompressed and the mixture was filtered through a Celite layer. The residue was washed with methanol. The solvent of the filtrate was removed under reduced pressure and the product was obtained in virtually quantitative yield as a colourless foam.