Phosphines chiraphos

In a related study using the chelating phosphine chiraphos, several species, 8-10, were recognized by P NMR (see Figure 1.3) [18]. Only a single diastereomer, 10, forms, indicating that the binding is stereospecific. 

The two square planar species of line 2 are diastereomers. They contain the same optically active chelating phosphine chiraphos, but the rhodium atom is coordinated to different sides of the prochiral olefin (re/si sides). The two diastereomers of line 2 are rapidly interconverting. In this equilibrium the isomer shown on the left-hand side (si-coordination of the olefin) is the minor isomer and the isomer shown on the right-hand side (re-coordination of the olefin) is the major isomer [91, 92]. 

Early work in the field of asymmetric hydroboration employed norbornene as a simple unsaturated substrate. A range of chiral-chelating phosphine ligands were probed (DIOP (5), 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl (BINAP) (6), 2,3-bis(diphenylphosphino)butane (CHIRAPHOS) (7), 2,4-bis(diphenylphosphino)pentane (BDPP) (8), and l,2-(bis(o-methoxyphenyl)(phenyl)phos-phino)ethane) (DIPAMP) (9)) in combination with [Rh(COD)Cl]2 and catecholborane at room temperature (Scheme 8).45 General observations were that enantioselectivities increased as the temperature was lowered below ambient, but that variations of solvent (THF, benzene, or toluene) had little impact. 

It was not until 1995 that a synthetically useful enantioselective hydroalumination was first described.123 The early attempts to develop enantioselective hydroalumination used chiral phosphines such as prophos, chiraphos 26, and BINAP 23 as ligands. The most successful of these was BINAP with ee s of 56% being obtained (entry 1, Table 11). 

In general, the chiral ligands are water-soluble variants of those already studied in purely organic solvents (e.g., the sulfonated chiraphos, A, cyclobutane-diop, C, BDPP, F, MeOBIPHEP-TS, Q, BIFAPS, R and the quaternary ammonium derivatives of diop, D, BDPP, E). Solubility in water could also be achieved by attaching the parent phosphine molecule to a water-soluble polymer (J, M, P). The chiral phosphinites and phosphines derived from carbohydrates (e.g., K and L) have intrinsic solubility in water. During studies of one-phase... 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

KP(C6H4-4-NMc2)2 reacts with the appropriate diol ditosylates yielding the chiral phosphines 77-79. These analogs of the well known Chiraphos, BDPP (Skewphos) and DIOP can be made water soluble by protonation or quatemization. Quatemization can be achieved with (CH3)30Bp4 with the phosphoms atoms protected by complexation to Rh(I) [76]. This method of quatemization was originally introduced [77] to prepare 81 in its rhodium complex. It is remarkable, that DIOP which is known to be acid sensitive survives all these manipulations. 

This NOE idea was then extended to Pd(ii) allyl complexes with bidentate phosphine auxiliaries [99-111], with the ortho P-phenyl protons acting as the reporters (see 81). Figure 1.17 shows a section of the H, H NOESY for [Pd(P-pinene allyl) (Chiraphos)](OTf) (Chiraphos = Ph2PCH(CH3)CH(CH3)PPh2), 81 [129], and reveals the numerous contacts from the chiral phenyl array to the allyl ligand. 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

The simplest and the least technically demanding method of isolating S,S-CHIRAPHOS from the reaction mixture is by the formation of its very insoluble bis(thiocyanato-N)nickel(II) complex. The nickel selectively separates the S.S-CHIRAPHOS from other phosphine species in the reaction mixture. 

Asymmetric hydroboration of prochiral alkenes has been achieved using transition metal catalysts and chiral phosphines as ligands to obtain enantiomerically pure alkyl boronates <1997CC173>. Catalysts such as Rh(COD)2+BF4 , Rh(COD)2+Cl, Rh+BF4 , etc., in combination with chiral phosphines like DIOP 71, BINAP 72, CHIRAPHOS 73, DIPAMP 74, BDPP 75, ferrocene-based diphosphines 76<1999TL4977>, etc., have been employed for the asymmetric hydroboration of prochiral alkenes with moderate to high ee (DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane BINAP = 2,2-bis(diphenyl-phos-phanyl)-l,1-binaphthyl CHIRAPHOS = 2,3-bis(diphenylphosphino)butane DIPAMP = l,2-bis[(2-methoxyphe-nyl)phenylphosphino]ethane BDPP = 2,4-bis(diphenylphosphino)pentane). 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

We previously prepared surface-bonded rhodium phosphine complexes in Al-MCM-41. In a solution of dichloromethane, [Rh(acac)(chiraphos)] and Al-MCM-41 react to a surface bonded [(Os)x-Rh(chiraphos)] complex due to an exchange reaction of the acetylacetonato ligand and surface oxygens of the acidic support12. Here we present a heterogeneous Rhodium diphosphine catalyst and its application in the enantioselective hydrogenation of dimethylitaconate. The results indicate the localisation of the complex inside of the mesoporous channel system. 

The type of procedure described in the protocol has been used to synthesize sulfonated versions of BINAP , DPPE , and Chiraphos ligands. An easily prepared class of water-soluble phosphines can all be made from the same precursor, 95 (Scheme 34). 

Handling, Storage, and Precautions is indefinitely stable in air in the solid state. Solutions of CHIRAPHOS are readily oxidized to the phosphine oxide and should be handled under N2 or Ar. ... 

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