Phosphite precursors

Examples of cyclic aliphatic or aryl phosphonate triesters, such as 36 or 37 have also been prepared by this HHT method from their cyclic phosphite precursors (2). 

Finally, new tricyclic hexacoordinated phosphoranes with internal P-N coordination were synthesized by Swamy and coworkers by oxidative addition of cyclic phosphite precursors with quinones or with a combination of diols and (z-Pr)2NCl [57, 58]. Various ring sizes from five to eight membered were obtained showing the generality of the approach. A selection of compounds (47a-47e) is presented in Fig. 8. 

A nitrogen-bridged diphosphorane is obtained by reacting the phosphite precursor with chlorine. The diphosphorane decomposes slowly to form 5 and a diazadiphosphetidine. 

Scheme 1.1 shows the two-stage oxidation of Ultranox 626 diphosphite to form the diphosphate compound, XR 2502. The LD/EI/FT/ICR mass spectrum in Figure 1.22b shows the predominant molecular ion signal (m/z 636) for the diphosphate, XR 2502, along with residual signals from incompletely oxidised phosphite precursor at m/z 620 (half-oxidised monophosphate) and m/z 604 (nnoxidised phosphite). 

More recently, a double reductive ring closure of the dinitrodiphenylbenzene 139 with triethyl phosphite as the reducing agent has been reported to produce 3 in 59% yield (Scheme 18). The precursor 139 was prepared in excellent yield by Pd(0)-catalyzed coupHng of phenylboronic acid and 1,3-dibromo-4,6-dinitro-benzene (92JHC1237). 

Under high pressure, the reaction favors the formation of kinetic adducts (10) and (11) with precursor (6). Further regioselectivity and yield enhancement could be achieved with the bidentate phosphite ligand tpdp (12) as illustrated in Scheme 2.3 [11]. 

The parent TMM precursor (1), now commercially available, has played a pivotal role in the execution of many synthetic plans directed at natural and unnatural targets. Reaction of (1) with 2-(methoxycarbonyl)cyclohexenone (14, R=C02Me) in the presence of palladium acetate and triethyl phosphite produced the adduct (15) in near quantitative yield. This cycloadduct is a critical intermediate in the total synthesis of a hydroxykempenone (16), a component of the defensive substances secreted by termites (Scheme 2.5) [12]. In accord with a previous observation by Trost that unactivated 2-cyclohexenone reacts poorly with TMM-Pd [13], the substrate (14, R=Me) was essentially inert in the cycloaddition. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

The first examples of five-coordinate platinum(II) complexes of the type [Pt(PR3)L]2+ (L = tris(2-(diphenylphosphino)ethyl)phosphine R = Et, OMe, OEt) (104) containing only P-donor atoms have been prepared by the reaction of [PtClL]+ with an appropriate monodentate tertiary phosphine or phosphite ligand.284 Triaryl phosphines and phosphites do not react with the precursor complex, even at elevated temperatures, most probably due to the considerable steric interactions that would occur upon the approach of the P-donor ligand to the platinum(II) center. 

Chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with /V-phenyl-TV -OS )-(l-phenylethyl)thiourea associated with a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved.387,388 Chiral polyether-phosphite ligands derived from (5)-binaphthol were prepared and combined with [Rh(cod)2]BF4. These systems showed high activity, chemo- and regio-selectivity for the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions. Ee values of up to 25% were obtained and recycling was possible without loss of enantioselectivity.389... 

The photolysis of carboxylic acids and derivatives as lactones, esters and anhydrides can yield decarboxylated products 253>. This reaction has been utilized in the synthesis of a-lactones from cyclic diacyl peroxides 254) (2.34) and in the synthesis of [2,2]paracyclophane by bis-decarboxylation of a lactone precursor (2.35) 255). This latter product was also obtained by photoinduced desulfurization of the analogous cyclic sulfide in the presence of triethyl phosphite 256). 

First we will describe the hydrocyanation of ethene as a model substrate. The catalyst precursor is a nickel(O) tetrakis(phosphite) complex which is protonated to form a nickel(II) hydride. Actually, this is an oxidative addition of HCN to nickel zero. In Figure 11.1 the hydrocyanation mechanism in a simplified form is given the basic steps are the same as for butadiene, the actual substrate, but the complications due to isomer formation are lacking. 

Propylene carbonate is a good solvent of the rhodium precursor [Rh(acac) (00)2] and the phosphite ligand BIPHEPHOS and can thus be used as the catalyst phase in the investigation of the isomerizing hydroformylation of trans-4-octene to n-nonanal in a biphasic system [24]. As already mentioned, the reaction products can be extracted with the hydrocarbon dodecane. Instead of an additional extraction after the catalytic reaction, we carried out in-situ extraction experiments, where the products are separated from the catalytic propylene carbonate phase while the reaction is still in progress. Conversion of 96% and selectivity of 72% was achieved under comparably mild conditions (p(CO/H2) = 10 bar, T = 125 °C, 4 h, substrate/Rh = 200 1). 

In order to assess structural influences on the catalytic performance of mono-cyclic PEG-modifled phosphites, the monocyclic phosphites 14a,c, 17a-e, and 19b,c were used in the rhodium-catalyzed hydroformylation of 1-octene (Scheme 6). The catalysts were prepared in situ by adding a solution of [Rh(acac)(CO)2] as a catalyst precursor in toluene to the phosphite ligands... 

The furanoside phosphite-phosphinite ligand 13, related to the previously diphosphinite ligands 12, was also applied to the asymmetric hydrogenation of N-arylimines, increasing the enantioselectivities up to 76% (Fig. 13) [26]. Precursors based on cationic [Ir(cod)2]BF4 provided better yields and enantioselectivities than... 

Scheme 8.9 Formation of a phosphite-phosphine bidentate ligand by simply mixing the porphyrin-functionalized phosphite and the 3-pyridyldiphenylphosphine ligand in the presence of a metal precursor.

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