BINOL phosphoramidite

Enantioselective catalysis of S alkylation has been achieved.91 A BINOL-phosphoramidite catalyst (o-methoxyphenyl analog) similar to that in Entry 3 in Scheme 8.7 gave good results. 

Although disubstituted alkynes are used successfully as two-carbon components in chromium-mediated and -catalyzed [6 + 2]-reactions, the use of terminal alkynes produces a [6 + 2 + 2]-reaction (Section 10.13.3.7). Buono and co-workers have discovered that when a cobalt catalyst is employed, several monosubstituted alkynes can be used in [6 + 2]-cycloadditions with cycloheptatriene (Scheme 35). The use of a chiral BINOL-phosphoramidite cobalt complex affords an enantioselective [6 + 2]-cycloaddition reaction (Equation (18)).121... 

Recsei C, McErlean CSP (2012) Synthesis of modified binol-phosphoramidites. Tetrahedron 68 464 80... 

Many other catalysts and ligands have been examined for the enantioselective reduction of a-acetamidoacrylates and related substrates. Phosphoramidites derived from BINOL and the cyclic amines piperidine and morpholine give excellent results.35... 

Platinum catalysts are also effective for the silaboration of 1,3-dienes.234 Although almost no stereoselectivitiy is observed in the silaboration of acyclic 1,3-dienes, 1,3-cyclohexadiene undergoes the stereoselective silaboration in fair yields (Equation (87)). Enantioselective silaboration of 1,3-cyclohexadiene has been achieved with 70% ee by using a platinum catalyst bearing a binol-based optically active phosphoramidite ligand.235... 

The use of monodentate phosphoramidites in enantioselective hydrogenation was first reported in 2000, together with reports on the use of phosphites and phospho-nites [15]. Phosphoramidites are prepared in a variety of ways, but the most common route is the treatment of a diol with PC13, followed by addition of an amine [60, 61]. MonoPhos (29a), the first reported phosphoramidite used as a ligand, is prepared from BINOL and HMPT in toluene [62]. Phosphoramidites, especially... 

The majority of the reported phosphoramidite ligands consist of BINOL and a diversity of readily available amines. Excellent enantioselectivities in the hydrogenation of a- and /fdehydroamirio acids, itaconates and enamides [63, 64] have been reported. In a recent full report, the group of Minnaard, De Vries and Fer-inga noted that especially the BINOL-derived ligands containing a piperidine or... 

Very recently, Reetz, Ma and Goddard reported phosphoramidites based on BINOL bearing a single ortho-substituent (Scheme 28.10) [69]. These ligands are also chiral on phosphorus, such that the synthesis results mostly in diastereo-mers which have to be separated. In several cases, however, one of the diaster-eomers was formed exclusively. Some of the ligands afford high ee-values in the hydrogenation of methyl N-acyl dehydroalanine and dimethyl itaconate. 

Phosphoramidite ligands based on TADDOL (36) and on D-mannitol (37) [74] have also been used (Scheme 28.11). However, the enantioselectivities reported for the hydrogenation of a-dehydroamino acids and itaconates were generally lower compared to the ligands based on BINOL. A different strategy is the use of ligands 38a-g based on the achiral diol catechol, and chiral amines [75]. 

Scheme 28.10 Monodentate phosphoramidites based on monosubstituted BINOL.

The hydrogenation of methyl-Z-3-acetamido-2-butenoate resulted in more surprises. Although ligand D7 based on a primary amine was known to give good results with these substrates [37], the library shows that in general all BINOL-based phosphoramidites that contain a primary amine with branching in the a-... 

Pfaltz introduced phosphite ligands 22, with BINOL and chiral oxazoline units, which gives excellent enantioselectivities [47]. In phosphoramidites 14 and 15 (Scheme 7.9) the structure of the amine moiety is crucial, but substituents at the 3,3 -positions of the BINOL unit had only minor influences on the enantiose-lectivity of the 1,4-addition to cyclohexenone. In contrast, the introduction of the two 3,3 -methyl substituents in ligand 22 increased the ee drastically from 54% to 90%. 

Bidentate phosphorus ligands based on BINOL, such as phosphonite 23, phosphites 24 and 25, and phosphoramidite 26 (Tab. 7.2), with various bridging units were introduced by the groups of Reetz, Chan, and Waldmann [48-50]. Excellent enantioselectivities - up to 96% for ligand 23, for instance - were found. 

Concurrent with studies on cyclometalation, studies on the effects of the structure of phosphoramidite ligand had been conducted. Several groups studied the effect of the stmcmre of ligand on the rate and selectivity of these iridium-catalyzed allylic substitutions. LI contains three separate chiral components - the two phenethyl moieties on the amine as well as the axially chiral BINOL backbone. These portions of the catalyst structure can control reaction rates by affecting the rate of cyclometalation, by inhibiting catalyst decomposition, or by forming a complex that reacts faster in the mmover-limiting step(s) of the catalytic cycle. 

BINOL-derived phosphoramidites are versatile ligands in palladium-catalysed umpol-ung allylation of aryl aldehydes mediated by diethylzinc.172 The possible roles of allyl-zinc and -palladium species in the mechanism are discussed in detail. 

Considerable success has been realized for asymmetric hydrogenations of carbon-carbon unsaturation with monodentate ligands, especially those derived from BINOL. The most popular class is the phosphoramidites (157) as used by DSM.200 20 Phosphites (158) have also been successfully used in a wide range of asymmetric hydrogenations,202 as have phosphonites (159).203 204 These ligands are discussed in depth in Chapter 14. 

Asymmetric Olefin Hydrogenation Using Monodentate BINOL- and Bisphenol-Based Ligands Phosphonites, Phosphites, and Phosphoramidites... 

The first method gives relatively pure phosphoramidites with most substrates. The second, reversed method is preferred with hindered amines.32 MonoPhos , the dimethyl analogue 24a, is made in a single step from BINOL and HMPT (hexamethylphosphoric triamide) in toluene.33 It... 

Recently, Reek et al. published the synthesis of a 9H,9 H- [4,4 ]bicarbazole-3,3r-diol (BICOL)-based chiral monodentate phosphoramidite ligand, which was functionalized with two different third-generation carbosilane dendritic wedges (Fig. 26) [57]. As reference reaction in the catalytic study, the rhodium-catalyzed asymmetric hydrogenation of Z-methyl-a-acetamido-cinnamate was chosen. Using a ligand-to-rhodium ratio of 2.2 led to enantio-selectivities which were comparable to the results obtained using the parent BINOL-derived monodentate phosphoramidite MonoPhos. 

Several other groups developed alternative BINOL-derived phosphoramidite ligands for highly enantioselective copper-catalyzed Michael additions. Whereas Waldmann et al. included a bicyclic bispidine in... 

Many other binaphthalene-derived phosphoramidites were used as chiral ligands in copper-catalyzed Michael additions as well. Whereas 2,2 -bis(diphenylphosphino)-l,T-binaphthalene (BINAP) gave unsatisfactory stereoselectivities in Cu(OTf)2-catalyzed 1,4-additions of diethylzinc to enones,226 BINOL oxazoline phosphites obtained by Pfaltz et al. turned out to be remarkably versatile and in some cases complementary to phosphorami-... 

Besides the very low stereosdectivities, a major problem encountered with this substrate is the low chemical yield (due to subsequent reaction between the resulting zinc enolate and the starting material) and the hi volatility of the product. Using TADDOL-phosphoramidite 27 in a tandem lj4-addition-aldol condensation to cydopentenone we were only able to obtain an ee of 37%, but the enantiosele-ctLvity was raised to 62% in the presence of wet powdered molecular sieves (4 A) [52]. This beneficial effect of water and molecular sieves in some catalytic 1,4-additions has been observed in other cases recently [52, 59]. Important to note is that the yidds in the tandem version are dramatically increased, presumably due to in situ trapping of the reactive enolate (vide infra). Pfaltz et al. reported a 72% ee in the addition of Et Zn to 44 when using BINOL-oxazoline phosphite ligand 22 [47]. 

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