Achiral Phosphine

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

Similar catalytic reactions allowed stereocontrol at either of the olefin carbons (Scheme 5-13, Eqs. 2 and 3). As in related catalysis with achiral diphosphine ligands (Scheme 5-7), these reactions proceeded more quickly for smaller phosphine substrates. These processes are not yet synthetically useful, since the enantiomeric excesses (ee s) were low (0-27%) and selectivity for the illustrated phosphine products ranged from 60 to 100%. However, this work demonstrated that asymmetric hydrophosphination can produce non-racemic chiral phosphines [13]. 

Another series of achiral iridium catalysts containing phosphine and heterocyclic carbenes have also been tested in the hydrogenation of unfunctionalized alkenes [38]. These showed similar activity to the Crabtree catalyst, with one analogue giving improved conversion in the hydrogenation of 11. 

Several examples of achiral biphasic hydrogenations are shown in Table 38.1. It can be seen, that the activity of the various phosphine complexes of precious metals rarely exceeds 100 h-1 under mild conditions. In many cases hydrogenation is accompanied by isomerization of the olefinic substrates. 

Hence, the concept that the process might be rendered enantioselective by simply incorporating chiral ligands in place of the achiral ones was relatively straightforward, and early results with chiral phosphines showed promise.131 However, the enantiomeric excesses obtained in this fashion were low, and it was clear that considerable improvements were required. 

To date, the most popular and successful designs for PN-ligands have incorporated an achiral phosphine with a substituted non-aromatic heterocyde as the primary chiral unit, largely due to the ease by which the P and N modules may be altered, and the ready availability of enantiopure, amino add-derived starting materials.1281 Considerable variation of the predse steric and electronic environment imposed on a chelated metal centre has been achieved by the synthesis of a substantial library of... 

The related chiral rhodium catalyst 4 has been used to effect kinetic resolution of these substrates.2 In this catalyst the achiral phosphine ligand of 1 is replaced by (R,R)-l,2-bis(o-anisylphenylphosphino)ethane (DIPAMP). Hydrogenation cat-... 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

Corey reported a catalytic enantioselective cyanosilylation of methyl ketones using combination of a chiral oxazaborolidinium and an achiral phosphine oxide, [Eq. (13.23)]. An intermolecular dual activation of a substrate by boron and TMSCN by the achiral phosphine oxide (MePh2PO) is proposed as a transition-state model (54). The same catalyst was also used for cyanosilylation of aldehydes ... 

The catalytic activity of phosphines 8 and 9 were first evaluated in the KR of ferf-butyl phenylcarbinol using the electron-deficient m-chlorobenzoic anhydride as the achiral acylating agent. Interestingly, not only did these two catalyst display... 

Another type of chiral alkene applied in 1,3-dipolar cycloadditions are vinyl groups attached to chiral phosphine oxides or sulfoxides. Brandi et al. (150,151) used chiral vinyl phosphine oxide derivatives as alkenes in 1,3-dipolar cycloadditions with chiral nitrones. This group also studied reactions of achiral nitrones with chiral vinyl phosphine oxide derivatives. Using this type of substrate, fair endo/exo-selectivities were obtained. In reactions involving optically pure vinyl phosphine oxides, diastereofacial selectivities of up to 42% de were obtained. Chiral vinyl... 

With the chiral diamine (S,S)-20 as a co-catalyst full conversion was obtained in all cases, indicating that the amine has a pronounced influence on reactivity and selectivity (entries 9-14). The combination (R Rc)-4ael/(S,S)-20 afforded 18a as an almost racemic mixture (entry 9). The value of 6% ee (R) obtained in this experiment reflects two opposite contributions. On one hand, the system chiral phosphorous ligand/achiral diamine (R Rc)-4ael/19 led to 18a with 65% ee (S) (entry 1). On the other hand, an ee value of 75% (R) in the hydrogenation of 1-acetonaphthone has been reported for the system achiral phosphine (PPh3)/(S,S)-20 [41] This indicates that two inductions are canceled in an almost additive way in the mixed system. 

Note that the cis isomer lacks an improper axis of rotation and is therefore chiral, but that the trans isomer has a plane of symmetry and will be achiral in the absence of an asymmetric carbon in the phosphine ligand-28 As in the case of the previously encountered cyclopentadienyl complex (page 476), it can be argued whether the coordination number is 5 or 9. In either semantic interpretation these compounds are of considerable interest since isomerism in nine-coordinate complexes Is even less well documented than in those with coordination number 5. 

Takemoto and coworkers extended their palladium-catalyzed asymmetric allylic alkylation strategy using allyl acetate and chiral phase-transfer catalyst to the quaternization of 13 [23b]. A correct choice of the achiral palladium ligand, (PhO P, was again crucial to achieve high enantioselectivity and hence, without chiral phosphine ligand on palladium, the desired allylation product 15 was obtained with 83% ee after hydrolysis of the imine moiety with aqueous citric acid and subsequent benzoylation (Scheme 2.12). 

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