Ferrocenyl alcohol

Since electron-donating substituents at the phosphorus atom favor addition reactions over olefination reactions, addition of 9 to aldehydes leads to the exclusive formation of the silyl-pro-tected allylic alcohols 10. No reaction products arising from Wittig alkenylation could be detected. The ylides (R,S)-9 and (S.S)-9 and their enantiomers were prepared from the corresponding optically pure l-[2-(diphenylphosphino)ferrocenyl]-A,A -dimethylethanamine diastereomers 7 via the phosphonium salts 8. 

The partitioning of ferrocenyl-stabilized carbocations [30] between nucleophile addition and deprotonation (Scheme 18) has been studied by Bunton and coworkers. In some cases the rate constants for deprotonation and nucleophile addition are comparable, but in others they favor formation of the nucleophile adduct. However, the alkene product of deprotonation of [30] is always the thermodynamically favored product.120. In other words, the addition of water to [30] gives an alcohol that is thermodynamically less stable than the alkene that forms by deprotonation of [30], but the reaction passes over an activation barrier whose height is equal to, or smaller than, the barrier for deprotonation of [30], These data require that the intrinsic barrier for thermoneutral addition of water to [30] (As) be smaller than the intrinsic barrier for deprotonation of [30] (Ap). It is not known whether the magnitude of (Ap — As) for the reactions of [30] is similar to the values of (Ap - As) = 4-6 kcal mol 1 reported here for the partitioning of a-methyl benzyl carbocations. 

The dithiophosphonic acid monoesters, RP(OR )(S)SH can be conveniently prepared by cleavage of dimeric, cyclic diphosphetane disulfides, [RP(S)S]2 with alcohols, silanols, or trialkylsilylalcohols180 and then can be converted into metal complexes M[SPR(OR )]2 without isolation.181 The substituted ferrocenyl anion, (N3C6H4CH20)(CpFeC5H4)PS2 has been prepared in two steps from P4Sio, ferrocene and hydroxymethylbenzotriazole (and its salt was used for the preparation of some nickel and rhodium complexes).182 Zwitter-ionic ferrocenylditiophosphonates,... 

Menthone and camphor undergo asymmetric hydrosilylation to give alkoxysilanes with up to 82% optical purity using neutral rhodium(I) catalysts containing DIOP or neomenthyl- or menthyl-diphenylphos-phine even triphenylphosphine gave about 65% ee (300). Hydrolysis to alcohols was not reported. The ferrocenyl ligands (28, 29) are similarly effective for asymmetric hydrosilylation (255), and could be used for production of the optically active alcohols. 

Ferrocenyl-substituted allenyl cations 28 were generated when 1,3-diferrocenyl-substituted secondary and ferrocenyl-substituted tertiary alcohols 29 were treated with trifluoroacetic acid27. These were rapidly converted into trifluoroacetoxyallylic ions by solvent addition the ions gave ferrocenyl-substituted enones by reaction with water (equation 8). 

Table 2 Fu s planar chiral ferrocenyl 4-DMAP catalyzed KR of sec-alcohols [80, 81] 16 (1-2 mol %)...

Fu s planar chiral ferrocenyl 4-DMAP derivative 16 is also the first organocatalyst that has been reported to efficiently perform the KR of certain propargylic ec-alcohols [83]. These KRs were achieved using 1 mol% of catalyst 16 and Acp as the acylating agent in tert-smyl alcohol at 0 °C in the absence of a stoichiometric auxiliary base... 

In addition to the planar chiral ferrocenyl catalysts 15-18, 24 developed by Fu, a number of other chiral derivatives of 4-DMAP and 4-PPY [4, 47, 48] have been explored by other groups as organocatalysts for KR of ec-alcohols. Contributions have been made by the groups of Vedejs [104, 105, 110, 111], Fuji and Kawabata... 

Addition of water (36) or alcohols (37—39) direcdy to butadiene at 40—100°C produces the corresponding unsaturated alcohols or ethers. Acidic ion exchangers have been used to catalyze these reactions. The yields for these latter reactions are generally very low because of unfavorable thermodynamics. At 50°C addition of acetic acid to butadiene produces the expected butenyl acetate with 60—100% selectivity at butadiene conversions of 50%. The catalysts are ion-exchange resins modified with quaternary ammonium, quaternary phosphonium, and ammonium substituted ferrocenyl ions (40). Addition of amines yields unsaturated alkyl amines. The reaction can be catalyzed by homogeneous catalysts such as Rh[P(C(5H5)3]3Q (41) or heterogeneous catalysts such as MgO and other solid bases (42). 

Pyridyl alkanol [41], diol [42], and ferrocenyl alcohol [43] were the first asymmetric autocatalysts found by Soai and co-workers in the enantioselective alkylation of pyridine-3-carbaldehyde, dialdehyde, and ferrocenecarbaldehyde, respectively, with dialkylzincs. 

The direct strong steric interaction between substrate substituents and ligand substituents has been demonstrated in asymmetric addition of diethylzinc to aldehydes catalysed by sterically congested ferrocenyl aziridino alcohol derivatives.114 In addi- tion, this non-bonded steric repulsion influenced enantioselectivities significantly, and even led to inversion of the absolute configuration. This fact was further confirmed by theoretical calculations and the design of a new chiral ferrocenyl aziridino alcohol ligand. 

Ferrocenyl amido complexes, with Zr(IV), 4, 773 Ferrocenyl amino alcohols, synthesis, 6, 202 Ferrocenylated ODN, characteristics, 1, 909—910 Ferrocenyl diols, chiral, synthesis, 6, 204 Ferrocenyldiphenylphosphines, preparation, 6, 202 Ferrocenylimines mercuration, 2, 433 into platinacycles, 8, 511... 

In other cases, the oxidation reaction may not be asymmetric, but stereogenic centers within the substrate are preserved in the product allowing for an asymmetric reaction. An example of this type of reaction is provided by ozonolysis, which is discussed in Chapter 11. The use of ozone also overcomes one of the major problems that has been associated with oxidations at scale—the use of toxic, heavy metals their separation from the reaction product and waste disposal. However, there are still some useful reactions that use metals without chiral ligands and provide stereodifferentiation. An example is provided by the manganese oxide oxidation of ferrocenyl amino alcohols (Scheme 9.2).14... 

Scheme 8.10 Fu s chiral planar ferrocenyl DMAP-catalyzed KR of sec-allylic alcohols [54].

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