Carbon acids. 9. The effects of divalent sulfur and divalent oxygen on carbanion stabilities

Bordwell, F. G.; Puy, Michael Van Der; Vanier, Noel R.

doi:10.1021/jo00872a050

Cited by 78 publications

(73 citation statements)

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“…We have proposed that the former system reacts mainly under nonchelation control due to the weak donor ability of the sulfonyl group. [37] Furthermore, the parent compound of LiEt 3 BH, lithium borohydride, [25] does not work at -78°C either with or without CeCl 3 . To eliminate complexity due to the disproportion of the reducing species, those having complex hydride anions of a less dissociative nature should have been the choice.…”

Section: Diastereoselective Reduction Of α-Alkyl-β-keto Sulfonesmentioning

confidence: 99%

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

et al. 2005

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A high stereochemical induction has often been observed in the reduction of ketones with stereogenic centres close to the carbonyl function. The level of stereoselectivity is particularly high when “hard” donor groups such as nitrogen or oxygen atoms are present in the molecule. These results have been commonly explained in terms of a chelate‐controlled process. However, this interpretation has recently been challenged, and in many cases an open‐chain mechanism can better account for the observed results. Thus, Lewis‐acid‐mediated chelation and nonchelation control is one of the most fundamental and practical concepts in transferring a nucleophile moiety to carbonyl compounds. In order to clarify the role of a hard Lewis acid in controlling the stereochemical outcome of a given reaction, the Lewis acid mediated reduction of a series of α‐alkyl‐β‐functionalized carbonyl compounds with metallic hydrides in various solvents was recently investigated. In particular, the results derived from the reductions in the presence of dry CeCl3 were compared to those obtained under the same experimental conditions but in the presence of TiCl4, whose ability to form chelation complexes is well established. These investigations show a stereochemical outcome that is fully consistent with a chelation‐controlled pathway in the case of titanium, and an open‐chain‐controlled pathway in the case of CeCl3. The strongly chelating TiCl4 led to the syn isomer in high diastereomeric excess in noncoordinating solvents at –78 °C with borane–Lewis base complex reducing agents, while nonchelating CeCl3 produced a high excess of the anti isomer in coordinating solvents at the same temperature with metal borohydrides as reducing agent. Therefore, our method based on the choice of CeCl3 or TiCl4 as a Lewis acid under the reported conditions represents a significant contribution to the development of new stereoselective reductions of α‐alkyl‐β‐functionalized ketones. These procedures are one of the best known ways to obtain an alcoholic moiety. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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Section: Diastereoselective Reduction Of α-Alkyl-β-keto Sulfonesmentioning

confidence: 99%

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

et al. 2005

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“…4) with pK a of 11-23, such as N-methyltosylamide (3, pK a 11.7), 5,6) (cyanomethyl)phenylsulfone (4, pK a 12.0 in DMSO 7) ), 5,6) (methylthiomethyl)tolylsulfone (5, pK a 23.4 in DMSO 8) ), 5,6) benzyl phenyl sulfone (6a, pK a 23.4 in DMSO 9) ), 10) 3-[(phenylsulfonyl)methyl]pyridine (6b, pK a 16.7 in DMSO 9) ) 10) and geranyl phenyl sulfone (7, pK a 22.5 in DMSO 9) ).…”

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confidence: 99%

Preparation of (Cyanomethylene)trimethylphosphorane as a New Mitsunobu-Type Reagent.

Izumi

Kaku

Tsunoda

2003

CHEMICAL & PHARMACEUTICAL BULLETIN

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The Mitsunobu reaction is a very versatile method for the alkylation of various nucleophiles (HA) with alcohols (ROH) to give RA, utilizing the redox system of diethyl azodicarboxylate (DEAD)-triphenylphosphine (TPP) (Fig. 1). 1,2) Without any prerequisite activation of the alcohol, it is a unique alkylation reaction and widely applied to various phases of organic synthesis. However, the reaction has a serious limitation; the acidic hydrogen in HA has to have pK a lower than 11 for the reaction to proceed satisfactorily. If HA has a pK a higher than 11, the yield of RA is considerably lower, and with HA having a pK a higher than 13, the desired reaction does not occur. In order to overcome these drawbacks and expand the versatility of the original Mitsunobu reaction, stabilized trialkylphosphoranes such as (cyanomethylene)tributylphosphorane (CMBP) 3) and (cyano-methylene)trimethylphosphorane (CMMP) 4) have been developed to replace the DEAD-TPP system.These new reagents were designed based on the structural similarity between the ylide 2 and the betaine 1 generated as an important intermediate in the Mitsunobu reaction (Fig. 2). The ylide 2, which is another resonance form of trialkylphosphorane, was expected to behave similarly with 1 towards a mixture of ROH and HA to yield RA along with the by-products, trialkylphosphine oxide and acetonitrile (Fig. 3).In fact, the trialkylphosphoranes, especially CMMP, mediate the Mitsunobu-type reactions of various kind of nucleophiles (Fig. 4) with pK a of 11-23, such as N-methyltosylamide (3, pK a 11.7), 5,6) (cyanomethyl)phenylsulfone (4, pK a 12.0 in DMSO 7) ), 5,6) (methylthiomethyl)tolylsulfone (5, pK a 23.4 in DMSO 8) ), 5,6) benzyl phenyl sulfone (6a, pK a 23.4 in DMSO 9) ), 10) 3-[(phenylsulfonyl)methyl]pyridine (6b, pK a 16.7 in DMSO 9) ) 10) and geranyl phenyl sulfone (7, pK a 22.5 in DMSO 9) ).11)The useful attributes of the new Mitsunobu reaction are as follows. 1) These reactions with secondary alcohols proceed with the complete Walden inversion of the stereochemistry at the carbinyl carbon.2) The reaction of the DEAD-TPP redox system (the traditional Mitsunobu reaction) is not normally effective at higher temperature, whereas the reaction of CMBP and CMMP even at higher temperature can be carried out satisfactorily, because they are quite thermally stable although very sensitive to air and moisture. Furthermore, 3) in the traditional Mitsunobu reaction, one problem is the laborious purification of the product from dihydro-DEAD and triphenylphosphine oxide, whereas the use of the phosphoranes leads to an easy workup. Acetonitrile produced in place of dihydro-DEAD can be easily evaporated, and the removal of tributyl-or trimethylphosphine oxide can be attained by SiO 2 column chromatography because of its high polarity. As an alternative workup for the reaction with CMMP, aqueous treatment of the reaction mixture is also quite effective because of the good aqueous solubility of trimethylphosphine oxide.Thus, the very high reactivity observed for the bond for...

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“…For conjugated carbanions, as in enolate ions that require the carbanion carbon to be planar, the cyclopropyl anions have lower stabilities than simple alkyl anions [17]. The crystal structure of a phenylsulfonylcyclopropyllithium shows a highly pyramidal carbanionic carbon [18]; yet the equilibrium acidity studies of Bordwell's group in DMSO show that cyclopropyl sulfones are generally less acidic than the corresponding methyl or isopropyl sulfones [19,20]. This result contrasts with kinetic acidity studies in which cyclopropyl sulfones undergo base-catalyzed hydrogen isotope exchange faster than the isopropyl analogs [21,22].…”

Section: Resultsmentioning

confidence: 99%

Carbon acidity. 97. Cesium ion pair acidity of some sulfones in cyclohexylamine

Streitwieser

Schriver

1997

Heteroatom Chem.

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Carbon acids. 9. The effects of divalent sulfur and divalent oxygen on carbanion stabilities

Cited by 78 publications

References 1 publication

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

Preparation of (Cyanomethylene)trimethylphosphorane as a New Mitsunobu-Type Reagent.

Carbon acidity. 97. Cesium ion pair acidity of some sulfones in cyclohexylamine

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