Conformational preferences of cyclohexyl Grignard reagents

Jensen, Frederick R.; Nakamaye, Kay L.

doi:10.1021/ja01014a055

Cited by 22 publications

(4 citation statements)

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“…Vinyl acetate (0.75 ml) was introduced through the No-Air stopper by use of a syringe, and the apparatus was pressurized to 100 psi. After 1.5 hr deuterium uptake stopped at = 18 psi. The reaction vessel was cooled in an ice bath and vented, and 4 ml (3.62 g, 2.55 mmol) of zra/zj-l-acetoxy-3,3-dimethylbut-l-ene was injected with a syringe.…”

Section: Methodsmentioning

confidence: 99%

“…Examination of diastereomeric organometallic compounds provides approaches to organometallic stereochemistry that are more practical than those based on enantiomeric substances. [16][17][18][19][20] We have developed an nmr technique, based on determination of the relative configurations at the CHD centers of the (CH3)3-CCHDCHD moiety, that is particularly convenient. 21 The usefulness of this alkyl group rests in the fact that threo and erythro diastereomers of most compounds having the composition (CH3)3CCHDCHDX are readily identified using spectroscopy.…”

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confidence: 99%

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Stereochemistry of reactions at carbon-transition metal .sigma. bonds. .pi.-Cyclopentadienyldicarbonyliron erythro-and threo-3,3-dimethylbutyl-1,2-d2

Bock¹,

Boschetto²,

Rasmussen³

et al. 1974

J. Am. Chem. Soc.

154

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Convenient, stereoselective syntheses of threo-and ¡?r>7/iro-3,3-dimethylbutan-l-oI-/,2-i/2 (1) are described. Reaction of the p-bromobenzenesulfonate esters of 1 with CsHsFeCCOjüMgBr, -Li, or -Na yields ircyclopentadienyldicarbonyliron erythro-and ?Areo-3,3-dimethylbutyl-/,2-d2 (erythro-and threo-2) with >95% inversion of configuration at carbon. Bromination of 2 in pentane, chloroform, carbon disulfide, and dimethylformamide, and iodination in carbon disulfide, all take place with >90% inversion of configuration at carbon. Reaction of 2 with triphenylphosphine, zm-butyl isocyanide, bromine (in methanol), oxygen, cerium(IV) ion, or chlorine (in chloroform) leads to derivatives of 4,4-dimethylpentanoic-2,J-flf2 acid with >90% retention of configuration at carbon. Reaction of 2 with sulfur dioxide in a variety of solvents yields the product of sulfur dioxide insertion, (CH3)3CCHDCHDS02Fe(C0)2Cp, with >95 % inversion of configuration at carbon. Insertion of dimethyl acetylenedicarboxylate into the carbon-iron bond of 2 takes place with >80% retention of configuration at carbon. Thermal decomposition of 2 produces a range of isotopicaUy substituted derivatives of 3,3-dimethyl-l-butene. The mechanistic significance of the results is discussed.The most valuable single type of information to have in characterizing the mechanism of a reaction that makes or breaks bonds at a tetrahedral carbon atom is the stereochemistry of the transformation at that carbon. With this information it is possible to judge whether a reaction involves intermediate free carbon radicals or ions or occurs by Sn2 or Se2 transition states. It is experimentally difficult to obtain stereochemical information about reactions involving transition metal alkyls. Traditional experimental procedures based on the preparation and reaction of enantiomeric organometallic compounds suffer from serious shortcomings. Enantiomerically pure organolithium and magnesium reagents are not generally available,6 and nucleophilic displacements by metallate anions and decarbonylation reactions are complicated by side reactions6 and by thermal instability of the products.12 Further, it is presently difficult to relate the stereochemistry of the starting materials and products in organometallic reactions, since reactions of "known" and reliable stereochemistry have not been established. [13][14][15]

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Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Stereochemistry of reactions at carbon-transition metal .sigma. bonds. .pi.-Cyclopentadienyldicarbonyliron erythro-and threo-3,3-dimethylbutyl-1,2-d2

Bock¹,

Boschetto²,

Rasmussen³

et al. 1974

J. Am. Chem. Soc.

154

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“…Surprisingly, we find that sterically large carboxylates X prefer NmnMgX over MenMgX. Additional sterical hindrance caused by RMgX(S) n dimerization or ligand-exchange equilibria might be responsible for this deviation from the usual steric interpretation of A values …”

Section: Discussionmentioning

confidence: 58%

“…This is not contradictory because the major chemical species in 1 differ from those in carboxylated reagent 1a I / b I by the presence of chloride vs menthyl carboxylate as counterion (Scheme b). However, the carboxylated reagent shows a surprising prevalence of the axially substituted Nmn diastereomer 1b I , which may be a consequence of secondary dimerization and solvation equilibria because MgX usually favors the equatorial position in cyclohexanes. , Independent confirmation that the change of conformational preference is caused by the carboxylate counterion was obtained by combining 1 with lithium salts of hindered carboxylic acids and determining the composition of the in situ generated (Men/Nmn)Mg(O 2 CR) species after thermal equilibration. With 2,4,6-trimethylbenzoate (mesitoate), the shift was from dr 57:43 ( 1a / b , rt) to 47:53 ( 1a II / 1b II , 60 °C; after 24 h).…”

Section: Resultsmentioning

confidence: 97%

Stereochemistry of the Menthyl Grignard Reagent: Generation, Composition, Dynamics, and Reactions with Electrophiles

et al. 2018

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Menthyl Grignard reagent 1 from either menthyl chloride (2) or neomenthyl chloride (3) consists of menthylmagnesium chloride (1a), neomenthylmagnesium chloride (1b), trans-p-menthane (4), 2-menthene (8), 3-menthene (9), and Wurtz coupling products including symmetrical bimenthyl 13. The diastereomeric ratio 1a/1b was determined in situ by 13 C NMR or after D 2 O quenching by 2 H NMR analysis. Hydrolysis of the C−Mg bond proceeds with retention of configuration at C-1. The kinetic ratio 1a/1b from Grignard reagent generation (dr 59:41 at 50 °C in THF) is close to the thermodynamic ratio (56:44 at 50 °C in THF). Carboxylation of 1 at −78 °C separates diastereomers 1a/b to give the anion of menthanecarboxylic acid (19) from 1a, which combines with unreactive 1b to give neomenthylmagnesium menthanecarboxylate (1b I ). The kinetics of epimerization for the menthyl/neomenthylmagnesium system was analyzed (ΔH ⧧ = 98.5 kJ/mol, ΔS ⧧ = −113 J/mol•K for 1b I → 1a I ). Reactions of 1 with phosphorus electrophiles proceed stereoconvergently at C-1 of 1a/b to give predominantly menthyl-configured substitution products: PCl 3 and 2 equiv of 1 give Men 2 PCl (6), which hydrolyzes to dimenthylphosphine P-oxide (7), whereas Ph 2 PCl with 1 equiv of 1 gave Pmenthyldiphenylphosphine oxide (27) after workup in air.

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Organoalkali Chemistry

Schlosser

2001

Organometallics in Synthesis: A Manual

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Conformational preferences of cyclohexyl Grignard reagents

Cited by 22 publications

References 1 publication

Stereochemistry of reactions at carbon-transition metal .sigma. bonds. .pi.-Cyclopentadienyldicarbonyliron erythro-and threo-3,3-dimethylbutyl-1,2-d2

Stereochemistry of reactions at carbon-transition metal .sigma. bonds. .pi.-Cyclopentadienyldicarbonyliron erythro-and threo-3,3-dimethylbutyl-1,2-d2

Stereochemistry of the Menthyl Grignard Reagent: Generation, Composition, Dynamics, and Reactions with Electrophiles

Organoalkali Chemistry

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