76
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Ma Y, Lobkovsky E, Collum DB. BF3-mediated additions of organolithiums to ketimines: X-ray crystal structures of BF3-ketimine complexes. J Org Chem 2005; 70:2335-7. [PMID: 15760225 DOI: 10.1021/jo0480895] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
[reaction: see text] Additions of lithium acetylides and n-BuLi to N-alkyl ketimines mediated by BF(3)-Et(2)O in THF afford hindered tert-alkylamines in moderate to good yields. Stereochemical results and crystal structures of three BF(3)-imine complexes suggest that allylic strain strongly influences conformation and may be an important determinant of reactivity and selectivity.
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77
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McNeil AJ, Collum DB. Reversible Enolization of β-Amino Carboxamides by Lithium Hexamethyldisilazide. J Am Chem Soc 2005; 127:5655-61. [PMID: 15826205 DOI: 10.1021/ja043470s] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The enolization of beta-amino carboxamides by lithium hexamethyldisilazide (LiHMDS) in THF/toluene and subsequent diastereoselective alkylation with CH(3)I are reported. In situ IR spectroscopic studies reveal that beta-amino carboxamides coordinate to LiHMDS at -78 degrees C before enolization. Comparison with structurally similar carboxamides suggests that the beta-amino group promotes the enolization. IR spectroscopic studies also show that the enolization is reversible. Efficient trapping of the enolate by CH(3)I affords full conversion to products. (6)Li and (15)N NMR spectroscopic studies reveal that lithium enolate-LiHMDS mixed dimers and trimers as well as a homoaggregated enolate are formed during the reaction. At ambient temperature, racemization of the beta-position through a putative reversible Michael addition was observed.
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78
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McNeil AJ, Toombes GES, Gruner SM, Lobkovsky E, Collum DB, Chandramouli SV, Vanasse BJ, Ayers TA. Diastereoselective Alkylation of β-Amino Esters: Structural and Rate Studies Reveal Alkylations of Hexameric Lithium Enolates. J Am Chem Soc 2004; 126:16559-68. [PMID: 15600361 DOI: 10.1021/ja045144i] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Alkylation of beta-amino ester enolates proceeds with high diastereoselectivity. Single crystal, powder, and solution X-ray diffraction studies of the enolate show that the racemic enolate forms prismatic hexamers. 6Li NMR spectroscopic studies on partially racemic enolates reveal complex mixtures of homo- and heterochiral hexamers. An implicit fit of the aggregate populations to the Boltzmann distribution provides the free energy differences and equilibrium constants for the ensemble. Rate studies show that enolate alkylation occurs directly from the hexamer with participation by THF. A mechanism based on the alkylation of a ladder-like aggregate is proposed.
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79
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Ramírez A, Candler J, Bashore CG, Wirtz MC, Coe JW, Collum DB. Formation of Benzynes from 2,6-Dihaloaryllithiums: Mechanistic Basis of the Regioselectivity. J Am Chem Soc 2004; 126:14700-1. [PMID: 15535677 DOI: 10.1021/ja044899m] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The key elimination step for the formation of 3-chloro- and 3-fluorobenzyne from 2-chloro-6-fluorophenyllithium displays a pronounced solvent-dependent regioselectivity. 6Li and 13C NMR spectroscopic studies on 2-chloro-6-fluorophenyllithium reveal a single monomeric aryllithium, suggested by DFT computational studies to be a trisolvate. Rate studies indicate that the elimination of LiCl and LiF proceeds via trisolvated and disolvated monomers, respectively.
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80
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McNeil AJ, Toombes GES, Chandramouli SV, Vanasse BJ, Ayers TA, O'Brien MK, Lobkovsky E, Gruner SM, Marohn JA, Collum DB. Characterization of beta-amino ester enolates as hexamers via 6Li NMR spectroscopy. J Am Chem Soc 2004; 126:5938-9. [PMID: 15137738 DOI: 10.1021/ja049245s] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Low-temperature 6Li NMR spectroscopic studies on a chiral beta-amino ester enolate reveal a complex mixture of homo- and heterochiral aggregates. Subsequent warming of the samples led to rapid intra-aggregate exchange, resulting in four distinct resonances consistent with an ensemble of hexamers. An implicit fit of the aggregate populations to the Boltzmann distribution provided the free energy differences and equilibrium constants. An X-ray crystal structure obtained from the racemic enolate is consistent with the predominant aggregate in solution.
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81
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Briggs TF, Winemiller MD, Collum DB, Parsons RL, Davulcu AH, Harris GD, Fortunak JM, Confalone PN. Structural and Rate Studies of the 1,2-Additions of Lithium Phenylacetylide to Lithiated Quinazolinones: Influence of Mixed Aggregates on the Reaction Mechanism. J Am Chem Soc 2004; 126:5427-35. [PMID: 15113214 DOI: 10.1021/ja0305813] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The 1,2-addition of lithium phenylacetylide (PhCCLi) to quinazolinones was investigated using a combination of structural and rate studies. (6)Li, (13)C, and (19)F NMR spectroscopies show that deprotonation of quinazolinones and phenylacetylene in THF/pentane solutions with lithium hexamethyldisilazide affords a mixture of lithium quinazolinide/PhCCLi mixed dimer and mixed tetramer along with PhCCLi dimer. Although the mixed tetramer dominates at high mixed aggregate concentrations and low temperatures used for the structural studies, the mixed dimer is the dominant form at the low total mixed aggregate concentrations, high THF concentrations, and ambient temperatures used to investigate the 1,2-addition. Monitoring the reaction rates using (19)F NMR spectroscopy revealed a first-order dependence on mixed dimer, a zeroth-order dependence on THF, and a half-order dependence on the PhCCLi concentration. The rate law is consistent with the addition of a disolvated PhCCLi monomer to the mixed dimer. Investigation of the 1,2-addition of PhCCLi to an O-protected quinazolinone implicates reaction via trisolvated PhCCLi monomers.
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82
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Zhao P, Condo A, Keresztes I, Collum DB. Reaction of Ketones with Lithium Hexamethyldisilazide: Competitive Enolizations and 1,2-Additions. J Am Chem Soc 2004; 126:3113-8. [PMID: 15012141 DOI: 10.1021/ja030582v] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Reaction of 2-methylcyclohexanone with lithium hexamethyldisilazide (LiHMDS, TMS(2)NLi) displays highly solvent-dependent chemoselectivity. LiHMDS in THF/toluene effect enolization. Rate studies using in situ IR spectroscopy are consistent with a THF concentration-dependent monomer-based pathway. LiHMDS in pyrrolidine/toluene affords exclusively 1,2-addition of the pyrrolidine fragment to form an alpha-amino alkoxide-LiHMDS mixed dimer shown to be a pair of conformers by using (6)Li, (15)N, and (13)C NMR spectroscopies. Rate studies are consistent with a monomer-based transition structure [(TMS(2)NLi)(ketone)(pyrrolidine)(3)](). The partitioning between enolization and 1,2-addition is kinetically controlled.
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83
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Wiedemann SH, Ramírez A, Collum DB. Lithium 2,2,6,6-Tetramethylpiperidide-Mediated α- and β-Lithiations of Epoxides: Solvent-Dependent Mechanisms. J Am Chem Soc 2003; 125:15893-901. [PMID: 14677981 DOI: 10.1021/ja0304087] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-mediated alpha- and beta-lithiations of epoxides are described. LiTMP displays a markedly higher reactivity than does lithium diisopropylamide, consistent with literature reports. Detailed rate studies of LiTMP/THF and LiTMP/Me(2)NEt mixtures reveal similar rates but significant mechanistic differences. LiTMP-mediated alpha-lithiation of cis-cyclooctene oxide with subsequent oxacarbenoid formation and transannular C-H insertion proceeds via monosolvated dimers in both THF and Me(2)NEt. LiTMP-mediated beta-lithiation of 2,3-dimethyl-2-butene oxide affords the corresponding allylic alcohol via a monosolvated monomer in THF and a monosolvated dimer in Me(2)NEt. We discuss how the solvent-dependent aggregation of LiTMP markedly influences the rate profile. The reaction transition structures are examined with density functional computations.
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84
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Liao S, Collum DB. Lithium Diisopropylamide-Mediated Lithiations of Imines: Insights into Highly Structure-Dependent Rates and Selectivities. J Am Chem Soc 2003; 125:15114-27. [PMID: 14653747 DOI: 10.1021/ja030409z] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Lithium diisopropylamide-mediated lithiations of N-alkyl ketimines derived from cyclohexanones reveal that simple substitutions on the N-alkyl side chain and the 2-position of the cyclohexyl moiety afford a 60,000-fold range of rates. Detailed rate studies implicate monosolvated monomers at the rate-limiting transition structures in all instances. Comparisons of experimentally derived regioselectivities and rates, taken in conjunction with density functional theory computational studies, reveal a number of factors that influence reactivities including: (a) axial versus equatorial disposition of the proton on the cyclohexane ring, (b) syn versus anti orientation of the lithiation relative to the N-alkyl group, (c) the presence or absence of a potentially chelating methoxy moiety on the N-alkyl group, (d) the presence of a 2-methyl substituent at the geminal or distal alpha-carbon, and (e) branching in the N-alkyl group. The isolated contributions are not large, yet they display a strong and predictable additivity leading to a kinetic resolution of imines derived from racemic 2-methylcyclohexanone.
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85
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Ramírez A, Lobkovsky E, Collum DB. Hemilabile Ligands in Organolithium Chemistry: Substituent Effects on Lithium Ion Chelation. J Am Chem Soc 2003; 125:15376-87. [PMID: 14664582 DOI: 10.1021/ja030322d] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The lithium diisopropylamide-mediated 1,2-elimination of 1-bromocyclooctene to provide cyclooctyne is investigated using approximately 50 potentially hemilabile polyethers and amino ethers. Rate laws for selected ligands reveal chelated monomer-based pathways. The dependence of the rates on ligand structure shows that anticipated rate accelerations based on the gem-dimethyl effect are nonexistent and that substituents generally retard the reaction. With the aid of semiempirical and DFT computational studies, the factors influencing chelation are discussed. It seems that severe buttressing within chelates of the substitutionally rich ligands precludes a net stabilization of the chelates relative to nonchelated (eta(1)-solvated) forms. One ligand-MeOCH(2)CH(2)NMe(2)-appears to promote elimination uniquely by a higher-coordinate monomer-based pathway.
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86
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Zhao P, Lucht BL, Kenkre SL, Collum DB. Lithium Hexamethyldisilazide-Mediated Ketone Enolization: The Influence of Hindered Dialkyl Ethers and Isostructural Dialkylamines on Reaction Rates and Mechanisms. J Org Chem 2003; 69:242-9. [PMID: 14725435 DOI: 10.1021/jo030221y] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Mechanistic studies of the enolization of 2-methylcyclohexanone mediated by lithium hexamethyldisilazide (LiHMDS; TMS(2)NLi) solvated by hindered dialkyl ethers (ROR') are described. Rate studies using in situ IR spectroscopy show that enolizations in the presence of i-Pr(2)O, 2,2,5,5-tetramethyltetrahydrofuran, and cineole proceed via dimer-based transition structures [(TMS(2)NLi)(2)(ROR')(ketone)]. Comparing the relative solvation energies and the corresponding solvent-dependent activation energies shows that the highly substituted ethers accelerate the enolizations by sterically destabilizing the reactants and stabilizing the transition structures. Comparisons of hindered dialkyl ethers with their isostructural dialkylamines reveal that the considerably higher rates elicited by the amines derive from an analogous relative destabilization of the reactants and relative stabilization of the transition structures.
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87
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Zhao P, Collum DB. Ketone Enolization by Lithium Hexamethyldisilazide: Structural and Rate Studies of the Accelerating Effects of Trialkylamines. J Am Chem Soc 2003; 125:14411-24. [PMID: 14624589 DOI: 10.1021/ja030168v] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Mechanistic studies of the enolization of 2-methylcyclohexanone mediated by lithium hexamethyldisilazide (LiHMDS; TMS2NLi) in toluene and toluene/amine mixtures are described. NMR spectroscopic studies of LiHMDS/ketone mixtures in toluene reveal the ketone-complexed cyclic dimer (TMS2NLi)2(ketone). Rate studies using in situ IR spectroscopy show the enolization proceeds via a dimer-based transition structure, [(TMS2NLi)2(ketone)]++. NMR spectroscopic studies of LiHMDS/ketone mixtures in the presence of relatively unhindered trialkylamines such as Me2NEt reveal the quantitative formation of cyclic dimers of general structure (TMS2NLi)2(R3N)(ketone). Rate studies trace a >200-fold rate acceleration to a dimer-based transition structure, [(TMS2NLi)2(R3N)(ketone)]++. Amines of intermediate steric demand, such as Et3N, are characterized by recalcitrant solvation, saturation kinetics, and exceptional (>3000-fold) accelerations traced to the aforementioned dimer-based pathway. Amines of high steric demand, such as i-Pr2NEt, do not observably solvate (TMS2NLi)2(ketone) but mediate enolization via [(TMS2NLi)2(R3N)(ketone)]++ with muted accelerations. The most highly hindered amines, such as i-Bu3N, do not influence the LiHMDS structure or the enolization rate. Overall, surprisingly complex dependencies of the enolization rates on the structures and concentrations of the amines derive from unexpectedly simple steric effects. The consequences of aggregation, mixed aggregation, and substrate-base precomplexation are discussed.
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88
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Zhao P, Collum DB. Lithium hexamethyldisilazide/triethylamine-mediated ketone enolization: remarkable rate accelerations stemming from a dimer-based mechanism. J Am Chem Soc 2003; 125:4008-9. [PMID: 12670196 DOI: 10.1021/ja021284l] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Mechanistic studies of the enolization of 2-methylcyclohexanone mediated by lithium hexamethyldisilazide (LiHMDS; TMS2NLi) in toluene and toluene/triethylamine (Et3N) mixtures are described. Structural studies of LiHMDS/ketone mixtures in toluene reveal cyclic dimer (TMS2NLi)2(ketone). Rate studies using in situ IR spectroscopy show the enolization proceeds via a dimer-based transition structure, [(TMS2NLi)2(ketone)]. NMR spectroscopic studies of LiHMDS/ketone mixtures in the presence of relatively unhindered trialkylamines such as Me2NEt reveal the quantitative formation of cyclic dimers of general structure (TMS2NLi)2(Et3N)(ketone). Rate studies trace a >3000-fold rate acceleration to a dimer-based transition structure, [(TMS2NLi)2(Et3N)(ketone)].
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89
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Lucht BL, Collum DB. Structure of Lithium Hexamethyldisilazide (LiHMDS): Spectroscopic Study of Ethereal Solvation in the Slow-Exchange Limit. J Am Chem Soc 2002. [DOI: 10.1021/ja00092a078] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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90
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Romesberg FE, Collum DB. Determination of structures of solvated lithium dialkylamides by semiempirical (MNDO) methods. Comparison of theory and experiment. J Am Chem Soc 2002. [DOI: 10.1021/ja00032a027] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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91
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Collum DB, Klang JA, Depue RT. Mechanistic studies of phosphorus-oxygen bond cleavages in group 7 dinuclear complexes of 2-pyridyl dimethylphosphinite. J Am Chem Soc 2002; 108:2333-40. [DOI: 10.1021/ja00269a032] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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92
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Abruna HD, Breikss AI, Collum DB. Improved synthesis of 4-vinyl-4'-methyl-2,2'-bipyridine. Inorg Chem 2002. [DOI: 10.1021/ic00201a003] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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93
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Hallock JS, Galiano-Roth AS, Collum DB. Organometallic chemistry of sulfinic acids. Highly stereo- and regioselective intramolecular hydroplatinations. X-ray crystal structure of (Ph3P)2Pt[trans-SO2CH(CH3)CH2CH(CH2CH3)]. Organometallics 2002. [DOI: 10.1021/om00102a010] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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94
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Collum DB, McDonald JH, Still WC. Synthesis of the polyether antibiotic monensin. 1. Strategy and degradations. J Am Chem Soc 2002. [DOI: 10.1021/ja00526a073] [Citation(s) in RCA: 72] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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95
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Romesberg FE, Gilchrist JH, Harrison AT, Fuller DJ, Collum DB. The structure of lithium tetramethylpiperidide and lithium diisopropylamide in the presence of hexamethylphosphoramide: structure-dependent distribution of cyclic and open dimers, ion triplets, and monomers. J Am Chem Soc 2002. [DOI: 10.1021/ja00015a032] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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96
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Romesberg FE, Collum DB. Mechanism of Lithium Dialkylamide-Mediated Ketone and Imine Deprotonations: An MNDO Study of Monomer and Open Dimer Pathways. J Am Chem Soc 2002. [DOI: 10.1021/ja00113a006] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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97
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Collum DB, McDonald JH, Still WC. Synthesis of the polyether antibiotic monensin. 2. Preparation of intermediates. J Am Chem Soc 2002. [DOI: 10.1021/ja00526a074] [Citation(s) in RCA: 118] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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98
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99
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Romesberg FE, Collum DB. Lithium Dialkylamide Mixed Aggregation: MNDO Computational Study of Salt and Solvent Dependencies. J Am Chem Soc 2002. [DOI: 10.1021/ja00099a038] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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100
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Lucht BL, Collum DB. Structure of Lithium 2,2,6,6-Tetramethylpiperidine (LiTMP) and Lithium 2,2,4,6,6-Pentamethylpiperidide (LiPMP) in Hydrocarbon Solution: Assignment of Cyclic Trimer and Tetramer Conformational Isomers. J Am Chem Soc 2002. [DOI: 10.1021/ja00096a083] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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