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1.
J Org Chem ; 89(8): 5555-5559, 2024 Apr 19.
Article in English | MEDLINE | ID: mdl-38592064

ABSTRACT

Commercially and readily available MSTFA [2,2,2-trifluoro-N-methyl-N-(trimethylsilyl)acetamide] was identified as a highly effective TMS (trimethylsilyl) source for the convenient preparation of cyclic acetals under modified Noyori's conditions. The reactions proceeded smoothly under mild conditions, affording a wide range of the corresponding cyclic acetals with excellent yields in the presence of catalytic TMSOTf (trimethylsilyl trifluoromethanesulfonate). The present method does not require a large excess of diols that can be valuable and does not require presynthesized silylated diols. In contrast to other silylating reagents such as BSA [N,O-bis(trimethylsilyl)acetamide] and BSTFA [N,O-bis(trimethylsilyl)trifluoroacetamide], the application of MSTFA avoided the inhibition of catalytic acetalization by the side product 2,2,2-trifluoro-N-methylacetamide.

2.
J Org Chem ; 86(6): 4877-4882, 2021 03 19.
Article in English | MEDLINE | ID: mdl-33686865

ABSTRACT

A mild and nonreversible tert-butylation of alcohols and phenols can be achieved in high yields using the noncoordinating acid-base catalyst [bis(trifluoromethane)sulfonimide and 2,6-lutidine] with a tert-butylation reagent, tert-butyl 2,2,2-trichloroacetimidate. This method allows the use of substrates containing acid sensitive groups such as ketal, Boc, and boronate esters.


Subject(s)
Alcohols , Phenols , Acids , Catalysis , Ethers
3.
Org Lett ; 20(5): 1333-1337, 2018 03 02.
Article in English | MEDLINE | ID: mdl-29461064

ABSTRACT

Enantioselective synthesis of α-aryl and α-heteroaryl piperidines is reported. The key step is an iridium-catalyzed asymmetric hydrogenation of substituted N-benzylpyridinium salts. High levels of enantioselectivity up to 99.3:0.7 er were obtained for a range of α-heteroaryl piperidines. DFT calculations support an outersphere dissociative mechanism for the pyridinium reduction. Notably, initial protonation of the final enamine intermediate determines the stereochemical outcome of the transformation rather than hydride reduction of the resultant iminium intermediate.


Subject(s)
Piperidines/chemical synthesis , Pyridinium Compounds/chemistry , Catalysis , Hydrogenation , Iridium , Models, Chemical , Molecular Structure , Oxidation-Reduction , Stereoisomerism
4.
Nat Chem ; 10(1): 99-108, 2018 01.
Article in English | MEDLINE | ID: mdl-29256506

ABSTRACT

A rapidly emerging set of catalytic reactions involves intermediates that contain a copper-substituted stereogenic carbon centre. Here, we demonstrate that an intimate understanding of this distinction provides ways for addressing limitations in reaction scope and explaining why unexpected variations in enantioselectivity often occur. By using catalytic enantioselective Cu-boryl addition to alkenes as the model process, we elucidate several key mechanistic principles. We show that higher electrophile concentration can lead to elevated enantioselectivity. This is because diastereoselective Cu-H elimination may be avoided and/or achiral Cu-boryl intermediates can be converted to allyl-B(pin) rather than add to an alkene. We illustrate that lower alkene amounts and/or higher chiral ligand concentration can minimize the deleterious influence of achiral Cu-alkyl species, resulting in improved enantiomeric ratios. Moreover, and surprisingly, we find that enantioselectivities are higher with the less reactive allylphenyl carbonates as chemoselective copper-hydride elimination is faster with an achiral Cu-alkyl species.


Subject(s)
Alkenes/chemistry , Boron Compounds/chemistry , Copper/chemistry , Organometallic Compounds/chemistry , Catalysis , Chemistry Techniques, Synthetic , Models, Chemical , Stereoisomerism
5.
Tetrahedron ; 73(33): 5011-5017, 2017 Aug 17.
Article in English | MEDLINE | ID: mdl-29973744

ABSTRACT

A practical, efficient and broadly applicable catalytic method for synthesis of easily differentiable vicinal diboronate compounds is presented. Reactions are promoted by a combination of PCy3 or PPh3, CuCl and LiOt-Bu and may be performed with readily accessible alkenyl boronate substrates. Through the use of an alkenyl-B(pin) (pin = pinacolato) or alkenyl- B(dan) (dan = naphthalene-1,8-diaminato) starting material and commercially available (pin)B- B(dan) or B2(pin)2 as the reagent, a range of vicinal diboronates, including those that contain a B-substituted quaternary carbon center, may be prepared in up to 91% yield and with >98% site selectivity. High enantioselectivities can be obtained (up to 96:4 er) through the use of commercially available chiral bis-phosphine ligands for reactions that afford mixed diboronate products.

6.
J Am Chem Soc ; 137(33): 10585-602, 2015 Aug 26.
Article in English | MEDLINE | ID: mdl-26263513

ABSTRACT

Broadly applicable enantioselective C-B and C-Si bond-forming processes catalyzed by an N-heterocyclic carbene (NHC) were recently introduced; these boryl and silyl conjugate addition reactions (BCA and SCA, respectively), which proceed without the need for a transition-metal complex, represent reaction pathways that are distinct from those facilitated by transition-metal-containing species (e.g., Cu, Ni, Pt, Pd, or Rh based). The Lewis-base-catalyzed (NHC) transformations are valuable to chemical synthesis, as they can generate high enantioselectivities and possess unique chemoselectivity profiles. Here, the results of investigations that elucidate the principal features of the NHC-catalyzed BCA and SCA processes are detailed. Spectroscopic evidence is provided illustrating why the presence of excess base and MeOH or H2O is required for efficient and enantioselective boryl and silyl addition reactions. It is demonstrated that the proton sources influence the efficiency and/or enantioselectivity of NHC-catalyzed enantioselective transformations in several ways. The positive, and at times adverse, impact of water (biphasic conditions) on catalytic enantioselective silyl addition reactions is analyzed. It is shown that a proton source can facilitate nonenantioselective background reactions and NHC decomposition, requiring the catalyst to surpass such complications. Stereochemical models are presented that account for the identity of the observed major enantiomers, providing a rationale for the differences in selectivity profiles of BCA and SCA processes.


Subject(s)
Boron/chemistry , Heterocyclic Compounds/chemistry , Ketones/chemistry , Methane/analogs & derivatives , Silanes/chemistry , Catalysis , Free Radicals/chemistry , Hydroxides/chemistry , Kinetics , Methane/chemistry , Methanol/chemistry , Models, Molecular , Molecular Conformation , Permeability , Stereoisomerism , Water/chemistry
7.
Angew Chem Int Ed Engl ; 53(13): 3387-91, 2014 Mar 24.
Article in English | MEDLINE | ID: mdl-24615958

ABSTRACT

The first examples of Lewis base catalyzed enantioselective boryl conjugate additions (BCAs) that generate products containing boron-substituted quaternary carbon stereogenic centers are disclosed. Reactions are performed in the presence of 1.0-5.0 mol% of a readily accessible chiral accessible N-heterocyclic carbene (NHC) and commercially available bis(pinacolato)diboron; cyclic or linear α,ß-unsaturated ketones can be used and rigorous exclusion of air or moisture is not necessary. The desired products are obtained in 63-95% yield and 91:9 to >99:1 enantiomeric ratio (e.r.). The special utility of the NHC-catalyzed approach is demonstrated in the context of an enantioselective synthesis of natural product antifungal (-)-crassinervic acid.


Subject(s)
Boron/chemistry , Carbon/chemistry , Catalysis , Molecular Structure , Protein Structure, Quaternary , Stereoisomerism
8.
J Am Chem Soc ; 134(19): 8277-85, 2012 May 16.
Article in English | MEDLINE | ID: mdl-22559866

ABSTRACT

The first broadly applicable metal-free enantioselective method for boron conjugate addition (BCA) to α,ß-unsaturated carbonyls is presented. The C-B bond forming reactions are promoted in the presence of 2.5-7.5 mol % of a readily accessible C(1)-symmetric chiral imidazolinium salt, which is converted, in situ, to the catalytically active diastereo- and enantiomerically pure N-heterocyclic carbene (NHC) by the common organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu). In addition to the commercially available bis(pinacolato)diboron [B(2)(pin)(2)], and in contrast to reactions with the less sterically demanding achiral NHCs, the presence of MeOH is required for high efficiency. Acyclic and cyclic α,ß-unsaturated ketones, as well as acyclic esters, Weinreb amides, and aldehydes, can serve as suitable substrates; the desired ß-boryl carbonyls are isolated in up to 94% yield and >98:2 enantiomer ratio (er). Transformations are often carried out at ambient temperature. In certain cases, such as when the relatively less reactive unsaturated amides are used, elevated temperatures are required (50-66 °C); nonetheless, reactions remain highly enantioselective. The utility of the NHC-catalyzed method is demonstrated through comparison with the alternative Cu-catalyzed protocols; in cases involving a polyfunctional substrate, unique profiles in chemoselectivity are exhibited by the metal-free approach (e.g., conjugate addition vs reaction with an alkyne, allene, or aldehyde).


Subject(s)
Aldehydes/chemistry , Amides/chemistry , Boron/chemistry , Heterocyclic Compounds/chemistry , Ketones/chemistry , Methane/analogs & derivatives , Catalysis , Copper/chemistry , Esters , Methane/chemistry , Stereoisomerism , Substrate Specificity
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