Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

Abstract

Analysis of 13 C chemical shifts and Li-C couplings showed that allenyllithium (5A-Li) was a mixture of monomer and dimer in THF, both with an allenyl structure. Similarly, the metalation products of 2-butyne (6A-Li), 4-methylpentyne (9A-Li), and 4,4-dimethylpentyne (21A-Li) in THF, as well as several 1, 3-dialkyl (32A-Li, 33A-Li, 34A-Li) propargyl-allenyllithiums formed by metalation or Li/Sn exchange were all monomeric lithioallenes in THF. Compound 6A-Li was shown to have less than 5% and 21A-Li less than 3% of the propargyllithium isomers (6P-Li, 21P-Li) present from analysis of residual broadening of the propargyl carbon by Li-C coupling. The reagent prepared by metalation of dicyclopropylacetylene (8P-Li) has a propargyl structure, but two related reagents with a cyclobutane spanning the 3,3-positions (37A-Li and 39A-Li) had allenyl structures. Several triorganosilyl-substituted reagents were also investigated. Those with silyl groups at the allenyl position (28A-Li, 31A-Li) are allenyllithiums, those with silyl groups at the propargyl position (22-Li to 26P-Li) showed chemical shifts intermediate between those of allenyl and propargyl isomers, and the shifts were strongly temperature-dependent under some conditions. These compounds are probably equilibrating mixtures of allenyl and propargyllithiums or equilibrating mixtures of unsymmetrically π-complexed structures, with barriers to interconversion (∆G q -150 ) below 4 kcal/mol. Several of the organolithium reagents studied had diastereotopic carbon signals (SiMe 2 , CMe 2 , or CPh 2 groups), which allowed determination of barriers to configurational inversion of the chiral allenyl fragment. Barriers from 6.1 kcal/mol (26A-Li in dimethyl ether) to 14.5 kcal/mol (39A-Li in 3:2 THF/ether) were measured.

Experimental and Theoretical Study of Tunable 1,3‐Lithium Shift of Propargylic/Allenylic Species, Transmetallation, and Pd‐Catalyzed Cross‐Coupling Reactions

“…It is noted that minor changes in the relative position of the solvent molecules and the steric effect would lead to dramatic changes in the optimized structures of the intermediates, which are consistent with Reichs conclusion. [31] Compared with the gas phase models, the solvated models have some features (Figure 2): in addition to the bridged allenyl structure (b) (THF-B1-b and THF-B2-b), the linear allenyl (a) (THF-B1-a)-and propargyl (p) (THF-B2-p)-type isomers have also been located for B-1 and THF-B1-a is the most stable; the p-type isomer is located for C-1 (THF-C1-p) in addition to the b type isomer (THF-C2-b), which is the most stable.…”

First Total Synthesis of the Potent Anticancer Natural Product Dideoxypetrosynol A: Preparation of the “Skipped” (Z)‐Enediyne Moiety by Oxidative Coupling of Homopropargylphosphonium Ylide

“…Propargylic CHs are well known to be acidic and amide bases have been used to remove propargylic protons,16–18 though there is little data to be found concerning the p K a of propargylic CH groups 19. Propargyllithium is known to have an allene‐like structure,20 suggesting that propargylic CH groups are less acidic than allenylic CH groups, which is consistent with gas phase acidity orders of allene and propyne 21. Thus the CH 2 bonded to the positively charged phosphorus atom in 9a should be more acidic than the propargylic CH 2 .…”

On the Nature of Organoindium Intermediates: the Formation of Readily Isolable Difluoropropargylindium Reagents and their Regioselectivity Towards Electrophilic Substitutions

“…The interconver- www.chemeurj.org sion of propargyl metal species and allenyl metal species has been well documented in literature. [19][20][21] A potential energy diagram for reaction of 6 a with weak electrophiles is shown in the Supporting Information. In this case, the energy of activation for the reaction of indium complex with electrophiles may be greater than the equilibrium of the two indium complexes 6 a and 7 a.…”