Ernst Peter Kundig

Naphthalene Complexes. V. Arene Exchange Reactions in Naphthalenechromium Complexes

Abstract Di-η 6 -naphthalenechromium(0) ( 1 ) reacts at 150°C with benzene to yield (η 6 -naphthalene)(η 6 -benzene)chromium(0) ( 3 ) in 76% yield. In the presence of THF, 1 undergoes Lewis base catalyzed arene exchange at 80°C. Reactions of 1 with substituted arenes yield the mixed sandwich complexes 4 and 6–10 (arene = 1,4-C 6 H 4 Me 2 , 1,3,5-C 6 H 3 Me 3 , C 6 Me 6 , 1,4-C 6 H 4 (OMe) 2 , 1,4-C 6 H 4 F 2 and 1,4-C 10 H 6 Me 2 ). In all but one case (with 1,4-dimethylnaphthalene) exchange of a single naphthalene ligand is observed. In marked contrast to the lability of 1 , dimesitylenechromium(0) ( 5 ) is inert to arene displacement in benzene up to 240°C. The molecular structure of 3 has been determined by X-ray crystallography. The crystal data are as follows: a 7.784(1), b 13.411(2), c 22.772(5) A, Z = 8, space group Pbca . The structure was refined to a R w value of 0.043. The naphthalene ligand in 3 is nearly planar and parallel to the approximately eclipsed benzene ring. Metal atom-ring distances are 1.631(9) and 1.611(4) A for naphthalene and benzene, respectively. Catalyzed and uncatalyzed naphthalene exchanges in the sandwich complex are compared to the analogous reactions with the Cr(CO) 3 complex 2 . Naphthalene exchange in 2 in benzene is 10 3 to 10 4 times faster than arene exchange in other arenetricarbonylchromium compounds. The mild conditions for Lewis base catalyzed naphthalene exchange make 2 a good precursor of other arenetricarbonylchromium compounds. Examples include the Cr(CO) 3 complexes of styrene, benzocyclobutene, 1-ethoxybenzocyclobutene, 1,8-dimethoxy-9,10-dihydroanthracene and 1,4-dimethylnaphthalene.

New Chiral N-Heterocyclic Carbene Ligands in Palladium-Catalyzed α-Arylations of Amides: Conformational Locking through Allylic Strain as a Device for Stereocontrol

New Enders/Herrmann-type chiral N-heterocyclic carbene (NHC) ligands have been developed and applied in asymmetric palladium-catalyzed intramolecular alpha-arylations of amides. The best ligands feature the bulky tert-butyl group and ortho-substituted aryl groups at the stereogenic centers. Aryl bromides readily react at room temperature and aryl chlorides at 50 degrees C. The highly enantiomerically enriched (up to 96% ee) 3-alkyl-3-aryloxindole products were obtained in generally high yields (>95%) except in cases of steric congestion. The critical roles both of the bulky alkyl group and of the ortho-aryl substituent at the stereogenic center of the ligand were revealed in the crystal structure of a [Pd(eta(3)-allyl)(NHC-L*)(I)] complex. The ligand aryl location and orientation is fixed by conformational locking that minimizes A(1,3)-strain and enables optimal transfer of chiral information.

Synthesis of (−)-Lasubine(I) via a Planar Chiral [(η6-arene)Cr(CO)3] Complex

Key steps of the synthesis of the Lythracaea alkaloid (--)-lasubine(I) are the formation of an enantiopure planar chiral arylaldehyde tricarbonylchromium complex and highly diastereoselective aza-Diels-Alder cycloaddition and intramolecular radical cyclization reactions to afford a quinolizidinone intermediate. Ketone reduction, desilylation, and decomplexation yield the enantiomerically pure product.

α-t-butyl- and α-i-propyl-ortho-hydroxybenzylamines: Racemic synthesis/resolution and asymmetric synthesis

Efficient routes to α-tert-butyl- and α-iso-propyl-ortho-hydroxybenzylamines 1a and 1b are described. Highly enantioenriched 1a and 1b were obtained by resolution of the methoxy derivatives 2 by recrystallization of the salts formed with mandelic acid followed by Lewis acid mediated demethylation. The chiral 1,3-amino alcohol 1a has also been obtained in an asymmetric synthesis with the key step a diastereoselective alkylation of the imine obtained by condensation of o-anisaldehyde with phenyl glycinol. The absolute stereochemistry of these 1,3-aminophenols was determined by CD spectroscopy of the salicylideneamines 12 and by an X-ray structure analysis of the salt formed between (R)-mandelic acid and (S)-α-tert-butyl-ortho-methoxybenzylamine ((S)-2a).

Chiral CO-emulating ligands: From arene chromium chemistry to enantioselective catalysis

A one pot nucleophile/electrophile additiodhydrogenation sequence was applied to ((benzene)Cr(CO)j) to give predominantly the 4,5-trans-disubstituted cy- clohexene. Several asymmemc modifications of the sequential addition of C-nu- cleophiles and C-elecuophiles to (arene)Cr(CO), complexes are discussed. A mechanistically intriguing route involves the use of chiral phosphorous ligands to control the diastereoselectivity in the migratory CO insertion step and/or the reductive elimination step in the sequence. Ephedrine and norephedrine derived ligands gave product ee of up to 69 9%. New C2-chiral bidentate ligands (L*) which emulate some of the bonding characteristics of CO were synthesized and briefly considered for this application. The main interest in these ligands concerns their potential in catalytic C-C bond forming reactions; a first application to the Lewis acid (CpFeL*)+ catalyzed Diels- Alder reaction between enals and dienes was successfully realized.

Chiral Ruthenium Lewis Acid Catalyzed Intramolecular Diels−Alder Reactions

Single point binding ruthenium Lewis acid catalysts [Ru(acetone)(S,S)-BIPHOP-F)Cp][SbF(6)] ((S,S)-1b) and [Ru(acetone)(S,S)-BIPHOP-F)(indenyl)][SbF(6)] ((S,S)-1c) efficiently catalyze intramolecular Diels-Alder (IMDA) reactions under mild conditions to afford the endo cycloaddition products as the major product in excellent yields with high diastereo- and enantioselectivities.

Regioselective Synthesis and Characterization of Multinuclear Convex-Bound Ruthenium-[n]Cycloparaphenylene (n=5 and 6) Complexes

Mono- and multinuclear complexes of ruthenium and [n]cycloparaphenylene (CPP, n = 5 and 6) were synthesized in excellent yields through ligand exchange of the cationic complex [(Cp)Ru(CH3CN)3](PF6) with CPP. In the multinuclear complexes, ruthenium selectively coordinated to alternate paraphenylene units to give bis- and tris-coordinated Ru complexes for [5] and [6]CPPs, respectively. Single-crystal X-ray analysis revealed the Ru was coordinated with η(6)-hapticity on the convex surface of CPP.

Palladium-NHC catalyzed enantioselective synthesis of fused indolines via inert C(sp(3))-H activation.

New sterically hindered chiral N-heterocyclic carbene (NHC) ligands were used in palladium catalysis to bring about a highly enantioselective C(sp(3))-H activation on the methylene site of a cycloalkane moiety. The intramolecular coupling reaction of a prochiral N-aryl-N-cycloalkyl methyl carbamate required high temperatures (140-160 °C) and afforded highly enantioenriched trans-2,3-fused indolines.

Diastereoselective and enantioselective reduction of tetralin-1,4-dione

Summary Background The chemistry of tetralin-1,4-dione, the stable tautomer of 1,4-dihydroxynaphthalene, has not been explored previously. It is readily accessible and offers interesting opportunities for synthesis. Results The title reactions were explored. L-Selectride reduced the diketone to give preferentially the cis-diol (d.r. 84 : 16). Red-Al gave preferentially the trans-diol (d.r. 13 : 87). NaBH4, LiAlH4, and BH3 gave lower diastereoselectivities (yields: 76–98%). Fractional crystallization allowed isolation of the cis-diol and the trans-diol (55% and 66% yield, respectively). Borane was used to cleanly give the mono-reduction product. Highly enantioselective CBS reductions afforded the trans-diol (72% yield, 99% ee) and the mono-reduction product (81%, 95% ee). Conclusion Diastereoselective and enantioselective reductions of the unexplored tetralin-1,4-dione provides a very convenient entry into a number of synthetically highly attractive 1,4-tetralindiols and 4-hydroxy-1-tetralone.

Stereoselective chromium- and molybdenum- mediated transformations of arenes*

Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [alpha] values ranging from −174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser–Claisen rearrangement and a regio- and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO)3 group have been realized. The reactions show strong analogies to the Cr(CO)3-mediated reactions, but exhibit also marked differences: the arene–Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs.