The (alkynylcarbyne)tungsten complexes [L 3 (CO) 2 W C C C R] (3a,b-6a,b) [L 3 = hydro[tris(3,5-dimethylpyrazol-1-yl)]borato (Tp%, 3), hydro[tris(pyrazol-1-yl)]borato (Tp, 4), cyclopentadienyl (Cp, 5), bis(3,5-dimethylpyrazol-1-yl)acetato (bdmpza, 6); R =SiMe 3 (a), Ph (b)] were prepared in a stepwise fashion from [W(CO) 6 ] and Li[C CR], (CF 3 CO) 2 O and M[L 3 ] (M=Na, K). The formation of 6a,b was highly selective, only complexes with a trans arrangement of the carboxylate group of bdmpza and the alkynylcarbyne ligand were detected. The reaction of [W(CO) 6 ] with Li[C CR], C 2 O 2 Cl 2 and tmeda afforded trans-[Cl(CO) 2 (tmeda)W C C C R] (7a,b). The electron-donating potential of the different tripodal ligands L 3 was studied by IR-and 13 C-NMR spectroscopy and compared to that of the ligand combination Cl/tmeda. The IR data suggest that in these complexes bdmpza is a weaker electron donor than Tp% and Tp but displays stronger electron-donating abilities than Cp. The structures of 6b and 7b were established by X-ray structural analyses.
The substitution of PR3 (R = C6H4CH3-p, C,H4C1-p, C6Hll, OCH,) for a CO ligand in chiral carbohydratocarbene complexes [(q5-C5H,)(CO),Mn=C(OR*)Ph] [OR* = a-( l a ) and P-mannofuranosyl ( l p ) , (-)-menthyloxy (9)] proceeds diastereoselectively. The diastereoselectivity depends on PR, and on the OR* substituent and ranges from 12% d e (R = OCH,) to 80% (R = CKH4CH3-p). In contrast, the reaction of i p with P(OPh), is non-selective. The diastereoselectivity generally increases with increasing nucleophilicity of PR, and decreases in the series 1p > l a > 9. The highest diastereoselectivity was observed in the reaction of 18 with P(C6H4CH3-p)3. Predominantly, the isomer with the (S) configuration at the metal [(SM,)-2p] was formed which could be separated from the diastereomeric mixture by chromatography and be obtained in a pure form. Subsequent reaction of (SM,)-2p with BF, afforded the carbyne-manganese complex (s,,)-[(rl"-C,H,)(P(C,H,CH,-p),)(CO)Mn..CPhlBF4. Chiral transition-metal complexes play a prominent role in enantioselective synthesis and catalysis. Although carbohydrates are available in a great variety from the chiral pool, the number of reports on their use as chiral auxiliaries in transition-metal-mediated transformations of organic molecules is rather restricted. There are only very few transition-metal carbene complexes known carrying carbohydrate substituents at the carbene carbon atom ['-'].Recently, we reported on the synthesis of the first carbohydratocarbene complexes of manganese and rheniumL41. These chiral carbene complexes, [Cp(C0)2M = C(Aryl)OR*] (M = Mn, Re; OR* = mannofuranosyl, glucofuranosyl, fructopyranosyl; Aryl = Ph, Tol), are obtained by addition of the anion of the corresponding protected carbohydrate to the cationic carbyne complexes [Cp( CO),M=C -Aryl]+. The carbohydrate is attached directly to the carbene carbon atom and thus in close proximity to the metal. Therefore, transfer of the chiral information of the carbohydrate substituent should render the substitution of a nucleophile for CO in the prochiral Cp(COj2Mn fragment diastereoselective. Separation of the diastereomers by column chromatography and subsequent OR* elimination by electrophiles such as boron trihalides should afford enantiomerically pure carbyne complexes which then could easily be transformed into other carbene complexes. In addition, oxidative displacement of the carbene ligand in the presence of a potential ligand L' should readily make a great variety of chiral-at-metal complexes of the type [Cp(CO)LMn -L'] accessible. Two different routes to diastereomerically enriched [Cp(CO)LMn=C(Aryl)OR*] compounds are conceivable: (a) addition of carbohydrate anions [OR*] to an excess of [Cp(CO)LMn=C-Aryl]+ cations and (b) replacement of a CO ligand in [Cp(CO)2Mn=C(Aryl)OR*] by L (Scheme 1). Scheme 1 Ph l+ Cp(CO),Mn = C I Cp(CO)(L)MnZC-Ph 'OR* -co Ph I Cp(CO)(L)Mn = C, OR* We recently observed that the kinetic resolution of [OR*]-addition to the carbyne carbon atom of [Cp(CO)LMn=C-Ph]+ [L = P(OMej3, PTo13] (route a)...
The cationic carbyne complexes [Cp(CO),M=CR]+ [BX4]-[M = Mn, X = F: R = Ph ( l ) , To1 (2); M = Re, X = 3 3 -CsH2(CF3)2: R = Ph (3)] add the anion of monodeprotonated protected mannofuranose (4a), glucofuranose (4b), and fructopyranose (4c) to the carbyne carbon atom to form the carbohydratocarbene complexes 5a-c, 6a, b, and ?a. With 5b, 512, and 6b the addition proceeds with retention of configuration at the anomeric center. Due to inversion of configuration in the deprotonation step the complexes 5a, 6a, and ?a are obtained as P-glycosides. The carbene ligand is oxidatively cleaved from the metal by trimethylamine N-oxide or air. Cleavage of the C(carbene)-0 bond with reformation of the Chiral transition-metal complexes play a prominent role in enantioselective synthesis and catalysis. In these complexes either the metal or one or more ligands carry the chiral information. In most transformations complexes having chiral ligands are employed.Transition metal-carbene complexes with a carbohydrate bound to the carbene carbon atom have been known until now only for gold and platinum [']. These compounds are obtained by addition of amines or alcohols to the isocyanide functionality of the corresponding carbohydratoisocyano ligand. Recently, alkenylcarbene complexes with a carbohydrate in the P-position to the carbene c a r b~n [~,~] were obtained by Aumann et al. by addition of sugar derivatives to alkynylcarbene complexes of chromium and tungsten.The cationic carbyne complexes [(C0)5Cr=C-NEt2]+ and [Cp(CO)2M=C-Aryl]+ (M = Mn, Re) add anionic and neutral nucleophiles such as alcohol ate^[^], a m i n e~[~,~] , and isocyanides['l to the carbyne carbon atom to form carbene Analogously, it should be possible to also add carbohydrates via a hydroxy, an amino, or an isocyano group. In the resulting carbene complexes the carbohydrate should be attached directly to the carbene carbon atom. When carbyne complexes of the type [Cp(CO)-(L)M=CR]+ are used the addition of carbohydrates gives diastereomers. Separation of the diastereomers should afford enantiomerically pure carbene complexes. These carbene complexes can be derivatized or easily be converted into other complexes, e.g. by displacement of the carbene ligand. Therefore, this route should render a great number of different types of chiral transition-metal complexes accessible. In this paper we report on the synthesis of the first cation of the carbyne complex 2 is achieved by reaction of 6a with BC13. Photolysis of [Cp(CO),Mn=C(Ph)OEt] in the presence of L affords the carbene complexes [Cp(CO)(L)Mn=C(Ph)OEt] [L = P(OMe)3 ( l l ) , P ( T o~)~ (12)]. Ethoxide abstraction from 11 and 12 by BF3 gives the chiral cationic carbyne complexes [ Cp (CO) (L)Mn=CPh] + [ BF4]-(13, 14) which add 4a to form the corresponding mannofuranosylcarbene complexes (15, 16). When 0.5 equivalents of 4a are employed in the reaction with 13,14 the ratio of diastereomers is 3 : 2, both for 15 and 16. Complex 11 was characterized by an X-ray structural analysis. carbohydratocarbene complexes of ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.