New Reactivity Modes of Chromium(0) Fischer Carbene Complexes:  Unprecedented Insertion of a Carbene Ligand into an Active B−H Bond

Abstract

The reaction of R,β-unsaturated chromium(0) complexes with hydrides leads to different products, depending on the electronic properties of the reducing agent and the carbene carbon. Electrophilic alkynylcarbene complexes react with NaCNBH 3 to produce compounds derived from the insertion of the carbene ligand into the B-H bond in a process that has no precedent in the literature. This reaction does not take place if the electrophilicity of the carbene carbon is diminished by the presence of a strong electron-donating group. However, the behavior of alkenylcarbene complexes toward NaCNBH 3 can be interpreted by initial addition of the hydride to the C-Cr bond followed by evolution of the intermediate thus formed through different reaction pathways. These include chromium 1,3-migration and alkoxide extrusion followed by dimerization or oxidation. This route also justifies the reactivity of alkenyl and alkynyl alkoxy Fischer carbene complexes toward other boron or aluminum hydrides. Finally, when a bulky reagent such as K-Selectride is employed, the 1,2-hydride addition is disfavored and only products derived from the reduction of the C-C multiple bonds are obtained.

Oxidative demetalation of Fischer alkoxy carbene complexes with stoichiometric pyridine N-oxide and NaBH4-promoted demetalation of Fischer iminocarbene complexes with sulfur and selenium

“…With metal hydrides or borohydrides Fischer carbene complexes can be reductively demetalated, depending on the controlled conditions [8]. Complex 23 cannot react with NaCNBH 3 in methanol [8a] and complicated products are formed in the reactions of 22 and 23 with NaBH 4 .…”

“…Complex 25b W-C(6) 2.207 (7) N(1)-C(6) 1.357 (9) N(1)-C(9) 1.307(8) C(6)-C (7) 1.418 (10 7) 118.4(6) C(6)-C(7)-C(8) 121.7 (7) Complex 28b W-Se 2.6568 (8) Se-C(6) 1.835(6) N(2)-C(9) 1.299 (8) (1) 119.0(6) C(6)-C(7)-C (8) 119.9 (6) Compound 29 Se(1)-C(6) 1.843(6) N(1)-C (9) 1.334 (9) N(1)-C(6) 1.387(9) N(2)-C (9) 1.352(9) N(2)-C(8) 1.388(9) C(6)-C(7) 1.383(10) C(7)-C (8) 1.363(9) N(1)-C(6)-Se (1) 118.0(5) C(6)-N(1)-C(9) 117.7(6) N(1)-C(9)-N(2) 123.8(7) C(9)-N(2)-C (8) 119.1(6) N(2)-C(8)-C(7) 118.2(6) C(8)-C(7)-C(6) 121.0(7) N(1)-C(6)-C (7) 119.4(6) moiety coordinating to the selenium atom, shortening the related bonds in 28b.…”

“…In these reactions, the carbene carbon is directly incorporated into the skeleton of the newly formed products and the metal moiety is removed during the reaction. Transition metal-catalyzed transmetalations [6], carbocyclization reactions [7], and reduction of Fischer carbene complexes with metal hydrides [8] belong to this category. In other reactions such as Diels-Alder reactions [9], 1,3-dipolar cycloadditions [10] and Micheal additions [11], the transition metal-carbene carbon bond still preserves in the formed complex products, and thus a further step is necessary to remove the metal moiety.…”

“…[8]. The reported oxidants usually effect to oxidatively demetalate Fischer alkoxy carbene complexes, but only limited success has been achieved to demetalate a few activated Fischer aminocarbene complexes using the oxidation methodology [18e].…”

Alkynylation of ferrocene by terminal alkynes. Part I. A simple one-step synthesis of ferrocenylacetylenes

“…However, the preparation of 1-ferrocenyl substituted 2-R-alkynes (R e substituent different from ferrocenyl) requires multi-step syntheses such as: the Cu/ Pd-catalyzed cross-coupling reactions of iodo derivatives of ferrocenes with 1-alkynes [4,5], modification of beforehand prepared ferrocenylacetylene [6] and syntheses based on the Mo-catalyzed metathesis of 1-ferrocenylpropyne [7,8] which is again prepared from ferrocenylacetylene. Particular concerns come up for multistep syntheses of ferrocenyl substituted alkynes which contain sensitive functional groups in the organic moiety (for example, Br [9], HO [10,11]). Therefore, the development of new synthetic approaches to 1-ferrocenyl 2-R-alkynes remains of indubitable interest.…”