Carbon–carbon bond formation via oxidative-addition processes of titanium(II) reagents with π-bonded organic substrates. Reactivity modifications by Lewis acids and Lewis bases

Mechanism of the Kulinkovich Cyclopropanol Synthesis: Transfer‐Epititanation of the Alkene in Generating the Key Titanacyclopropane Intermediate

“…[8,12] Sato's approach uses mainly the isopropyl Grignard reagent to generate the putative Kulinkovich intermediate 11, via 10, with subsequent transfer of Ti(OiPr) 2 (8) presumably by ligand transfer to an allylic halide, [7] olefin [13] or to acetylene, [14] to form the titanometallics 12, 13 or 14, respectively (Scheme 2). Although the final titanium-free products formed by the hydrolysis or oxidation of, or the insertion of unsaturated substrates into, the σ(CϪTi) bonds are consistent with the presence of 12, 13 or 14 as precursors, the pathway of their formation via intermediates 10 and 11, and subsequent li- (15) [11,15,16] or indirectly by transfer-epititanation with R 2 TiL 2 (16) [17] to form three-membered titanocycles 17 have increased our understanding of the formation and reactivity of such metallocyclic intermediates (Scheme 3). Accordingly, in the present work we sought to employ such insights for determining the nature of the reactive intermediates in the Kulinkovich reaction and their possible pathways of generation.…”

“…In modifications of the Kulinkovich procedure, where the Grignard reagent [7] (or an analogous organolithium reagent [11] ) does not serve as the source of the olefin or acetylene but only as a reductant for the Ti(OiPr) 4 , it has been assumed without proof that the dialkyltitanium() dialkoxide formed decomposes analogously to that mentioned above, namely 10 to 11 as does 7 to 9, but then the added allylic halide, olefin or acetylene undergoes ligand exchange with the intermediate titanacyclopropane 11 to form the derived titanium reagents 12, 13 or 14 (Scheme 2). Scheme 2 This viewpoint is exemplified by the work of both the Sato group [7] and the Cha group.…”

The McMurry Coupling and Related Reactions

“…199 Fully cross-conjugated pentaene 66 is obtained by the coupling of 4,4-dimethylcyclohexa-2,5-diene. β-Carotene (67) and lycopene (68) are synthesized by treatment of retinal and γ-retinal in 84 and 75% yields, respectively, with the TiCl 3 -LiAlH 4 system. 201 Other titanium reagents such as TiCl 4 -Zn, 202 titanium powder-TMSCl, 83 and TiCl 3 -Li 27 have been used for the β-carotene synthesis.…”

Vanadium(I) Chloride and Lithium Vanadium(I) Dihydride as Epimetallating Reagents for Unsaturated Organic Substrates: Constitution and Mode of Reaction

“…Strong support for the intermediacy of 7 in the Kulinkovich reaction was brought forward from our group by preparing 7 separately and then showing that the two apparent Ti-C bonds of 7b can be detected by stepwise insertions of benzonitrile and then of CO 2 (Scheme 4). [16] Products 10 and 11 could only arise via intermediates 7b and 9. Scheme 3. Such oxidative additions to π-bonds of C=C, CϵC, C=O and C=N linkages, leading to three-membered metallocyclic intermediates, as are involved in the reactions depicted in Equations (2)- (4), and the oxidative additions to sigma-covalent bonds leading to bond cleavage and C-C bond coupling, as illustrated in Equations (5)-(7), have been termed by our group as epimetallations (Greek: literally metallations upon a bond, by analogy with epoxidation of an olefin).…”

“…The two-step process depicted in Scheme 1, known as alkylative reduction, has subsequently been applied to preparing subvalent reductants having empirical formulas of Ti(OiPr) 2 from Ti(OiPr) 4 , [7,10] ZrCl 2 from ZrCl 4 , [7,11] HfCl 2 from HfCl 4 , [7,11] and CrCl (4) from CrCl 3 . [12] Adding significantly to potential reductants obtainable through alkylative reduction was the surprising finding that anionic hydride complexes could be generated by an excess of BuLi over the replaceable chlorides on the transition metal (Scheme 2).…”