Ti -alkene complex

Scheme 49. Alkenyl complexes derived from parent Ti-alkene complexes. Conditions (i) A or hv,...

It should be noted that while a number iridapyrroles exist (Section II-E) their protonation does not universally afford isolable alkylidenes, particularly those complexes comprising an alkyl ligand in place of the hydride, which typically afford chelating Ti-alkene complexes. 

Ti -alkene) ] complexes was characterised by C and P NMR spectroscopy and for [PtCPPha) 2 ti -C(C02CH3) 2 CH(CX52CH3) )1 by X-ray crystallography . The preparation and X-ray cjrystal structure of the vinylcyclobutene complex (8) have been described . Ab initio calculations have been performed on both... 

Applications in organic synthesis make particular use of [(ri -Cp)Fe(CO)2(ti -alkene)] complexes and those involving alkenes coordinated to Pd(II) centres. The latter is the basis for the Wacker process (see Fig. 25.2) and for the Heck reaction (eq. 24.87). The Heck reaction is a... 

Presumably, the ti -alkene complex is formed initially, and then it is attacked by Pt(CNt-Bu)2 from the opposite side to cause the C-C bond cleavage. 

The synthesis and crystal structures of ionic Ti -alkene complexes of copper(I) of the type [Cu2X2(CH2=CHCH2NH3)2(H20)]2+[S04l2- (X = Cl, Br) have been described >9. 

In the absence of alkene, alcohols lose hydrogen to the Ti—H202 complex to form water and ketone or aldehyde as shown in Fig. 6—IO47,49... 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

A tentative mechanism involving the heterolytic cleavage of the 0-0 bond along with electron transfer from the alkene to the electrophilic oxygen of the Ti(02H) complex is shown in Scheme 27. 

Another class of organylzirconocene derivatives capable of reacting with ketones are 1,1-dimetalloalkanes and -alkenes containing Zr and Al, as well as their Ti and Zn analogues [120,121] (Scheme 1.30). These serve as Zr- or Ti-carbene complex equivalents, but mechanistic details are not clear. 

The versatile Ti(II) chemistry available using preformed (alkene)Ti(OiPr)2 species was opened up by the discovery of the Kulinkovich cyclopropanation reaction [55]. Since 1995, Sato and collaborators have developed a wide range of elegant and synthetically useful reactions based on the Ti(OiPr)4/iPrMgCl reagent [56]. In particular, it was reported that the Ti(II) complex (q2-propene)Ti(OiPr)2, preformed from Ti(OiPr)4 and 2 equivalents of iPrMgCl, reacts with allylic compounds, such as halide, acetate, carbonate, phosphate, sulfonate, and aryl ether derivatives, to afford allyltitanium compounds as depicted in Scheme 13.27 [57]. 

Replacement of the Ti in these catalysts with Zr, Hf, or Ta does not improve the chiral efficiency (5S). Dibenzyl tartramide in place of tartrates is also effective as chiral auxiliary of the Ti complex and results in high ee (59). The catalyst systems formed from Ti(0-/-C3H7)4 and the tartramide in 2 2 and 2 1 mole ratio leads to opposite enantioselec-tion as shown in Scheme 24. The ene reaction of singlet oxygen and alkenes in the presence of a Ti-DET complex has been used to prepare chiral epoxy alcohols in up to 72% ee (Scheme 25) (60). 

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

Indenyl-fluorenyl systems, propylene polymerization, 4, 1068 Indenylidenes, in ROMP initiation, 11, 633 7]5-Indenyl ligand, in molybdenocene dihalides, 5, 573 Indenyl ligands, in cobalt(III) complexes, 7, 20 7]5-Indenyl ligands, in rhodium alkene complexes, 7, 197 2-(Indenyl)—phenoxo complexes, with mono-Cp Ti(IV),... 

Palladium-carbon Ti-bonded alkene complexes characteristics, 8, 328... 

In 2002, however, it was observed that water did substantially affect radical cyclizations mediated by Ti(III) complexes. Thus, for example, when epoxide 20 was treated with Cp2TiCl in dry THF, the exocyclic alkene 21 was obtained. When the same epoxide was treated with CpaTiCl in the presence of water, however, the corresponding reduction product 22 was formed. Moreover, with deuterium oxide instead of water, the deuterium labeled isotopomer 23 (Scheme 18) was isolated [72],... 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

Isomerization of safrole and eugenol is reported to occur when these alkenes are irradiated in the presence of iron carbonyls. Co-ordinatively unsaturated iron carbonyls are first formed and subsequently give iron carbonyl-alkene complexes whose alkene ligands are then isomerized. A further reported photoinduced isomerization involves Fe(fnb-Ti -cot)(CO)3 (cot = cyclo-octatetraene), which on irradiation in a CH4 matrix at 12 K is converted into Fe(cfiair-Ti -cot)(CO)3 via Fe(T -cot)(CO)3 as intermediate. The results of these studies are related to the fluxional behaviour of Fe(i7 -cot)(CO)3 in solution. ... 

In Section 3.11.1.4 it was pointed out that salts of certain transition metals, lanthanides and actinides promote the hydroalumination reaction. Since such metal salts are introduced into the reaction in their high oxidation states it can be assumed that the metal ions are rapidly reduced to a lower oxidation state and that this state is the active catalyst. For nickel(II) salts, Wilke has shown conclusively that the active agent is a nickel(0)-alkene complex. Analogously, for titanium(IV) salts, such as TiCU, Ti(OR>4 and Cp2TiCl2, it is most likely that a titanium(III) state is involved. The possible role of such metal centers in accelerating hydroalumination will be considered in the next section. 

Reductive coupling of alkynes or alkenes may be carried out with Cp2ZrCl2, Ti(0-i-Pr)4, or other Ti(IV) and Zr(IV) complexes. The reaction is often carried out in an intramolecular fashion to achieve better control of regiochemistry. The diyne or other polyunsaturated compound is added to a mixture of the Zr(IV) or Ti(IV) complex and a reducing agent such as BuLi. After formation of the intermediate metallacycle, the C-M bonds are cleaved by treatment with an electrophile such as H+ or I2 to give the organic product. 

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