Insertion stereospecific

Carbon atom insertion. Stereospecific formation of cw-epoxide (81) from nopinone (80) using dimethylsulphonium methylide, has been reported. The exclusive formation of (81) is considered to be a result of steric interactions favouring the intermediate conformation shown. The isolation of... 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Photolysis of the parent compound (44) yields singlet methylene, as evidenced by its stereospecific addition to ( )-butene. The cyclopropane is formed together with the characteristic insertion products of methylene (62MI50800, 64PAC(9)527)... 

In the past few years metal deuterides have become commercially available at reasonable prices. This has encouraged the use of these reagents for reactions involving deuteride displacements of suitable leaving groups. The attractive feature of these reactions is the stereospecificity of the deuterium insertion. 

Photolysis of phenyldiazomethane in cis- or trans-butene leads to nearly stereospecific cyclopropane formation, although some C—H insertion occurs<2W ... 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Treatment of the olefin 49 with Zeise s dimer leads to the chloroplatination complex 50 [26], The addition adduct 50 is hydrogenated stereospecifically to the trans-disubstituted chlorocyclohexane 51. The insertion of carbon monoxide into 50, in the presence of methanol, yields the ester 52 stereoselectively. 

Cationic Fp (olefin) complexes [Fp = f/5-C5H5Fe(CO)2] undergo regio-specific addition of heteroatomic nucleophiles.32 Subsequent ligand transfer (carbonyl insertion) occurs with retention of configuration at the migrating center (R—Fe—CO -> RCOFe).33 A combination of these processes has provided a novel stereospecific azetidinone synthesis which can also be applied to condensed systems.34... 

The stereochemistry of the insertion by (phenylthio)carbene to the a C-H bond of trans- and c/s-4-terr-butylcyclohexyloxides 16 was investigated19 to find that the reaction proceeds stereospecifically giving trans and cw-4-rert-butyl-l-methylcyclohexanol 19, respectively, after desulfurization of the primary insertion products 17 (Scheme 9). 

When the reaction of DMFL with alcohols, cyclohexane, or a-methylstyrene is initiated by triplet senitization, the outcome is virtually the same as it is for the direct irradiation. Thus ethers are formed in high yield with the alcohols, direct insertion accounts for the major product in cyclohexane, and the olefin cyclopropanation is stereospecific. 

The signature application for the G-H insertion in synthesis is probably the total synthesis of (—)-tetrodotoxin 126 by Du Bois and Hinman.233 Two stereospecific G-H activation steps, rhodium-catalyzed carbene G-H insertion and carbamate-based nitrene C-H insertion, have been used to install the two tetrasubstituted centers C6 and C8a (Scheme 12). Diazoketone 122 was treated with 1.5mol% Rh2(HNCOCPh3)4, and cyclic ketone 123 was selectively formed in high yield without purification. The reaction of carbamate 124 with 10mol% Rh2(HNCOCF3)4, PhI(OAc)4, and MgO in C6H6 solvent furnished the insertion product 125 in 77% yield. 

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