Insertions propyne

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

The reactivity of OsHCl(CO)(P Pr3)2 toward alkynes depends on the type of alkyne used. Whereas phenylacetylene, propyne, and acetylene react by insertion to give the five-coordinate alkenyl derivatives Os ( >CI I=CHR Cl(CO)(PIPr3)2 (R = Ph, Me, H),31,33 the reaction with methylpropiolate affords a mixture of Os C(=CH2)C(OMe)0 Cl(CO)(P,Pr3)2 and 0s ( )-CH=CHC02Me Cl(C0) (P Pr3)234 (Scheme 3), and tert-butyl acetylene and diphenylacetylene are inert. 

Some of these complexes have also been used as suitable precursors of related allenylidenes obtained through substitution or insertion reactions (see below). Other synthetic approaches to achieve selectively C/C substituted allenylidenes from reactions of dilithium derivatives Li2[C=CCR20]> obtained by deprotonation of 2-propyn-l-ols, with [M(CO)5(THF)] (M = Cr, W) and subsequent deoxygenation with phosgene are not always straightforward [10]. 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

An analogous mechanistic scheme (equation 87) has been proposed for the flash vacuum pyrolysis of dimethylsilyl(trimethylsilyl)thioketene148 (256). The pyrolysis of bis(trimethylsilyl)thioketene (257) leads to a more complicated product mixture (equation 88). With 47% conversion, a mixture of trimethylsilylacetylene, 1-trimethylsilyl-1-propyne, bis(trimethylsilyl)acetylene, (trimethylsilyl)thioketene, 2,2,4,4-tetramethyl-2,4-disila-l-thietane and 2,2,4,4-tetramethyl-2,4-disila-l,3-dithietane was obtained. All products can be rationalized, however, by the assumption that carbene 258 undergoes not only a silylcarbene-to-silene rearrangement (as in the preceding two cases) but also isomerization to 2-thiirene and insertion into a methyl-C, H bond. 

Formation of the tricyclo[3.3.0.0.]decane 209 by the reaction of [3.2.0]bicyclo-heptadiene 205 with propyne complex (206) is an example [81], The Pauson Khand reaction is explained by the following simplified mechanism. At first the oxidative cyclization of 205 and 206 generates the cobaltacyclopentene 207, to which insertion of CO gives 208. Finally, reductive elimination of208 affords the cyclopentenone 209. 

Ans. Insertion of propyne into Pd-H in a Markovnikov manner followed by CO insertion into the Pd-C bond followed by methanolysis. 

Methyl methacrylate can be formed via two different possible catalytic cycles. One starts with a Pd-hydride species, undergoing propyne insertion in the 2,1 regio-mode, followed by carbon monoxide insertion in the Pd-alkenyl bond and subsequent termination by alcoholysis to give MMA and a regenerated Pd-hydride species (Scheme lA). Likewise, methyl crotonate is formed via insertion in the 1,2 regio-mode. 

Remarkably, the rate of propyne insertion via the MMA pathway in absolute terms is not significantly affected by the presence of medium-sized substituents, such as a methyl group (Figure 2) at the 6-position of the pyridyl group, whereas the insertion rate to methyl crotonate is very seriously suppressed (e. g., by a factor 20, with a methyl group substituent). 

An example of reversible insertion of alkynes into M-H bonds has been reported in the context of the reactivity summarized in Scheme 6.14 [64]. The three products framed are successively formed. The formation of the two zirconacy-clopentenes from the bispropenyl initial kinetic product requires /1-H elimination followed hy reductive elimination of propene, its coordination to give a propene-propyne complex, and finally oxidative coupling of the two unsaturated ligands. 

In contrast to propyne, allene hydroformylation presents a kinetic and thermodynamic preference for the anfi-Markovnikov insertion, as indicated by the lower activation free energy (20.1 vs. 33.5 kJ/mol) and the higher stability of the insertion product (—122.2 vs. -54.0 kJ/mol) for anfi-Markovnikov path with respect to Markovnikov path. In terms of these energy differences, a ratio of 98 2 for anti-Markovnikov to Markovnikov products is predicted kinetically. Furthermore, from... 

Fried and Goyal (342) studied the permeabihty of poly[l-(trimethylsilyl)-1-propyne] to several gases, and foimd good agreement with experiment. An insertion method was used to determine the solubility of He, O2, N2, CH4, and CO2 in the pol5uner, and diffusion coefficients were obtained from the Einstein relation. The DREIDING force field was used for the simulations. This polymer is... 

Pyrone synthesis by cycloaddition of CO2 to terminal alkynes (1-hexyne, 1-propyne) has also been investigated. This process can be catalytically promoted, albeit with low yield and selectivity, by Co [74] and Rh [75] complexes. Rh(dppe) (Ti -BPh4), in acetonitrile, at 390 K, catalyzed the formation of 4,6-dimethyl-2-pyrone from 1-propyne and CO2 (1 MPa) with a TON of 50 [75]. The Rh-catalyzed reaction has been proposed to proceed through a mechanism (Scheme 5.15) not involving an oxametallacycle intermediate species. The CO2 insertion into the Rh-C(sp )-o-bond of a Rh-alkenyl intermediate, obtained upon propyne dimerization, affords a linear unsaturated carboxylate which is converted into the pyrone. 

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