Uncatalyzed reactions alkynes

The regiochemistry of Al-H addition to unsymmetrically substituted alkynes can be significantly altered by the presence of a catalyst. This was first shown by Eisch and Foxton in the nickel-catalyzed hydroalumination of several disubstituted acetylenes [26, 32]. For example, the product of the uncatalyzed reaction of 1-phenyl-propyne (75) with BujAlH was exclusively ds-[3-methylstyrene (76). Quenching the intermediate organoaluminum compounds with DjO revealed a regioselectivity of 82 18. In the nickel-catalyzed reaction, cis-P-methylstyrene was also the major product (66%), but it was accompanied by 22% of n-propylbenzene (78) and 6% of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (77). The selectivity of Al-H addition was again studied by deuterolytic workup a ratio of 76a 76b = 56 44 was found in this case. Hydroalumination of other unsymmetrical alkynes also showed a decrease in the regioselectivity in the presence of a nickel catalyst (Scheme 2-22). 

Unlike the late metal chemistry reviewed above, these reactions did not require Michael acceptor substrates, but the reactions were rather slow (turnover frequencies range from 2 to 13 h at 22°C). For phosphino-alkenes (Scheme 5-15, Eqs. 1-3), a competing uncatalyzed reaction gave six-membered phosphorinane rings (Scheme 5-15, Eq. 6) this could be minimized by avoiding light and increased temperature. For phosphino-alkynes (Scheme 5-15, Eqs. 4 and 5), the products were unstable and could not be isolated [14]. 

These results prompted us to ask how do these catalysts compare with enzymes What is the rate of uncatalyzed alkyne hydration Our search of the literature thus far has not revealed a study of the uncatalyzed reaction. 

The catalyzed hydroboration did not attract much attention until Sneddon in 1980 and Noth in 1985 reported that rhodium complexes significantly accelerate the addition of B-H bond to alkenes or alkynes. The protocol was proved to be an interesting strategy to realize the different chemo-, regio-, diastereo-, and enantioselectivities, relative to the uncatalyzed reaction. The reaction has been reviewed.132-135... 

Alkene- and alkyne-substituted Fischer carbenes participate as dienophiles in Diels-Alder reactions. The conditions are usually mild and the reaction proceeds smoothly at room temperature. Similar isomeric ratio and rate acceleration is observed to that of Lewis acid-promoted Diels-Alder reactions between methyl acrylates and dienes when compared to the uncatalyzed reactions. The reactions are endo-selective. Asymmetric Diels-Alder reactions are... 

The addition of H2O to alkynes is catalyzed by Hg(II) under acidic conditions. (The uncatalyzed reaction is extremely difficult to carry out due to the instability of alkenyl cations.)... 

Hydrostannylations. Hydrostannanes add to alkynes in uncatalyzed reactions at 60 °C. Phenylacetylene, for instance gives a mixture of ( )- and (Z)-vinylstannanes, wherein the tin atom has added to the terminal carbons. In the presence of Wilkinson s catalyst, however, the hydrostannylation proceeds at 0 °C to give mostly the regioisomeric vinylstannanes (eq 23). Terminal stannanes in the latter process seem to result from competing free radical additions. This may not be a complication with some other catalysts the complexes PdCl2(PPh3)2 and Mo( j -allyl) (CO)2(NCMe)2 also mediate hydrostannylations of alkynes, and they are reported to be 100% cis selective. Hydrostannanes and thiols react in a similar way to silanes and alcohols (eq 24). ... 

Sharpless has called for the need to develop a series of highly reproducible, high-yield reactions, click chemistry, that can be relied on to couple molecules in a variety of media. Catalysis plays its usual role in mediating such reactions. In one of the best examples, a I-alkyne can be coupled with an organic azide to give a 4-substituted 1,2,3-triazole. A Cu(II)/ascorbate or [Cu(I)(NCMe)4]Bp4 catalyst ensures that 1,4-selectivity is achieved (the 1,5-substituted product also forms in uncatalyzed reactions). The mechanism goes via the Cu(I) acetylide,... 

When using aliphatic alkynes the lactam products are also preferentially obtained in good yields. Through modification of the phosphoramidite ligand, high levels of enantio-selectivity, regioselectivity and product selectivities are obtained The uncatalyzed reaction of heterocyclic formamidines 201 with two equivalents of phenyl isocyanate affords the [2+2+2] cycloadduct 202 in 48 % yield... 

Sharpless [9] and Meldal [10] independently reported a Cu(I)-catalyzed version of the cycioaddition reaction between azides and terminal alkynes, which is 10 times faster than the uncatalyzed reaction. The interaction between Cu(I) and terminal alkynes makes the latter a better 1,3-dipolarophile, enhancing its reaction... 

We are applying the principles of enzyme mechanism to organometallic catalysis of the reactions of nonpolar and polar molecules for our early work using heterocyclic phosphines, please see ref. 1.(1) Here we report that whereas uncatalyzed alkyne hydration by water has a half-life measured in thousands of years, we have created improved catalysts which reduce the half-life to minutes, even at neutral pH. These data correspond to enzyme-like rate accelerations of >3.4 x 109, which is 12.8 times faster than our previously reported catalyst and 1170 times faster than the best catalyst known in the literature without a heterocyclic phosphine. In some cases, practical hydration can now be conducted at room temperature. Moreover, our improved catalysts favor anti-Markovnikov hydration over traditional Markovnikov hydration in ratios of over 1000 to 1, with aldehyde yields above 99% in many cases. In addition, we find that very active hydration catalysts can be created in situ by adding heterocyclic phosphines to otherwise inactive catalysts. The scope, limitations, and development of these reactions will be described in detail. 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

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