Alkynes uncatalyzed

Sulfonyl bromides and iodides react similarly217-218-225 copper-salt catalysis in these cases facilitates the additions but is not absolutely necessary however, it influences the stereochemistry of the additions. Addition of sulfonyl iodides226 as well as the uncatalyzed thermal addition of sulfonyl bromides227 to alkynes leads to an exclusive trans-addition, whereas CuBr2 catalysis in the latter case causes the formation of cis-addition products to some extent (11 16%) correspondingly, copper-salt catalysis in sulfonyl chloride additions to alkynes leads to the formation of a mixture of Z,E-isomers228-229 (equation 40). 

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]. 

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. 

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. 

Arylzinc reagents are completely inert towards alkenes and alkynes in the absence of any added catalyst, whereas the reported examples of uncatalyzed intermolecular carbozincations involving alkylzincs appear to be restricted to the more nucleophilic di(tert-butyl)zinc. 

Whereas the intermolecular uncatalyzed allylzincations of unactivated monosubstituted alkenes or disubstituted alkynes do not proceed readily, successful examples of intramolecular additions have been reported. 

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 ... 

A most useful haloboration agents, B-bromo-9-borabicyclo[3.3.1]nonane and BBr3, react readily with terminal alkynes via Markovnikov syn addition of the Hlg-B moiety to the carbon-carbon triple bond in an uncatalyzed process. The haloboration reaction occurs regio- and chemoselectively at terminal triple bonds, but for other types of unsaturated compounds it is nonselective 639... 

As a last example of an uncatalyzed C,C coupling of a neutral organocopper compound Figure 16.9 depicts the alkynylation of a copper acetylide with a bromoalkyne which is easily accessible via bromination of a terminal alkyne ... 

The same group reported additions of tetrahydrofuran or dioxolane 243 to alkynes 242b catalyzed by 10 mol% CuBr and using ferf-butyl hydroperoxide as the oxidant [337]. The reaction does not proceed in the absence of tert-butyl hydroperoxide however, CuBr was not essential as a catalyst, since the additions also proceeded in its absence, although reduced yields of products 244b were obtained. In the catalyzed process a two-electron cycle was proposed to operate since the reaction was not inhibited by BHT, while the uncatalyzed was as expected inhibited. It remains thus unclear what the role of CuBr is and how it is regenerated under the oxidative conditions (see above). 

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... 

Tandem intramolecular silylformylation-allylation reaction of diallylhydrosilyl ethers derived from homoallyl alcohols is convenient for rapid, stereoselective synthesis of 1,3,5-triols convertible to more oxygen-functionalized compounds (Scheme 12).142,142a,142b 143 [ he second uncatalyzed allylation step would be facilitated by the formation of a strained silacycle intermediate, which has enough Lewis acidity to activate the formyl group. A similar tandem reaction via alkyne silylformylation has been reported.144... 

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... 

Finally, in uncatalyzed hydroaluminations with organosubstituted alkynes, the initially formed syn adduct can isomerize to the more stable anti adduct by way of the dialuminoalkane (equation 24). ... 

The additions proceed regioselectively in favor of terminal boron adducts to produce (Z)-l-alkenylboron compounds through a syn addition of the X-B bond to 1-alkynes. The mechanism is fundamentally different from the uncatalyzed process and is postulated to proceed through the oxidative addition of the X-B bonds (X = H, RS, Y2B) to the transition-metal complex [M(0)] to form X-M-BYj species (28), followed by the migratory cis insertion... 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

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