Alkanes Lewis acid formation

Homoallyl ethers or sulfides.1 gem-Methoxy(phenylthio)alkanes (2), prepared by reaction of 1 with alkyl halides, react with allyltributyltin compounds in the presence of a Lewis acid to form either homoallyl methyl ethers or homoallyl phenyl sulfides. Use of BF3 etherate results in selective cleavage of the phenylthio group to provide homoallyl ethers, whereas TiCl effects cleavage of the methoxy group with formation of homoallyl sulfides. 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

The oxidative dehydrogenation (ODH) of lower alkanes is an attractive process for the formation of alkenes. The ODH of propane to produce propene has been particularly studied, given its high demand for the production of polypropene, acrylonitrile and propene oxide. There is a combined influence of the redox and acid-base properties of the surface of the oxides used for propane ODH. Intermediate reducibility, weak Lewis acid centers and oxygen mobility represent the essential requirements for selective ODH, as they are consistent with the trends in ODH rates observed in VO, MoO and WO based catalysts. 

This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have reactivity comparable to organometallic alkane complexes. In addition, we have studied the co-ordination of COj to metal centres in supercritical CO2 (scCOj) and shown that v(C-O) bands provide a very sensitive probe for the oxidation state of the metal centre. We found evidence, albeit circumstantial, for the formation and reactivity of ri -O bound metal COj complexes in solution at or above room temperature and found these highly reactive COj complexes have similar reactivity to the analogous Xe complexes [11-12]. We have also used TRIR to examine the reactivity of CpMofCO), radicals in scCOj and found evidence for an interaction, possibly Lewis Acid/Base, between CpMo(CO), and scCO [13]. 

Carbon-Carbon Bond Formation. Various types of Bronsted and Lewis acids and superacids may be applied to perform the reaction of alkanes or arenes with different alkylating agents (alkenes, alkynes, alkyl halides, alcohols, ethers, esters). Acid-catalyzed alkylations, particularly alkane-alkene reactions, are of great practical importance in upgrading motor fuels. Acid-catalyzed alkylation produces alkylaromatics for manufacture of plastics (styrenes), detergents, and chemicals. 

The acylation of alkenes in the presence of Lewis acid may be an interesting and selective reaction. In the course of our works, we always observed the formation of two carbon-carbon bonds during the condensation. The functionalization of alkanes leads to an alkene generated in situ The very hight stereoselectivity observed with the disilyloctadiene 22 is particularly striking and will induce new studies with the aim of best understanding the factors involved in the acylation reaction. 

Carbocations are formed by several reactions. One example has been discussed already in the context of the SnI reaction (Scheme 2.2.8a). Other important options include the addition of protons to double bonds, for example, the addition of a Br0nsted acid to an alkene or ketone (Scheme 2.2.8b and c, respectively). The addition of a Lewis acid to a carbonyl group can also lead to a type of carbocation, an effect that is exploited in all kinds of technical Friedel-Crafts acylation reactions (Scheme 2.2.8d). Finally, in high-temperature refinery processes, the formation of carbocations from alkanes is of highest relevance. Here acidic catalysts are usually applied that abstract a hydride from the alkane to form hydrogen and a carbocation at the alkane substrate (Scheme 2.2.8e). 

In a series of seminal papers, summarized beautifully in an authoritative review [9], Piers demonstrated that the reduction by hydrosilanes of carbonyls (Fig. 2a), thiocar-bonyls, imines, and other functional groups could be catalyzed by catalytic (but not insignificant 5-10mol%) quantities of the hydrophobic Lewis acid B(C6F5)3. As part of his careful synthetic and mechanistic studies. Piers noted that over-reduction of the intermediate alkoxysilane could lead to complete reduction to the alkane and the formation of a siloxane by-product (Fig. 2b) [10]. 

The reactions typically proceed at 150°C with n-octane and di-pinacolboronate for 5-24 h, and the amount of catalyst typically is 1-5%. The yields of 1-octylboronate ester are good and the reaction is regioselective in the terminal alkyl position (second equation below, top of p. 418). The proposed mechanism involves oxidative addition of the B-B or B-H bond, followed by a-bond metathesis between the M-B and R-H bonds, which is driven towards formation of the B-R bond by the Lewis-acid property of boron. Note that 16e species (see Scheme p. 418) could also be involved in oxidative addition of the alkane to give an 18e intermediate M(Bpin)2(H)(R) or M(Bpin)(H)2(R) that would provide R-Bpin as well by reductive elimination. Calculations showed, however, that the o-bond metathesis path is preferred by about 10 kcal mol" over this alkane oxidative-addition path. 

The insertion of isocyanates into C-H bonds is also well known. Olefins, alkanes, aromatic and heteroaromatic compounds are known to react with isocyanates to give N-substituted carboxylic acid amides. Often the formation of the linear adduct is the result of a [2+2] cycloaddition reaction and subsequent rearrangement. Electron donating groups on the aromatic nucleus on the one side and electron withdrawing groups on the isocyanate enhance the reactivity of both components. Lewis acids, such as aluminum chloride, are supplied successfully as catalysts... 

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

Brpnsted acids have shown good-to-excellent levels of selectivity in C S bond formations. This chapter aims to review the stereoselectivity aspect of C—S bond formations that use commodity chemicals such as alkynes, alkenes, and alkanes. The stereoselective functionalization of alkynes is reviewed in section 1 using hydrothiolation, bisthiolation, and carbothiolation that necessitate transition metal and Brpnsted acid catalysis. The next section covers sulfethera-tion and conjugate addition reactions, where electron-rich and electron-poor alkenes are exploited under Lewis acid and Brpnsted acid catalysis. The last section is centered on the use of alkanes in the stereoselective 5-alkylation catalyzed by transition metal and organocatalysis. 

---