Mechanism alkyne addition reactions

Mechanisms of some alkyne addition reactions are complex. 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

The mechanism of alkyne additions is similar but not identical to that of alkene additions. When an electrophile such as HBr adds to an alkene (Sections 6.7 and 6.8), the reaction takes place in two steps and involves an alkyl carbocation intermediate. If HBr were to add by the same mechanism to an alkyne, an analogous vinylic carbocation would be formed as the intermediate. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

The complexes [Cu(NHC)(MeCN)][BF ], NHC = IPr, SIPr, IMes, catalyse the diboration of styrene with (Bcat) in high conversions (5 mol%, THF, rt or reflux). The (BcaO /styrene ratio has also an important effect on chemoselectivity (mono-versus di-substituted borylated species). Use of equimolecular ratios or excess of BCcat) results in the diborylated product, while higher alkene B(cat)j ratios lead selectively to mono-borylated species. Alkynes (phenylacetylene, diphenylacety-lene) are converted selectively (90-95%) to the c/x-di-borylated products under the same conditions. The mechanism of the reaction possibly involves a-bond metathetical reactions, but no oxidative addition at the copper. This mechanistic model was supported by DFT calculations [68]. 

Imino-1,2,4-thiadiazoles such as 27 react with electron-deficient alkynes to afford arylimino thiazoles such as 28. There has been some speculation as to the mechanism of this reaction, which may involve a 1,3-dipolar cycloaddition or a stepwise nucleophilic addition (Equation 6) <1996CHEC-II(4)307>. 

Alkyne hydrosilylation continues as a focus of current research. Despite the relative simplicity of the transformation, it is becoming increasingly clear that different catalysts often utilize unique mechanisms. In addition, the demands placed by the need to access vinylsilanes of differing substitution patterns, stereochemistries, and functional groups require a diverse, complementary set of methodologies. This discussion covers hydrosilylation reactions... 

Hydrostannation can be catalyzed by Lewis acids. If a mixture of an alkyne, tributyltin hydride, and Z1CI4 or HfCl4 is stirred in hexane, the product of ////-addition is formed (Equation (19)) 98 the hydride can be generated in situ from Bu3SnCl and EtjSiH, and B(C6F5)3 is an alternative Lewis acid.99 100 The mechanism of these reactions is still a matter of speculation. 

A proposed mechanism of this reaction was reported by Magnus and Principle [10], which is nowadays widely accepted (Scheme 1). Recently, negative-ion electrospray collision experiments have confirmed this mechanism in detail [11]. Starting with the formation of the alkyne-Co2(CO)6 complex 2, olefin 3 coordination and subsequent insertion takes place at the less hindered end of the alkyne. The in situ formed metallacycle 4 reacts rapidly under insertion of a CO ligand 5 and reductive elimination of 6 proceeds to liberate the desired cyclopentenone 7. It is important to note that all the bond-forming steps occur on only one cobalt atom. The other cobalt atom of the complex is presumed to act as an anchor which has additional electronic influences on the bond-forming metal atom via the existing metal-metal bond [12]. 

In fact, a mechanism for this reaction can be drawn that does not involve Pd at all, but let s assume that Pd is required for it to proceed. Cl- must be nucleophilic. It can add to Cl of the alkyne if the alkyne is activated by coordination to Pd(II). (Compare Hg-catalyzed addition of water to alkynes.) Addition of Cl- to an alkyne-Pd(II) complex gives a o-bound Pd(II) complex. Coordination and insertion of acrolein into the C2-Pd bond gives a new a-bound Pd(II) complex. In the Heck reaction, this complex would undergo P-hydride elimination, but in this case the Pd enolate simply is protonated to give the enol of the saturated aldehyde. 

In 1994, Thomas reported146,147 that alkenes also underwent an addition reaction with vinylketene complexes that differed crucially in the loss of the ketene carbonyl fragment. Complexes 252.a-252.d were isolated as yellow crystalline solids. Clearly this suggests that the process occurs by a mechanism different from the alkyne insertion, and this will be discussed... 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

One of the most general and useful reactions of alkenes and alkynes for synthetic purposes is the addition of electrophilic reagents. This chapter is restricted to reactions which proceed through polar intermediates or transition states. Several other classes of addition reactions are also of importance, and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes were covered in Chapter 1, and cycloadditions involving concerted mechanisms will be encountered in Chapter 6. Free-radical addition reactions are considered in Chapter 10. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

A novel gold catalyzed example of three-component addition was recently reported by Shi et al. (Equation 8.44) [106]. Terminal aryl alkynes, alcohols and 2-(arylmethy-lene) cyclopropylcarbinols provided an intermolecular tandem hydroalkoxylation/ Prins-type reaction to form 3-oxabicyclo[3.1.0]hexanes from simple materials and under mild conditions, catalyzed by the system AuClPPh3/AgOTf. The proposed mechanism for this reaction is shown in Scheme 8.19. 

Pyridazines and their partially saturated analogs have been prepared on insoluble supports by Diels-Alder reaction of electron-rich alkenes or alkynes with 1,2,4,5-tetrazines (Entries 1-3, Table 15.27). The mechanism of this reaction is outlined in Figure 15.15. An additional approach, also based on the Diels-Alder reaction, is the cycloaddition of azo compounds to 1,3-dienes (Entries 4 and 5, Table 15.27). The resulting tetrahydropyridazines (Entry 4) have been used as constrained 3-strand mimetics for the discovery of new protease inhibitors [323], An example of the N-alkylation of hexahydropyridazines on solid phase is given in Section 10.3. 

---