Cationic alkyne complexes

Addition of 2-butyne to [CpMo(dppe)2][PF6] displaces one dppe chelate to yield the cationic alkyne complex, [CpMo(dppe)(MeC=CMe)][PF6]-[Eq. (14)] (66). The replacement of a bidentate phosphorus ligand with a single 2-butyne donor is a comment on the propensity of Mo2+ to bind alkynes. 

Only severely sterically hindered alkynes, such as [(OC)9Co3CC=CC Co3(CO)9],6 fail to react with octacarbonyldicobalt. This steric effect is also illustrated by the reactions of [Cp(OC)2FePPh2C=CR]+ (R = H, Me, Ph, p-tolyl, t-Bu) with octacarbonyldicobalt.7 All of the cationic alkyne complexes form hexacarbonylalkyne-dicobalt derivatives except when the substituent is the bulky t-Bu group. 

Transition metal alkyne complexes also react with nucleophiles, in this case to generate CT-vinyl complexes. There are fewer stable alkyne complexes of higher oxidation state or cationic metals than olefin complexes. Because these types of alkyne complexes are most susceptible to nucleophilic attack, less information is available on tfiis reaction than on nucleophilic attack on coordinated alkenes. Nevertheless, reactions of several cationic alkyne complexes with nucleophiles have been reported, and a few examples are presented here. 

The yellow compound is soluble in polar solvents such as CHjClj, acetone, or acetonitrile. Although prolonged exposure to air leads to decomposition, the compound can be handled in air for short periods of time. Its IR spectrum in Nujol shows one Vuco vibration at 2088 cm" and a weak vuc band at 2040 cm ". Also a weak absorption at 1741 cm occurs, which may be due to the V(c=c) band of the coordinated alkyne. The H NMR spectrum in CH2CI2 has a sharp singlet for the CjHs protons at = 6.20 ppm, besides the broad resonance of the phenyl protons of the diphenylacetylene. Interestingly, KBr pellets of the compound several hours after initially formed, show a bathochromic shift of the V(co) band, which is also observed with other cationic alkyne complexes. ... 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

Rhodium complexes facilitate the reductive cydization of diyne species in good yield, although the product olefin geometry depends on the catalysts used. Moderate yields of -dialkylideneclopentane 169 resulted if a mixture of diyne 146 and trialkylsilane was added to Wilkinson s catalyst ClRh[PPh3]3 (Eq. 33) [101]. If, however, the diyne followed by silane were added to the catalyst, a Diels-Alder derived indane 170 was produced (Eq. 34). Cationic Rh complex, (S-BINAP)Rh(cod) BF4, provides good yields of the Z-dialkylidenecyclopentane derivatives, although in this case, terminal alkynes are not tolerated (Eq. 35) [102]. 

Clark and co-workers have reported reactions of Ir(III) cations with terminal alkynes in methanol in which alkoxycarbene complexes are formed (60). By analogy with a more extensively studied Pt(II) system (61), it has been concluded that cationic vinylidene complexes, e.g., 35, are reaction intermediates, e.g.,... 

In 1985, Eisch et al. isolated a cationic alkenyltitanium complex (55) by the insertion of an alkyne into the cationic Ti-C bond generated from titanocene dichloride and methylaluminum dichloride (Eq. 2) [77], Similarly, a mixture of Cp2TiCl(CH2SiMe3) and A1C13 afforded the solvent-separated ion pairs,... 

Recently, Ohe and IJemura reported a novel approach to the catalytic cyclopropanation of alkenes via 2-furyl178 179 or 2-pyrrolyl carbenoids180 that originate from the intramolecular nucleophilic attack of a carbonyl oxygen or an imine nitrogen (ene-yne-ketone and ene-yne-imine precursor, respectively) on a 7t-alkyne complex or a cationic cr-vinyl complex. Initially, the group 6 complexes like Cr(CO)s were used. Soon it was found that a series of late transition... 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

Recently, Shibita et al. reported catalysis of alkyne insertion into an arylamide sp C-H bond to give allylamides (42) by a cationic iridium complex [118]. An interesting aspect of this work is the unusually selective cleavage of an sp C-H bond over sp aromatic C-H bonds so that the alkenyl arylamide (43) is only a very minor product (30). The carbonyl group is required for the reaction as no coupling... 

The cationic gold complex with CAAC ligand 29 can also catalyze the unprecedented hydroamination reaction of alkynes and allenes using ammonia [59, 60]. It was also demonstrated that it can catalyze the simple hydroamination reaction. It... 

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