Cumulene electrophilic addition

As already commented in the introduction of this chapter, regardless of its substitution pattern, the main trends of allenylidene reactivity are governed by the electron deficient character of the C and Cy carbon atoms of the cumulenic chain, the Cp being a nucleophilic center [9-15]. Thus, as occurs with their allcarbon substituted counterparts, electrophilic additions on 7i-donor-substituted allenylidene complexes are expected to take place selectively at Cp, while nucleophiles can add to both C and Cy atoms. However, the extensive 71-conjugation present in these molecules results in a reduced reactivity of the cumulenic chain and, in some cases, in marked differences in the regioselectivity of the nucleophilic additions when compared to the all-carbon substituted allenylidenes. In the following subsections updated reactivity studies on 7i-donor-substituted allenylidene complexes are presented by Periodic Group. 

The alkenyl(amino)aUenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4 OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

The electrophilic additions discussed above also occur with alk)Ties and cumulenes, but there can be significant differences in reactivity between alkenes and these other compounds. The reaction of bromine and 1-phenylpropyne (56) in acetic acid led to the formation of four primary products, 57, 58, 59, and 60 (equation 9.56). ... 

Besides the classical additions of carbon-centered nucleophiles to the electrophilic sites of the cumulenic chain, transition-metal allenyhdenes are able to promote... 

Addition of water to the electrophilic Co. of the cumulenic chain explains the formation of the a, (3-unsaturated aldehydes. 

Although the chemistry of pentatetraenylidene complexes [M]=C(=C)3=CR R has not received as much attention as that of aUenylidenes, there is ample experimental evidence to confirm the electrophilic character of the C, Cy and carbons of the cumulenic chain [26-29, 31]. Thus, treatment of tra s-[RuCl(=C=C=C=C=CPh2) (dppe)2][PFg] (132) with alcohols or secondary amines resulted in addition of the nucleophilic solvent across the Cy=Cs double bond to give alkenyl-allenylidenes 138 (Scheme 48) [358]. In chloroform, electrophilic cyclization with one of the Ph groups occurred to give 139. This transformation is actually the parent of the later observed allenylidene to indenylidene intramolecular rearrangement (Scheme 15). 

Here, we shall focus on ruthenium-catalyzed nucleophilic additions to alkynes. These additions have the potential to give a direct access to unsaturated functional molecules - the key intermediates for fine chemicals and also the monomers for polymer synthesis and molecular multifunctional materials. Ruthenium-catalyzed nucleophilic additions to alkynes are possible via three different basic activation pathways (Scheme 8.1). For some time, Lewis acid activation type (i), leading to Mar-kovnikov addition, was the main possible addition until the first anfi-Markovnikov catalytic addition was pointed out for the first time in 1986 [6, 7]. This regioselectiv-ity was then explained by the formation of a ruthenium vinylidene species with an electron-deficient Ru=C carbon site (ii). Although currently this methodology is the most often employed, nucleophilic additions involving ruthenium allenylidene species also take place (iii). These complexes allow multiple synthetic possibilities as their cumulenic backbone offers two electrophilic sites (hi). 

Figure 2.59 collects several [2t-2]-cycloadditions that can be carried out with C )-The reaction with dehydrobenzene is instructive as it reveals electronic properties of the fullerene The dehydrobenzene generated in situ from anthranilic acid reacts with Cso exclusively in a [2-i-2]-cycloaddition, although in principle a [4-f2]-reaction would also be possible, and dehydrobenzene usually enters into the latter when adding to electron-rich dienes. The nonoccurrence of this reaction clearly shows the electron-deficient character of C,so. For the same reason it never constitutes the diene part in a Diels-Alder reaction. Furthermore, the [24-2]-cycloaddition may be thermally effected, for example, the addition of long-chain cumulenes, allene amides or quadricyclan. The addition of ketenes as well occurs without irradiating the reaction mixture. Normally a reduced reactivity toward C,so should be expected for the electrophilic ketenes, but in reality the products of a [2-1-2]-cycloaddition are even found to be the major product. 

According to the proposed mechanism, the electrophilic arylpaUadium iodide 326, formed by an oxidative addition of Ar-1 to a Pd(0) species, was proposed to activate the central carbon-carbon double bond in the cyclic cumulene intermediate 317b to form the complex 327. A subsequent attack by the Zr-C(sp ) nucleophilic center produces the aUcenylpalladium intermediate 325, which upon reductive elimination furnishes the corresponding alkenylzirconium species 324. Hydrolysis of the latter finally produces the 3-methylenecyclopentene 319. To support the mechanistic hypothesis, deuterolysis was carried out to provide the deuterated compound 319-D in 60% yield with a high level of deuterium incorporation [87]. 

Substituents attached to the cumulenes can influence the mechanism of the cycloaddition reactions by rendering one molecule more nucleophilic and thereby deciding the course of the addition, i.e. determine which molecule is the electron donor to attack the electrophilic center of the other molecule . 

---