Palladium dibromide

The only known example is hexahydro-lH-arsolo[l,2-a]arsole (17) (63JCS725). This unusual compound is prepared when the arsonium bromide (18) is heated to 200 °C (equation 6). Cyclization is accompanied by loss of methyl bromide to give (17 b.p. 100-102 °C/17 mmHg) which was characterized as the red palladium dibromide adduct (m.p. 150 °C). No analogous derivatives of antimony or bismuth have been reported. 

These are prepared by the cyclization of either an arsenous chloride (29) (23JCS2489) or an arsenic acid (30) (58JCS1719). These routes have been used to prepare both the 1-methyl-and the 1-phenyl-arsindolines. The arsindolines show the normal properties of tertiary arsines and form addition compounds with alkyl halides or palladium dibromide. Attempts to dehydrogenate arsindolines to the parent arsindoles (31) have not been successful (Scheme 7). 

Grigg et al. also introduced another Heck-type reaction. 2,6-Dibromo-hepta-1,6-dienes 80 cyclize to the same products 83 (n = 5) as do 2-bromo-1,6-dienes 78 (n = 5) when treated with the usual precatalyst mixture, yet containing a stoichiometric amount of triphenylphosphine [63,64], In this case, palladium dibromide rather than hydridopalladium bromide is eliminated in the final step of the cross-coupling reaction, and the palladium(II) salt is reduced by the phosphine to regenerate the reactive palladium(O) species. Completely selective exo-trig cyclizations occur in these examples, however, the respective cyclohexane derivatives with n = 6 are formed in poor yields. Additionally, it is sometimes difficult to separate the product from the phosphine oxide after aqueous work-up. This latter difficulty was circum-... 

Another direct route to polycarbonate was reported by Okuyama et al. [36]. The process consists of an oxidative carbonylation procedure. It is catalyzed with a Pd complex system. l,l -Di-tert-butyl-3,3 -methylenediiimidazolin-2,2, dylidene [palladium dibromide and produces a highest molecular weight polymer, (M = 9,600, Mw = 24,000) in 80% yields. 

While such a process had initially been observed as an undesired side-reaction in transformations where copper salts were employed as re-oxidants [13], Chemler demonstrated that various aminohalogenation reactions proceed in THF or acetonitrile in the presence of potassium carbonate as base [14]. These reactions employ palladium trifluoroacetate or palladium dibromide as catalyst source and require a moderate excess of the copper oxidant (3-4 equiv) giving moderate to excellent yields. However, they usually suffer from rather low selectivity, either in the initial aminopalladation or via subsequent rearrangement pathways to provide mixtures of pyrrolidines and piperazines (Scheme 4.2, Eq. (4.3)). A stoichiometric control reaction in the presence of palladium bromide led only to the Wacker cydization together with an alkene isomerization product, suggesting that the presence of copper(II) salts is crucial for the overall process. The exact role of the copper(II) salts has not yet been darified and palladium intermediates of different oxidation states may be involved in the final stage of carbon-halogen bond formation. 

In support of the involvement of [Pd(PR3)j in the oxidative addition, Pd(I) dimers 13a,b have been found to catalyze the room-temperature amination and the Suzuki-Miyaura couplings of aryl chlorides and bromides (Scheme 1.15) [162]. These palladium dimers decompose to form the palladium dibromide [Pd(PRj)Br2] and a highly reactive Pd(0) complex [Pd(PR3)j [162, 163]. 

Although a metal exchange reaction is not often thought of as a preparative method, such reactions can be useful since they generally proceed in good yield. Palladium is replaced by iron (2-8) by conversion of tetraphenylcyclo-butadiene palladium dibromide to tetraphenylcyclobutadiene iron tricarbonyl. The driving force involves the formation of the more stable tetraphenylcyclobutadiene iron tricarbonyl complex. 

Alternate Names palladium bromide palladium dibromide. Physical Data mp 717 2 °C X-ray structure. ... 

The synthesis of 2- and 2,3-substituted pyrazolo[l,5-a]pyridines proceed via a two-step procedure. For instance, to form 2-[4-(benzyloxy)phenyl]pyrazolo[l,5-a]pyridine, the stepwise method involves (i) the activation of pyridine to generate iV-benzoylitninopyridinium ylide as already described and (ii) a palladium-mediated C2 alkenylation of the iminopyridinium ylide in the presence of (. -(2-iodovinyl)(4-benzyloxy)benzene, silver benzoate (3 equiv), and palladium dibromide (5 mol %)/tris(4-methoxyphenyl)phosphine (10 mol%) as a catalytic system (eq 38). The second step (i.e., the C-H alkenylation step) delivers the desired product in 87% step after 16 h at 125 °C in 1,4-dioxane. 

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