Bromo bridges

The chloro- and bromo-bridged complexes were first prepared by Werner (7). Werner originally formulated these complexes as singly bridged complexes (structure IX), but the /i-amido-/<-chloro and fi-amido-/i-bromo structures have recently been demonstrated by crystallographic studies (387, 388). The equilibrium constant Kl2 = Kt x K2 has been determined spectrophotometrically and the formation of structure X for X"- = Br is less favorable than its formation for the chloro analog (376, 379) (Table XXXVI). The equilibrium between... 

Early attempts to prepare carbonyl halides of molybdenum failed,15-17 until pioneering work by Colton and Tomkins in 196618 described the reaction of [Mo(CO)6] with liquid chlorine at -78°C to yield the tetracar-bonyl dimer, [ Mo(,u.-C1)C1(CC))4 2] [Eq. (1)]. In the same year19 the bromo-bridged dimer, [ Mo(p.-Br)Br(CO)4 2J, was prepared in an analogous manner. 

Reaction of dimeric, bromo-bridged Co2Br2(NO)4 with NaCN (1 4 mole ratio) in EtOH produces Na[Co(CN)2(NO)2], which may be obtained in pure form by evaporating the solution to dryness, followed by recrystallization from THF-petroleum ether. This compound is also prepared from CoBr(L)(NO) (L = PPh3, SPPhj, SePPhj) with xs NaCN in EtOH at 40-60°C. The same starting materials in a 1 1 mole ratio give instead single bromide-by-cyanide substitution, with formation of Co(CN)L(NO) °. 

Other spectroscopic techniques also may be applied to the resolution of molecular structure. For now, one general method may be illustrated by the Mossbauer elucidation of the structure of UCl fBr,. By analogy with IjClf, (Fig. 6.26a), we might expect a bridged structure with either chloro or bromo bridges (Fig. 6.26b-d). The experimen-... 

Aquation of /ra j-[PtBr(gly)(NH3)] is catalysed by Hgaq. The mechanism is of an equilibrium formation of a Pt—Br—Hg intermediate, followed by slow generation of the /ranj-[Pt(gly)(NH3)(OHa)]+ product. The enthalpy and entropy of formation of the bromo-bridged dinuclear intermediate, its u.v.-visible spectrum, and its kinetics of decomposition, have been measured. ... 

Planar chiral cyclopalladated ferrocenylhydrazones and di-/x-bromo-bridged dimers such as 86 have been synthesized in enantiomerically pure form with a high level of diastereoselectivity and X-ray structures have been obtained. Optically active cyclopalladated compounds derived from chiral arylimines have been obtained. 

Heterobimetallic complexes in which a half-sandwich complex of rhodium ill) is cormected by three bromo-bridges to Re(CO)3 were prepared from metathesis reactions of [Cp RhBr2]2 and [Re( -Br)(C0)3(C4H80)]2- The crystal structure of Cp Rh(/t-Br)3Re(CO)3 was determined by X-ray analysis. The treatment of a mixture of [(rf-CjH8)RuCl2]2 and [Cp RhCl2]2 with silver carboxylates and subsequent hydrogenation afforded mixed metal... 

The A -ray structure of [Ru3(CO)9(CgHe)HgBr]2 shows that it is a bromo-bridged dimer linked via the HggBra-unit in which each Hg atom bridges the Ru-Ru edge of Rug-triangles. ... 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

This synthesis came shortly after one by Prelog, Kohlberg, Cerkovnikov, Rezek and Piantanida (1937) based on a series of reactions which, with modifications and extensions. Prelog and his colleagues have applied to the syntheses of bridged heterocyclic nuclei, of which this is an example. 4-Hydroxymethyltetrahydropyran (VI R =. OH) is converted via the bromo-compound (VI R = Br) and the nitrile (VI R = CN) into tetrahydropyran-4-acetic acid of which the ethyl ester (VII) is reduced to 4-()3-hydroxyethyl)-tetrahydropyTan (VIII). This is converted by fuming hydrobromic acid into 3-(2-bromoethyl)-l 5-dibromopentane (IX) which with ammonia in methyl alcohol yields quinuclidine (V). 

In /r-bridged dimers, two phthalocyanine moieties are linked by one or more bridging atoms such as oxygen or halogens. Niobium(V) oxide tribromide (NbOBr,) when treated with phthalonitrile in 1-chloronaphthalene at 230CC for 3 hours produces /r-bromo-/t-dioxobis[nio-bium(V) phthalocyanine] tribromide in 52%.232... 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

Jakle and Wagner have communicated the synthesis of bromo-substituted, boranediyl-bridged poly(ferrocenylene)s (40) (Fig. 28) by the reaction of Fc(BBr2)2 with three equivalents of HSiEt3. The polymer 40 was transformed into the corresponding mesityl-substituted polymer (40Mes) by treating its slurry in toluene with... 

Figure 28 Synthesis of bromo-substituted, boranediyl-bridged poly(ferrocenylene)s (40) from Fc(BBr2)2. (Adapted from ref. 62.)...

The construction of the central ring can also be accomplished by a radical cyclization of a non-fused heterobiaryl system composed of two six-membered heterocycles and linked by a two-atom bridge. An example is the synthesis of 106 from the bromo derivative 105 in the presence of AIBN and Bu3SnH (Equation 8) <1999TL4073>. 

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