Stepwise substitution

In general, Grignard reagents are useful in the synthesis of mixed hydridochlorosilanes because these reagents can effect stepwise substitution of the halogen, eg,... 

Numerous carbonyl halides, of which the best known are octahedral compounds of the type [M(C0)4X2] are obtained by the action of halogen on Fe(CO)5, or CO on MX3 (M = Ru, Os). Stepwise substitution of the remaining CO groups is possible by X or other ligands such as N, P and As donors. 

Reaction of OsCl - with BrF3 affords stepwise substitution... 

We found a new route for preparing larger amounts of these disilanes. The stepwise substitution of the phenyl groups at the silicon atoms with triflic acid and the additional conversion to Si-H and Si-C1 functions at low temperatures leads to pure chloro-hydrogen disilanes. The synthetic routes to 1,1,1-trichlorodisilane 2 and 1,1-dichlorodisilane 3 [7] are shown in Eq.(4). 

Stepwise substitution and elimination reactions 67 Insight from recent work 69... 

It has been proposed that the first stage of alkali metal complex formation by dibenzo-30-crown-10 in methanol involves a fast ligand conformational change which is then followed by a stepwise substitution of the coordinated solvent by the ligand (Chock, 1972) ... 

Spectral changes (Fig. 2) obtained on addition of pyridine to an aqueous solution of la indicate stepwise substitution of both aqueous ligands (Eq. (1)). 

Calculated Activation Energies (B3LYP/6-311+G ) for the Stepwise Substitution of Ammonia by Water (91)... 

Eaton and co-workers also reported the synthesis of 1,3,5-trinitrocubane and 1,3,5,7-tetranitrocubane (39) ° The required tri- and tetra-substituted cubane precursors were initially prepared via stepwise substitution of the cubane core using amide functionality to permit ort/jo-lithiation of adjacent positions. The synthesis of precursors like cubane-1,3,5,7-tetracarboxylic acid was long and inefficient by this method and required the synthesis of toxic organomercury intermediates. Bashir-Hashemi reported an ingenious route to cubane-1,3,5,7-tetracarboxylic acid chloride (35) involving photochemical chlorocarbonylation of cubane carboxylic acid chloride (34) with a mercury lamp and excess oxalyl chloride. Under optimum conditions this reaction is reported to give a 70 8 22 isomeric mixture of 35 36 37... 

A series of Tc complexes with the monobasic ligands 2-methyl-3-oxy-4-pyronate (maltolate, Hma (78)) or l,2-dimethyl-3-oxy-pyridinonate, (Hdpp, (79)) were prepared in nonaqueous solution by chloride substitution in [TcOCU]. Stepwise substitution is observed. The substitution of one... 

A chlorine-bromine exchange in fluorodichloromethanesulfenyl chloride by means of hydrogen bromide provides a route to additional sulfenyl bromides (109). After the initial halogen exchange at the sulfur, further addition of hydrogen bromide causes stepwise substitution at the methyl group ... 

The carrier molecules have sufficient flexibility to allow for a stepwise substitution of the solvent molecules, resulting in a low activation barrier and fast complexation reaction rates (75—77). 

The ammine complexes of Co3+ are prepared by adding excess ammonia to a solution of cobalt salt followed by air oxidation and boding. The brown solution turns pink on boiling. The cyanide complexes are made by adding excess potassium cyanide to a solution of cobalt salt. Acidification of the solution with a small amount of acetic or hydrochloric acid followed by boiling yields K3Co(CN)6. The aquo-halo mixed complexes are formed by stepwise substitution of H2O molecule with halide ion in the coordination sphere. In general, a mixed complex may be prepared by substitution with a specific anion. 

The high electrophilic character of the 2- and 4-positions in quinazoline makes hydrolysis to oxo derivatives relatively easy, and both 2- and 4-chloro substituents can be hydrolyzed in either alkaline or acid solution. The significantly higher reactivity in the 4-position simplifies stepwise substitutions, and the synthesis of 2-chloro-4(3//)-quinazoli-nones 173 is readily performed from the dichloro compounds 172 with sodium or potassium hydroxide at room temperature <2003BMC2439, 20050PD80, 2006H(67)489, 20060PD391>. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

Figure 5 Electronic spectral changes (in aqueous solution) accompanying stepwise substitution in [Cr(NCS)fi]3 by CN" (adapted from Z. Artorg. Allg. Chem., 1975, 417, 55)...

Most of the alkyls of the other early transition metals demonstrate similar reactivity.101102 Hydridic metal hydrides will similarly react with the weakly acidic OH groups of alcohols and phenols, liberating hydrogen and forming alkoxides or phenoxides. Aluminum hydride will undergo stepwise substitution by alcohols (equation 29).103... 

A convenient method for the stepwise substitution of halide groups by alkoxide ligands involves the use of alkoxysilanes. By this method, Handy and coworkers have synthesized and studied a large number of mixed alkoxy halides of molybdenum and tungsten (equation 39).131-134 The method has also been used for the preparation of phenoxy and substituted phenoxy derivatives.133... 

In addition to influencing the rate of a reaction, pH may also control the products where alternate or sequential pH-dependent reactions take place. An example of this type of reaction is the chlorination of phenol. Lee and Morris (37) have shown that the chlorination of phenol proceeds by the stepwise substitution at the 2, 4, and 6 positions of the aromatic ring. The rate of each of these reactions depends on the product of phenate or chlorophenate anion and the hypochlorous acid concentrations. Since each phenolic compound has a slightly different acid dissociation constant, the species of chlorophenols that are formed depend on the pH of the solution. 

Naturally, the ideal source of starting materials for homoleptic metal isocyanide compounds is via metal carbonyl complexes, but previously only with the two carbonyls Ni(CO)4 (24) and Co2(CO)g (25) has direct substitution of all carbonyl groups been effected. Recently, however, remarkable discoveries by Coville and co-workers (26-31) on the transition-metal-catalyzed substitution of carbonyl groups in monomeric and cluster compounds have shown that Fe(CNR)s, Mo(CNR)6, and Ir4(CO)5(CNR)7 (32) can be prepared in high yield by stepwise substitution from the parent carbonyl. 

In contrast, stepwise substitution reactions on M(CO)6 (M = Cr, Mo, W) have been achieved with a series of heterogeneous catalysts including co-balt(ll) chloride (27), activated charcoal (159), and platinum metals dispersed on oxide or carbon supports (31), to give mono-, di-, tri-, and complete substitution (124) in yields > 90%. Representative reaction times are given in Table II (159). The efficiency of the method was further demonstrated by the stepwise synthesis of the mixed isocyanide complexes m-Mo(CO)4(CNMe)(CNBu ) and /ac-Mo(CO)3(CNMeXCNBu )2 from Mo(CO)6 in <25 min in 85 and 95% yields, respectively (159). 

The discovery that additions of catalytic amounts of CoCl2, activated carbon, or metals on metal oxide or carbon supports are extremely effective in labilizing carbonyl groups in Fe(CO)5 is thus particularly noteworthy. Stepwise substitution products Fe(CO)5 x(CNR)x (x = 1-5) have been obtained in high yield with short reaction times using these catalysts (see Table III) (759). 

Rhodium(I) or polymer supported rhodium(I) compounds catalyzed the formation ofFefCO - CNR L (x = 1 - 3 R = Bu, xylyl L = MA, citra-conic anhydride, acrylamide) (29, 30), and the dimer [CpFe(CO)2]2 catalyzed the stepwise substitution of carbonyl groups in CpFeI(CO)2 to give CpFeKCO - CNR) (x = 1,2 R = Bu, xylyl) in 60-80% yields. A nonchain free-radical mechanism was proposed for the latter reaction (28). The compounds CpFeX(CO)2 x(CNR)x (x = 1,2 X = halide, SiMe3) are known for a range of alkyl and aryl isocyanides (169-171). 

The energy diagram Fig. 20 explains that mono-substitution of OMCTS with fluoroboranes, -phophanes and -silanes is possible and that a symmetrically substituted four-membered ring is formed in stepwise substitution (C ). 

Formation of Ni(CO)4 or Co(CO)4- by the cyanide method depends upon the stepwise substitution of the anion of the cyano complex by the iso-electronic carbon monoxide molecule. By treating Co2(CO)8 with potassium cyanide we obtained cyanocarbonyls of cobalt of low oxidation number (83). In reactions of the nitrosyl carbonyls of iron and cobalt, Behrens (86) substituted all the CO groups with CN to give K3[Co(NO)(CN)3] or... 

In borazine compounds having branched alkyl groups (other than t-butyl) at the N-atoms, a stepwise substitution of the (B)—H atoms in Grignard alkylation 84> is observed rather than the statistical substitution which occurs on alkylation of N-methylborazines with n-alkyl Grignard reagents 34>. It has been reported 49-85> that base dehydrochlorination of isobutylammonium trichlorophenylborate as well as that of isobutyl-amine-dichlorophenylborane results in a mixture of the trimeric and tetrameric N-isobutyl-B-phenylborazines [Eq. (20)]. 

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