C-Cl Additions

In contrast, chlorocarbons undergo exclusive activation of a terminal methyl C-H, with no evidence for C-Cl addition to the metal.Moreover, with 2-chloropentane only the 4-chloropentyl product (410) was obtained, the absence of the 2-chloro isomer being attributed to rapid S-chloride elimination to afford Tp RhHCl(CNCH2 Bu) (422) (28% by-product) and pent-l-ene. 

To gain unambiguous mechanistic support despite the computational shortcomings outlined above, we turned to experiments. If the anionic palladium species were indeed reactive in a polar solvent in the presence of coordinating additives (such as KF or ArB(OH)2), then one should also be able to observe C-Cl addition in a polar solvent, in the absence of those additives, as Pd(0)L... 

Polytetrafluoroethylene contains only C—C and C—F bonds. These are both very stable and the polymer is exceptionally inert. A number of other fluorine-containing polymers cU e available which may contain in addition C—H and C—Cl bonds. These are somewhat more reactive and those containing C—H bonds may be cross-linked by peroxides and certain diamines and di-isocyanates. 

HSI anodes are subject to severe pitting by halide ions and this precludes their use in seawater or other environments in which these ions may be present in quantity. They are ideal for fresh-water applications (below 2(X)p.p.m. Cl"), although not for temperatures above 38°C. The addition of Mo or Cr to the alloy can improve performance under these conditions, with an upper limit of temperature of which may be affected by the... 

Trimethylsilyl halides can also be used for analogous reactions with arenediazo-nium tetrafluoroborates, as shown by Keumi et al. (1989). These authors treated 2-fluorenediazonium tetrafluoroborate in A/,Af-dimethylformamide or -acetamide with trimethylsilylchloride, -bromide, or -iodide in the presence of an excess of N-chlorosuccinimide, Af-bromosuccinimide, or methyl iodide, respectively, at 60 °C (Cl, Br) or at room temperature (I). The yields of the 2-halofluorenes were good in addition fluorene, the product of hydro-de-diazoniation, was obtained, if the reaction was run in tetrahydrofuran/Af,7V-dimethylformamide mixtures. The mechanism of these reactions, as well as that of the corresponding azido-de-diazoniation, is uncertain (see also Secs. 10.2 and 10.7). 

One other point to note in regard to this study (141) is that any evidence of oxidative addition, particularly with the chloro-olefins, was absent. The similarity of the spectra, coupled with the nonobservation of any bands in the visible region, as well as the observation of vc-c in the region commonly associated with 7r-complexation of an olefin (141, 142), all argue in favor of normal ir-coordination, rather than oxidative insertion of the metal atom into, for example, a C-Cl bond. Oxidative, addition reactions of metal atoms will be discussed subsequently. 

The presence at the catalyst s surface of active sites which made possible the dehydrofluorination reaction (C-F rupture) and the chlorine-fluorine substitution (C-Cl rupture) was then supposed There would also exist active sites which would only allow the second reaction (C-Cl nipture). The addition of nickel, by suppressing the sites which allow the two reactions (fluorination and dehydrofluorination) decreases the total activity. 

Little is known about the tolerance of 1 with unsaturated (poly)halogen compounds. Hydrozirconation of chloroalkenes can lead to competitive cycUzation, and simple reduction of both C=C and C-Cl bonds [98, 222], However addition of 1 to an alkenyl- or propargyl bromide led to the expected product as the sole product of the reaction in excellent yield (Scheme 8-30) [134, 223]. 

Restoration of the decayed species to its active valence is thus the key to reactivating the catalyst. It has been known that organic halides with activated C—Cl bonds can add to lower-valent transition metals and convert them to their higher oxidation states by oxidative addition (12-16) ... 

The effectiveness of an organic chloride in activating the catalyst appears to be related to the lability of the C—Cl bonds and probably also to their coordinating ability with the Rh. For example, CH30—CH2C1 and CH2=CH—CH2C1 were found to be extremely effective activators presumably, the presence of the ether or the allylic donor sites adds to the ease of the oxidative addition reaction ... 

Addition of up to 0.42 M TME did not much affect the yields, even though up to 37% of the cyclopropane adduct of carbene 20 formed, presumably at the expense of freon-carbene (C-Cl) insertion products which form in the absence of TME. The yield of 30, but not f-butylethene, was also minimally affected by TME when the diazirine was thermally decomposed at 100°C.46... 

The well-known addition of tert-butyl chloride to ethylene by means of A1C13 without skeletal rearrangement is simply another example of the initiation step of a /-cat polymerisation, involving the insertion of the alkene into an ester, namely the C-Cl bond, which is activated by the A1C13, via a six-centred transition state (I), as shown in equation (ii) ... 

There are two possible pathways for formation of biphenyl. One is cleavage of C-Cl bond followed by H addition ... 

Chloroprene (2-chloro-l,3-butadiene 105), which is a mass-produced, inexpensive industrial material, is an excellent precursor to a variety of terminal allenes 107 [97]. The palladium-catalyzed reaction of 105 with pronucleophiles 106 in the presence of an appropriate base gave the terminal allenes 107 in good yields (Scheme 3.53). The palladium species generated from Pd2(dba)3-CHC13 and DPEphos was a good catalyst for these reactions of chloroprene. In contrast, (Z)-l-Phenyl-2-chloro-l,3-buta-diene, which is isostructural with the bromo-substrate 101, was nearly inert under these conditions. There is no substituent at the vicinal ris-position to the chloride in 105, which allows oxidative addition of the C-Cl bond in 105 to the Pd(0) species. 

PdCl2-promoted stoichiometric dichlorocarbonylation of acetylene (Eq. 20) is the first example of oxidative carbonylation of an alkyne that appeared in the literature [69,70], and presumably occurs through the mechanism shown in Scheme 13, involving addition of PdC to the triple bond followed by CO insertion, reductive elimination, oxidative addition to the C - Cl bond, further CO insertion and reductive elimination (Scheme 13, path a). 

Again, it proceeds by the standard mechanism for cross-coupling reactions oxidative addition of Pd(0) to the C-Cl bond, transmetallation (can also be viewed as ligand substitution) to give the N-Pd(II)-C compound, and reductive elimination. 

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