Aliphatic terminal

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

The above-postulated overall mechanism considers two alternative pathways depending on the nature of the acetylene derivative. Region A outlines a proposal in which the formation of the a-complex intermediate is supported by the fact that the treatment of aliphatic terminal acetylenes with FeCl3 led to 2-chloro-l-alkenes or methyl ketones (Scheme 12). The catalytic cycle outlined in region B invoked the formation of the oxetene. Any attempt to control the final balance of the obtained... 

Various metal complexes catalyze the addition of catecholborane and pinacolbo-rane to aliphatic terminal alkenes (Table 1-2). Neither the borane reagents nor the catalysts alter the high terminal selectivity, but a titanium catalyst does (entry 3). Although Cp2TiMe2 [30] exhibits high terminal selectivity for vinylarenes, aliphatic alkenes afford appreciable amounts of internal products, whereas an analogous Cp 2Sm(THF) [31] allows selective addition of catecholborane to the terminal car-... 

Berkessel, A., Rollmann, C., Chamouleau, F. et al. (2007) Practical two-step synthesis of an enantiopure aliphatic terminal (S)-epoxide based on reduction of haloalkanones with designer cells . Advanced Synthesis and Catalysis, 349 (17-18), 2697-2704. 

The displacement of less activated halogen groups by silver(I) tetrafluoroborate to the corresponding fluorides is not so widespread. Although silver(I) tetrafluoroborate cannot be used to transform an aliphatic terminal dichloromethyl or trichloromethyl group to the corresponding fluoro compound, it has been known to work in many other cases (Table 13).70... 

The aldol (23) on treatment with benzenesulphonyl chloride yields the oxetanone ((3-lactone) (24) which is an intermediate in the synthesis of the butenolides (25) (95SC479). Aliphatic terminal alkynes or arylalkynes react with nitrones in the presence of a copper based catalyst system to give 1,3,4-trisubstituted [3-lactones (95JOC4999). 

Tetrasubstituted silanes are also sources of silylene. Suginome and coworkers reported that palladium catalyzed the transfer of dimethylsilylene, formed from silylborane 44, to alkynes [equation (7.8)], 60 Exposure of silylborane 48 and alkyne 49 to substoichiometric amounts of palladium and P(7-Bu)2(2-biphenyl) afforded 2,4-disubstituted silole 50. This process tolerates a variety of functionality including silyl ether-, dimethylamino-, and trifluoromethyl groups. In addition to aliphatic terminal acetylenes, arylacetylenes were also competent substrates. For... 

We have studied homopolycyclotrimerizations of aliphatic terminal diynes with various alkylene spacer lengths catalyzed by tantalum and niobium halides or by binary mixtures of the metal halides and tetraphenyltin (Scheme 18) [64-66]. Under optimized conditions, completely soluble diyne homopolymers with high molecular weights (Mw up to 1.4 x 106) and pre-... 

Under these conditions, a variety of linear aliphatic terminal alkynes were transformed into aldehydes with good selectivity. The efficiency, regioselec-tivity of the addition and substituent tolerance were improved by using RuCl(Cp)(phosphine)2, where Cp is cyclopentadienyl, or RuCl(Cp)(diphos-phine) as catalyst precursors [30]. The best results were obtained with diphenylphosphinomethane as a ligand, which made possible the preparation of aldehydes from bulky aliphatic alkynes (tert-BuC=CH), aromatic alkynes (PhC=CH), diynes [HC=C(CH2)6C=CH] and functional terminal alkynes [NC(CH2)3C=CH, PhCH20(CH2)2C=CH,...]. The mechanism of this reaction was investigated in details by isolation of intermediates, deuterium-... 

Tetrahydrofuran was coupled to a variety of aromatic and aliphatic terminal alkynes under microwave irradiation to provide a mixture of cis- and t/ an5-2-vinyltetrahydrofuran. A representative example is shown below <04TL7581>. 

Similarly, aliphatic terminal alkynes (76) can also be oxygenated (272,273],... 

However, all types of alkenes are not always used in the peroxidation. The most efficient alkene is the 1,1-disubstituted alkene due to the stability of the intermediate tertiary radicals B in Scheme 31. Aliphatic terminal alkenes such as n-hexene are a poor substrate. In addition, ambient air is the best atmosphere for the catalytic autoxidation since overoxidation occurs under an oxygen atmosphere. 

By contrast, chloroperoxidase-catalyzed epoxidation of alkenes proceeds with excellent enantioselectivites [1333, 1334]. For styrene oxide it was demonstrated that all the oxygen in the product is derived from hydrogen peroxide, which implies a true oxygen-transfer reaction (path 3, Scheme 2.174) [1335]. As depicted in Scheme 2.178, unfunctionalized c/s-alkenes [1336] and 1,1-disubstituted olehns [1337, 1338] were epoxidized with excellent selectivities. On the other hand, aliphatic terminal and irans-l,2-disubstituted alkenes were epoxidized in low yields and moderate enantioselectivities [1339]. 

In more recent work, Noyori and coworkers found conditions for the selective epoxidation of aliphatic terminal alkenes either in toluene, or using a completely solvent-free reaction setup [23, 24]. One of the disadvantages with the previous... 

In further optimizations, Beller and coworkers examined various benzyl amines as replacement for pyrrolidine in the FeCl3-H2pydic catalyst system. They found thatthe use of different benzyl amines resulted in higher yields and better selectivity for the formation of epoxides, predominantly from aliphatic alkenes [111]. As seen in Table 2.8, the epoxidation of trans-l,2-disubstituted alkenes such as frans-2-octene and fruns-5-decene resulted in high yields, whereas aliphatic terminal alkenes appear to be problematic substrates. The epoxidation of aromatic alkenes using this catalytic system gave similar results to those obtained using pyrrolidine as base. 

The first examples of linear co-dimerization of acetylenes with buta-l,3-diene have been reported. Aliphatic terminal acetylenes (196) react with buta-1,3-diene in the presence of a catalytic amount of an appropriate ruthenium complex at 60-80 C to give almost quantitative yields of E-conjugated enynes (197). ... 

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