Olefin with carbamates

Photochemical aryloxymethylation of enones was expected to proceed by employing a-stannyl ethers in a similar fashion to stannyl sulfides. During our study, Steckhan and co-workers reported the alkoxymethylation of electron-deficient olefins with a-silyl ethers in the presence of sensitizers. On irradiation of a mixture of a-silyl ether 70 and strongly electron deficient alkene 71 in the presence of DCA and BP as sensitizers, alkoxymethyl product 72 was obtained (Scheme 29). This reaction mechanism is explained in a similar manner as mentioned in the reaction of a-silyl carbamates 57 (Scheme 24). 

A similarly tethered phosphonate was used to prepare the exocyclic olefin in Danishefsky s synthesis of calicheamicinone 347 [147]. Chemoselective acylation of the advanced intermediate 348 provided the olefination precursor 349, and subsequent cyclization gave 5-lactone 350, enforcing the desired stereochemistry of the double bond. The additional conjugative stabilization of the exocyclic olefin with the ester also facilitated subsequent transformations of the azide to the methyl carbamate functionality, and further manipulations ultimately led to the first synthesis of calicheamicinone 347 (Scheme 10-117). 

Luxenburger and coworkers reported a base-free, intermolecu-lar asymmetric aminohydroxylation (AA) reaction of olefins with alkyl 4-chlorobenzoyloxycarbamates as a nitrogen source that is readily prepared7 Generally, the reoxidant for an AA reaction is typically carbamate salt prepared in situ by treatment of NaOH with r-BuOCl. The addition of a base was mandatory for the promotion of the AA reaction. However, Luxenburger s reagent is found to be effective under base-free reaction conditions. The reaction proceeds under neutral conditions and various base-sensitive functional substituents were not affected (eq 55). 

Widenhoefer and Han have reported an effective protocol for the intramolecular hydroamination of unactivated C=C bonds with carbamates [52]. As an example of this protocol, treatment of the N-y-alkenyl carbamate 76 with a catalytic 1 1 mixture of [P(f-Bu)2(o-biphenyl)]AuCl and AgOTf (5 mol%) in dioxane at 60 C for 22 h formed pyrrolidine 77 in 91% isolated yield as a 3.6 1 mixture of diastereomers (Eq. (11.43)). The protocol tolerated substitution at the internal olefinic carbon atom and along the alkyl backbone and the method was applied to the synthesis of both heterobicyclic compounds and piperidine derivatives. This protocol was subsequently expanded to include the intramolecular hydroamination of N-alkenyl carboxamides including 2-allyl aniline derivatives (Eq. (11.44)) [53]. 

From a practical point of view, isocyanates, together with carbamates and ureas (Chapter 3), are the most important organic products discussed in this book. Their synthesis from nitroarenes has indeed been the subject of many patents. There are also limited examples of aliphatic isocyanates obtained by this route. Organic mono- and diisocyanates may be prepared in a continues liquid phase method by treating the appropriate amine with phosgene. However, the reaction is rather complex [6] and, besides the use of the dangerous phosgene, the formation of the corrosive hydrochloric acid creates several problems. Aliphatic isocyanates can also be obtained from olefins with isocyanate ion in the presence of a salt of a coordination compound of palladium or platinum [7], from olefins with isocyanic acid in the vapour phase over Pt/ALOs [8], and from formamides, by oxidation over a silver catalyst [9]. Apparently only the last reaction seems to have some potential practical applications [10]. 

For example (Scheme 13.44), treatment of y-hydroxy olefins with a catalytic amount of PdCl2—(CH3CN)2 and a stoichiometric amount of CUCI2 in methanol under CO leads to intramolecular alkoxylation/carboalkoxylation sequence and formation of substituted tetrahydrofurans 61 [80-83]. Similarly, 4-pentenyl carbamates undergo palladium-catalyzed intramolecular amination/carboalkoxylation to form substituted pyrrolidine derivatives 62 [84,85]. 

An alternative approach involves the reaction of an alkyl carbamate with a tertiary olefin (89,90). The resultant carbamates are thermally cracked at temperatures of 150—350°C to yield the isocyanate. The isocyanate is generally purified via distillation. 

Carbamates. Lower alkyl A/-halo- and W,W-dihalocarbamates are distillable Hquids (70,112). A/-Halo-A/-metallocarbamates are crystalline hygroscopic soHds. A/-Chloro-A/-sodiourethane [17510-52-0] C2H OCONQNa, does not decompose on heating to 250°C (113), but violent decompositions have occurred at room temperature (114). A/-Halocarbamates react with a variety of organic substrates, eg, the free-radical addition of W-chlorourethane [16844-21 -6] C2H OCONHCl, and A,A-dichlorourethane [15698-16-5], C2H OCONCl2, to olefins provides a convenient route to... 

P-chlorocarbamates which can be converted to aziridines and alkyloxazoUdones (93,115). A/-Chloro-A/-sodiourethane reacts with organoboranes forming A/-alkylcarbamates (114), and with olefins, catalyzed by Os, forming vicinal hydroxy carbamates (116). 

Following Uskokovic s seminal quinine synthesis [40], Jacobsen has very recently reported the first catalytic asymmetric synthesis of quinine and quinidine. The stereospecific construction of the bicyclic framework, introducing the relative and absolute stereochemistry at the Cg- and expositions, was achieved by way of the enantiomerically enriched trans epoxide 87, prepared from olefin 86 by SAD (AD-mix (3) and subsequent one-pot cyclization of the corresponding diol [2b], The key intramolecular SN2 reaction between the Ni- and the Cg-positions was accomplished by removal of the benzyl carbamate with Et2AlCl/thioanisole and subsequent thermal cyclization to give the desired quinudidine skeleton (Scheme 8.22) [41],... 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Peterson olefination, a silicon variant of the Wittig reaction, has been used to convert a-silyl benzyl carbamates (78) into trisubstituted vinyl carbamates (79) in moderate-to-good yields and with some ii/Z-selectivity. ... 

When olefins bearing a carbamate group at a suitable position are submitted to treatment with phenyltellurinyl acetate in the presence of BFj EtjO (previously described for... 

Other useful dehydrating agents are dimethylaminosulfur trifluoride (DAST), methyl A -(triethylammoniosulfonyl)carbamate (Burgess salt), acetic anhydride, oxalyl chloride, and phosphorous oxychloride, each one in combination with triethylamine (89). Dehydration of O-sUylated hydroxamic acids using trifluoro-methanesulfonic anhydride and triethylamine under mild conditions also gave nitrile oxides, which in the presence of olefins led to the formation of 2-isoxazolines in moderate to good yields (90). In view of the less readily available starting materials, this method probably will be of limited use. 

Hosokawa, Murahashi, and coworkers demonstrated the ability of Pd" to catalyze the oxidative conjugate addition of amide and carbamate nucleophiles to electron-deficient alkenes (Eq. 42) [177]. Approximately 10 years later, Stahl and coworkers discovered that Pd-catalyzed oxidative amination of styrene proceeds with either Markovnikov or anti-Markovnikov regioselectivity. The preferred isomer is dictated by the presence or absence of a Bronsted base (e.g., triethylamine or acetate), respectively (Scheme 12) [178,179]. Both of these reaction classes employ O2 as the stoichiometric oxidant, but optimal conditions include a copper cocatalyst. More recently, Stahl and coworkers found that the oxidative amination of unactivated alkyl olefins proceeds most effectively in the absence of a copper cocatalyst (Eq. 43) [180]. In the presence of 5mol% CUCI2, significant alkene amination is observed, but the product consists of a complicated isomeric mixture arising from migration of the double bond into thermodynamically more stable internal positions. 

Hudson et a/.156 have shown that N,N-dialkylcarbamates decompose in strongly acidic media to carbon dioxide, olefin, alkyl halide and alcohol, the rate of reaction of the secondary esters closely following h0. This fact, together with the variation in the rate of hydrolysis of carbamates of cyclic alcohols with the ring size154, shows that these reactions involve the intermediate formation of carbonium ions. 

Nitrogen-transfer reactions of osmium imido species lead to cis diamines or amino alcohols. Reaction of 0s02(N-/-C4H9)2 [63174-13-0] with olefins produces vicinal diamines after reductive cleavage. Catalytic oxyamination of olefins using chloramine-T or AJ-chlor-AJ-argenocarbamates yields vicinal hydroxy toluenesulfonamides (112) or carbamates (113), respectively, which maybe deprotected to vicinal amino alcohols. 

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