Alkenes sterically hindered

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

Alkenes of all types can be converted to cyclopropane derivatives by this reaction (though difficulty may be encountered with sterically hindered ones). Even tetracyanoethylene, which responds very poorly to electrophilic attack, gives cyclopropane derivatives with carbenes.Conjugated dienes give 1,2 addition ... 

There are actually three reactions called by the name Schmidt reaction, involving the addition of hydrazoic acid to carboxylic acids, aldehydes and ketones, and alcohols and alkenes. The most common is the reaction with carboxylic acids, illustrated above.Sulfuric acid is the most common catalyst, but Lewis acids have also been used. Good results are obtained for aliphatic R, especially for long chains. When R is aryl, the yields are variable, being best for sterically hindered compounds like mesi-toic acid. This method has the advantage over 18-13 and 18-14 that it is just one laboratory step from the acid to the amine, but conditions are more drastic. Under the acid conditions employed, the isocyanate is virtually never isolated. 

ANSWER Let s first consider the expected regiochetnical outcome of the reaction. The reaction does not employ a sterically hindered base, so we expect formation of the more substituted alkene (the Zaitsev product) ... 

The more-substituted alkene (Zaitsev product) is the major product. However, there is one critical difference between the regiochemical outcomes of El and E2 reactions. Specifically, we have seen that the regiochemical outcome of an E2 reaction can often be controlled by carefully choosing the base (sterically hindered or not sterically hindered), hi contrast, the regiochemical outcome of an El process cannot be controlled. The Zaitsev product will generally be obtained. 

We can control which product we get by carefully choosing our base. If we use a strong base (like methoxide or ethoxide), then we will get the more substituted alkene. However, if we use a strong, sterically hindered base, such as tert-butoxide, then we will get the less substituted alkene. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

As the steric bulk of the propargylic substituents increased, the preference for the formation of the seven-membered ring increased as well. Formation of a ruthenacyclopentene intermediate with sterically hindered substrates involves a large amount of A(1,3) strain, leading to preferential formation of a 7r-allyl species. This novel cycloisomerization process is very sensitive to alkene substitution the requirement for a m-methyl group was evidenced by the failure of 70 to give... 

The 1,3-dipolar cycloaddition of imidazolinone 123 with ethyl m-4,4,4-trifluorocrotonate 124 provided, after 36 h at reflux, the regio- and stereoisomer 125 (90%), accompanied with traces of three other unidentified cycloadducts (10%) <2001JFC275>. Compound 125 was isolated in 70% yield (Scheme 15). The structures of the final product were elucidated by nuclear Overhauser effect (NOE) experiments. This high selectivity is the result of a preferred ///////-orientation of both ester and CF3 groups in the transition state and of an impeded ////////-approach of the CFj-substituted terminus of the alkene to the sterically hindered ct-site of 123. 

There are few reports of the hydrostannation of simple alkenes with metal catalysts in homogeneous solution, but steric strain in the ring causes cyclopropenes to be reactive even at —78 °C, with addition of Sn-H to the less sterically hindered face (Equation (25)). Distannation with Me3SnSnMe3 and silastannation with Me3SiSnBu3 could similarly be achieved with Pd(OH)2 as catalyst.107... 

As observed with alkenes, bromine addition to sterically hindered dienes shows a peculiar behavior. Highly substituted dienes, existing predominantly in non-planar conformations, often present a chemical reactivity distinctly different from that of planar... 

In 2008, the Ackennann group reported on the use of phosphoric acid 3r (10 mol%, R = SiPhj) as a Brpnsted acid catalyst in the unprecedented intramolecular hydroaminations of unfunctionaUzed alkenes alike 144 (Scheme 58) [82], BINOL-derived phosphoric acids with bulky substituents at the 3,3 -positions showed improved catalytic activity compared to less sterically hindered representatives. Remarkably, this is the first example of the activation of simple alkenes by a Brpnsted acid. However, the reaction is limited to geminally disubstituted precursors 144. Their cyclization might be favored due to a Thorpe-Ingold effect. An asymmetric version was attempted by means of chiral BINOL phosphate (R)-3( (20 mol%, R = 3,5-(CF3)2-CgH3), albeit with low enantioselectivity (17% ee). 

The nucleophilic attack of f-butyllithium on lithium vinylidene carbenoids has also been used for synthetic purposes in as far as the reaction permits to generate sterically hindered alkenes. Thus, treatment of the dibromoalkene 78 generated from adamantanone with an excess of f-butyllithium results in the formation of the alkene 79 that contains three bulky substituents at the double bond (equation 43) . In an analogous way, a f-butyl residue is introduced into chloroenamine 80 (equation 44) . 

The methods described above are applicable to almost any alcohol. Procedure A is best for sterically hindered alcohols and procedure B is especially useful for sensitive alcohols where rearrangement or alkene formation is likely. The submitter s... 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

In contrast, the a-peroxy lactones, also members of the dioxetane family, display a higher reactivity toward nucleophiles, in view of the inherent polarization of the peroxide bond by the carbonyl functionality. Consequently, the nucleophilic attack is expected to take place at the more sterically hindered but more electrophilic alkoxy-type oxygen atom of the peroxide bond. A recent detailed study of the oxidation of various di-, tri-and tetrasubstituted alkenes 6 with dimethyl a-peroxy lactone (7) revealed, however, much complexity, as illustrated in Scheme 7 for R = CH3, since cycloaddition (8), ene-reaction (9 and 10) and epoxidation (11) products were observed. In the presence of methanol, additionally the trapping products 12 and 13 were obtained, at the expense of the polyester 14. The preferred reaction mode is a sensitive function of the steric demand imposed by the attacking alkene nucleophile. 

Employment of the less sterically hindered yttrocene catalyst [(Cp )2YMe]2 or the more reactive zwitterionic zirconocene catalyst Cp 2ZrMe(/x-Me)B(C6E5)3 allowed cascade cyclization/hydrosilylation of trienes that possessed one or more 1,1-disubstituted alkene. As examples, reaction of 2-(3-butenyl)-l,6-hexadiene and phenylsilane catalyzed by [(Gp )2YMe]2 gave silylated spirocycle 74 in 88% yield. Likewise, the reaction of the dialkenyl alkylidene cyclopentane 75 gave silylated propellane 76 in good yield (Equations (50) and (51)). 

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