Dienes boron-substituted

Due to their great versatility, boron-substituted dienophiles and dienes have emerged as attractive building blocks in cycloaddition reactions. The early works in the field have been reviewed.1... 

IntermolecularDiels-Alder Reactions 4.1 J.4 Silicon-, Tin- and Boron-substituted Dienes... 

L. Eberlin, F. Tripoteau, F. Carreaux, A. Whiting, B. Carboni, Beilstein J. Org. Chem. 2014,10, 237-250. Boron-substituted 1,3-dienes and heterodienes as key elements in multieompo-nent processes. 

Wolfe and Wagener have developed main-chain boronate polymers (59) (Fig. 38) by the acyclic diene metathesis (ADMET) polymerization of symmetrical ,oj-dienes, containing both methyl- and phenyl-substituted boronate functionalities using Mo and Ru catalysts.84 The ring-opening metathesis polymerization (ROMP) of several norbornene monomers containing methyl- and phenyl-substituted boronates into... 

The cobalt(I) complex CoBr(PPh3)3 as a boron trifluoride etherate selectively hydrogenates conjugated dienes to monoenes via an unusual 1,2-hydrogen addition at the more-substituted double bond (186). 

Regents which are active upon irradiation with benzene are olefins and dienes None of these react photochemically with borazine. Recently, the photolysis of benzene at X = 184.9 run with D2 has been shown to produce a small amount of CgHsD (4> = 0.01) This is contrast to the very efficient deuteration at the boron site of borazine (4 = 0.90). Hexafluoroacetone as the absorbing species reacts with borazine to produce a B-alkoxyborazine substitution product. Reaction of this reagent with benzene, on the other hand, involves the CF3 radicals and the products are addition rather than substitution products ss)... 

Reaction of pentafluoro-substituted benzene derivatives with xenon difluoride/boron trifluoride results in 1,4- and 1,2-fluorine addition with regiospecificity depending on the substituent.41 For example, the heptafluorocyclohexa-1,4-dienes 1 can be obtained in this way in high yield. 

Germyl or silyl derivatives of pentafluoro-substituted benzenes react in the same manner with boron trifluoride-diethyl ether complex without cleavage of the C — Ge or C — Si bonds to yield the 1-substituted heptafluorocyclohexa-1,4-dienes l.43... 

The chiral piperidinones 46 formed by the reaction between Danishefsky s diene and the glycosyl imines are valuable synthons for the synthesis of higher substituted piperidine derivatives. 2,6-Disubstituted piperidinones 49 are obtained by addition of organocuprates complexes with boron trifluoride [53]. The reaction pathway is illustrated in Scheme 30. 

Asymmetric Diels-Alder Reaction of Unsaturated Aldehydes . The boron atom of acyloxyborane is activated by the electron-withdrawing acyloxy groups, and consequently acyloxyborane derivatives are sufficiently Lewis acidic to catalyze certain reactions. Thus, asymmetric Diels-Alder reactions of a,p-enals with dienes using (1) as a Lewis acid catalyst have been developed. For example, the reaction of cyclopentadiene and methacrolein gives the adduct in 85% yield (endo exo= 11 89) and 96% ee (major exo isomer) (eq 3). Some additional examples are listed in Figure 1. The a-substituent on the dienophile increases the enantioselectivity, while p-substitution dramatically decreases the selectivity. In the case of a substrate having substituents in both a- and p-positions, high enantioselectivity is observed thus the a-substituent effect overcomes that of the p-substituent. 

The process is quite general for simple dienes and aldehydes. For example, the reaction of acrolein with cyclopentadiene, cyclohexadiene, or 2,3-dimethyl-l,3-butadiene gives cycloadducts with 8(F-84 % ee and exolendo = 12/88-< 1/99. The a-substituent on the dienophile increases the enantioselectivity (acrolein compared with methacro-lein). When there is /3-substitution in the dienophile, as in crotonaldehyde, the cycloadduct is almost racemic. On the other hand, for a substrate with substituents at both a and ji positions, high ee is observed, as for 2-methylcrotonaldehyde and cyclopentadiene (90 % ee, exolendo = 97/3). The active boron catalyst is beheved to have the structure shown in Eq. (8), with a five-membered ring and a free carboxyl group. The latter seems not to be crucial for the enantioselectivity because eomparable results are obtained when the carboxylic group is transformed into an ester. 

BLA 4 (Fig. 5) is one of the best catalysts for the enantio- and earo-selective cycloaddition of a-substituted a,/3-enals with highly reactive dienes such as cyclopen-tadiene. The corresponding reactions of a-unsubstituted a,)8-enals such as acrolein and crotonaldehyde are, however, characterized by low enantioselectivity and/or reactivity. The range of dienophiles that can be used with less reactive dienes is quite limited. The use of arylboronic acid with electron-withdrawing substituents such as [3,5-bis(trifluoromethyl)phenyl]boronic acid in the preparation of BLA greatly enhances... 

The most important interaction in hydroboration is between the HOMO of the alkene (here i diene) and the empty orbital (LUMO) on boron (p. 1279). Here the ends of the diene have th larger coefficients. The reaction occurs on the opposite face of the diene to the one occupied by tbt substituent, especially as it must occur next to that substituent. In the oxidation, the less substitute group migrates with retention. 

R = H) [26]. However, the turnover number was very limited (about 100). The strong incentive for further developments lies in the fact that both HCN and acetylene are cheap bulk chemicals in industry. The introduction of boron into the carbocyclic ligand attached to the cobalt enhances the catalytic activity considerably, probably via the suppression of the protolytic 1,4-addition of HCN to the olefinic cobaltacycle the resulting cyaino-substituted 1,3- dienes cannot be displaced from the cobalt center by acetylene and the catalytic cycle is stopped (eq. (4)). 

Similar reactions of norbornadiene with substituted buta-1,3-dienes in the presence of cobalt catalysts lead to the corresponding 1,4-adducts in 92-96% yield.In the presence of chiral phosphane ligands, this reaction has been brought about with up to 79% enantiomeric excess. The head-to-head dimer of norbornadiene, 1,2,4 5,6,8-dimethano-5-indacene (3, Binor-S ) was formed in quantitative yield on dimerizing norbornadiene with catalysts such as co-balt(II) bromide/triphenylphosphane, cobalt(ll) iodide/triphenylphosphane, or (triphenylphos-phanejrhodium chloride, in the presence of boron trifluoride-diethyl ether complex. ... 

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