Intramolecular allylboration reactions

Among the possible intramolecular allylmetallation reactions the intermolecular allylstannation (2) has been most widely studied by the group of Y. Yamamoto. (/) These reactions can be carried out under Lewis acid catalysis to proceed via transition states of the type 3, thermally via transition states of the type 4, and under Broensted acid catalysis, a reaction for which the nature of the transition state is not yet clear. In these reactions high to very high stereoselectivity has been attained. But there is not yet a uniform solution to reach predictable stereoselectivity in terms of attaining each of a number of possible stereoisomers selectively. It is for this reason that we wanted to evaluate the scope of the intramolecular allylboration reaction as a route to substituted tetrahydropyrans and piperidines. 

These results augured well for applying the intramolecular allylboration reaction to the synthesis of multisubstituted tetrahydropyrans and piperidines. 

Scheme 5 Asymmetric induction in intramolecular allylboration reactions...

Liberation of the aldehyde from 31 was followed by cyclization to give 32 as a single stereoisomer in up to 60% yield. However, the yields were compromized by partial decomposition of 31 due to the higher acid lability of the y-oxyallylboronate moiety. This induced us to develop an alternate route, in which the allylboronate moiety was generated under in situ protection of the aldehyde function. To this end, the N-methoxy-methyl amide was added to the aldehyde 33 followed by metallation with s-butyllithium. Upon addition of the mixed borate ester the intermediate 34 was formed. Hydrolysis of the reaction mixture with a pH7-buffer solution liberated both the aldehyde and the allylboronate function which immediately underwent the intramolecular allylboration reaction. This resulted in a 74% overall yield of the cyclized product 32. (14)... 

Kruger, J. and Hoffmann, R.W. (1997) Substituted oxocanes by intramolecular allylboration reactions. Entry to an efficient synthesis of (-l-)-laurencin.y. Am. Chem. Soc., 119, 7499-7504. 

Finally, several examples of intramolecular allylboration have been reported 51,92 -95. These reactions are highly stereoselective and afford only a single diastereomer in each case. 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

In a similar fashion, allylboronates can be used as allylation reagents under hydroformylation conditions. Thus condensed 1,5-oxazadecalin systems are achieved via tandem hydroformylation/allylboration/hydroformylation sequences starting from an N-allyl-y-amidoallylboronate (Scheme 23) [77,78]. The aldehyde obtained from a regioselective hydroformylation undergoes diastereoselective intramolecular allylboration to give an intermediate al-lylic alcohol derivative. The reaction does not stop at this stage, since this... 

Hydroformylation reactions have been shown to be amenable to use in tandem or domino reaction sequences. In one elegant example, alkene 36 was subjected to rho-dium(I)-catalyzed hydroformylation, and the resulting aldehyde underwent smooth intramolecular allylboration (Scheme 5.14) [19]. This produced a new terminal alkene which underwent a second hydroformylation to provide, after workup,lactols 37 in 80% yield and with excellent diastereoselectivity. 

Intramolecular Allylboration. In one rare but impressive example involving a masked aldehyde, a domino hydroformylation/allylboration/hydroformyla-tion reaction cascade has been designed to generate bicyclic annulated... 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

The reaction tolerated various functional groups, thus allowing the in situ preparation of allylboron compounds possessing a carbonyl group (Equation (32)).236 The tandem diboration-intramolecular allylboration provided a diastereoselective access to the cycloalkanes bearing 1,3-diols. 

Selected allyl boronates synthesized according to Equation (57) are shown in Scheme 18. The reaction provided a convenient access to allylboranes possessing a carbonyl group 168 and 169,310,311 which underwent in situ intramolecular allylboration. A variety of 5-5, 6-5, and 7-5 m-fused exomethylene cyclopentanols were synthesized... 

Abstract Intramolecular allylboration of a>oxo-2-alkenylboronates allows the stereoselective formation of 2-vinyl-cycloalkanols. When heteroatoms are present in the chain linking the aldehyde and allylboronate functions a variety of saturated heterocycles with a stereodefined pattern of substituents may be prepared in this manner. Rapid assembly of anellated heterocycles becomes possible by domino-hydroformylation-allylboration-hydroformylalion cascade reactions. 

The thermal reaction of dipropynylsilanes with AII3B also involves 1,1- and 1,2-allylboration of the triple bond (or vice versa) (Scheme 50). l-Sila-3-bora-2-ethylidene-4-cyclopentene derivatives 119 are formed quantitatively at room temperature. They undergo an intramolecular 1,2-allylboration on heating for 3h to give the corresponding bicyclic compounds 120 quantitatively <2002JOM(649)232>. 

Wrackmeyer et al. obtained ( )-3,4-diallyl-2-(but-3-enylidene)-l,l,5-trimethyl-2,3-dihydro-l/7-l,3-silaborole 28 selectively by the reaction of dimethyldi(prop-l-ynyl)silane 63 with triallylborane in CDCI3 at room temperature for several days (Equation 7) <2002JOM146>. The reaction was monitored by H and Si NMR spectroscopy and is believed to take place via an intermolecular 1,1-allylboration followed by an intramolecular 1,2-allylboration. 

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