Hydroamination titanium catalyst

Fischer indolization. Odom and coworkers performed this chemistry with 1,1-disubstituted hydrazines a selection is shown in Table 4 (entries 1-4) [94, 95]. In a similar approach to aryl hydrazones and indoles, Seller and colleagues anployed a titanium catalyst to hydroaminate terminal alkynes leading to indoles (Table 4, entries 5-7) [96-98]. Seller s protocol is particularly valuable for the synthesis of tryptamines, tryptophols, and their homo-logues. Interestingly, Seller found that 1-alkyl-l-phenylhy-drazines react with acetylenedicarboxylates in the absence of titanium to give the corresponding indole-2,3-dicarbox-ylates and 2-arylindole-3-carboxylates after treatment with zinc chloride [99]. This reaction was apparently first discovered by Diels and Reese in 1935 [100] and by others subsequently [101, 102]. Some examples of Seller s work are shown in Scheme 11 (equations 1-3) [99]. 

Carbon-nitrogen bond formation is an important subject in the organic synthesis [301], and hydroamination is an atom-efficient process for the generation of amines and imines from olefins, allenes, and alkynes. Titanium-mediated hydroamination was among the most useful protocols thus far developed for this reaction. By using unsymmetrical olefins and alkynes, the addition of HNR2 can in principle lead to two isomeric products, where the isomeric ratio is usually dependent on the type of titanium catalyst used. 

Reactions of unsymmetric internal alkynes are more challenging, since two hydroamination products can be formed. The feasibility to control regioselectivity depends on the steric properties of both substrate and catalyst and a universal regioselective catalyst remains to be elaborated. When anilines are employed as reactants, high a t/-Markovnikov selectivity is obtained with titanocene catalysts 47 and 48 (Table 11) [182, 183] while aliphatic amines gave poor results. Again, the bis(indenyl)titanium catalyst 49 showed superior a t/-Markovnikov selectivity... 

Although hydroamination of allenes can be easily achieved with group 4 and group 5 metal catalysts, the stereoselectivity of these systems is rather limited. Several attempts to perform asymmetric hydroamination/cyclization of aminoallenes employing chiral aminoalcohols [260, 261] and sulfonamide alcohols [262] as chiral proligands for titanium- and tantalum-based catalyst systems have produced vinyl pyrrolidines with low selectivities only. While the titanium catalysts were... 

The formation of a bis(guanidinate)-supported titanium imido complex has been achieved in different ways, two of which are illustrated in Scheme 90. The product is an effective catalyst for the hydroamination of alkynes (cf. Section V.B). It also undergoes clean exchange reactions with other aromatic amines to afford new imide complexes such as [Me2NC(NPr )2]2Ti = NC6F5. ... 

The guanidinate-supported titanium imido complex [Me2NC(NPr02l2Ti = NAr (Ar = 2,6-Me2C6H3) (cf. Section IILB.2) was reported to be an effective catalyst for the hydroamination of alkynes. The catalytic activity of bulky amidinato bis(alkyl) complexes of scandium and yttrium (cf. Section III.B.l) in the intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentenylamine has been investigated and compared to the activity of the corresponding cationic mono(alkyl) derivatives. 

The hydroamination of alkynes is an efficient way to obtain aldimines with the advantage of avoiding formation of by-products. As shown in Scheme 8.65, the method has been developed into a multicomponent synthesis of a-branched amines. Aldimines 154 are formed using a titanium derivative as catalyst and reacted in situ with an organolithium reagent [141]. 

Hydroamination reactions involving alkynes and enantiomerically pure chiral amines can produce novel chiral amine moieties after single pot reduction of the Schiffbase intermediate 82 (Scheme 11.27) [123]. Unfortunately, partial racemiza tion ofthe amine stereocenter was observed with many titanium based hydroamina tion catalysts, even in the absence of an alkyne substrate. No racemization was observed when the sterically hindered Cp 2TiMe2 or the constrained geometry catalyst Me2Si(C5Me4)(tBuN)Ti(NMe2)2 was used in the catalytic reaction. Also, the addition of pyridine suppressed the racemization mostly. 

To this end, a titanium bis(2-pyridonate) complex (17) has been developed. [22c] These catalytic systems promote catalyst-controlled selectivity for hydroaminoalkylation over hydroamination and for the first time amine-substituted cyclopentanes can be prepared preferentially over piperidine hydroamination products (Scheme 25). 

Hydroamination reactions of alkynes provide an alternate route to arylhydrazones that can be utilized in the Fischer indole synthesis. Treatment of arylhydrazine 99 with alkyne 100 in the presence of catalyst system comprised of titanium tetrachloride and /-butylamine afforded arylhydrazone intermediate 101 which underwent a Fischer cyclization to give 1,2,3-trisubstituted indole 102 as a single regioisomer <04TL9541>. A similar titanium-catalyzed hydroamination reaction was utilized to prepare tryptamine derivatives <04TL3123>. 

Titanium-crafted catalysts have been employed by several groups to effect hydroamination of alkynes with hydrazines to afford hydrazones en route to indoles via... 

Schafer found that the bulky bis(amidate) complex is an effective catalyst for intermolecular hydroamination of terminal alkyl alkynes with alkylamines, giving exclusively the anti-Markovnikov aldimine product [309]. The same titanium complexes can also be utilized in the hydroamination of substituted allenes in good yields (Scheme 14.132). Under the catalysis of an imidotitanium complex, the highly strained methylenecyclopropane can undergo hydroamination reaction with either aromatic or aliphatic amines, to give ring-opened imine products in good to excellent yields and chemoselectivities [310]. 

A catalytic asymmetric intramolecular hydroamination of aminoallenes has been carried out in the presence of titanium complexes prepared by the in situ reaction of Ti(NMe3)4 with chiral amino alcohols [319]. The ring-closing reaction of hepta-4,5-dienylamine at 110 °C with 5 mol% catalyst gives a mixture of 6-ethyl-2,3,4,5-tetrahydropyridine (14-33%) and both Z- and T-2-propenylpyrroli-dine (67-86%), whereas the same reaction with 6-methylhepta-4,5-dienylamine under similar conditions gives exclusively 2-(2-methylpropenyl)pyrrolidine with modest enantiomeric excess values (<16%) (Scheme 14.139). 

In recent years, synthesis of pyrroles has drawn the attention of chemists. Traditional methods used for pyrrole synthesis include the Hantzsch reaction [45] and the Paal-Knorr condensation reaction [46,47], The latter is the most widely used method, which involves the cyclocondensation reaction of 1,4-dicarbonyl compounds with primary amines to produce substituted pyrroles. In addition, there are several methods such as 1,3-dipolar cydoaddition reaction, aza-Wittig reaction, reductive coupling, and titanium-catalyzed hydroamination of diynes. Scheme 1 shows several catalysts used in this type of reaction [44]. 

As discussed in Sect. 5, the intermolecular hydroamination of alkynes catalyzed by group 4 metal complexes is a well-documented process. The less challenging intramolecular transformation can be achieved efficiently with various titanium-based catalysts [51, 125-130]. The cyclization proceeds analogously to the rare earth metal-catalyzed process with exclusive ej o-selectivity and often requires elevated temperatures. However, the homoleptic titanium tetraamide Ti(NMe2)4 catalyzes the cyclization of both terminal and internal aminoalkynes at room temperature (7) [126, 127]. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

The increased Markovnikov selectivity in the hydroamination of aliphatic terminal alkynes with aniline derivatives seems to be universal for a number of titanium-based hydroamination catalysts, such as Ind2TiMe2 (49) [184], the di-(pyrrolyl) amine complex 50 [186, 187], and the di(pyrrolyl)methane complex 51 [188]. The bis(amidate) titanium complex 43 exhibited enhanced catalytic activity compared to titanocene catalysts, thus combining high a ri-Markovnikov selectivity with high catalytic activity [191]. 

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