Next, we explored the structural diversity of quinolines-bearing substituents directly on the reducible ring (Scheme 2). Simple and methyl substituents at the 2-,3-, and 4-position of quinoline gave products 26, 27, 28, and 29 in good to high yields, respectively. Interestingly, bromo and chloro substitution on the 3-position of quinoline ring were tolerated under standard procedure, providing the corresponding products in high yields (30 and 31), which can be used for further cross-coupling reactions involving oxidative addition of an aryl-X (X=Cl, Br). Sensitive functional groups such as bromo and chloro (30 and 31) were untouched under our optimized conditions, showing notable significance in organic synthesis, and such compounds have scarcely been documented. Alcohol derivatives such as 2-quinolinylmethanol and 3-quinolinylmethanol were amenable to this transformation and afforded the quinoline-reduced product 32 and 33 in good yields. It is noteworthy that no hydrodeoxygenation products were observed, underscoring the high selectivity. Even free NH2-group on quinoline ring is tolerated and gave 1,2,3,4-tetrahydroquinolin-3-amine (34) in high yield without any observed byproduct. Encouragingly, quinolines featuring 3-phenyl, and 4-phenyl groups turned out to be effective substrates, being transformed into the targeted tetrahydroquinoline products in good to high yields (35-36). Selective saturation of the N-heteroarene ring in complex molecules can quickly influence the structural and functional diversities. In this respect, several nitrogen-based complex molecules were demonstrated under standard conditions. For instance, 3-(p-tolyl)quinoline, 4-(quinolin-3-yl)phenol, and 3-(4-methoxy-phenyl)quinoline were cleanly reduced into the corresponding N-saturated products in good yields with titanium catalysis (37-39) while retaining the aromaticity of other phenyl ring systems. Although organosilicon compounds constitute useful synthetic precursors in Hiyama couplings and Brook rearrangement reactions, the reduction of silyl-substituted substrates tends to be more challenging due to competing desilylation reactions. Our optimal conditions enable selective quinoline reduction of 3-(4-(trimethylsilyl)phenyl)quinoline smoothly and provide a yield of 75% (40). In this substrate, no desilylation occurred. The incorporation of 4-trifluoromethylphenyl and 4-fluorophenyl substituents into the 3-position of quinoline has no influence on the reduction of N-heterocycle ring; 3-(4-(trifluoromethyl)phenyl)-1,2,3,4-tetrahydroquinoline and 3-(4-fluorophenyl)-1,2,3,4-tetrahydroquinoline congeners (43 and 44) were obtained in excellent yields, which are useful compounds in drug discovery. The chemoselective reduction of N-heterocycles in the presence of a carbon-carbon double bond constitutes an additional challenge in organic synthesis. Thus, we explored the selective reduction of quinolines comprising double bonds. A series of styryl-N-heteroarenes bearing methyl, methoxy, fluoro, and -CF3 were subjected to this procedure, and exclusively the corresponding quinoline-reduced products 45-47 were obtained and isolated in high yields (75-82%) without further double bond reduction, which revealed that a variety of styryl-N-heteroarenes could operate as substrates for our chemoselective N-heteroarene reduction reaction. Despite varying the reaction conditions, when using 3-(4-(trimethylsilyl)phenethyl)quinoline, both double bond and quinoline ring were synchronously reduced and afforded the quinoline-reduced product 48 in good yield. We believe the reduction approach of complex molecules, as well as access to such types of functionalized tetrahydroquinolinebased products, will provide an excellent platform for generating structural analogs to meet the demand for optimizing drug activities. Although there are several catalysts available for the hydrogenation of N-heterocycles, sulfur-containing quinolines are still a formidable challenge because sulfur atoms are capable of binding to metal complexes and may poison catalysts. Very recently, glorius and coworkers made a significant contribution in this area and reported a Rubased heterogeneous catalyst for the hydrogenation of sulfur-containing quinolines with molecular hydrogen as a reductant. However, still, a few molecules with sulfur substituents were not undertaken. In this respect, we subjected five challenging sulfur-containing quinolines with Ti catalyst under the standard reaction conditions that are scarcely reported in the literature. To our delight, the incorporation of 2-methylthiophenyl, 4-methylthiophenyl, and 4-methylsulfonylphenyl substituents on the 3-position of quinoline did not diminish the reaction yield and furnished the corresponding sulfur-containing 1,2,3,4-tetrahydroquinoline in high yields (49-51). It is worth noting that compound 51 is used for the medical treatment of ferroptosis inhibitory. Similar results were obtained with 4-methylthiophenyl and thiomethyl substituents on the 6-position of quinoline to deliver the targeted products 52 and 53 in high yields, and the success of these examples highlights the Ti catalyst's remarkable sulfur resistance in organic synthesis. Tetrahydroquinoxaline skeletons are present in the structures of various biologically relevant compounds. Utilizing our catalytic protocol, three quinoxaline substrates were reduced with full conversion and yielded the desired products 54, 55, and 56 in very good yields. In addition, unprotected simple indole and its derivatives bearing methyl, bromo, fluoro, trifluoromethyl, trifluoromethoxy, and methylsulfonyl functional groups were selectively transformed to the corresponding indolines in excellent yields under standard reaction conditions (57-64). This provides a substantial impact on reduction chemistry, as indolines are widely used in both organic synthesis and drug discovery. The selective reduction of functionalized benzofurans to the corresponding dihydrobenzofuran derivatives is very attractive and remains a challenge. Under the standard conditions, benzofuran derivatives, including 2,5-methylfuran and 2-furanmethanol, were also able to reduce very selectively and give access to the corresponding dihydrobenzofurans products in good to high yields (65-70). Late-stage saturation of medicinally relevant compounds without disturbing the core structure is a powerful method to quickly generate new analogues. In this regard, we applied our titanium catalyst for the quinoline reduction of some selected bioactive molecules (Scheme 2) Firstly, we have synthesized 2-methyl-N-phenyl-1,2,3,4-tetrahydroquinolin-4-amine (71) in 44% isolated yield from 2-methyl-N-phenylquinolin-4-amine as a starting material. Interestingly, compound 71 has significant applications in medicinal chemistry. Similarly, an intermediate of galipeine (72) was also proved successful, leading to their corresponding reduced derivative with good yield. Remarkably, we also prepared α-(4-piperidyl)benzhydrol in 53% isolated yield, which serves as a crucial building block for the manufacture of terfenadine (73). N-heteroarenes, such as hybrid tetrahydroquinoline derivatives with phosphorated functionality, possess antileishmanial activity, and a multi-component approach is essential to prepare such compounds. By keeping this aspect in mind, we attempted diphenyl(4-(quinolin-3yl)phenyl)phosphine oxide for this transformation, and selectively quinoline ring was reduced in order to generate the corresponding product (74) in high yields while preserving the aromaticity of other phenyl ring systems. These results clearly indicated the wide generality of our Ti catalytic system to afford structurally diverse saturated N-heterocycles, thereby making important contributions to the field of medicinal chemistry and drug discovery Scheme 2. Substrate scope for the reduction of functionalized quinolines under optimized conditions. Reaction conditions: 0.5 mmol of quinoline, 3 equiv. NH3•BH3, 10 mol % Cp2TiCl2, 1.0 equiv of NaOMe, 3 mL of dry THF in 21 mL sealed tube and stirred at 120 °C for 24 h, isolated yields.